51
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Guo M, Sulc F, Ribbe MW, Farmer PJ, Burgess BK. Direct assessment of the reduction potential of the [4Fe-4S](1+/0) couple of the Fe protein from Azotobacter vinelandii. J Am Chem Soc 2002; 124:12100-1. [PMID: 12371842 DOI: 10.1021/ja026478f] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Recently, it has been demonstrated that the [4Fe-4S] cluster of the Fe protein of nitrogenase from Azotobacter vinelandii can be reduced to an unprecedented all-ferrous state. In this work, the reduction potential for the formation of the all-ferrous state is measured by the reactions of the reduced and oxidized Fe protein with a variety of chemical redox active agents, and by mediated spectroelectrochemical titration. Redox titrations obtain a potential ca. -790 mV/NHE for the formation of the all-ferrous state, a value consistent with the chemical reactivity experiments and with recent theoretical calculations. At present, no known redox protein in A. vinelandii is capable of generating the all-ferrous Fe protein.
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Affiliation(s)
- Maolin Guo
- Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-2025, USA
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52
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Tewari BB. Electrophoretic studies on the chelating tendency of bioactive sulphur-containing amino acids. The metal-methylcysteine-cysteine system. J Chromatogr A 2002; 962:233-7. [PMID: 12198967 DOI: 10.1016/s0021-9673(02)00432-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Stability constants of binary Fe(III)-methylcysteine, Cr(III)-methylcysteine and mixed Fe(III)-methylcysteine-cysteine, Cr(III)-methylcysteine-cysteine complexes have been determined by paper electrophoresis at 0.1 M ionic strength and a temperature of 35 degrees C. The stability constants of Fe(III)-methylcysteine-cysteine and Cr(III)-methylcysteine-cysteine mixed complexes were found to be 6.00 +/- 0.07 and 5.05 +/- 0.15 (logarithm of stability constant values), respectively.
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Affiliation(s)
- Brij Bhushan Tewari
- Department of Chemistry, Faculty of Natural Sciences, University of Guyana, P.O. Box 10 1110, Georgetown, Guyana.
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53
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Ribbe MW, Hu Y, Guo M, Schmid B, Burgess BK. The FeMoco-deficient MoFe protein produced by a nifH deletion strain of Azotobacter vinelandii shows unusual P-cluster features. J Biol Chem 2002; 277:23469-76. [PMID: 11978793 DOI: 10.1074/jbc.m202061200] [Citation(s) in RCA: 64] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The His-tag MoFe protein expressed by the nifH deletion strain Azotobacter vinelandii DJ1165 (Delta(nifH) MoFe protein) was purified in large quantity. The alpha(2)beta(2) tetrameric Delta(nifH) MoFe protein is FeMoco-deficient based on metal analysis and the absence of the S = 3/2 EPR signal, which arises from the FeMo cofactor center in wild-type MoFe protein. The Delta(nifH) MoFe protein contains 18.6 mol Fe/mol and, upon reduction with dithionite, exhibits an unusually strong S = 1/2 EPR signal in the g approximately 2 region. The indigo disulfonate-oxidized Delta(nifH) MoFe protein does not show features of the P(2+) state of the P-cluster of the Delta(nifB) MoFe protein. The oxidized Delta(nifH) MoFe protein is able to form a specific complex with the Fe protein containing the [4Fe-4S](1+) cluster and facilitates the hydrolysis of MgATP within this complex. However, it is not able to accept electrons from the [4Fe-4S](1+) cluster of the Fe protein. Furthermore, the dithionite-reduced Delta(nifH) MoFe can be further reduced by Ti(III) citrate, which is quite unexpected. These unusual catalytic and spectroscopic properties might indicate the presence of a P-cluster precursor or a P-cluster trapped in an unusual conformation or oxidation state.
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Affiliation(s)
- Markus W Ribbe
- Department of Molecular Biology and Biochemistry and the Program in Macromolecular Structure, University of California, Irvine, California 92697-3900, USA.
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54
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Noodleman L, Lovell T, Liu T, Himo F, Torres RA. Insights into properties and energetics of iron-sulfur proteins from simple clusters to nitrogenase. Curr Opin Chem Biol 2002; 6:259-73. [PMID: 12039013 DOI: 10.1016/s1367-5931(02)00309-5] [Citation(s) in RCA: 117] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Some of the principal physical features of iron-sulfur clusters in proteins are analyzed, including metal-ligand covalency, spin polarization, spin coupling, valence delocalization, valence interchange and small reorganization energies, with emphasis on recent spectroscopic and theoretical work. The current state of structural, spectroscopic, and computational knowledge for the iron-sulfur clusters in the nitrogenase iron and iron-molybdenum proteins is examined by comparison and contrast to 'simpler' ironclusters. The differing interactions of the nitrogenase iron and iron-molybdenum clusters compared with those of other iron-sulfur clusters with the protein and solvent environment are also explored.
