1
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Siegbahn PEM, Wei WJ. The energetics of N 2 reduction by vanadium containing nitrogenase. Phys Chem Chem Phys 2024; 26:1684-1695. [PMID: 38126534 DOI: 10.1039/d3cp04698b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2023]
Abstract
The main class of nitrogenases has a molybdenum in its cofactor. A mechanism for Mo-nitrogenase has recently been described. In the present study, another class of nitrogenases has been studied, the one with a vanadium instead of a molybdenum in its cofactor. It is generally believed that these classes use the same general mechanism to activate nitrogen. The same methodology has been used here as the one used for Mo-nitrogenase. N2 activation is known to occur after four reductions in the catalytic cycle, in the E4 state. The main features of the mechanism for Mo-nitrogenase found in the previous study are an activation process in four steps prior to catalysis, the release of a sulfide during the activation steps and the formation of H2 from two hydrides in E4, just before N2 is activated. The same features have been found here for V-nitrogenase. A difference is that five steps are needed in the activation process, which explains why the ground state of V-nitrogenase is a triplet (even number) and the one for Mo-nitrogenase is a quartet (odd number). The reason an additional step is needed for V-nitrogenase is that V3+ can be reduced to V2+, in contrast to the case for Mo3+ in Mo-nitrogenase. The fact that V3+ is Jahn-Teller active has important consequences. N2H2 is formed in E4 with reasonably small barriers.
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Affiliation(s)
- Per E M Siegbahn
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden.
| | - Wen-Jie Wei
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden.
- Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
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2
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Jiang H, Ryde U. H 2 formation from the E 2-E 4 states of nitrogenase. Phys Chem Chem Phys 2024; 26:1364-1375. [PMID: 38108422 DOI: 10.1039/d3cp05181a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2023]
Abstract
Nitrogenase is the only enzyme that can cleave the strong triple bond in N2, making nitrogen available for biological lifeforms. The active site is a MoFe7S9C cluster (the FeMo cluster) that binds eight electrons and protons during one catalytic cycle, giving rise to eight intermediate states E0-E7. It is experimentally known that N2 binds to the E4 state and that H2 is a compulsory byproduct of the reaction. However, formation of H2 is also an unproductive side reaction that should be avoided, especially in the early steps of the reaction mechanism (E2 and E3). Here, we study the formation of H2 for various structural interpretations of the E2-E4 states using combined quantum mechanical and molecular mechanical (QM/MM) calculations and four different density-functional theory methods. We find large differences in the predictions of the different methods. B3LYP strongly favours protonation of the central carbide ion and H2 cannot form from such structures. On the other hand, with TPSS, r2SCAN and TPSSh, H2 formation is strongly exothermic for all structures and En and therefore need strict kinetic control to be avoided. For the E2 state, the kinetic barriers for the low-energy structures are high enough to avoid H2 formation. However, for both the E3 and E4 states, all three methods predict that the best structure has two hydride ions bridging the same pair of Fe ions (Fe2 and Fe6) and these two ions can combine to form H2 with an activation barrier of only 29-57 kJ mol-1, corresponding to rates of 7 × 102 to 5 × 107 s-1, i.e. much faster than the turnover rate of the enzyme (1-5 s-1). We have also studied H-atom movements within the FeMo cluster, showing that the various protonation states can quite freely be interconverted (activation barriers of 12-69 kJ mol-1).
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Affiliation(s)
- Hao Jiang
- Department of Computational Chemistry, Lund University, Chemical Centre, P. O. Box 124, SE-221 00 Lund, Sweden.
| | - Ulf Ryde
- Department of Computational Chemistry, Lund University, Chemical Centre, P. O. Box 124, SE-221 00 Lund, Sweden.
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3
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Einsle O. On the Shoulders of Giants-Reaching for Nitrogenase. Molecules 2023; 28:7959. [PMID: 38138449 PMCID: PMC10745432 DOI: 10.3390/molecules28247959] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2023] [Revised: 11/14/2023] [Accepted: 11/27/2023] [Indexed: 12/24/2023] Open
Abstract
Only a single enzyme system-nitrogenase-carries out the conversion of atmospheric N2 into bioavailable ammonium, an essential prerequisite for all organismic life. The reduction of this inert substrate at ambient conditions poses unique catalytic challenges that strain our mechanistic understanding even after decades of intense research. Structural biology has added its part to this greater tapestry, and in this review, I provide a personal (and highly biased) summary of the parts of the story to which I had the privilege to contribute. It focuses on the crystallographic analysis of the three isoforms of nitrogenases at high resolution and the binding of ligands and inhibitors to the active-site cofactors of the enzyme. In conjunction with the wealth of available biochemical, biophysical, and spectroscopic data on the protein, this has led us to a mechanistic hypothesis based on an elementary mechanism of repetitive hydride formation and insertion.
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Affiliation(s)
- Oliver Einsle
- Institute of Biochemistry, Albert-Ludwigs-Universität Freiburg, Albertstrasse 21, 79104 Freiburg im Breisgau, Germany
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4
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Jiang H, Lundgren KJM, Ryde U. Protonation of Homocitrate and the E 1 State of Fe-Nitrogenase Studied by QM/MM Calculations. Inorg Chem 2023; 62:19433-19445. [PMID: 37987624 DOI: 10.1021/acs.inorgchem.3c02329] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2023]
Abstract
Nitrogenase is the only enzyme that can cleave the strong triple bond in N2, making nitrogen available for biological life. There are three isozymes of nitrogenase, differing in the composition of the active site, viz., Mo, V, and Fe-nitrogenase. Recently, the first crystal structure of Fe-nitrogenase was presented. We have performed the first combined quantum mechanical and molecular mechanical (QM/MM) study of Fe-nitrogenase. We show with QM/MM and quantum-refinement calculations that the homocitrate ligand is most likely protonated on the alcohol oxygen in the resting E0 state. The most stable broken-symmetry (BS) states are the same as for Mo-nitrogenase, i.e., the three Noodleman BS7-type states (with a surplus of β spin on the eighth Fe ion), which maximize the number of nearby antiferromagnetically coupled Fe-Fe pairs. For the E1 state, we find that protonation of the S2B μ2 belt sulfide ion is most favorable, 14-117 kJ/mol more stable than structures with a Fe-bound hydride ion (the best has a hydride ion on the Fe2 ion) calculated with four different density-functional theory methods. This is similar to what was found for Mo-nitrogenase, but it does not explain the recent EPR observation that the E1 state of Fe-nitrogenase should contain a photolyzable hydride ion. For the E1 state, many BS states are close in energy, and the preferred BS state differs depending on the position of the extra proton and which density functional is used.
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Affiliation(s)
- Hao Jiang
- Department of Computational Chemistry, Lund University, Chemical Centre, P.O. Box 124, Lund SE-221 00, Sweden
| | - Kristoffer J M Lundgren
- Department of Computational Chemistry, Lund University, Chemical Centre, P.O. Box 124, Lund SE-221 00, Sweden
| | - Ulf Ryde
- Department of Computational Chemistry, Lund University, Chemical Centre, P.O. Box 124, Lund SE-221 00, Sweden
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5
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Zhai H, Lee S, Cui ZH, Cao L, Ryde U, Chan GKL. Multireference Protonation Energetics of a Dimeric Model of Nitrogenase Iron-Sulfur Clusters. J Phys Chem A 2023; 127:9974-9984. [PMID: 37967028 PMCID: PMC10694817 DOI: 10.1021/acs.jpca.3c06142] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2023] [Revised: 10/31/2023] [Accepted: 11/01/2023] [Indexed: 11/17/2023]
Abstract
Characterizing the electronic structure of the iron-sulfur clusters in nitrogenase is necessary to understand their role in the nitrogen fixation process. One challenging task is to determine the protonation state of the intermediates in the nitrogen fixing cycle. Here, we use a dimeric iron-sulfur model to study relative energies of protonation at C, S, or Fe. Using a composite method based on coupled cluster and density matrix renormalization group energetics, we converge the relative energies of four protonated configurations with respect to basis set and correlation level. We find that accurate relative energies require large basis sets as well as a proper treatment of multireference and relativistic effects. We have also tested ten density functional approximations for these systems. Most of them give large errors in their relative energies. The best performing functional in this system is B3LYP, which gives mean absolute and maximum deviations of only 10 and 13 kJ/mol with respect to our correlated wave function estimates, respectively, comparable to the uncertainty in our correlated estimates. Our work provides benchmark results for the calibration of new approximate electronic structure methods and density functionals for these problems.
