1
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Willard D, Arellano JJ, Underdahl M, Lee TM, Ramaswamy AS, Fumes G, Kliman A, Wong EY, Owens CP. Mutational Analysis of the Nitrogenase Carbon Monoxide Protective Protein CowN Reveals That a Conserved C-Terminal Glutamic Acid Residue Is Necessary for Its Activity. Biochemistry 2024; 63:152-158. [PMID: 38091601 PMCID: PMC10765410 DOI: 10.1021/acs.biochem.3c00421] [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: 08/10/2023] [Revised: 11/21/2023] [Accepted: 11/27/2023] [Indexed: 01/03/2024]
Abstract
Nitrogenase is the only enzyme that catalyzes the reduction of nitrogen gas into ammonia. Nitrogenase is tightly inhibited by the environmental gas carbon monoxide (CO). Many nitrogen fixing bacteria protect nitrogenase from CO inhibition using the protective protein CowN. This work demonstrates that a conserved glutamic acid residue near the C-terminus of Gluconacetobacter diazotrophicus CowN is necessary for its function. Mutation of the glutamic acid residue abolishes both CowN's protection against CO inhibition and the ability of CowN to bind to nitrogenase. In contrast, a conserved C-terminal cysteine residue is not important for CO protection by CowN. Overall, this work uncovers structural features in CowN that are required for its function and provides new insights into its nitrogenase binding and CO protection mechanism.
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Affiliation(s)
- Dustin
L. Willard
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Joshuah J. Arellano
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Mitch Underdahl
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Terrence M. Lee
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Avinash S. Ramaswamy
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Gabriella Fumes
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Agatha Kliman
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Emily Y. Wong
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
| | - Cedric P. Owens
- Department of Chemistry and
Biochemistry, Schmid College, Chapman University, Orange, California 92866, United States
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2
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Darnajoux R, Inomura K, Zhang X. A diazotrophy-ammoniotrophy dual growth model for the sulfate reducing bacterium Desulfovibrio vulgaris var. Hildenborough. Comput Struct Biotechnol J 2023; 21:3136-3148. [PMID: 37293241 PMCID: PMC10244686 DOI: 10.1016/j.csbj.2023.05.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2023] [Revised: 05/04/2023] [Accepted: 05/05/2023] [Indexed: 06/10/2023] Open
Abstract
Sulfate reducing bacteria (SRB) comprise one of the few prokaryotic groups in which biological nitrogen fixation (BNF) is common. Recent studies have highlighted SRB roles in N cycling, particularly in oligotrophic coastal and benthic environments where they could contribute significantly to N input. Most studies of SRB have focused on sulfur cycling and SRB growth models have primarily aimed at understanding the effects of electron sources, with N usually provided as fixed-N (nitrate, ammonium). Mechanistic links between SRB nitrogen-fixing metabolism and growth are not well understood, particularly in environments where fixed-N fluctuates. Here, we investigate diazotrophic growth of the model sulfate reducer Desulfovibrio vulgaris var. Hildenborough under anaerobic heterotrophic conditions and contrasting N availabilities using a simple cellular model with dual ammoniotrophic and diazotrophic modes. The model was calibrated using batch culture experiments with varying initial ammonium concentrations (0-3000 µM) and acetylene reduction assays of BNF activity. The model confirmed the preferential usage of ammonium over BNF for growth and successfully reproduces experimental data, with notably clear bi-phasic growth curves showing an initial ammoniotrophic phase followed by onset of BNF. Our model enables quantification of the energetic cost of each N acquisition strategy and indicates the existence of a BNF-specific limiting phenomenon, not directly linked to micronutrient (Mo, Fe, Ni) concentration, by-products (hydrogen, hydrogen sulfide), or fundamental model metabolic parameters (death rate, electron acceptor stoichiometry). By providing quantitative predictions of environment and metabolism, this study contributes to a better understanding of anaerobic heterotrophic diazotrophs in environments with fluctuating N conditions.
