1
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Silwal S, Shrestha D, Neupane G, Rana R, Bhurtel S, Adhikari P, Khadka N. Awake tracheal intubation in a patient with a post-burn contracture performed via direct laryngoscopy in a resource-limited setting. Anaesth Rep 2023; 11:e12265. [PMID: 38058474 PMCID: PMC10696405 DOI: 10.1002/anr3.12265] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/20/2023] [Indexed: 12/08/2023] Open
Affiliation(s)
- S. Silwal
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - D. Shrestha
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - G. Neupane
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - R. Rana
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - S. Bhurtel
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - P. Adhikari
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
| | - N. Khadka
- Department of AnaesthesiologyBharatpur HospitalChitwanNepal
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2
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Silwal S, Shrestha D, Neupane G, Rana R, Bhurtel S, Adhikari P, Khadka N. Awake tracheal intubation in a patient with a post‐burn contracture performed via direct laryngoscopy in a resource‐limited setting. Anaesth Rep 2023; 11. [DOI: https:/doi.org/10.1002/anr3.12265] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/20/2023] [Indexed: 12/27/2023] Open
Affiliation(s)
- S. Silwal
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - D. Shrestha
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - G. Neupane
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - R. Rana
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - S. Bhurtel
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - P. Adhikari
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
| | - N. Khadka
- Department of Anaesthesiology Bharatpur Hospital Chitwan Nepal
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3
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Bandara S, Thomas LD, Ramkumar S, Khadka N, Kiser PD, Golczak M, von Lintig J. The Structural and Biochemical Basis of Apocarotenoid Processing by β-Carotene Oxygenase-2. ACS Chem Biol 2021; 16:480-490. [PMID: 33600157 DOI: 10.1021/acschembio.0c00832] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
In mammals, carotenoids are converted by two carotenoid cleavage oxygenases into apocarotenoids, including vitamin A. Although knowledge about β-carotene oxygenase-1 (BCO1) and vitamin A metabolism has tremendously increased, the function of β-carotene oxygenase-2 (BCO2) remains less well-defined. We here studied the role of BCO2 in the metabolism of long chain β-apocarotenoids, which recently emerged as putative regulatory molecules in mammalian biology. We showed that recombinant murine BCO2 converted the alcohol, aldehyde, and carboxylic acid of a β-apocarotenoid substrate by oxidative cleavage at position C9,C10 into a β-ionone and a diapocarotenoid product. Chain length variation (C20 to C40) and ionone ring site modifications of the apocarotenoid substrate did not impede catalytic activity or alter the regioselectivity of the double bond cleavage by BCO2. Isotope labeling experiments revealed that the double bond cleavage of an apocarotenoid followed a dioxygenase reaction mechanism. Structural modeling and site directed mutagenesis identified amino acid residues in the substrate tunnel of BCO2 that are critical for apocarotenoid binding and catalytic processing. Mice deficient for BCO2 accumulated apocarotenoids in their livers, indicating that the enzyme engages in apocarotenoid metabolism. Together, our study provides novel structural and functional insights into BCO2 catalysis and establishes the enzyme as a key component of apocarotenoid homeostasis in mice.
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Affiliation(s)
| | | | | | | | - Philip D. Kiser
- Department of Physiology and Biophysics, University of California, Irvine, California 92697, United States
- Research Service, Veterans Affairs Long Beach Healthcare System, Long Beach, California 90822, United States
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4
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York NJ, Lockart MM, Sardar S, Khadka N, Shi W, Stenkamp RE, Zhang J, Kiser PD, Pierce BS. Structure of 3-mercaptopropionic acid dioxygenase with a substrate analog reveals bidentate substrate binding at the iron center. J Biol Chem 2021; 296:100492. [PMID: 33662397 PMCID: PMC8050391 DOI: 10.1016/j.jbc.2021.100492] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Revised: 02/18/2021] [Accepted: 02/26/2021] [Indexed: 12/20/2022] Open
Abstract
Thiol dioxygenases are a subset of nonheme iron oxygenases that catalyze the formation of sulfinic acids from sulfhydryl-containing substrates and dioxygen. Among this class, cysteine dioxygenases (CDOs) and 3-mercaptopropionic acid dioxygenases (3MDOs) are the best characterized, and the mode of substrate binding for CDOs is well understood. However, the manner in which 3-mercaptopropionic acid (3MPA) coordinates to the nonheme iron site in 3MDO remains a matter of debate. A model for bidentate 3MPA coordination at the 3MDO Fe-site has been proposed on the basis of computational docking, whereas steady-state kinetics and EPR spectroscopic measurements suggest a thiolate-only coordination of the substrate. To address this gap in knowledge, we determined the structure of Azobacter vinelandii 3MDO (Av3MDO) in complex with the substrate analog and competitive inhibitor, 3-hydroxypropionic acid (3HPA). The structure together with DFT computational modeling demonstrates that 3HPA and 3MPA associate with iron as chelate complexes with the substrate-carboxylate group forming an additional interaction with Arg168 and the thiol bound at the same position as in CDO. A chloride ligand was bound to iron in the coordination site assigned as the O2-binding site. Supporting HYSCORE spectroscopic experiments were performed on the (3MPA/NO)-bound Av3MDO iron nitrosyl (S = 3/2) site. In combination with spectroscopic simulations and optimized DFT models, this work provides an experimentally verified model of the Av3MDO enzyme-substrate complex, effectively resolving a debate in the literature regarding the preferred substrate-binding denticity. These results elegantly explain the observed 3MDO substrate specificity, but leave unanswered questions regarding the mechanism of substrate-gated reactivity with dioxygen.
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Affiliation(s)
- Nicholas J York
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Molly M Lockart
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Sinjinee Sardar
- Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas, USA
| | - Nimesh Khadka
- Department of Pharmacology, Case Western Reserve University, Cleveland, Ohio, USA
| | - Wuxian Shi
- National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, New York, USA
| | - Ronald E Stenkamp
- Departments of Biological Structure and Biochemistry, University of Washington, Seattle, Washington, USA
| | - Jianye Zhang
- Department of Ophthalmology, School of Medicine, University of California, Irvine, Irvine, California, USA
| | - Philip D Kiser
- Department of Ophthalmology, School of Medicine, University of California, Irvine, Irvine, California, USA; Department of Physiology & Biophysics, School of Medicine, University of California, Irvine, Irvine, California, USA; Research Service, VA Long Beach Healthcare System, Long Beach, California, USA.
| | - Brad S Pierce
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA.