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Affiliation(s)
- Huanchen Zhai
- Division
of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
| | - Seunghoon Lee
- Division
of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
| | - Zhi-Hao Cui
- Division
of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
| | - Lili Cao
- Department
of Theoretical Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Ulf Ryde
- Department
of Theoretical Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Garnet Kin-Lic Chan
- Division
of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
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6
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Pang Y, Bjornsson R. The E3 state of FeMoco: one hydride, two hydrides or dihydrogen? Phys Chem Chem Phys 2023; 25:21020-21036. [PMID: 37522223 DOI: 10.1039/d3cp01106b] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/01/2023]
Abstract
Hydrides are present in the reduced states of the iron-molybdenum cofactor (FeMoco) of Mo nitrogenase and are believed to play a key mechanistic role in the dinitrogen reduction reaction catalyzed by the enzyme. Two hydrides are present in the E4 state according to 1H ENDOR and there is likely a single hydride in the E2 redox state. The 2-hydride E4 state has been experimentally observed to bind N2 and it has been speculated that E3 may bind N2 as well. However, the E3 state has not been directly observed and very little is known about its molecular and electronic structure or reactivity. In recent computational studies, we have explored the energy surfaces of the E2 and E4 by QM/MM modelling, and found that the most stable hydride isomers contain bridging or partially bridging hydrides with an open protonated belt sulfide-bridge. In this work we systematically explore the energy surface of the E3 redox state, comparing single hydride and two-hydride isomers with varying coordination and bridging vs. terminal sulfhydryl groups. We also include a model featuring a triply protonated carbide. The results are only mildly dependent on the QM-region size. The three most stable E3 isomers at the r2SCAN level of theory have in common: an open belt sulfide-bridge (terminal sulfhydryl group on Fe6) and either 2 bridging hydrides (between Fe2 and Fe6), 1 bridging-1-terminal hydride (around Fe2 and Fe6) or a dihydrogen ligand bound at the Fe2 site. Analyzing the functional dependency of the results, we find that functionals previously found to predict accurate structures of spin-coupled Fe/Mo dimers and FeMoco (TPSSh, B97-D3, r2SCAN, and B3LYP*) are in generally good agreement about the stability of these 3 E3 isomers. However, B3LYP*, similar to its parent B3LYP method, predicts a triply protonated carbide isomer as the most stable isomer, an unlikely scenario in view of the lack of experimental evidence for carbide protonation occurring in reduced FeMoco states. Distinguishing further between the 3 hydride isomers is difficult and this flexible coordination nature of hydrides suggests that multiple hydride isomers could be present during experimental conditions. N2 binding was explored and resulted in geometries with 2 bridging hydrides and N2 bound to either Fe2 or Fe6 with a local low-spin state on the Fe. N2 binding is predicted to be mildly endothermic, similar to the E2 state, and it seems unlikely that the E3 state is capable of binding N2.
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Affiliation(s)
- Yunjie Pang
- College of Chemistry, Beijing Normal University, 100875, Beijing, China
- Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Ragnar Bjornsson
- Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
- Univ. Grenoble Alpes, CNRS, CEA, IRIG, Laboratoire de Chimie et Biologie des Métaux, 17 Rue des Martyrs, F-38054 Grenoble, Cedex, France.
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7
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Jodts RJ, Wittkop M, Ho MB, Broderick WE, Broderick JB, Hoffman BM, Mosquera MA. Computational Description of Alkylated Iron-Sulfur Organometallic Clusters. J Am Chem Soc 2023; 145:13879-13887. [PMID: 37307050 PMCID: PMC10573082 DOI: 10.1021/jacs.3c03062] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
The radical S-adenosyl methionine (SAM) enzyme superfamily has widespread roles in hydrogen atom abstraction reactions of crucial biological importance. In these enzymes, reductive cleavage of SAM bound to a [4Fe-4S]1+ cluster generates the 5'-deoxyadenosyl radical (5'-dAdo•) which ultimately abstracts an H atom from the substrate. However, overwhelming experimental evidence has surprisingly revealed an obligatory organometallic intermediate Ω exhibiting an Fe-C5'-adenosyl bond, whose properties are the target of this theoretical investigation. We report a readily applied, two-configuration version of broken symmetry DFT, denoted 2C-DFT, designed to allow the accurate description of the hyperfine coupling constants and g-tensors of an alkyl group bound to a multimetallic iron-sulfur cluster. This approach has been validated by the excellent agreement of its results both with those of multiconfigurational complete active space self-consistent field computations for a series of model complexes and with the results from electron nuclear double-resonance/electron paramagnetic resonance spectroscopic studies for the crystallographically characterized complex, M-CH3, a [4Fe-4S] cluster with a Fe-CH3 bond. The likewise excellent agreement between spectroscopic results and 2C-DFT computations for Ω confirm its identity as an organometallic complex with a bond between an Fe of the [4Fe-4S] cluster and C5' of the deoxyadenosyl moiety, as first proposed.
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Affiliation(s)
- Richard J. Jodts
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208
| | - M Wittkop
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
| | - Madeline B. Ho
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208
| | - William E. Broderick
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
| | - Joan B. Broderick
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
| | - Brian M. Hoffman
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208
| | - Martín A. Mosquera
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
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8
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Badding ED, Srisantitham S, Lukoyanov DA, Hoffman BM, Suess DLM. Connecting the geometric and electronic structures of the nitrogenase iron-molybdenum cofactor through site-selective 57Fe labelling. Nat Chem 2023; 15:658-665. [PMID: 36914792 PMCID: PMC10710871 DOI: 10.1038/s41557-023-01154-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2022] [Accepted: 01/26/2023] [Indexed: 03/16/2023]
Abstract
Understanding the chemical bonding in the catalytic cofactor of the Mo nitrogenase (FeMo-co) is foundational for building a mechanistic picture of biological nitrogen fixation. A persistent obstacle towards this goal has been that the 57Fe-based spectroscopic data-although rich with information-combines responses from all seven Fe sites, and it has therefore not been possible to map individual spectroscopic responses to specific sites in the three-dimensional structure. Here we have addressed this challenge by incorporating 57Fe into a single site of FeMo-co. Spectroscopic analysis of the resting state informed on the local electronic structure of the terminal Fe1 site, including its oxidation state and spin orientation, and, in turn, on the spin-coupling scheme for the entire cluster. The oxidized resting state and the first intermediate in nitrogen fixation were also characterized, and comparisons with the resting state provided molecular-level insights into the redox chemistry of FeMo-co.
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Affiliation(s)
- Edward D Badding
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA
| | | | | | - Brian M Hoffman
- Department of Chemistry, Northwestern University, Evanston, IL, USA
| | - Daniel L M Suess
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA.
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9
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Torbjörnsson M, Hagemann MM, Ryde U, Hedegård ED. Histidine oxidation in lytic polysaccharide monooxygenase. J Biol Inorg Chem 2023; 28:317-328. [PMID: 36828975 PMCID: PMC10036459 DOI: 10.1007/s00775-023-01993-4] [Citation(s) in RCA: 9] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2022] [Accepted: 01/10/2023] [Indexed: 02/26/2023]
Abstract
The lytic polysaccharide monooxygenases (LPMOs) comprise a super-family of copper enzymes that boost the depolymerisation of polysaccharides by oxidatively disrupting the glycosidic bonds connecting the sugar units. Industrial use of LPMOs for cellulose depolymerisation has already begun but is still far from reaching its full potential. One issue is that the LPMOs self-oxidise and thereby deactivate. The mechanism of this self-oxidation is unknown, but histidine residues coordinating to the copper atom are the most susceptible. An unusual methyl modification of the NE2 atom in one of the coordinating histidine residues has been proposed to have a protective role. Furthermore, substrate binding is also known to reduce oxidative damage. We here for the first time investigate the mechanism of histidine oxidation with combined quantum and molecular mechanical (QM/MM) calculations, with outset in intermediates previously shown to form from a reaction with peroxide and a reduced LPMO. We show that an intermediate with a [Cu-O]+ moiety is sufficiently potent to oxidise the nearest C-H bond on both histidine residues, but methylation of the NE2 atom of His-1 increases the reaction barrier of this reaction. The substrate further increases the activation barrier. We also investigate a [Cu-OH]2+ intermediate with a deprotonated tyrosine radical. This intermediate was previously proposed to have a protective role, and we also find it to have higher barriers than the corresponding a [Cu-O]+ intermediate.
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Affiliation(s)
- Magne Torbjörnsson
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden
| | - Marlisa M Hagemann
- Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden.
| | - Erik Donovan Hedegård
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden.
- Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark.
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10
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Sato A, Hori Y, Shigeta Y. Characterization of the Geometrical and Electronic Structures of the Active Site and Its Effects on the Surrounding Environment in Reduced High-Potential Iron-Sulfur Proteins Investigated by the Density Functional Theory Approach. Inorg Chem 2023; 62:2040-2048. [PMID: 36695190 DOI: 10.1021/acs.inorgchem.2c03617] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The high-potential iron-sulfur protein (HiPIP) is an electron-transporting protein that functions in the photosynthetic electron-transfer system and possesses a cubane-type [4Fe-4S] cluster in the active center. Characterization of the geometrical and electronic structures of the [4Fe-4S] cluster leads to an understanding of the functions in HiPIP, which are expected to be influenced by the environment surrounding the [4Fe-4S] cluster. This work characterized the geometrical and electronic structures of the [4Fe-4S] cluster in the reduced HiPIP and evaluated their effects on the protein environment using the density functional theory (DFT) approach. DFT calculations showed that the structural asymmetry and spin delocalization between iron atoms allowed for the acquisition of a unique stable geometrical and electronic structure in the open-shell singlet. In addition, the formation of an Fe-Fe bond accompanying the spin delocalization was found to depend on the interatomic distance. A comparison of the calculated stable structures with and without consideration of the amino acids around the [4Fe-4S] cluster demonstrated that the surrounding amino acids stabilized the unique geometrical and electronic structure of the [4Fe-4S] cluster in HiPIP.