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Affiliation(s)
- Romain Darnajoux
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- High Meadow Environmental Institute, Princeton University, Princeton, NJ 08544, USA
| | - Keisuke Inomura
- Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA
| | - Xinning Zhang
- Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
- High Meadow Environmental Institute, Princeton University, Princeton, NJ 08544, USA
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3
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Medina MS, Bretzing KO, Aviles RA, Chong KM, Espinoza A, Garcia CNG, Katz BB, Kharwa RN, Hernandez A, Lee JL, Lee TM, Lo Verde C, Strul MW, Wong EY, Owens CP. CowN sustains nitrogenase turnover in the presence of the inhibitor carbon monoxide. J Biol Chem 2021; 296:100501. [PMID: 33667548 PMCID: PMC8047169 DOI: 10.1016/j.jbc.2021.100501] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2020] [Revised: 01/28/2021] [Accepted: 03/01/2021] [Indexed: 11/28/2022] Open
Abstract
Nitrogenase is the only enzyme capable of catalyzing nitrogen fixation, the reduction of dinitrogen gas (N2) to ammonia (NH3). Nitrogenase is tightly inhibited by the environmental gas carbon monoxide (CO). Nitrogen-fixing bacteria rely on the protein CowN to grow in the presence of CO. However, the mechanism by which CowN operates is unknown. Here, we present the biochemical characterization of CowN and examine how CowN protects nitrogenase from CO. We determine that CowN interacts directly with nitrogenase and that CowN protection observes hyperbolic kinetics with respect to CowN concentration. At a CO concentration of 0.001 atm, CowN restores nearly full nitrogenase activity. Our results further indicate that CowN's protection mechanism involves decreasing the binding affinity of CO to nitrogenase's active site approximately tenfold without interrupting substrate turnover. Taken together, our work suggests CowN is an important auxiliary protein in nitrogen fixation that engenders CO tolerance to nitrogenase.
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Affiliation(s)
- Michael S Medina
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Kevin O Bretzing
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Richard A Aviles
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Kiersten M Chong
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Alejandro Espinoza
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Chloe Nicole G Garcia
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Benjamin B Katz
- Department of Chemistry, University of California, Irvine, Irvine, California, USA
| | - Ruchita N Kharwa
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Andrea Hernandez
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Justin L Lee
- Department of Chemistry, University of California, Irvine, Irvine, California, USA
| | - Terrence M Lee
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Christine Lo Verde
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Max W Strul
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Emily Y Wong
- Schmid College of Science and Technology, Chapman University, Orange, California, USA
| | - Cedric P Owens
- Schmid College of Science and Technology, Chapman University, Orange, California, USA.
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4
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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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5
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Yoon KS, Nguyen NT, Tran KT, Tsuji K, Ogo S. Nitrogen Fixation Genes and Nitrogenase Activity of the Non-Heterocystous Cyanobacterium Thermoleptolyngbya sp. O-77. Microbes Environ 2017; 32:324-329. [PMID: 29176306 PMCID: PMC5745016 DOI: 10.1264/jsme2.me17015] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Cyanobacteria are widely distributed in marine, aquatic, and terrestrial ecosystems, and play an important role in the global nitrogen cycle. In the present study, we examined the genome sequence of the thermophilic non-heterocystous N2-fixing cyanobacterium, Thermoleptolyngbya sp. O-77 (formerly known as Leptolyngbya sp. O-77) and characterized its nitrogenase activity. The genome of this cyanobacterial strain O-77 consists of a single chromosome containing a nitrogen fixation gene cluster. A phylogenetic analysis indicated that the NifH amino acid sequence from strain O-77 was clustered with those from a group of mesophilic species: the highest identity was found in Leptolyngbya sp. KIOST-1 (97.9% sequence identity). The nitrogenase activity of O-77 cells was dependent on illumination, whereas a high intensity of light of 40 μmol m−2 s−1 suppressed the effects of illumination.