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Thomas LD, Bandara S, Parmar VM, Srinivasagan R, Khadka N, Golczak M, Kiser PD, von Lintig J. The human mitochondrial enzyme BCO2 exhibits catalytic activity toward carotenoids and apocarotenoids. J Biol Chem 2020; 295:15553-15565. [PMID: 32873706 DOI: 10.1074/jbc.ra120.015515] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2020] [Revised: 08/28/2020] [Indexed: 12/14/2022] Open
Abstract
The enzyme β-carotene oxygenase 2 (BCO2) converts carotenoids into more polar metabolites. Studies in mammals, fish, and birds revealed that BCO2 controls carotenoid homeostasis and is involved in the pathway for vitamin A production. However, it is controversial whether BCO2 function is conserved in humans, because of a 4-amino acid long insertion caused by a splice acceptor site polymorphism. We here show that human BCO2 splice variants, BCO2a and BCO2b, are expressed as pre-proteins with mitochondrial targeting sequence (MTS). The MTS of BCO2a directed a green fluorescent reporter protein to the mitochondria when expressed in ARPE-19 cells. Removal of the MTS increased solubility of BCO2a when expressed in Escherichia coli and rendered the recombinant protein enzymatically active. The expression of the enzymatically active recombinant human BCO2a was further improved by codon optimization and its fusion with maltose-binding protein. Introduction of the 4-amino acid insertion into mouse Bco2 did not impede the chimeric enzyme's catalytic proficiency. We further showed that the chimeric BCO2 displayed broad substrate specificity and converted carotenoids into two ionones and a central C14-apocarotendial by oxidative cleavage reactions at C9,C10 and C9',C10'. Thus, our study demonstrates that human BCO2 is a catalytically competent enzyme. Consequently, information on BCO2 becomes broadly applicable in human biology with important implications for the physiology of the eyes and other tissues.
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Affiliation(s)
- Linda D Thomas
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Sepalika Bandara
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Vipulkumar M Parmar
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Ramkumar Srinivasagan
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Nimesh Khadka
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Marcin Golczak
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA; Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA
| | - Philip D Kiser
- Department of Physiology and Biophysics, University of California, Irvine, California, USA; Research Service, Veterans Affairs Long Beach Healthcare System, Long Beach, California, USA
| | - Johannes von Lintig
- Department of Pharmacology, Cleveland Center for Membrane and Structural Biology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, USA.
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6
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Khadka N, Farquhar ER, Hill HE, Shi W, von Lintig J, Kiser PD. Evidence for distinct rate-limiting steps in the cleavage of alkenes by carotenoid cleavage dioxygenases. J Biol Chem 2019; 294:10596-10606. [PMID: 31138651 DOI: 10.1074/jbc.ra119.007535] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2019] [Revised: 05/24/2019] [Indexed: 11/06/2022] Open
Abstract
Carotenoid cleavage dioxygenases (CCDs) use a nonheme Fe(II) cofactor to split alkene bonds of carotenoid and stilbenoid substrates. The iron centers of CCDs are typically five-coordinate in their resting states, with solvent occupying an exchangeable site. The involvement of this iron-bound solvent in CCD catalysis has not been experimentally addressed, but computational studies suggest two possible roles. 1) Solvent dissociation provides a coordination site for O2, or 2) solvent remains bound to iron but changes its equilibrium position to allow O2 binding and potentially acts as a proton source. To test these predictions, we investigated isotope effects (H2O versus D2O) on two stilbenoid-cleaving CCDs, Novosphingobium aromaticivorans oxygenase 2 (NOV2) and Neurospora crassa carotenoid oxygenase 1 (CAO1), using piceatannol as a substrate. NOV2 exhibited an inverse isotope effect (k H/k D ∼ 0.6) in an air-saturated buffer, suggesting that solvent dissociates from iron during the catalytic cycle. By contrast, CAO1 displayed a normal isotope effect (k H/k D ∼ 1.7), suggesting proton transfer in the rate-limiting step. X-ray absorption spectroscopy on NOV2 and CAO1 indicated that the protonation states of the iron ligands are unchanged within pH 6.5-8.5 and that the Fe(II)-aquo bond is minimally altered by substrate binding. We pinpointed the origin of the differential kinetic behaviors of NOV2 and CAO1 to a single amino acid difference near the solvent-binding site of iron, and X-ray crystallography revealed that the substitution alters binding of diffusible ligands to the iron center. We conclude that solvent-iron dissociation and proton transfer are both associated with the CCD catalytic mechanism.
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Affiliation(s)
- Nimesh Khadka
- From the Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106
| | - Erik R Farquhar
- National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, New York 11973.,Center for Proteomics and Bioinformatics, Center for Synchrotron Biosciences, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4988, and
| | - Hannah E Hill
- From the Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106
| | - Wuxian Shi
- National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, New York 11973.,Center for Proteomics and Bioinformatics, Center for Synchrotron Biosciences, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4988, and
| | - Johannes von Lintig
- From the Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106
| | - Philip D Kiser
- From the Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, .,Research Service, Louis Stokes Cleveland Veterans Affairs Medical Center, Cleveland, Ohio 44106
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7
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Khadka N, Zannou A, Truong D, Zhang T, Esteller R, Hersey B, Bikson M. Generation 2 kilohertz spinal cord stimulation (kHz-SCS) bioheat multi-physics model. Brain Stimul 2019. [DOI: 10.1016/j.brs.2018.12.876] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022] Open
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Choi EH, Suh S, Sander CL, Hernandez CJO, Bulman ER, Khadka N, Dong Z, Shi W, Palczewski K, Kiser PD. Insights into the pathogenesis of dominant retinitis pigmentosa associated with a D477G mutation in RPE65. Hum Mol Genet 2019; 27:2225-2243. [PMID: 29659842 DOI: 10.1093/hmg/ddy128] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2018] [Accepted: 04/06/2018] [Indexed: 12/31/2022] Open
Abstract
RPE65 is the essential trans-cis isomerase of the classical retinoid (visual) cycle. Mutations in RPE65 give rise to severe retinal dystrophies, most of which are associated with loss of protein function and recessive inheritance. The only known exception is a c.1430G>A (D477G) mutation that gives rise to dominant retinitis pigmentosa with delayed onset and choroidal and macular involvement. Position 477 is distant from functionally critical regions of RPE65. Hence, the mechanism of D477G pathogenicity remains unclear, although protein misfolding and aggregation mechanisms have been suggested. We characterized a D477G knock-in mouse model which exhibited mild age-dependent changes in retinal structure and function. Immunoblot analysis of protein extracts from the eyes of these knock-in mice demonstrated the presence of ubiquitinated RPE65 and reduced RPE65 expression. We observed an accumulation of retinyl esters in the knock-in mice as well as a delay in rhodopsin regeneration kinetics and diminished electroretinography responses, indicative of RPE65 functional impairment induced by the D477G mutation in vivo. However, a cell line expressing D477G RPE65 revealed protein expression levels, cellular localization and retinoid isomerase activity comparable to cells expressing wild-type protein. Structural analysis of an RPE65 chimera suggested that the D477G mutation does not perturb protein folding or tertiary structure. Instead, the mutation generates an aggregation-prone surface that could induce cellular toxicity through abnormal complex formation as suggested by crystal packing analysis. These results indicate that a toxic gain-of-function induced by the D477G RPE65 substitution may play a role in the pathogenesis of this form of dominant retinitis pigmentosa.