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Affiliation(s)
- Ayaka Sato
- Center for Computational Sciences, University of Tsukuba, Ibaraki305-8577, Japan.,Master's Program in Physics, Degree Programs in Pure and Applied Sciences, Graduate School of Science and Technology, University of Tsukuba, Ibaraki305-8577, Japan
| | - Yuta Hori
- Center for Computational Sciences, University of Tsukuba, Ibaraki305-8577, Japan
| | - Yasuteru Shigeta
- Center for Computational Sciences, University of Tsukuba, Ibaraki305-8577, Japan
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11
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Quantum Mechanical Calculations of Redox Potentials of the Metal Clusters in Nitrogenase. MOLECULES (BASEL, SWITZERLAND) 2022; 28:molecules28010065. [PMID: 36615260 PMCID: PMC9822455 DOI: 10.3390/molecules28010065] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/18/2022] [Revised: 12/14/2022] [Accepted: 12/19/2022] [Indexed: 12/24/2022]
Abstract
We have calculated redox potentials of the two metal clusters in Mo-nitrogenase with quantum mechanical (QM) calculations. We employ an approach calibrated for iron-sulfur clusters with 1-4 Fe ions, involving QM-cluster calculations in continuum solvent and large QM systems (400-500 atoms), based on structures from combined QM and molecular mechanics (QM/MM) geometry optimisations. Calculations on the P-cluster show that we can reproduce the experimental redox potentials within 0.33 V. This is similar to the accuracy obtained for the smaller clusters, although two of the redox reactions involve also proton transfer. The calculated P1+/PN redox potential is nearly the same independently of whether P1+ is protonated or deprotonated, explaining why redox titrations do not show any pH dependence. For the FeMo cluster, the calculations clearly show that the formal oxidation state of the cluster in the resting E0 state is MoIIIFe3IIFe4III , in agreement with previous experimental studies and QM calculations. Moreover, the redox potentials of the first five E0-E4 states are nearly constant, as is expected if the electrons are delivered by the same site (the P-cluster). However, the redox potentials are insensitive to the formal oxidation states of the Fe ion (i.e., whether the added protons bind to sulfide or Fe ions). Finally, we show that the later (E4-E8) states of the reaction mechanism have redox potential that are more positive (i.e., more exothermic) than that of the E0/E1 couple.
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12
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Jiang H, Svensson OKG, Ryde U. QM/MM Study of Partial Dissociation of S2B for the E 2 Intermediate of Nitrogenase. Inorg Chem 2022; 61:18067-18076. [DOI: 10.1021/acs.inorgchem.2c02488] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Hao Jiang
- Department of Theoretical Chemistry, Lund University, Chemical Centre, SE-221 00Lund, Sweden
| | - Oskar K. G. Svensson
- Department of Theoretical Chemistry, Lund University, Chemical Centre, SE-221 00Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, SE-221 00Lund, Sweden
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13
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Eschenbach P, Neugebauer J. Subsystem density-functional theory: A reliable tool for spin-density based properties. J Chem Phys 2022; 157:130902. [PMID: 36209003 DOI: 10.1063/5.0103091] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Subsystem density-functional theory compiles a set of features that allow for efficiently calculating properties of very large open-shell radical systems such as organic radical crystals, proteins, or deoxyribonucleic acid stacks. It is computationally less costly than correlated ab initio wave function approaches and can pragmatically avoid the overdelocalization problem of Kohn-Sham density-functional theory without employing hard constraints on the electron-density. Additionally, subsystem density-functional theory calculations commonly start from isolated fragment electron densities, pragmatically preserving a priori specified subsystem spin-patterns throughout the calculation. Methods based on subsystem density-functional theory have seen a rapid development over the past years and have become important tools for describing open-shell properties. In this Perspective, we address open questions and possible developments toward challenging future applications in connection with subsystem density-functional theory for spin-dependent properties.
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Affiliation(s)
- Patrick Eschenbach
- Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and Simulation, Westfälische Wilhelms-Universität Münster, Corrensstraße 36, 48149 Münster, Germany
| | - Johannes Neugebauer
- Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and Simulation, Westfälische Wilhelms-Universität Münster, Corrensstraße 36, 48149 Münster, Germany
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14
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Benediktsson B, Bjornsson R. Analysis of the Geometric and Electronic Structure of Spin-Coupled Iron-Sulfur Dimers with Broken-Symmetry DFT: Implications for FeMoco. J Chem Theory Comput 2022; 18:1437-1457. [PMID: 35167749 PMCID: PMC8908755 DOI: 10.1021/acs.jctc.1c00753] [Citation(s) in RCA: 11] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
![]()
The open-shell electronic
structure of iron–sulfur clusters
presents considerable challenges to quantum chemistry, with the complex
iron–molybdenum cofactor (FeMoco) of nitrogenase representing
perhaps the ultimate challenge for either wavefunction or density
functional theory. While broken-symmetry density functional theory
has seen some success in describing the electronic structure of such
cofactors, there is a large exchange–correlation functional
dependence in calculations that is not fully understood. In this work,
we present a geometric benchmarking test set, FeMoD11, of synthetic
spin-coupled Fe–Fe and Mo–Fe dimers, with relevance
to the molecular and electronic structure of the Mo-nitrogenase FeMo
cofactor. The reference data consists of high-resolution crystal structures
of metal dimer compounds in different oxidation states. Multiple density
functionals are tested on their ability to reproduce the local geometry,
specifically the Fe–Fe/Mo–Fe distance, for both antiferromagnetically
coupled and ferromagnetically coupled dimers via the broken-symmetry
approach. The metal–metal distance is revealed not only to
be highly sensitive to the amount of exact exchange in the functional
but also to the specific exchange and correlation functionals. For
the antiferromagnetically coupled dimers, the calculated metal–metal
distance correlates well with the covalency of the bridging metal–ligand
bonds, as revealed via the corresponding orbital analysis, Hirshfeld
S/Fe charges, and Fe–S Mayer bond order. Superexchange via
bridging ligands is expected to be the dominant interaction in these
dimers, and our results suggest that functionals that predict accurate
Fe–Fe and Mo–Fe distances describe the overall metal–ligand
covalency more accurately and in turn the superexchange of these systems.
The best performing density functionals of the 16 tested for the FeMoD11
test set are revealed to be either the nonhybrid functionals r2SCAN and B97-D3 or hybrid functionals with 10–15% exact
exchange: TPSSh and B3LYP*. These same four functionals are furthermore
found to reproduce the high-resolution X-ray structure of FeMoco well
according to quantum mechanics/molecular mechanics (QM/MM) calculations.
Almost all nonhybrid functionals systematically underestimate Fe–Fe
and Mo–Fe distances (with r2SCAN and B97-D3 being
the sole exceptions), while hybrid functionals with >15% exact
exchange
(including range-separated hybrid functionals) overestimate them.
The results overall suggest r2SCAN, B97-D3, TPSSh, and
B3LYP* as accurate density functionals for describing the electronic
structure of iron–sulfur clusters in general, including the
complex FeMoco cluster of nitrogenase.
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Affiliation(s)
- Bardi Benediktsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland
| | - Ragnar Bjornsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland.,Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
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15
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Jiang H, Ryde U. Thermodynamically Favourable States in the Reaction of Nitrogenase without Dissociation of any Sulfide Ligand. Chemistry 2022; 28:e202103933. [PMID: 35006641 PMCID: PMC9305431 DOI: 10.1002/chem.202103933] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2021] [Indexed: 12/16/2022]
Abstract
We have used combined quantum mechanical and molecular mechanical (QM/MM) calculations to study the reaction mechanism of nitrogenase, assuming that none of the sulfide ligands dissociates. To avoid the problem that there is no consensus regarding the structure and protonation of the E4 state, we start from a state where N2 is bound to the cluster and is protonated to N2H2, after dissociation of H2. We show that the reaction follows an alternating mechanism with HNNH (possibly protonated to HNNH2) and H2NNH2 as intermediates and the two NH3 products dissociate at the E7 and E8 levels. For all intermediates, coordination to Fe6 is preferred, but for the E4 and E8 intermediates, binding to Fe2 is competitive. For the E4, E5 and E7 intermediates we find that the substrate may abstract a proton from the hydroxy group of the homocitrate ligand of the FeMo cluster, thereby forming HNNH2, H2NNH2 and NH3 intermediates. This may explain why homocitrate is a mandatory component of nitrogenase. All steps in the suggested reaction mechanism are thermodynamically favourable compared to protonation of the nearby His‐195 group and in all cases, protonation of the NE2 atom of the latter group is preferred.