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Affiliation(s)
- Ki-Seok Yoon
- International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University.,Center for Small Molecule Energy, Kyushu University
| | - Nga T Nguyen
- International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University
| | - Kien Trung Tran
- Center for Small Molecule Energy, Kyushu University.,Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University
| | - Kohsei Tsuji
- Center for Small Molecule Energy, Kyushu University.,Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University
| | - Seiji Ogo
- International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University.,Center for Small Molecule Energy, Kyushu University.,Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University
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6
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7
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Kästner J, Blöchl PE. Model for Acetylene Reduction by Nitrogenase Derived from Density Functional Theory. Inorg Chem 2005; 44:4568-75. [PMID: 15962963 DOI: 10.1021/ic0500311] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The catalytic cycle of acetylene reduction at the FeMo cofactor of nitrogenase has been investigated on the basis of density functional theory. C2H2 binds to the same site as N2, but it binds to a less reduced state of the cofactor. In a manner similar to that of N2 binding, one of the sulfur bridges opens during acetylene binding. The model explains the strong noncompetitive inhibition of N2 reduction by C2H2 and the weak competitive inhibition of C2H2 reduction by N2. Our proposed mechanism is consistent with experimentally observed stereoselectivity and the ability of C2H2 to suppress H2 production by nitrogenase.
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Affiliation(s)
- Johannes Kästner
- Institute for Theoretical Physics, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany
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8
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Durrant MC. An atomic-level mechanism for molybdenum nitrogenase. Part 2. Proton reduction, inhibition of dinitrogen reduction by dihydrogen, and the HD formation reaction. Biochemistry 2002; 41:13946-55. [PMID: 12437351 DOI: 10.1021/bi025624r] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Quantum calculations have been used to examine the energetics of the reactions of diazene and isodiazene with H(2) and the properties of the Fe and Mo sites of the nitrogenase iron-molybdenum cofactor with respect to the binding of H and H(2). The results have been used to extend the model for N(2) reduction by nitrogenase given in the preceding paper to describe the formation of HD from D(2). The proposed mechanism for HD formation invokes a combination of two well-established chemical reactions, namely, competitive protonation of metal N(2) species at either the metal or at N(2), followed by scrambling of D(2) at a metal hydride. The model is evaluated against the available biochemical data for the nitrogenase HD formation reaction and extended to account for H(2) inhibition of N(2) reduction and the reduction of H(+) in the absence of other substrates.
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Affiliation(s)
- Marcus C Durrant
- Computational Biology Group, John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK.
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9
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Christiansen J, Dean DR, Seefeldt LC. MECHANISTIC FEATURES OF THE MO-CONTAINING NITROGENASE. ANNUAL REVIEW OF PLANT PHYSIOLOGY AND PLANT MOLECULAR BIOLOGY 2001; 52:269-295. [PMID: 11337399 DOI: 10.1146/annurev.arplant.52.1.269] [Citation(s) in RCA: 130] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Nitrogenase is the complex metalloenzyme responsible for biological dinitrogen reduction. This reaction represents the single largest contributor to the reductive portion of the global nitrogen cycle. Recent developments in understanding the mechanism of the Mo-based nitrogenase are reviewed. Topics include how nucleotide binding and hydrolysis are coupled to electron transfer and substrate reduction, how electrons are accumulated and transferred within the MoFe-protein, and how substrates bind and are reduced at the active site metal cluster.