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Affiliation(s)
- Elliot H Choi
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Susie Suh
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Christopher L Sander
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Christian J Ortiz Hernandez
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA.,University of Puerto Rico at Humacao, Humacao, PR, USA
| | - Elizabeth R Bulman
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA.,Research Service, Louis Stokes Cleveland VA Medical Center, Cleveland, OH 44106, USA
| | - Nimesh Khadka
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Zhiqian Dong
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA.,Polgenix Inc., Cleveland, OH 44106, USA
| | - Wuxian Shi
- National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, NY 11973, USA
| | - Krzysztof Palczewski
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA.,Cleveland Center for Membrane and Structural Biology, Case Western Reserve University, Cleveland, OH 44106, USA
| | - Philip D Kiser
- Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA.,Research Service, Louis Stokes Cleveland VA Medical Center, Cleveland, OH 44106, USA.,Cleveland Center for Membrane and Structural Biology, Case Western Reserve University, Cleveland, OH 44106, USA
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Lukoyanov DA, Khadka N, Yang ZY, Dean DR, Seefeldt LC, Hoffman BM. Hydride Conformers of the Nitrogenase FeMo-cofactor Two-Electron Reduced State E 2(2H), Assigned Using Cryogenic Intra Electron Paramagnetic Resonance Cavity Photolysis. Inorg Chem 2018; 57:6847-6852. [PMID: 29575898 PMCID: PMC6008734 DOI: 10.1021/acs.inorgchem.8b00271] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Early studies in which nitrogenase was freeze-trapped during enzymatic turnover revealed the presence of high-spin ( S = 3/2) electron paramagnetic resonance (EPR) signals from the active-site FeMo-cofactor (FeMo-co) in electron-reduced intermediates of the MoFe protein. Historically denoted as 1b and 1c, each of the signals is describable as a fictitious spin system, S' = 1/2, with anisotropic g' tensor, 1b with g' = [4.21, 3.76, ?] and 1c with g' = [4.69, ∼3.20, ?]. A clear discrepancy between the magnetic properties of 1b and 1c and the kinetic analysis of their appearance during pre-steady-state turnover left their identities in doubt, however. We subsequently associated 1b with the state having accumulated 2[e-/H+], denoted as E2(2H), and suggested that the reducing equivalents are stored on the catalytic FeMo-co cluster as an iron hydride, likely an [Fe-H-Fe] hydride bridge. Intra-EPR cavity photolysis (450 nm; temperature-independent from 4 to 12 K) of the E2(2H)/1b state now corroborates the identification of this state as storing two reducing equivalents as a hydride. Photolysis converts E2(2H)/1b to a state with the same EPR spectrum, and thus the same cofactor structure as pre-steady-state turnover 1c, but with a different active-site environment. Upon annealing of the photogenerated state at temperature T = 145 K, it relaxes back to E2(2H)/1b. This implies that the 1c signal comes from an E2(2H) hydride isomer of E2(2H)/1b that stores its two reducing equivalents either as a hydride bridge between a different pair of iron atoms or an Fe-H terminal hydride.
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Affiliation(s)
- Dmitriy A Lukoyanov
- Departments of Chemistry and Molecular Biosciences , Northwestern University , Evanston , Illinois 60208 , United States
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry , Utah State University , Logan , Utah 84322 , United States
| | - Zhi-Yong Yang
- Department of Chemistry and Biochemistry , Utah State University , Logan , Utah 84322 , United States
| | - Dennis R Dean
- Department of Biochemistry , Virginia Polytechnic Institute and State University , Blacksburg , Virginia 24061 , United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry , Utah State University , Logan , Utah 84322 , United States
| | - Brian M Hoffman
- Departments of Chemistry and Molecular Biosciences , Northwestern University , Evanston , Illinois 60208 , United States
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10
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Mao Z, Liou SH, Khadka N, Jenney FE, Goodin DB, Seefeldt LC, Adams MWW, Cramer SP, Larsen DS. Cluster-Dependent Charge-Transfer Dynamics in Iron-Sulfur Proteins. Biochemistry 2018; 57:978-990. [PMID: 29303562 DOI: 10.1021/acs.biochem.7b01159] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Photoinduced charge-transfer dynamics and the influence of cluster size on the dynamics were investigated using five iron-sulfur clusters: the 1Fe-4S cluster in Pyrococcus furiosus rubredoxin, the 2Fe-2S cluster in Pseudomonas putida putidaredoxin, the 4Fe-4S cluster in nitrogenase iron protein, and the 8Fe-7S P-cluster and the 7Fe-9S-1Mo FeMo cofactor in nitrogenase MoFe protein. Laser excitation promotes the iron-sulfur clusters to excited electronic states that relax to lower states. The electronic relaxation lifetimes of the 1Fe-4S, 8Fe-7S, and 7Fe-9S-1Mo clusters are on the picosecond time scale, although the dynamics of the MoFe protein is a mixture of the dynamics of the latter two clusters. The lifetimes of the 2Fe-2S and 4Fe-4S clusters, however, extend to several nanoseconds. A competition between reorganization energies and the density of electronic states (thus electronic coupling between states) mediates the charge-transfer lifetimes, with the 2Fe-2S cluster of Pdx and the 4Fe-4S cluster of Fe protein lying at the optimum leading to them having significantly longer lifetimes. Their long lifetimes make them the optimal candidates for long-range electron transfer and as external photosensitizers for other photoactivated chemical reactions like solar hydrogen production. Potential electron-transfer and hole-transfer pathways that possibly facilitate these charge transfers are proposed.