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Affiliation(s)
- Hao Jiang
- Department of Theoretical Chemistry, Lund University Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden
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16
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Donnan PH, Mansoorabadi SO. Broken-Symmetry Density Functional Theory Analysis of the Ω Intermediate in Radical S-Adenosyl-l-methionine Enzymes: Evidence for a Near-Attack Conformer over an Organometallic Species. J Am Chem Soc 2022; 144:3381-3385. [PMID: 35170316 DOI: 10.1021/jacs.2c00678] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Radical S-adenosyl-l-methionine (SAM) enzymes are found in all domains of life and catalyze a wide range of biochemical reactions. Recently, an organometallic intermediate, Ω, has been experimentally implicated in the 5'-deoxyadenosyl radical generation mechanism of the radical SAM superfamily. In this work, we employ broken-symmetry density functional theory to evaluate several structural models of Ω. The results show that the calculated hyperfine coupling constants (HFCCs) for the proposed organometallic structure of Ω are inconsistent with the experiment. In contrast, a near-attack conformer of SAM bound to the catalytic [4Fe-4S] cluster, in which the distance between the unique iron and SAM sulfur is ∼3 Å, yields HFCCs that are all within 1 MHz of the experimental values. These results clarify the structure of the ubiquitous Ω intermediate and suggest a paradigm shift reversal regarding the mechanism of SAM cleavage by members of the radical SAM superfamily.
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Affiliation(s)
- Patrick H Donnan
- Department of Chemistry and Biochemistry, Auburn University, 179 Chemistry Building, Auburn, Alabama 36849, United States
| | - Steven O Mansoorabadi
- Department of Chemistry and Biochemistry, Auburn University, 179 Chemistry Building, Auburn, Alabama 36849, United States
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17
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Comas-Vilà G, Salvador P. Accurate 57Fe Mössbauer Parameters from General Gaussian Basis Sets. J Chem Theory Comput 2021; 17:7724-7731. [PMID: 34806886 PMCID: PMC8675134 DOI: 10.1021/acs.jctc.1c00722] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
![]()
The prediction of
isomer shifts in 57Fe Mossbauer spectra
is typically achieved by building calibration lines using the values
of the density at the nuclear position. Using Slater-type orbital
basis or large and specific Gaussian-type orbital basis has been thus
far mandatory to achieve accurate predictions with density functional
theory methods. In this work, we show that replacing the value of
the density at the nucleus by the density integrated in a sphere of
radius 0.06 au centered on the Fe nuclei yields excellent calibration
lines (r2 = 0.976) with a high predictive
power (q2 = 0.975, MAE = 0.055 mm·s–1) while using the conventional def2-TZVP basis set
and X-ray geometrical parameters. Our data set comprises 69 57Fe-containing compounds and 103 signals. We also find B3LYP performing
significantly better than the PW91 functional.
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Affiliation(s)
- Gerard Comas-Vilà
- Institute of Computational Chemistry and Catalysis, Chemistry Department, University of Girona, Montilivi Campus, Girona, Catalonia 17003, Spain
| | - Pedro Salvador
- Institute of Computational Chemistry and Catalysis, Chemistry Department, University of Girona, Montilivi Campus, Girona, Catalonia 17003, Spain
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18
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Spiller N, Bjornsson R, DeBeer S, Neese F. Carbon Monoxide Binding to the Iron-Molybdenum Cofactor of Nitrogenase: a Detailed Quantum Mechanics/Molecular Mechanics Investigation. Inorg Chem 2021; 60:18031-18047. [PMID: 34767349 PMCID: PMC8653219 DOI: 10.1021/acs.inorgchem.1c02649] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Carbon monoxide (CO) is a well-known inhibitor of nitrogenase activity. Under turnover conditions, CO binds to FeMoco, the active site of Mo nitrogenase. Time-resolved IR measurements suggest an initial terminal CO at 1904 cm-1 that converts to a bridging CO at 1715 cm-1, and an X-ray structure shows that CO can displace one of the bridging belt sulfides of FeMoco. However, the CO-binding redox state(s) of FeMoco (En) and the role of the protein environment in stabilizing specific CO-bound intermediates remain elusive. In this work, we carry out an in-depth analysis of the CO-FeMoco interaction based on quantum chemical calculations addressing different aspects of the electronic structure. (1) The local electronic structure of the Fe-CO bond is studied through diamagnetically substituted FeMoco. (2) A cluster model of FeMoco within a polarizable continuum illustrates how CO binding may affect the spin-coupling between the metal centers. (3) A QM/MM model incorporates the explicit influence of the amino acid residues surrounding FeMoco in the MoFe protein. The QM/MM model predicts both a terminal and a bridging CO in the E1 redox state. The scaled calculated CO frequencies (1922 and 1716 cm-1, respectively) are in good agreement with the experimentally observed IR bands supporting CO binding to the E1 state. Alternatively, an E2 state QM/MM model, which has the same atomic structure as the CO-bound X-ray structure, features a semi-bridging CO with a scaled calculated frequency (1718 cm-1) similar to the bridging CO in the E1 model.
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Affiliation(s)
- Nico Spiller
- Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
| | - Ragnar Bjornsson
- Max Planck Institute for Chemical Energy Conversion, Stiftstr 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Serena DeBeer
- Max Planck Institute for Chemical Energy Conversion, Stiftstr 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Frank Neese
- Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
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19
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Thorhallsson AT, Bjornsson R. The E 2 state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H 2 Evolution. Chemistry 2021; 27:16788-16800. [PMID: 34541722 PMCID: PMC9293435 DOI: 10.1002/chem.202102730] [Citation(s) in RCA: 16] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2021] [Indexed: 11/27/2022]
Abstract
The iron‐molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E0, reduced states of FeMoco are much less well characterized. The E2 state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E2 state can, however, relax back the E0 state via a H2 side‐reaction, implying a hydride intermediate prior to H2 formation. This E2→E0 pathway is one of the primary mechanisms for H2 formation under low‐electron flux conditions. In this study we present an exploration of the energy surface of the E2 state. Utilizing both cluster‐continuum and QM/MM calculations, we explore various classes of E2 models: including terminal hydrides, bridging hydrides with a closed or open sulfide‐bridge, as well as models without. Importantly, we find the hemilability of a protonated belt‐sulfide to strongly influence the stability of hydrides. Surprisingly, non‐hydride models are found to be almost equally favorable as hydride models. While the cluster‐continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E2 state. These models feature either i) a bridging hydride between Fe2 and Fe6 and an open sulfide‐bridge with terminal SH on Fe6 (E2‐hyd) or ii) a double belt‐sulfide protonated, reduced cofactor without a hydride (E2‐nonhyd). We suggest both models as contenders for the E2 redox state and further calculate a mechanism for H2 evolution. The changes in electronic structure of FeMoco during the proposed redox‐state cycle, E0→E1→E2→E0, are discussed.
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Affiliation(s)
- Albert Th Thorhallsson
- Science Institute, University of Iceland, Dunhagi 3, 107, Reykjavik, Iceland.,Department of Inorganic Spectroscopy, Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany
| | - Ragnar Bjornsson
- Science Institute, University of Iceland, Dunhagi 3, 107, Reykjavik, Iceland.,Department of Inorganic Spectroscopy, Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany
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20
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Martín-Fernández C, Harvey JN. On the Use of Normalized Metrics for Density Sensitivity Analysis in DFT. J Phys Chem A 2021; 125:4639-4652. [PMID: 34018759 DOI: 10.1021/acs.jpca.1c01290] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
In the past years, there has been a discussion about how the errors in density functional theory might be related to errors in the self-consistent densities obtained from different density functional approximations. This, in turn, brings up the discussion about the different ways in which we can measure such errors and develop metrics that assess the sensitivity of calculated energies to changes in the density. It is important to realize that there cannot be a unique metric in order to look at this density sensitivity, simultaneously needing size-extensive and size-intensive metrics. In this study, we report two metrics that are widely applicable to any density functional approximation. We also show how they can be used to classify different chemical systems of interest with respect to their sensitivity to small variations in the density.
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Affiliation(s)
| | - Jeremy N Harvey
- Department of Chemistry, KU Leuven, Celestijnenlaan, 200F 3001 Leuven, Belgium
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21
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Postbiosynthetic modification of a precursor to the nitrogenase iron-molybdenum cofactor. Proc Natl Acad Sci U S A 2021; 118:2015361118. [PMID: 33836573 DOI: 10.1073/pnas.2015361118] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Nitrogenases utilize Fe-S clusters to reduce N2 to NH3 The large number of Fe sites in their catalytic cofactors has hampered spectroscopic investigations into their electronic structures, mechanisms, and biosyntheses. To facilitate their spectroscopic analysis, we are developing methods for incorporating 57Fe into specific sites of nitrogenase cofactors, and we report herein site-selective 57Fe labeling of the L-cluster-a carbide-containing, [Fe8S9C] precursor to the Mo nitrogenase catalytic cofactor. Treatment of the isolated L-cluster with the chelator ethylenediaminetetraacetate followed by reconstitution with 57Fe2+ results in 57Fe labeling of the terminal Fe sites in high yield and with high selectivity. This protocol enables the generation of L-cluster samples in which either the two terminal or the six belt Fe sites are selectively labeled with 57Fe. Mössbauer spectroscopic analysis of these samples bound to the nitrogenase maturase Azotobacter vinelandii NifX reveals differences in the primary coordination sphere of the terminal Fe sites and that one of the terminal sites of the L-cluster binds to H35 of Av NifX. This work provides molecular-level insights into the electronic structure and biosynthesis of the L-cluster and introduces postbiosynthetic modification as a promising strategy for studies of nitrogenase cofactors.