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Affiliation(s)
- Jason Christiansen
- Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061; e-mail: , Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84332; e-mail:
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10
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Lin-Vien D, Fateley WG, Davis LC. Estimation of nitrogenase activity in the presence of ethylene biosynthesis by use of deuterated acetylene as a substrate. Appl Environ Microbiol 1989; 55:354-9. [PMID: 2655535 PMCID: PMC184114 DOI: 10.1128/aem.55.2.354-359.1989] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
Nitrogenase reduces deuterated acetylene primarily to cis dideuterated ethylene. This can be distinguished from undeuterated ethylene by the use of Fourier transform infrared spectroscopy. Characteristic bands in the region from 800 to 3,500 cm-1 can be used to identify and quantitate levels of these products. This technique is applicable to field studies of nitrogen fixation where ethylene biosynthesis by plants or bacteria is occurring. We have verified the reaction stoichiometry by using Klebsiella pneumoniae and Bradyrhizobium japonicum in soybeans. The most useful bands for quantitation of substrate purity and product distribution are as follows: acetylene-d0, 3,374 cm-1; acetylene-d1, 2,584 cm-1; acetylene-d2, 2,439 cm-1; cis-ethylene-d2, 843 cm-1; trans-ethylene-d2, 988 cm-1; ethylene-d1, 943 cm-1; ethylene-d0, 949 cm-1. (The various deuterated ethylenes and acetylenes are designated by a lowercase d and subscript to indicate the number, but not the position, of deuterium atoms in the molecule.) Mass spectrometry coupled to a gas chromatograph system has been used to assist in quantitation of the substrate and product distributions. Significant amounts of trans-ethylene-d2 were produced by both wild-type and nifV mutant K. pneumoniae. Less of this product was observed with the soybean system.
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Affiliation(s)
- D Lin-Vien
- Department of Chemistry, Kansas State University, Manhattan 66506
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11
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Jones GT, Davis LC, Ghosh Hajra AK, Erickson LE. Modeling and analysis of diffusion and reaction in legume nodules. Biotechnol Bioeng 1987; 29:279-88. [DOI: 10.1002/bit.260290219] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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12
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Urban JE, Davis LC, Brown SJ. Rhizobium trifolii
0403 Is Capable of Growth in the Absence of Combined Nitrogen. Appl Environ Microbiol 1986; 52:1060-7. [PMID: 16347203 PMCID: PMC239173 DOI: 10.1128/aem.52.5.1060-1067.1986] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Rhizobium trifolii
0403 was treated with 16.6 mM succinate and other nutrients and thereby induced to grow in nitrogen-free medium. The organism grew microaerophilically on either semisolid or liquid medium, fixing atmospheric nitrogen to meet metabolic needs. Nitrogen fixation was measured via
15
N incorporation (18%
15
N enrichment in 1.5 doublings) and acetylene reduction. Nitrogen-fixing cells had a
K
m
for acetylene of 0.07 atm (ca. 7.09 kPa), required about 3% oxygen for optimum growth in liquid medium, and showed a maximal specific activity of 5 nmol of acetylene reduced per min per mg of protein at 0.04 atm (ca. 4.05 kPa) of acetylene. The doubling time on N-free liquid medium ranged from 1 to 5 days, depending on oxygen tension, with an optimum temperature for growth of about 30°C. Nodulation of white clover by the cultures showing in vitro nitrogenase activity indicates that at least part of the population maintained identity with wild-type strain 0403.
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Affiliation(s)
- J E Urban
- Microbiology Section, Division of Biology, and Department of Biochemistry, Kansas State University, Manhattan, Kansas 66506
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13
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Eady RR, Robson RL. Characteristics of N2 fixation in Mo-limited batch and continuous cultures of Azotobacter vinelandii. Biochem J 1984; 224:853-62. [PMID: 6596950 PMCID: PMC1144521 DOI: 10.1042/bj2240853] [Citation(s) in RCA: 31] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Steady-state chemostat cultures of Azotobacter vinelandii were established in a simple defined medium that had been chemically purified to minimize Mo and that contained no utilizable combined N source. Growth was dependent on N2 fixation, the limiting nutrient being the Mo contaminating the system. The Mo content of the organisms was at least 100-fold lower than that of Mo-sufficient cultures, and they lacked the characteristic g = 3.7 e.p.r. feature of the MoFe-protein of nitrogenase. A characteristic of nitrogenase activity in vivo in Mo-limited populations was a disproportionately low activity for acetylene reduction, which was 0.3 to 0.1 of that expected from the rate of N2 reduction. Acetylene was also a poor substrate in comparison with protons as a substrate for nitrogenase, and did not markedly inhibit H2 evolution, in contrast with Mo-sufficient populations. In batch cultures in similar medium or 'spent' chemostat medium inoculated with Mo-limited organisms, the addition of Mo elicited a biphasic increased growth response at concentrations as low as 2.5 nM, provided that sufficient Fe was supplied. In this system V did not substitute for Mo, and Mo-deficient cultures ceased growth at a 25-fold lower population density compared with cultures supplemented with Mo. Nitrogenase component proteins could not be unequivocally detected by visual inspection of fractionated crude extracts of Mo-limited organisms. 35SO42-pulse-labelling studies also showed that the rate of synthesis of the MoFe-protein component of nitrogenase was too low to be quantified. However, the Fe-protein of nitrogenase was apparently synthesized at high rates. The discussion includes an evaluation of the possibility that A. vinelandii possesses an Mo-independent N2-fixation system.