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Affiliation(s)
- Ziliang Mao
- Department of Chemistry, University of California at Davis , One Shields Avenue, Davis, California 95616, United States
| | - Shu-Hao Liou
- Department of Chemistry, University of California at Davis , One Shields Avenue, Davis, California 95616, United States
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University , 0300 Old Main Hill, Logan, Utah 84322, United States
| | - Francis E Jenney
- Georgia Campus, Philadelphia College of Osteopathic Medicine , Suwanee, Georgia 30024, United States
| | - David B Goodin
- Department of Chemistry, University of California at Davis , One Shields Avenue, Davis, California 95616, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University , 0300 Old Main Hill, Logan, Utah 84322, United States
| | - Michael W W Adams
- Department of Biochemistry, The University of Georgia , Athens, Georgia 30602, United States
| | - Stephen P Cramer
- Department of Chemistry, University of California at Davis , One Shields Avenue, Davis, California 95616, United States
| | - Delmar S Larsen
- Department of Chemistry, University of California at Davis , One Shields Avenue, Davis, California 95616, United States
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11
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Khadka N, Milton RD, Shaw S, Lukoyanov D, Dean DR, Minteer SD, Raugei S, Hoffman BM, Seefeldt LC. Mechanism of Nitrogenase H 2 Formation by Metal-Hydride Protonation Probed by Mediated Electrocatalysis and H/D Isotope Effects. J Am Chem Soc 2017; 139:13518-13524. [PMID: 28851217 DOI: 10.1021/jacs.7b07311] [Citation(s) in RCA: 37] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Nitrogenase catalyzes the reduction of dinitrogen (N2) to two ammonia (NH3) at its active site FeMo-cofactor through a mechanism involving reductive elimination of two [Fe-H-Fe] bridging hydrides to make H2. A competing reaction is the protonation of the hydride [Fe-H-Fe] to make H2. The overall nitrogenase rate-limiting step is associated with ATP-driven electron delivery from Fe protein, precluding isotope effect measurements on substrate reduction steps. Here, we use mediated bioelectrocatalysis to drive electron delivery to the MoFe protein allowing examination of the mechanism of H2 formation by the metal-hydride protonation reaction. The ratio of catalytic current in mixtures of H2O and D2O, the proton inventory, was found to change linearly with the D2O/H2O ratio, revealing that a single H/D is involved in the rate-limiting step of H2 formation. Kinetic models, along with measurements that vary the electron/proton delivery rate and use different substrates, reveal that the rate-limiting step under these conditions is the H2 formation reaction. Altering the chemical environment around the active site FeMo-cofactor in the MoFe protein, either by substituting nearby amino acids or transferring the isolated FeMo-cofactor into a different peptide matrix, changes the net isotope effect, but the proton inventory plot remains linear, consistent with an unchanging rate-limiting step. Density functional theory predicts a transition state for H2 formation where the S-H+ bond breaks and H+ attacks the Fe-hydride, and explains the observed H/D isotope effect. This study not only reveals the nitrogenase mechanism of H2 formation by hydride protonation, but also illustrates a strategy for mechanistic study that can be applied to other oxidoreductase enzymes and to biomimetic complexes.
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Affiliation(s)
- Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Ross D Milton
- Department of Chemistry, University of Utah , Salt Lake City, Utah 84112, United States
| | - Sudipta Shaw
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Dmitriy Lukoyanov
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
| | - Dennis R Dean
- Department of Biochemistry, Virginia Tech , Blacksburg, Virginia 24061, United States
| | - Shelley D Minteer
- Department of Chemistry, University of Utah , Salt Lake City, Utah 84112, United States
| | - Simone Raugei
- Pacific Northwest National Laboratory , Richland, Washington 99352, United States
| | - Brian M Hoffman
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
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12
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Zareen N, Shinozaki M, Ryan D, Alexander H, Amer A, Truong DQ, Khadka N, Sarkar A, Naeem S, Bikson M, Martin JH. Motor cortex and spinal cord neuromodulation promote corticospinal tract axonal outgrowth and motor recovery after cervical contusion spinal cord injury. Exp Neurol 2017; 297:179-189. [PMID: 28803750 DOI: 10.1016/j.expneurol.2017.08.004] [Citation(s) in RCA: 43] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2017] [Revised: 08/04/2017] [Accepted: 08/09/2017] [Indexed: 01/15/2023]
Abstract
Cervical injuries are the most common form of SCI. In this study, we used a neuromodulatory approach to promote skilled movement recovery and repair of the corticospinal tract (CST) after a moderately severe C4 midline contusion in adult rats. We used bilateral epidural intermittent theta burst (iTBS) electrical stimulation of motor cortex to promote CST axonal sprouting and cathodal trans-spinal direct current stimulation (tsDCS) to enhance spinal cord activation to motor cortex stimulation after injury. We used Finite Element Method (FEM) modeling to direct tsDCS to the cervical enlargement. Combined iTBS-tsDCS was delivered for 30min daily for 10days. We compared the effect of stimulation on performance in the horizontal ladder and the Irvine Beattie and Bresnahan forepaw manipulation tasks and CST axonal sprouting in injury-only and injury+stimulation animals. The contusion eliminated the dorsal CST in all animals. tsDCS significantly enhanced motor cortex evoked responses after C4 injury. Using this combined spinal-M1 neuromodulatory approach, we found significant recovery of skilled locomotion and forepaw manipulation skills compared with injury-only controls. The spared CST axons caudal to the lesion in both animal groups derived mostly from lateral CST axons that populated the contralateral intermediate zone. Stimulation enhanced injury-dependent CST axonal outgrowth below and above the level of the injury. This dual neuromodulatory approach produced partial recovery of skilled motor behaviors that normally require integration of posture, upper limb sensory information, and intent for performance. We propose that the motor systems use these new CST projections to control movements better after injury.