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22
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Bergmann J, Oksanen E, Ryde U. Critical evaluation of a crystal structure of nitrogenase with bound N 2 ligands. J Biol Inorg Chem 2021; 26:341-353. [PMID: 33713183 PMCID: PMC8068654 DOI: 10.1007/s00775-021-01858-8] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2021] [Accepted: 02/10/2021] [Indexed: 12/19/2022]
Abstract
Recently, a 1.83 Å crystallographic structure of nitrogenase was suggested to show N2-derived ligands at three sites in the catalytic FeMo cluster, replacing the three [Formula: see text] bridging sulfide ligands (two in one subunit and the third in the other subunit) (Kang et al. in Science 368: 1381-1385, 2020). Naturally, such a structure is sensational, having strong bearings on the reaction mechanism of the enzyme. Therefore, it is highly important to ensure that the interpretation of the structure is correct. Here, we use standard crystallographic refinement and quantum refinement to evaluate the structure. We show that the original crystallographic raw data are strongly anisotropic, with a much lower resolution in certain directions than others. This, together with the questionable use of anisotropic B factors, give atoms an elongated shape, which may look like diatomic atoms. In terms of standard electron-density maps and real-space Z scores, a resting-state structure with no dissociated sulfide ligands fits the raw data better than the interpretation suggested by the crystallographers. The anomalous electron density at 7100 eV is weaker for the putative N2 ligands, but not lower than for several of the [Formula: see text] bridging sulfide ions and not lower than what can be expected from a statistical analysis of the densities. Therefore, we find no convincing evidence for any N2 binding to the FeMo cluster. Instead, a standard resting state without any dissociated ligands seems to be the most likely interpretation of the structure. Likewise, we find no support that the homocitrate ligand should show monodentate binding.
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Affiliation(s)
- Justin Bergmann
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden
| | - Esko Oksanen
- European Spallation Source ESS ERIC, Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P. O. Box 124, 221 00, Lund, Sweden.
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23
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Bergmann J, Oksanen E, Ryde U. Quantum-refinement studies of the bidentate ligand of V‑nitrogenase and the protonation state of CO-inhibited Mo‑nitrogenase. J Inorg Biochem 2021; 219:111426. [PMID: 33756394 DOI: 10.1016/j.jinorgbio.2021.111426] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2020] [Revised: 02/19/2021] [Accepted: 03/10/2021] [Indexed: 10/21/2022]
Abstract
Nitrogenase is the only enzyme that can cleave the triple bond in N2, making nitrogen available to plants (although the enzyme itself is strictly microbial). It has been studied extensively with both experimental and computational methods, but many details of the reaction mechanism are still unclear. X-ray crystallography is the main source of structural information for biomacromolecules, but it has problems to discern hydrogen atoms or to distinguish between elements with the same number of electrons. These problems can sometimes be alleviated by introducing quantum chemical calculations in the refinement, providing information about the ideal structure (in the same way as the empirical restraints used in standard crystallographic refinement) and comparing different interpretations of the structure with normal crystallographic and quantum mechanical quality measures. We have performed such quantum-refinement calculations to address two important issues for nitrogenase. First, we show that the bidentate ligand of the active-site FeV cluster in V‑nitrogenase is carbonate, rather than bicarbonate or nitrate. Second, we study the CO-inhibited structure of Mo‑nitrogenase. CO binds to a reduced and protonated state of the enzyme by replacing one of the sulfide ions (S2B) in the active-site FeMo cluster. We examined if it is possible to deduce from the crystal structure the location of the protons. Our results indicates that the crystal structure is best modelled as fully deprotonated.
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Affiliation(s)
- Justin Bergmann
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Esko Oksanen
- European Spallation Source ESS ERIC, Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden.
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24
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Massolle A, Neugebauer J. Subsystem density-functional theory for interacting open-shell systems: spin densities and magnetic exchange couplings. Faraday Discuss 2020; 224:201-226. [PMID: 33000819 DOI: 10.1039/d0fd00063a] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
We investigate the possibility of describing interacting open-shell systems in high-spin and broken-symmetry (BS) states with subsystem density-functional theory (sDFT). This subsystem method typically starts from the electronic-structure results obtained for individual systems, for which the spin states can be individually defined. Through the confining effect of the embedding potential and/or the use of monomer basis sets, these individual spin states can be preserved in sDFT calculations. This offers the possibility of easy convergence to broken-symmetry states with arbitrary local spin patterns. We show that the resulting spin densities are in very good agreement with successfully converged broken-symmetry Kohn-Sham density-functional theory (KS-DFT) calculations. Yet sDFT can even cure those BS cases where KS-DFT suffers from convergence problems or convergence to undesired spin states. In contrast to KS-DFT, the sDFT-results only show a mild exchange-correlation functional dependence. We also show that magnetic coupling constants from sDFT are not satisfactory with standard approximations for the non-additive kinetic energy. When this component is evaluated "exactly", i.e. based on potential reconstruction, however, the magnetic coupling constants derived from spin-state energy differences are greatly improved. Hence, the interacting radicals studied here represent cases where even (semi-)local approximations for the non-additive kinetic-energy potential work well, while the parent energy functionals do not yield satisfactory results for spin-state energy differences.
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Affiliation(s)
- Anja Massolle
- Theoretische Organische Chemie, Organisch-Chemisches Institut, Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 36, 48149 Münster, Germany.
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25
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Cao L, Ryde U. Putative reaction mechanism of nitrogenase after dissociation of a sulfide ligand. J Catal 2020. [DOI: 10.1016/j.jcat.2020.08.028] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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26
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Cao L, Ryde U. Quantum refinement with multiple conformations: application to the P-cluster in nitrogenase. Acta Crystallogr D Struct Biol 2020; 76:1145-1156. [PMID: 33135685 PMCID: PMC7604908 DOI: 10.1107/s2059798320012917] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2020] [Accepted: 09/21/2020] [Indexed: 11/17/2022] Open
Abstract
X-ray crystallography is the main source of atomistic information on the structure of proteins. Normal crystal structures are obtained as a compromise between the X-ray scattering data and a set of empirical restraints that ensure chemically reasonable bond lengths and angles. However, such restraints are not always available or accurate for nonstandard parts of the structure, for example substrates, inhibitors and metal sites. The method of quantum refinement, in which these empirical restraints are replaced by quantum-mechanical (QM) calculations, has previously been suggested for small but interesting parts of the protein. Here, this approach is extended to allow for multiple conformations in the QM region by performing separate QM calculations for each conformation. This approach is shown to work properly and leads to improved structures in terms of electron-density maps and real-space difference density Z-scores. It is also shown that the quality of the structures can be gauged using QM strain energies. The approach, called ComQumX-2QM, is applied to the P-cluster in two different crystal structures of the enzyme nitrogenase, i.e. an Fe8S7Cys6 cluster, used for electron transfer. One structure is at a very high resolution (1.0 Å) and shows a mixture of two different oxidation states, the fully reduced PN state (Fe82+, 20%) and the doubly oxidized P2+ state (80%). In the original crystal structure the coordinates differed for only two iron ions, but here it is shown that the two states also show differences in other atoms of up to 0.7 Å. The second structure is at a more modest resolution, 2.1 Å, and was originally suggested to show only the one-electron oxidized state, P1+. Here, it is shown that it is rather a 50/50% mixture of the P1+ and P2+ states and that many of the Fe-Fe and Fe-S distances in the original structure were quite inaccurate (by up to 0.8 Å). This shows that the new ComQumX-2QM approach can be used to sort out what is actually seen in crystal structures with dual conformations and to give locally improved coordinates.