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14
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Davis LC. Diffusion of Gases through Plant Tissues : Entry of Acetylene into Legume Nodules. PLANT PHYSIOLOGY 1984; 76:854-7. [PMID: 16663958 PMCID: PMC1064393 DOI: 10.1104/pp.76.4.854] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
I have measured acetylene diffusion through plant tissues including nodules from several species of legume-vetch, peas, soybeans, and Sesbania rostrata. The observed half-time for reequilibration of internal and external concentration is less than 1 minute for typical nodules. Inward diffusion of acetylene in air is rapid relative to the use of acetylene by nitrogenase so that diffusion of acetylene would not be a significant limiting factor for nitrogenase activity in air. However, under an atmosphere of Ar:O(2) where there is no N(2) reduction, the inward diffusion rate of acetylene into larger nodules could produce a measurable limitation of observed nitrogenase activity at low acetylene concentrations.
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Affiliation(s)
- L C Davis
- Department of Biochemistry, Kansas State University, Manhattan, Kansas 66506
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15
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Murry MA, Fontaine MS, Tjepkema JD. Oxygen protection of nitrogenase in Frankia sp. HFPArI3. Arch Microbiol 1984; 139:162-6. [PMID: 6595968 DOI: 10.1007/bf00401993] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
O2 protection of nitrogenase in a cultured Frankia isolate from Alnus rubra (HFPArI3) was studied in vivo. Evidence for a passive gas diffusion barrier in the vesicles was obtained by kinetic analysis of in vivo O2 uptake and acetylene reduction rates in response to substrate concentration. O2 of NH4+-grown cells showed an apparent KmO2 of approximately 1 microM O2. In N2-fixing cultures a second Km O2 of about 215 microM O2 was observed. Thus, respiration remained unsaturated by O2 at air-saturation levels. In vivo, the apparent Km for acetylene was more than 10-fold greater than reported in vitro values. These data were interpreted as evidence for a gas diffusion barrier in the vesicles but not vegetative filaments of Frankia sp. HFPArI3.
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16
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Denison RF, Weisz PR, Sinclair TR. Analysis of acetylene reduction rates of soybean nodules at low acetylene concentrations. PLANT PHYSIOLOGY 1983; 73:648-51. [PMID: 16663275 PMCID: PMC1066523 DOI: 10.1104/pp.73.3.648] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
It has been previously proposed that acetylene reduction data at subsaturating acetylene concentrations could be interpreted by use of the Michaelis-Menten equation, based on the acetylene concentration external to the nodules. One difficulty of this view is that the assumption that the system is not diffusion limited is violated when studying intact nodules. The presence of a gas diffusion barrier in the nodule cortex leads to an alternate expression for the gas exchange rates at subsaturating gas concentrations. A theoretical comparison of the ;apparent' Michaelis-Menten model and diffusion model illustrated the difficulties observed in the former model of overestimating the Michaelis-Menten coefficient and yielding a correlation between the Michaelis-Menten coefficient and the maximum rate. On the other hand, use of a diffusion model resulted in (a) estimates of the Michaelis-Menten coefficient consistent with enzyme studies, (b) stability of the estimates of the Michaelis-Menten coefficient independent of treatment, and (c) a sensitivity of the diffusion barrier conductance to plant drought stress. It was concluded that all studies of nodule gas exchange need to consider possible effects caused by the presence of a diffusion barrier.