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Affiliation(s)
- N Zareen
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - M Shinozaki
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - D Ryan
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - H Alexander
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - A Amer
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA; CUNY Graduate Center, New York, NY 10031, USA
| | - D Q Truong
- Department of Biomedical Engineering, City College of NY, 10031, USA
| | - N Khadka
- Department of Biomedical Engineering, City College of NY, 10031, USA
| | - A Sarkar
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - S Naeem
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA
| | - M Bikson
- Department of Biomedical Engineering, City College of NY, 10031, USA
| | - J H Martin
- Department of Molecular, Cellular, and Biomedical Sciences, City University of NY School of Medicine, New York, NY 10031, USA; CUNY Graduate Center, New York, NY 10031, USA.
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13
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Lukoyanov D, Khadka N, Dean DR, Raugei S, Seefeldt LC, Hoffman BM. Photoinduced Reductive Elimination of H 2 from the Nitrogenase Dihydride (Janus) State Involves a FeMo-cofactor-H 2 Intermediate. Inorg Chem 2017; 56:2233-2240. [PMID: 28177622 DOI: 10.1021/acs.inorgchem.6b02899] [Citation(s) in RCA: 36] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
N2 reduction by nitrogenase involves the accumulation of four reducing equivalents at the active site FeMo-cofactor to form a state with two [Fe-H-Fe] bridging hydrides (denoted E4(4H), the Janus intermediate), and we recently demonstrated that the enzyme is activated to cleave the N≡N triple bond by the reductive elimination (re) of H2 from this state. We are exploring a photochemical approach to obtaining atomic-level details of the re activation process. We have shown that, when E4(4H) at cryogenic temperatures is subjected to 450 nm irradiation in an EPR cavity, it cleanly undergoes photoinduced re of H2 to give a reactive doubly reduced intermediate, denoted E4(2H)*, which corresponds to the intermediate that would form if thermal dissociative re loss of H2 preceded N2 binding. Experiments reported here establish that photoinduced re primarily occurs in two steps. Photolysis of E4(4H) generates an intermediate state that undergoes subsequent photoinduced conversion to [E4(2H)* + H2]. The experiments, supported by DFT calculations, indicate that the trapped intermediate is an H2 complex on the ground adiabatic potential energy suface that connects E4(4H) with [E4(2H)* + H2]. We suggest that this complex, denoted E4(H2; 2H), is a thermally populated intermediate in the catalytically central re of H2 by E4(4H) and that N2 reacts with this complex to complete the activated conversion of [E4(4H) + N2] into [E4(2N2H) + H2].
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Affiliation(s)
- Dmitriy Lukoyanov
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Dennis R Dean
- Department of Biochemistry, Virginia Tech , 110 Fralin Hall, Blacksburg, Virginia 24061, United States
| | - Simone Raugei
- Pacific Northwestern National Laboratory , Richland, Washington 99352, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Brian M Hoffman
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
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Abstract
The reduction of N2 to NH3 by Mo-dependent nitrogenase at its active-site metal cluster FeMo-cofactor utilizes reductive elimination of Fe-bound hydrides with obligatory loss of H2 to activate the enzyme for binding/reduction of N2. Earlier work showed that wild-type nitrogenase and a nitrogenase with amino acid substitutions in the MoFe protein near FeMo-cofactor can catalytically reduce CO2 by two or eight electrons/protons to carbon monoxide (CO) and methane (CH4) at low rates. Here, it is demonstrated that nitrogenase preferentially reduces CO2 by two electrons/protons to formate (HCOO(-)) at rates >10 times higher than rates of CO2 reduction to CO and CH4. Quantum mechanical calculations on the doubly reduced FeMo-cofactor with a Fe-bound hydride and S-bound proton (E2(2H) state) favor a direct reaction of CO2 with the hydride ("direct hydride transfer" reaction pathway), with facile hydride transfer to CO2 yielding formate. In contrast, a significant barrier is observed for reaction of Fe-bound CO2 with the hydride ("associative" reaction pathway), which leads to CO and CH4. Remarkably, in the direct hydride transfer pathway, the Fe-H behaves as a hydridic hydrogen, whereas in the associative pathway it acts as a protic hydrogen. MoFe proteins with amino acid substitutions near FeMo-cofactor (α-70(Val→Ala), α-195(His→Gln)) are found to significantly alter the distribution of products between formate and CO/CH4.
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Affiliation(s)
- Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
| | - Dennis R. Dean
- Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061
| | | | - Brian M. Hoffman
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208
| | - Simone Raugei
- Pacific Northwestern National Laboratory, Richland, Washington 99352
| | - Lance C. Seefeldt
- Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
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Lukoyanov D, Khadka N, Yang ZY, Dean DR, Seefeldt LC, Hoffman BM. Reductive Elimination of H2 Activates Nitrogenase to Reduce the N≡N Triple Bond: Characterization of the E4(4H) Janus Intermediate in Wild-Type Enzyme. J Am Chem Soc 2016; 138:10674-83. [PMID: 27529724 DOI: 10.1021/jacs.6b06362] [Citation(s) in RCA: 108] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
We proposed a reductive elimination/oxidative addition (re/oa) mechanism for reduction of N2 to 2NH3 by nitrogenase, based on identification of a freeze-trapped intermediate of the α-70(Val→Ile) MoFe protein as the Janus intermediate that stores four reducing equivalents on FeMo-co as two [Fe-H-Fe] bridging hydrides (denoted E4(4H)). The mechanism postulates that obligatory re of the hydrides as H2 drives reduction of N2 to a state (denoted E4(2N2H)) with a moiety at the diazene (HN═NH) reduction level bound to the catalytic FeMo-co. EPR/ENDOR/photophysical measurements on wild type (WT) MoFe protein now establish this mechanism. They show that a state freeze-trapped during N2 reduction by WT MoFe is the same Janus intermediate, thereby establishing the α-70(Val→Ile) intermediate as a reliable guide to mechanism. Monitoring the Janus state in WT MoFe during N2 reduction under mixed-isotope condition, H2O buffer/D2, and the converse, establishes that the bridging hydrides/deuterides do not exchange with solvent during enzymatic turnover, thereby solving longstanding puzzles. Relaxation of E4(2N2H) to the WT resting-state is shown to occur via oa of H2 and release of N2 to form Janus, followed by sequential release of two H2, demonstrating the kinetic reversibility of the re/oa equilibrium. Relative populations of E4(2N2H)/E4(4H) freeze-trapped during WT turnover furthermore show that the reversible re/oa equilibrium between [E4(4H) + N2] and [E4(2N2H) + H2] is ∼ thermoneutral (ΔreG(0) ∼ -2 kcal/mol), whereas, by itself, hydrogenation of N2(g) is highly endergonic. These findings demonstrate that (i) re/oa accounts for the historical Key Constraints on mechanism, (ii) that Janus is central to N2 reduction by WT enzyme, which (iii) indeed occurs via the re/oa mechanism. Thus, emerges a picture of the central mechanistic steps by which nitrogenase carries out one of the most challenging chemical transformations in biology.