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Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Lund University, PO Box 124, 221 00 Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, PO Box 124, 221 00 Lund, Sweden
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27
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Hait D, Haugen EA, Yang Z, Oosterbaan KJ, Leone SR, Head-Gordon M. Accurate prediction of core-level spectra of radicals at density functional theory cost via square gradient minimization and recoupling of mixed configurations. J Chem Phys 2020; 153:134108. [DOI: 10.1063/5.0018833] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- Diptarka Hait
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Eric A. Haugen
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Zheyue Yang
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Katherine J. Oosterbaan
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Stephen R. Leone
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - Martin Head-Gordon
- Department of Chemistry, University of California, Berkeley, California 94720, USA
- Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
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28
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Cao L, Caldararu O, Ryde U. Does the crystal structure of vanadium nitrogenase contain a reaction intermediate? Evidence from quantum refinement. J Biol Inorg Chem 2020; 25:847-861. [PMID: 32856107 PMCID: PMC7511287 DOI: 10.1007/s00775-020-01813-z] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2020] [Accepted: 08/14/2020] [Indexed: 12/16/2022]
Abstract
Recently, a crystal structure of V-nitrogenase was presented, showing that one of the µ2 sulphide ions in the active site (S2B) is replaced by a lighter atom, suggested to be NH or NH2, i.e. representing a reaction intermediate. Moreover, a sulphur atom is found 7 Å from the S2B site, suggested to represent a storage site for this ion when it is displaced. We have re-evaluated this structure with quantum refinement, i.e. standard crystallographic refinement in which the empirical restraints (employed to ensure that the final structure makes chemical sense) are replaced by more accurate quantum-mechanical calculations. This allows us to test various interpretations of the structure, employing quantum-mechanical calculations to predict the ideal structure and to use crystallographic measures like the real-space Z-score and electron-density difference maps to decide which structure fits the crystallographic raw data best. We show that the structure contains an OH--bound state, rather than an N2-derived reaction intermediate. Moreover, the structure shows dual conformations in the active site with ~ 14% undissociated S2B ligand, but the storage site seems to be fully occupied, weakening the suggestion that it represents a storage site for the dissociated ligand.
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Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden
| | - Octav Caldararu
- Department of Theoretical Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden.
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29
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Benediktsson B, Bjornsson R. Quantum Mechanics/Molecular Mechanics Study of Resting-State Vanadium Nitrogenase: Molecular and Electronic Structure of the Iron-Vanadium Cofactor. Inorg Chem 2020; 59:11514-11527. [PMID: 32799489 PMCID: PMC7458435 DOI: 10.1021/acs.inorgchem.0c01320] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2020] [Indexed: 12/18/2022]
Abstract
The nitrogenase enzymes are responsible for all biological nitrogen reduction. How this is accomplished at the atomic level, however, has still not been established. The molybdenum-dependent nitrogenase has been extensively studied and is the most active catalyst for dinitrogen reduction of the nitrogenase enzymes. The vanadium-dependent form, on the other hand, displays different reactivity, being capable of CO and CO2 reduction to hydrocarbons. Only recently did a crystal structure of the VFe protein of vanadium nitrogenase become available, paving the way for detailed theoretical studies of the iron-vanadium cofactor (FeVco) within the protein matrix. The crystal structure revealed a bridging 4-atom ligand between two Fe atoms, proposed to be either a CO32- or NO3- ligand. Using a quantum mechanics/molecular mechanics model of the VFe protein, starting from the 1.35 Å crystal structure, we have systematically explored multiple computational models for FeVco, considering either a CO32- or NO3- ligand, three different redox states, and multiple broken-symmetry states. We find that only a [VFe7S8C(CO3)]2- model for FeVco reproduces the crystal structure of FeVco well, as seen in a comparison of the Fe-Fe and V-Fe distances in the computed models. Furthermore, a broken-symmetry solution with Fe2, Fe3, and Fe5 spin-down (BS7-235) is energetically preferred. The electronic structure of the [VFe7S8C(CO3)]2- BS7-235 model is compared to our [MoFe7S9C]- BS7-235 model of FeMoco via localized orbital analysis and is discussed in terms of local oxidation states and different degrees of delocalization. As previously found from Fe X-ray absorption spectroscopy studies, the Fe part of FeVco is reduced compared to FeMoco, and the calculations reveal Fe5 as locally ferrous. This suggests resting-state FeVco to be analogous to an unprotonated E1 state of FeMoco. Furthermore, V-Fe interactions in FeVco are not as strong compared to Mo-Fe interactions in FeMoco. These clear differences in the electronic structures of otherwise similar cofactors suggest an explanation for distinct differences in reactivity.
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Affiliation(s)
- Bardi Benediktsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland
| | - Ragnar Bjornsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland
- Max-Planck Institute
for Chemical Energy Conversion, Stiftstrasse 34−36, 45470 Mülheim an der Ruhr, Germany
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30
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Van Stappen C, Decamps L, Cutsail GE, Bjornsson R, Henthorn JT, Birrell JA, DeBeer S. The Spectroscopy of Nitrogenases. Chem Rev 2020; 120:5005-5081. [PMID: 32237739 PMCID: PMC7318057 DOI: 10.1021/acs.chemrev.9b00650] [Citation(s) in RCA: 105] [Impact Index Per Article: 26.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2019] [Indexed: 01/08/2023]
Abstract
Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.
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Affiliation(s)
- Casey Van Stappen
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Laure Decamps
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - George E. Cutsail
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Ragnar Bjornsson
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Justin T. Henthorn
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - James A. Birrell
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
| | - Serena DeBeer
- Max Planck Institute for
Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany
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31
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Affiliation(s)
- Oliver Einsle
- Institute for Biochemistry, Albert-Ludwigs-University Freiburg, 79104 Freiburg, Germany
| | - Douglas C. Rees
- Division of Chemistry and Chemical Engineering, Howard Hughes Medical Institute, California Institute of Technology, Pasadena California 91125, United States
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32
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Miyagawa K, Shoji M, Isobe H, Yamanaka S, Kawakami T, Okumura M, Yamaguchi K. Theory of chemical bonds in metalloenzymes XXIV electronic and spin structures of FeMoco and Fe-S clusters by classical and quantum computing. Mol Phys 2020. [DOI: 10.1080/00268976.2020.1760388] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
Affiliation(s)
- Koichi Miyagawa
- The Institute for Scientific and Industrial Research, Osaka University, Ibaraki, Japan
| | - Mitsuo Shoji
- Center of Computational Sciences, Tsukuba University, Tsukuba, Japan
| | - Hiroshi Isobe
- Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan
| | - Shusuke Yamanaka
- Graduate School of Science, Osaka University, Toyonaka, Japan
- Division of Quantum Information and Quantum Biology (QIQB), Osaka University, Toyonaka, Japan
| | - Takashi Kawakami
- Graduate School of Science, Osaka University, Toyonaka, Japan
- RIKEN Center for Computational Science, Kobe, Japan
| | | | - Kizashi Yamaguchi
- The Institute for Scientific and Industrial Research, Osaka University, Ibaraki, Japan
- Division of Quantum Information and Quantum Biology (QIQB), Osaka University, Toyonaka, Japan
- RIKEN Center for Computational Science, Kobe, Japan
- Institute for Nanoscience Design, Osaka University, Toyonaka, Japan
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33
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Cao L, Ryde U. N 2H 2 binding to the nitrogenase FeMo cluster studied by QM/MM methods. J Biol Inorg Chem 2020; 25:521-540. [PMID: 32266560 PMCID: PMC7186253 DOI: 10.1007/s00775-020-01780-5] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2019] [Accepted: 03/25/2020] [Indexed: 12/15/2022]
Abstract
We have made a systematic combined quantum mechanical and molecular mechanical (QM/MM) investigation of possible structures of the N2 bound state of nitrogenase. We assume that N2 is immediately protonated to a N2H2 state, thereby avoiding the problem of determining the position of the protons in the cluster. We have systematically studied both end-on and side-on structures, as well as both HNNH and NNH2 states. Our results indicate that the binding of N2H2 is determined more by interactions and steric clashes with the surrounding protein than by the intrinsic preferences of the ligand and the cluster. The best binding mode with both the TPSS and B3LYP density-functional theory methods has trans-HNNH terminally bound to Fe2. It is stabilised by stacking of the substrate with His-195 and Ser-278. However, several other structures come rather close in energy (within 3-35 kJ/mol) at least in some calculations: The corresponding cis-HNNH structure terminally bound to Fe2 is second best with B3LYP. A structure with HNNH2 terminally bound to Fe6 is second most stable with TPSS (where the third proton is transferred to the substrate from the homocitrate ligand). Structures with trans-HNNH, bound to Fe4 or Fe6, or cis-HNNH bound to Fe6 are also rather stable. Finally, with the TPSS functional, a structure with cis-HNNH side-on binding to the Fe3-Fe4-Fe5-Fe7 face of the cluster is also rather low in energy, but all side-on structures are strongly disfavoured by the B3LYP method.
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Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Chemical Centre, Lund University, P. O. Box 124, 221 00, Lund, Sweden.
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34
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Abstract
![]()
Nitrogenase
is the only enzyme that can cleave the strong triple
bond in N2. The active site contains a complicated MoFe7S9C cluster. It is believed that it needs to accept
four protons and electrons, forming the E4 state, before
it can bind N2. However, there is no consensus on the atomic
structure of the E4 state. Experimental studies indicate
that it should contain two hydride ions bridging two pairs of Fe ions,
and it has been suggested that both hydride ions as well as the two
protons bind on the same face of the cluster. On the other hand, density
functional theory (DFT) studies have indicated that it is energetically
more favorable with either three hydride ions or with a triply protonated
carbide ion, depending on the DFT functional. We have performed a
systematic combined quantum mechanical and molecular mechanical (QM/MM)
study of possible E4 states with two bridging hydride ions.