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Affiliation(s)
- R F Denison
- Environmental Physiology Project, United States Department of Agriculture-Agricultural Research Service, University of Florida, Gainesville, Florida 32611
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17
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Abstract
The MoFe protein of nitrogenase from Klebsiella pneumoniae nifV mutants, NifV- Kp1 protein, in combination with the Fe protein from wild-type cells, catalysed CO-sensitive H2 evolution, in contrast with the CO-insensitive reaction catalysed by the wild-type enzyme. The decrease in H2 production was accompanied by a stoicheiometric decrease in dithionite (reductant) utilization, implying that CO was not reduced. However, CO did not affect the rate of phosphate release from ATP. Therefore the ATP/2e ratio increased, indicating futile cycling of electrons between the Fe protein and the MoFe protein. The inhibition of H2 evolution by CO was partial; it increased from 40% at pH6.3 to 82% at pH 8.6. Inhibition at pH7.4 (maximum 73%) was half-maximal at 3.1 Pa (0.031 matm) CO. The pH optimum of the mutant enzyme was lower in the presence of CO. Steady-state kinetic analysis of acetylene reduction indicated that CO was a linear, intersecting, non-competitive inhibitor of acetylene reduction with Kii = 2.5 Pa and Kis = 9.5 Pa. This may indicate that a single high-affinity CO-binding site in the NifV- Kp1 protein can cause both partial inhibition of H2 evolution and total elimination of acetylene reduction. Various models to explain the data are discussed.
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19
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Rice D, Mazur BJ, Haselkorn R. Isolation and physical mapping of nitrogen fixation genes from the cyanobacterium Anabaena 7120. J Biol Chem 1982. [DOI: 10.1016/s0021-9258(18)33636-6] [Citation(s) in RCA: 133] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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20
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McLean PA, Dixon RA. Requirement of nifV gene for production of wild-type nitrogenase enzyme in Klebsiella pneumoniae. Nature 1981; 292:655-6. [PMID: 7019729 DOI: 10.1038/292655a0] [Citation(s) in RCA: 78] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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21
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Davis LC. Hydrazine as a substrate and inhibitor of Azotobacter vinelandii nitrogenase. Arch Biochem Biophys 1980; 204:270-6. [PMID: 6932825 DOI: 10.1016/0003-9861(80)90033-8] [Citation(s) in RCA: 38] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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22
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Davis LC. Estimating Kinetic Parameters when the Amount of Enzyme Added to an Assay Is Not a Controlled Variable: NITROGENASE ACTIVITY OF INTACT LEGUMES . PLANT PHYSIOLOGY 1980; 66:126-9. [PMID: 16661372 PMCID: PMC440544 DOI: 10.1104/pp.66.1.126] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Reliable estimates of Michaelis constants (K(m)) and inhibitor constants may be obtained, in the absence of control over the amount of enzyme being added to any assay system, provided the following constraints are met. Michaelis-Menten kinetics are obeyed. Two rate measurements must be made with the same sample of enzyme: at low and high substrate concentration for determining K(m) or minus and plus an inhibitor for determining inhibitor constants. The Michaelis constant may be calculated from the equation [Formula: see text] Inhibitor constants are derived graphically from Lineweaver-Burk or Dixon plots, once the K(m) has been calculated. The above technique has been applied to study of the acetylene-reducing ability of intact legume plants. The apparent K(m) for acetylene reduction by nitrogenase in legume nodules is approximately 1/100 atmosphere in the absence of nitrogen and approximately 1/40 atmosphere in its presence.
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Affiliation(s)
- L C Davis
- Department of Biochemistry, Kansas State University, Manhattan, Kansas 66506
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23
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An Evaluation of N2 Fixation and H2 Production in Fermentation Culture. ACTA ACUST UNITED AC 1980. [DOI: 10.1016/b978-0-12-040304-2.50017-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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