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Affiliation(s)
- Dmitriy Lukoyanov
- Departments of Chemistry and Molecular Biosciences, Northwestern University , Evanston, Illinois 60208, United States
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Zhi-Yong Yang
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Dennis R Dean
- Department of Biochemistry, Virginia Tech , Blacksburg, Virginia 24061, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Brian M Hoffman
- Departments of Chemistry and Molecular Biosciences, Northwestern University , Evanston, Illinois 60208, United States
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Davydov R, Khadka N, Yang ZY, Fielding AJ, Lukoyanov D, Dean DR, Seefeldt LC, Hoffman BM. Exploring Electron/Proton Transfer and Conformational Changes in the Nitrogenase MoFe Protein and FeMo-cofactor Through Cryoreduction/EPR Measurements. Isr J Chem 2016; 56:841-851. [PMID: 27777444 DOI: 10.1002/ijch.201600026] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
We combine cryoreduction/annealing/EPR measurements of nitrogenase MoFe protein with results of earlier investigations to provide a detailed view of the electron/proton transfer events and conformational changes that occur during early stages of [e-/H+] accumulation by the MoFe protein. This includes reduction of (i) the non-catalytic state of the iron-molybdenum cofactor (FeMo-co) active site that is generated by chemical oxidation of the resting-state cofactor (S = 3/2)) within resting MoFe (E0), and (ii) the catalytic state that has accumulated n =1 [e-/H+] above the resting-state level, denoted E1(1H) (S ≥ 1) in the Lowe-Thorneley kinetic scheme. FeMo-co does not undergo a major change of conformation during reduction of oxidized FeMo-co. In contrast, FeMo-co undergoes substantial conformational changes during the reduction of E0 to E1(1H), and of E1(1H) to E2(2H) (n = 2; S = 3/2). The experimental results further suggest that the E1(1H) → E2(2H) step involves coupled delivery of a proton and electron (PCET) to FeMo-co of E1(H) to generate a non-equilibrium S = ½ form E2(2H)*. This subsequently undergoes conformational relaxation and attendant change in FeMo-co spin state, to generate the equilibrium E2(2H) (S = 3/2) state. Unexpectedly, these experiments also reveal conformational coupling between FeMo-co and P-cluster, and between Fe protein binding and FeMo-co, which might play a role in gated ET from reduced Fe protein to FeMo-co.
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Affiliation(s)
- Roman Davydov
- Department of Chemistry, Northwestern University, Evanston, IL 60208
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322
| | - Zhi-Yong Yang
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322
| | - Andrew J Fielding
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322
| | - Dmitriy Lukoyanov
- Department of Chemistry, Northwestern University, Evanston, IL 60208
| | - Dennis R Dean
- Department of Biochemistry, Virginia Tech, 110 Fralin Hall, Blacksburg, VA 24061
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322
| | - Brian M Hoffman
- Department of Chemistry, Northwestern University, Evanston, IL 60208
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17
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Brown KA, Harris DF, Wilker MB, Rasmussen A, Khadka N, Hamby H, Keable S, Dukovic G, Peters JW, Seefeldt LC, King PW. Light-driven dinitrogen reduction catalyzed by a CdS:nitrogenase MoFe protein biohybrid. Science 2016; 352:448-50. [PMID: 27102481 DOI: 10.1126/science.aaf2091] [Citation(s) in RCA: 390] [Impact Index Per Article: 48.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2016] [Accepted: 03/23/2016] [Indexed: 01/24/2023]
Abstract
The splitting of dinitrogen (N2) and reduction to ammonia (NH3) is a kinetically complex and energetically challenging multistep reaction. In the Haber-Bosch process, N2 reduction is accomplished at high temperature and pressure, whereas N2 fixation by the enzyme nitrogenase occurs under ambient conditions using chemical energy from adenosine 5'-triphosphate (ATP) hydrolysis. We show that cadmium sulfide (CdS) nanocrystals can be used to photosensitize the nitrogenase molybdenum-iron (MoFe) protein, where light harvesting replaces ATP hydrolysis to drive the enzymatic reduction of N2 into NH3 The turnover rate was 75 per minute, 63% of the ATP-coupled reaction rate for the nitrogenase complex under optimal conditions. Inhibitors of nitrogenase (i.e., acetylene, carbon monoxide, and dihydrogen) suppressed N2 reduction. The CdS:MoFe protein biohybrids provide a photochemical model for achieving light-driven N2 reduction to NH3.
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Affiliation(s)
- Katherine A Brown
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Derek F Harris
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA
| | - Molly B Wilker
- Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80309, USA
| | - Andrew Rasmussen
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA
| | - Hayden Hamby
- Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80309, USA
| | - Stephen Keable
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, USA
| | - Gordana Dukovic
- Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO 80309, USA
| | - John W Peters
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, USA
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA
| | - Paul W King
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA.