Our calculations suggest that the most favorable structure has hydride
ions bridging the Fe2/6 and Fe3/7 ion pairs. In fact, such structures
are 14 kJ/mol more stable than structures with three hydride ions,
showing that pure DFT functionals give energetically most favorable
structures in agreement with experiments. An important reason for
this finding is that we have identified a new type of broken-symmetry
state that involves only two Fe ions with minority spin, in contrast
to the previously studied states with three Fe ions with minority
spin. The energetically best structures have the two hydride ions
on different faces of the FeMo cluster, whereas better agreement with
ENDOR data is obtained if they are on the same face; such structures
are only 6–22 kJ/mol less stable.
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Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
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35
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Dance I. Computational Investigations of the Chemical Mechanism of the Enzyme Nitrogenase. Chembiochem 2020; 21:1671-1709. [DOI: 10.1002/cbic.201900636] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2019] [Indexed: 12/15/2022]
Affiliation(s)
- Ian Dance
- School of Chemistry UNSW Sydney Sydney 2052 Australia
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36
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Van Stappen C, Thorhallsson AT, Decamps L, Bjornsson R, DeBeer S. Resolving the structure of the E 1 state of Mo nitrogenase through Mo and Fe K-edge EXAFS and QM/MM calculations. Chem Sci 2019; 10:9807-9821. [PMID: 32055350 PMCID: PMC6984330 DOI: 10.1039/c9sc02187f] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2019] [Accepted: 09/03/2019] [Indexed: 11/21/2022] Open
Abstract
Biological nitrogen fixation is predominately accomplished through Mo nitrogenase, which utilizes a complex MoFe7S9C catalytic cluster to reduce N2 to NH3. This cluster requires the accumulation of three to four reducing equivalents prior to binding N2; however, despite decades of research, the intermediate states formed prior to N2 binding are still poorly understood. Herein, we use Mo and Fe K-edge X-ray absorption spectroscopy and QM/MM calculations to investigate the nature of the E1 state, which is formed following the addition of the first reducing equivalent to Mo nitrogenase. By analyzing the extended X-ray absorption fine structure (EXAFS) region, we provide structural insight into the changes that occur in the metal clusters of the protein when forming the E1 state, and use these metrics to assess a variety of possible models of the E1 state. The combination of our experimental and theoretical results supports that formation of E1 involves an Fe-centered reduction combined with the protonation of a belt-sulfide of the cluster. Hence, these results provide critical experiment and computational insight into the mechanism of this important enzyme.
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Affiliation(s)
- Casey Van Stappen
- Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . ;
| | - Albert Thor Thorhallsson
- Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . ;
| | - Laure Decamps
- Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . ;
| | - Ragnar Bjornsson
- Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . ;
| | - Serena DeBeer
- Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . ;
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37
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Li Z, Guo S, Sun Q, Chan GKL. Electronic landscape of the P-cluster of nitrogenase as revealed through many-electron quantum wavefunction simulations. Nat Chem 2019; 11:1026-1033. [PMID: 31570817 DOI: 10.1038/s41557-019-0337-3] [Citation(s) in RCA: 55] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2019] [Accepted: 08/20/2019] [Indexed: 11/09/2022]
Abstract
The electronic structure of the nitrogenase metal cofactors is central to nitrogen fixation. However, the P-cluster and FeMo cofactor, each containing eight Fe atoms, have eluded detailed characterization of their electronic properties. We report on the low-energy electronic states of the P-cluster in three oxidation states through exhaustive many-electron wavefunction simulations enabled by new theoretical methods. The energy scales of orbital and spin excitations overlap, yielding a dense spectrum with features that we trace to the underlying atomic states and recouplings. The clusters exist in superpositions of spin configurations with non-classical spin correlations, complicating interpretation of magnetic spectroscopies, whereas the charges are mostly localized from reorganization of the cluster and its surroundings. On oxidation, the opening of the P-cluster substantially increases the density of states, which is intriguing given its proposed role in electron transfer. These results demonstrate that many-electron simulations stand to provide new insights into the electronic structure of the nitrogenase cofactors.
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Affiliation(s)
- Zhendong Li
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA.
| | - Sheng Guo
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Qiming Sun
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
| | - Garnet Kin-Lic Chan
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA.
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38
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Cao L, Börner MC, Bergmann J, Caldararu O, Ryde U. Geometry and Electronic Structure of the P-Cluster in Nitrogenase Studied by Combined Quantum Mechanical and Molecular Mechanical Calculations and Quantum Refinement. Inorg Chem 2019; 58:9672-9690. [DOI: 10.1021/acs.inorgchem.9b00400] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Melanie C. Börner
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
- Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany
| | - Justin Bergmann
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Octav Caldararu
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
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39
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Cao L, Ryde U. Extremely large differences in DFT energies for nitrogenase models. Phys Chem Chem Phys 2019; 21:2480-2488. [PMID: 30652711 DOI: 10.1039/c8cp06930a] [Citation(s) in RCA: 48] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Nitrogenase is the only enzyme that can cleave the triple bond in N2, making nitrogen avaiable for other organisms. It contains a complicated MoFe7S9C(homocitrate) cluster in its active site. Many computational studies with density-functional theory (DFT) of the nitrogenase enzyme have been presented, but they do not show any consensus - they do not even agree where the first four protons should be added, forming the central intermediate E4. We show that the prime reason for this is that different DFT methods give relative energies that differ by almost 600 kJ mol-1 for different protonation states. This is 4-30 times more than what is observed for other systems. The reason for this is that in some structures, the hydrogens bind to sulfide or carbide ions as protons, whereas in other structures they bind to the metals as hydride ions, changing the oxidation state of the metals, as well as the Fe-C, Fe-S and Fe-Fe distances. The energies correlate with the amount of Hartree-Fock exchange in the method, indicating a variation in the amount of static correlation in the structures. It is currently unclear which DFT method gives the best results for nitrogenase. We show that non-hybrid DFT functionals and TPSSh give the most accurate structures of the resting active site, whereas B3LYP and PBE0 give the best H2 dissociation energies. However, no DFT method indicates that a structure of E4 with two bridging hydride ions is lowest in energy, as spectroscopic experiments indicate.
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Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Lund University, P. O. Box 124, SE-221 00 Lund, Sweden.
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40
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Survey of the Geometric and Electronic Structures of the Key Hydrogenated Forms of FeMo-co, the Active Site of the Enzyme Nitrogenase: Principles of the Mechanistically Significant Coordination Chemistry. INORGANICS 2019. [DOI: 10.3390/inorganics7010008] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
The enzyme nitrogenase naturally hydrogenates N2 to NH3, achieved through the accumulation of H atoms on FeMo-co, the Fe7MoS9C(homocitrate) cluster that is the catalytically active site. Four intermediates, E1H1, E2H2, E3H3, and E4H4, carry these hydrogen atoms. I report density functional calculations of the numerous possibilities for the geometric and electronic structures of these poly-hydrogenated forms of FeMo-co. This survey involves more than 100 structures, including those with bound H2, and assesses their relative energies and most likely electronic states. Twelve locations for bound H atoms in the active domain of FeMo-co, including Fe–H–Fe and Fe–H–S bridges, are studied. A significant result is that transverse Fe–H–Fe bridges (transverse to the pseudo-threefold axis of FeMo-co and shared with triply-bridging S) are not possible geometrically unless the S is hydrogenated to become doubly-bridging. The favourable Fe–H–Fe bridges are shared with doubly-bridging S. ENDOR data for an E4H4 intermediate trapped at low temperature, and interpretations in terms of the geometrical and electronic structure of E4H4, are assessed in conjunction with the calculated possibilities. The results reported here yield a set of 24 principles for the mechanistically significant coordination chemistry of H and H2 on FeMo-co, in the stages prior to N2 binding.
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41
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Abstract
Nitrogen in the air is turned into biologically useful ammonia by the nitrogenase enzyme. The leading member of this group has a cofactor with one molybdenum and seven irons linked together by sulfurs. The structure that binds N2 has a triply protonated carbide and a rotated homocitrate. Both these structural changes are necessary for the activation.
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Affiliation(s)
- Per E. M. Siegbahn
- Department of Organic Chemistry
- Arrhenius Laboratory
- Stockholm University
- Stockholm
- Sweden
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42
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Abstract
Metalloproteins are challenging objects if we want to investigate their chemical reactivity with theoretical approaches such as density functional theory (DFT). The complexity of these biomolecules often requires us to find a compromise between accuracy and feasibility, one that is tailored to the questions we set out to answer. In this chapter, we discuss computational approaches to studying chemical reactions in metalloproteins and how to utilize the information hidden in homologous proteins.
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Affiliation(s)
- Martin T Stiebritz
- Department of Molecular Biology and Biochemistry, University of California, Irvine, Irvine, CA, USA.
| | - Yilin Hu
- Department of Molecular Biology and Biochemistry, University of California, Irvine, Irvine, CA, USA.