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18
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Lukoyanov D, Khadka N, Yang ZY, Dean DR, Seefeldt LC, Hoffman BM. Reversible Photoinduced Reductive Elimination of H2 from the Nitrogenase Dihydride State, the E(4)(4H) Janus Intermediate. J Am Chem Soc 2016; 138:1320-7. [PMID: 26788586 DOI: 10.1021/jacs.5b11650] [Citation(s) in RCA: 51] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
We recently demonstrated that N2 reduction by nitrogenase involves the obligatory release of one H2 per N2 reduced. These studies focus on the E4(4H) "Janus intermediate", which has accumulated four reducing equivalents as two [Fe-H-Fe] bridging hydrides. E4(4H) is poised to bind and reduce N2 through reductive elimination (re) of the two hydrides as H2, coupled to the binding/reduction of N2. To obtain atomic-level details of the re activation process, we carried out in situ 450 nm photolysis of E4(4H) in an EPR cavity at temperatures below 20 K. ENDOR and EPR measurements show that photolysis generates a new FeMo-co state, denoted E4(2H)*, through the photoinduced re of the two bridging hydrides of E4(4H) as H2. During cryoannealing at temperatures above 175 K, E4(2H)* reverts to E4(4H) through the oxidative addition (oa) of the H2. The photolysis quantum yield is temperature invariant at liquid helium temperatures and shows a rather large kinetic isotope effect, KIE = 10. These observations imply that photoinduced release of H2 involves a barrier to the combination of the two nascent H atoms, in contrast to a barrierless process for monometallic inorganic complexes, and further suggest that H2 formation involves nuclear tunneling through that barrier. The oa recombination of E4(2H)* with the liberated H2 offers compelling evidence for the Janus intermediate as the point at which H2 is necessarily lost during N2 reduction; this mechanistically coupled loss must be gated by N2 addition that drives the re/oa equilibrium toward reductive elimination of H2 with N2 binding/reduction.
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Affiliation(s)
- Dmitriy Lukoyanov
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
| | - Nimesh Khadka
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Zhi-Yong Yang
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Dennis R Dean
- Department of Biochemistry, Virginia Tech , Blacksburg, Virginia 24061, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University , Logan, Utah 84322, United States
| | - Brian M Hoffman
- Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States
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Lukoyanov D, Yang ZY, Khadka N, Dean DR, Seefeldt LC, Hoffman BM. Identification of a key catalytic intermediate demonstrates that nitrogenase is activated by the reversible exchange of N₂ for H₂. J Am Chem Soc 2015; 137:3610-5. [PMID: 25741750 DOI: 10.1021/jacs.5b00103] [Citation(s) in RCA: 81] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Freeze-quenching nitrogenase during turnover with N2 traps an S = ½ intermediate that was shown by ENDOR and EPR spectroscopy to contain N2 or a reduction product bound to the active-site molybdenum-iron cofactor (FeMo-co). To identify this intermediate (termed here EG), we turned to a quench-cryoannealing relaxation protocol. The trapped state is allowed to relax to the resting E0 state in frozen medium at a temperature below the melting temperature; relaxation is monitored by periodically cooling the sample to cryogenic temperature for EPR analysis. During -50 °C cryoannealing of EG prepared under turnover conditions in which the concentrations of N2 and H2 ([H2], [N2]) are systematically and independently varied, the rate of decay of EG is accelerated by increasing [H2] and slowed by increasing [N2] in the frozen reaction mixture; correspondingly, the accumulation of EG is greater with low [H2] and/or high [N2]. The influence of these diatomics identifies EG as the key catalytic intermediate formed by reductive elimination of H2 with concomitant N2 binding, a state in which FeMo-co binds the components of diazene (an N-N moiety, perhaps N2 and two [e(-)/H(+)] or diazene itself). This identification combines with an earlier study to demonstrate that nitrogenase is activated for N2 binding and reduction through the thermodynamically and kinetically reversible reductive-elimination/oxidative-addition exchange of N2 and H2, with an implied limiting stoichiometry of eight electrons/protons for the reduction of N2 to two NH3.
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Affiliation(s)
- Dmitriy Lukoyanov
- ‡Departments of Chemistry and Molecular Biosciences, Northwestern University, Evanston, Illinois 60208, United States
| | - Zhi-Yong Yang
- †Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
| | - Nimesh Khadka
- †Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
| | - Dennis R Dean
- §Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
| | - Lance C Seefeldt
- †Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
| | - Brian M Hoffman
- ‡Departments of Chemistry and Molecular Biosciences, Northwestern University, Evanston, Illinois 60208, United States
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Banskota N, Jha R, Khadka N, Sharma GR, Bista P, Kumar P. Surgical Management of Spinal Dysraphism: Five -year Experience in a Central Hospital. J Nepal Paedtr Soc 2014. [DOI: 10.3126/jnps.v34i1.9378] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Introduction: Spinal dysraphism is a heterogeneous group of congenital spinal anomalies resulting from defective closure of the neural tube early in fetal life and anomalous development of the caudal cell mass. Meningomyelocele is common among Neural tube defects. Patients with myelomeningocele present with a spectrum of impairments, including primary functional deficits like are lower limb paralysis, sensory loss, bladderbowel dysfunction and cognitive dysfunction. Medical, surgical management and rehabilitation have helped patients with neural tube defects to participate and be productive in mainstream society. The aims of this study were to review the clinical presentation, surgical management and their outcome in the patient with spinal dysraphism. Materials and Methods: This is a retrospective study of Forty-one cases of spinal dysraphism managed during a period of five years from January 2008 to December 2012 in Department of Neurosurgery, National Academy of Medical Sciences (NAMS), Bir Hospital. Demographic profiles, clinical presentation of patients with spinal dysraphism, associated hydocephalus, surgical management and outcome were studied. Results: Out of total 41 cases studied, male patients outnumbered female with 58.5% to 41.5%. Age ranged from 5 days to 29 years and mean age was 2.71 years whereas. Lump (97%) and paraparesis (88%) were frequent mode of presentation. Lumbar lesion (65%) was commonest followed by lumbosacral (29%). Hydrocephalus was present in 51% of cases and in 24% cases developed hydrocephalus later after repair. Total 75.6% of cases were treated with VP shunt. Conclusion: Spinal dysraphism is debilitating entity and management is challenging. Lump on back and weakness of limb are major factor for children and their parents seek medical service. Lesion in low back (lumbar and lumbosacral) were most common location. Besides repair, majority of them needed CSF diversion surgery for hydocephalus. Aim of surgical management was to prevent further deterioration, control of hydrocephalus or leak. DOI: http://dx.doi.org/10.3126/jnps.v34i1.9378 J Nepal Paediatr Soc 2014;34(1):34-38
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Kc A, Thapa K, Pradhan YV, Kc NP, Upreti SR, Adhikari RK, Khadka N, Acharya B, Dhakwa JR, Aryal DR, Aryal S, Starbuck E, Paudel D, Khanal S, Devkota MD. Developing community-based intervention strategies and package to save newborns in Nepal. J Nepal Health Res Counc 2011; 9:107-118. [PMID: 22929839] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
In Nepal, the proportion of under 5 deaths that are neonatal (0-28 days) has been increasing in the last decade, due to faster declines in infant and child mortality than in neonatal mortality. This trend is likely due to a focus on maternal and child survival programs that did not adequately address newborn health needs. Policy and actions to save newborn lives resulted from increased attention to newborn deaths in 2001, culminating in the endorsement of the National Neonatal Health Strategy in 2004, a milestone that established newborn health and survival as a national priority. Operationalization of the National Neonatal Health Strategy took place in 2007 with the development of the Community-Based Newborn Care Package (CB-NCP). This paper describes how national stakeholders used global, regional and in-country research and policies to develop the CB-NCP, thus outlining key ingredients to make newborn health programming a reality in Nepal. A technical working group was constituted to review existing evidence on interventions to improve newborn survival, develop a tool to prioritize neonatal interventions, and conduct program learning visits to identify key components appropriate to the Nepal context that should be included in the Community Based Integrated Newborn Care Package. The group identified interventions based on the evidence of impact on newborn survival, potential mechanisms within the existing health system to deliver the interventions, and linkages with existing programs and different tiers of the health system. Not only was Nepal one of the first countries in south-east Asia where government adopted a national strategy to reduce neonatal deaths, but it was also one of the first to endorse a package of neonatal interventions for pilot testing and scaling up through existing community-based health systems that provide basic health services throughout the country. CB-NCP was designed to be gradually scaled up throughout the country by integration with Safe Motherhood and Child survival programs that are currently operating at scale. Under Ministry of health and Population leadership, a network of academia, professional bodies and partners developed a common vision for improving newborn health and survival, and launched district-level pilot programs to demonstrate and learn how newborn health interventions could be effectively and efficiently delivered and scaled up in Nepal.