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43
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Cao L, Caldararu O, Ryde U. Protonation and Reduction of the FeMo Cluster in Nitrogenase Studied by Quantum Mechanics/Molecular Mechanics (QM/MM) Calculations. J Chem Theory Comput 2018; 14:6653-6678. [DOI: 10.1021/acs.jctc.8b00778] [Citation(s) in RCA: 36] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Lili Cao
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Octav Caldararu
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
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44
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Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N 2 reduction. Proc Natl Acad Sci U S A 2018; 115:E10521-E10530. [PMID: 30355772 DOI: 10.1073/pnas.1810211115] [Citation(s) in RCA: 82] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Recent spectroscopic, kinetic, photophysical, and thermodynamic measurements show activation of nitrogenase for N2 → 2NH3 reduction involves the reductive elimination (re) of H2 from two [Fe-H-Fe] bridging hydrides bound to the catalytic [7Fe-9S-Mo-C-homocitrate] FeMo-cofactor (FeMo-co). These studies rationalize the Lowe-Thorneley kinetic scheme's proposal of mechanistically obligatory formation of one H2 for each N2 reduced. They also provide an overall framework for understanding the mechanism of nitrogen fixation by nitrogenase. However, they directly pose fundamental questions addressed computationally here. We here report an extensive computational investigation of the structure and energetics of possible nitrogenase intermediates using structural models for the active site with a broad range in complexity, while evaluating a diverse set of density functional theory flavors. (i) This shows that to prevent spurious disruption of FeMo-co having accumulated 4[e -/H+] it is necessary to include: all residues (and water molecules) interacting directly with FeMo-co via specific H-bond interactions; nonspecific local electrostatic interactions; and steric confinement. (ii) These calculations indicate an important role of sulfide hemilability in the overall conversion of E 0 to a diazene-level intermediate. (iii) Perhaps most importantly, they explain (iiia) how the enzyme mechanistically couples exothermic H2 formation to endothermic cleavage of the N≡N triple bond in a nearly thermoneutral re/oxidative-addition equilibrium, (iiib) while preventing the "futile" generation of two H2 without N2 reduction: hydride re generates an H2 complex, but H2 is only lost when displaced by N2, to form an end-on N2 complex that proceeds to a diazene-level intermediate.
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45
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Rohde M, Sippel D, Trncik C, Andrade SLA, Einsle O. The Critical E 4 State of Nitrogenase Catalysis. Biochemistry 2018; 57:5497-5504. [PMID: 29965738 DOI: 10.1021/acs.biochem.8b00509] [Citation(s) in RCA: 51] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The reaction catalyzed by the nitrogenase enzyme involves breaking the stable triple bond of the dinitrogen molecule and is consequently considered among the most challenging reactions in biology. While many aspects regarding its atomic mechanism remain to be elucidated, a kinetic scheme established by David Lowe and Roger Thorneley has remained a gold standard for functional studies of the enzyme for more than 30 years. Recent three-dimensional structures of ligand-bound states of molybdenum- and vanadium-dependent nitrogenases have revealed the actual site of substrate binding on the large active site cofactors of this class of enzymes. The binding mode of an inhibitor and a reaction intermediate further substantiate a hypothesis by Seefeldt, Hoffman, and Dean that the activation of N2 is made possible by a reductive elimination of H2 that leaves the cofactor in a super-reduced state that can bind and reduce the inert N2 molecule. Here we discuss the immediate implications of the structurally observed mode of binding of small molecules to the enzyme with respect to the early stages of the Thorneley-Lowe mechanism of nitrogenase. Four consecutive single-electron reductions give rise to two bridging hydrides at the cluster surface that can recombine to eliminate H2 and enable the reduced cluster to bind its substrate in a bridging mode.
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Affiliation(s)
- Michael Rohde
- Institute for Biochemistry , Albert-Ludwigs-University Freiburg , Albertstrasse 21 , 79104 Freiburg , Germany
| | - Daniel Sippel
- Institute for Biochemistry , Albert-Ludwigs-University Freiburg , Albertstrasse 21 , 79104 Freiburg , Germany
| | - Christian Trncik
- Institute for Biochemistry , Albert-Ludwigs-University Freiburg , Albertstrasse 21 , 79104 Freiburg , Germany
| | - Susana L A Andrade
- Institute for Biochemistry , Albert-Ludwigs-University Freiburg , Albertstrasse 21 , 79104 Freiburg , Germany.,BIOSS Centre for Biological Signalling Studies , Schänzlestrasse 1 , 79104 Freiburg , Germany
| | - Oliver Einsle
- Institute for Biochemistry , Albert-Ludwigs-University Freiburg , Albertstrasse 21 , 79104 Freiburg , Germany.,BIOSS Centre for Biological Signalling Studies , Schänzlestrasse 1 , 79104 Freiburg , Germany
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46
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Sippel D, Rohde M, Netzer J, Trncik C, Gies J, Grunau K, Djurdjevic I, Decamps L, Andrade SLA, Einsle O. A bound reaction intermediate sheds light on the mechanism of nitrogenase. Science 2018; 359:1484-1489. [PMID: 29599235 DOI: 10.1126/science.aar2765] [Citation(s) in RCA: 183] [Impact Index Per Article: 30.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2017] [Accepted: 01/31/2018] [Indexed: 01/26/2023]
Abstract
Reduction of N2 by nitrogenases occurs at an organometallic iron cofactor that commonly also contains either molybdenum or vanadium. The well-characterized resting state of the cofactor does not bind substrate, so its mode of action remains enigmatic. Carbon monoxide was recently found to replace a bridging sulfide, but the mechanistic relevance was unclear. Here we report the structural analysis of vanadium nitrogenase with a bound intermediate, interpreted as a μ2-bridging, protonated nitrogen that implies the site and mode of substrate binding to the cofactor. Binding results in a flip of amino acid glutamine 176, which hydrogen-bonds the ligand and creates a holding position for the displaced sulfide. The intermediate likely represents state E6 or E7 of the Thorneley-Lowe model and provides clues to the remainder of the catalytic cycle.
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Affiliation(s)
- Daniel Sippel
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Michael Rohde
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Julia Netzer
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Christian Trncik
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Jakob Gies
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Katharina Grunau
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Ivana Djurdjevic
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Laure Decamps
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany
| | - Susana L A Andrade
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany.,BIOSS Centre for Biological Signalling Studies, Schänzlestraße 1, 79104 Freiburg, Germany
| | - Oliver Einsle
- Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany. .,BIOSS Centre for Biological Signalling Studies, Schänzlestraße 1, 79104 Freiburg, Germany.,Freiburg Institute for Advanced Studies, 79104 Freiburg, Germany
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47
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Li Q, Qiu S, He L, Zhang X, Sun C. Impact of H-termination on the nitrogen reduction reaction of molybdenum carbide as an electrochemical catalyst. Phys Chem Chem Phys 2018; 20:23338-23343. [DOI: 10.1039/c8cp04474k] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
H-Terminals can remarkably affect the performance of catalysts in nitrogen reduction.
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Affiliation(s)
- Qinye Li
- School of Chemical Engineering
- Monash University
- Clayton
- VIC 3800
- Australia
| | - Siyao Qiu
- Science & Technology Innovation Institute
- Dongguan University of Technology
- Dongguan
- China
| | - Lizhong He
- School of Chemical Engineering
- Monash University
- Clayton
- VIC 3800
- Australia
| | - Xiwang Zhang
- School of Chemical Engineering
- Monash University
- Clayton
- VIC 3800
- Australia
| | - Chenghua Sun
- Science & Technology Innovation Institute
- Dongguan University of Technology
- Dongguan
- China
- Department of Chemistry and Biotechnology
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48
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Benediktsson B, Bjornsson R. QM/MM Study of the Nitrogenase MoFe Protein Resting State: Broken-Symmetry States, Protonation States, and QM Region Convergence in the FeMoco Active Site. Inorg Chem 2017; 56:13417-13429. [DOI: 10.1021/acs.inorgchem.7b02158] [Citation(s) in RCA: 57] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Affiliation(s)
- Bardi Benediktsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland
| | - Ragnar Bjornsson
- Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland
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49
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Cao L, Caldararu O, Ryde U. Protonation States of Homocitrate and Nearby Residues in Nitrogenase Studied by Computational Methods and Quantum Refinement. J Phys Chem B 2017; 121:8242-8262. [DOI: 10.1021/acs.jpcb.7b02714] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Lili Cao
- Department of Theoretical
Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Octav Caldararu
- Department of Theoretical
Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
| | - Ulf Ryde
- Department of Theoretical
Chemistry, Lund University, Chemical Centre, P.O. Box 124, SE-221 00 Lund, Sweden
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50
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Grunenberg J. The Interstitial Carbon of the Nitrogenase FeMo Cofactor is Far Better Stabilized than Previously Assumed. Angew Chem Int Ed Engl 2017; 56:7288-7291. [DOI: 10.1002/anie.201701790] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2017] [Revised: 03/16/2017] [Indexed: 01/08/2023]
Affiliation(s)
- Jörg Grunenberg
- TU Braunschweig; Fakultät für Lebenswissenschaften; Institut für Organische Chemie, Abteilung Computerchemie; Hagenring 30 38106 Braunschweig Germany
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