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Affiliation(s)
- A Kc
- Save the Children Nepal, Kathmandu, Nepal.
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Sharma GR, Jha R, Khadka N, Adhikari DR, Bista P, Sultaniya PK. Bilateral, Mirror-imaged, Postero-inferior Cerebellar Artery Aneurysms: Report of a Rare Case. JNMA J Nepal Med Assoc 2011. [DOI: 10.31729/jnma.251] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/01/2022] Open
Abstract
A 60-year-old right-handed lady presented with the features of subarachnoid haemorrhage. The CT angiogram showed a pair of very rare bilateral, mirror-imaged distal postero-inferior cerebellar artery aneurysms. Both aneurysms were clipped via the midline posterior fossa craniectomy under general anaesthesia. The literatures is reviewed on the incidence, presentation, management and outcome of bilateral distal posterior-inferior cerebellar artery aneurysms. Keywords: bilateral mirror image, distal postero-inferior cerebellar artery aneurysms, posterior fossa craniectomy, microsurgical clipping
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23
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Sharma GR, Jha R, Khadka N, Adhikari DR, Bista P, Sultaniya PK. Bilateral, mirror-imaged, postero-inferior cerebellar artery aneurysms: report of a rare case. JNMA J Nepal Med Assoc 2011; 51:90-93. [PMID: 22916520] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/01/2023] Open
Abstract
A 60-year-old right-handed lady presented with the features of subarachnoid haemorrhage. The CT angiogram showed a pair of very rare bilateral, mirror-imaged distal postero-inferior cerebellar artery aneurysms. Both aneurysms were clipped via the midline posterior fossa craniectomy under general anaesthesia. The literatures is reviewed on the incidence, presentation, management and outcome of bilateral distal posterior-inferior cerebellar artery aneurysms.
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Affiliation(s)
- G R Sharma
- Department of Neurosurgery, National Academy of Medical sciences, Bir Hospital, Kathmandu, Nepal.
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Syed U, Khadka N, Khan A, Wall S. Care-seeking practices in South Asia: using formative research to design program interventions to save newborn lives. J Perinatol 2008; 28 Suppl 2:S9-13. [PMID: 19057572 DOI: 10.1038/jp.2008.165] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The purpose of this paper is to summarize the formative research findings of newborn care practices in poor and rural districts of Bangladesh, Nepal, and Pakistan and to explain how these findings were used to design behavior change communication elements of newborn care programs. In-depth interviews and focus group discussions regarding newborn care practices were conducted with mothers, mothers-in-law, delivery attendants, health care providers, husbands/fathers, male and female community leaders, religious leaders and elderly influential persons between 2002-2003 in three countries supported by Save the Children's Saving Newborn Lives program. Key findings from each country are summarized according to time periods and care-seeking practices: antenatal care, birth and emergency preparedness/care-seeking, postnatal care and care-seeking for newborn illness. All country reports indicated cultural and religious barriers to seeking care as well as limited societal knowledge about the importance of care-seeking and recognition of maternal and newborn danger signs. Routine care-seeking, especially during the postnatal period, was universally low. When families did seek care, they preferred remedies from traditional healers rather than skilled health workers because of cultural and religious beliefs, poor access to health facilities, and financial barriers. Findings from the country reports were used to design behavior change communication strategies that addressed the underlying reasons why newborn care practices were sub-optimal. Cultural and religious barriers, though strong, were not insurmountable in implementing effective behavior change communication strategies. Formative research from South Asian countries has proved crucial to program approaches to improve care-seeking for maternal and newborn care, increasing availability and access of key services, and expanding family and community knowledge and demand for these services.
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Affiliation(s)
- U Syed
- Saving Newborn Lives, Save the Children, Washington, DC 20036, USA
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Karmacharya BG, Khadka N, Joshi MR, Sharma VK. Cervical Schwannoma. JNMA J Nepal Med Assoc 2004. [DOI: 10.31729/jnma.497] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/01/2022] Open
Abstract
Schwannomas are solitary, slow growing benign tumors arising from the Schwann cells that form the myelin sheath around the peripheral nerves. Head and neck is the commonest site of these tumors in the body. We present a case of schwannoma in a 35 years old man who had a painless slowly growing tumor in the right side of his neck.Key Words: Schwannoma, FNAC, parapharyngeal space.
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Ruchal SP, Butlin CR, Khadka N, Mijar K. Extra depth shoes made on special last. LEPROSY REV 1999; 70:363-5. [PMID: 10603727] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/14/2023]
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