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Affiliation(s)
- Louis Noodleman
- Department of Molecular Biology, TPC15, The Scripps Research Institute, La Jolla, California 92037, USA.
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55
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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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56
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Durrant MC. Controlled protonation of iron-molybdenum cofactor by nitrogenase: a structural and theoretical analysis. Biochem J 2001; 355:569-76. [PMID: 11311117 PMCID: PMC1221770 DOI: 10.1042/bj3550569] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Qualitative molecular modelling has been used to identify possible routes for transfer of protons from the surface of the nitrogenase protein to the iron-molybdenum cofactor (FeMoco) and to substrates during catalysis. Three proton-transfer routes have been identified; a water-filled channel running from the protein exterior to the homocitrate ligand of FeMoco, and two hydrogen-bonded chains to specific FeMoco sulphur atoms. It is suggested that the water channel is used for multiple proton deliveries to the substrate, as well as in diffusion of products and substrates between FeMoco and the bulk solvent, whereas the two hydrogen-bonded chains each allow a single proton to be added to, and subsequently depart from, FeMoco during the catalytic cycle. Possible functional differences in the proton-transfer channels are discussed in terms of assessment of the protein environment and specific hydrogen-bonding effects. The implications of these observations are discussed in terms of the suppression of wasteful production of dihydrogen by nitrogenase and the Lowe-Thorneley scheme for dinitrogen reduction.
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Affiliation(s)
- M C Durrant
- Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK.
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57
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Strop P, Takahara PM, Chiu H, Angove HC, Burgess BK, Rees DC. Crystal structure of the all-ferrous [4Fe-4S]0 form of the nitrogenase iron protein from Azotobacter vinelandii. Biochemistry 2001; 40:651-6. [PMID: 11170381 DOI: 10.1021/bi0016467] [Citation(s) in RCA: 111] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The structure of the nitrogenase iron protein from Azotobacter vinelandii in the all-ferrous [4Fe-4S](0) form has been determined to 2.25 A resolution by using the multiwavelength anomalous diffraction (MAD) phasing technique. The structure demonstrates that major conformational changes are not necessary either in the iron protein or in the cluster to accommodate cluster reduction to the [4Fe-4S](0) oxidation state. A survey of [4Fe-4S] clusters coordinated by four cysteine ligands in proteins of known structure reveals that the [4Fe-4S] cluster of the iron protein has the largest accessible surface area, suggesting that solvent exposure may be relevant to the ability of the iron protein to exist in three oxidation states.
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Affiliation(s)
- P Strop
- Biochemistry Option, California Institute of Technology, Mail Code 147-75, Pasadena, California 91125, USA
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58
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Nyborg AC, Johnson JL, Gunn A, Watt GD. Evidence for a two-electron transfer using the all-ferrous Fe protein during nitrogenase catalysis. J Biol Chem 2000; 275:39307-12. [PMID: 11005818 DOI: 10.1074/jbc.m007069200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The nitrogenase-catalyzed H(2) evolution and acetylene-reduction reactions using Ti(III) and dithionite (DT) as reductants were examined and compared under a variety of conditions. Ti(III) is known to make the all-ferrous Fe protein ([Fe(4)S(4)](0)) and lowers the amount of ATP hydrolyzed during nitrogenase catalysis by approximately 2-fold. Here we further investigate this behavior and present results consistent with the Fe protein in the [Fe(4)S(4)](0) redox state transferring two electrons ([Fe(4)S(4)](2+)/[Fe(4)S(4)](0)) per MoFe protein interaction using Ti(III) but transferring only one electron ([Fe(4)S(4)](2+)/[Fe(4)S(4)](1+)) using DT. MoFe protein specific activity was measured as a function of Fe:MoFe protein ratio for both a one- and a two-electron transfer reaction, and nearly identical curves were obtained. However, Fe protein specific activity curves as a function of MoFe:Fe protein ratio showed two distinct reactivity patterns. With DT as reductant, typical MoFe inhibition curves were obtained for operation of the [Fe(4)S(4)](2+)/[Fe(4)S(4)](1+) redox couple, but with Ti(III) as reductant the [Fe(4)S(4)](2+)/[Fe(4)S(4)](0) redox couple was functional and MoFe inhibition was not observed at high MoFe:Fe protein ratios. With Ti(III) as reductant, nitrogenase catalysis produced hyperbolic curves, yielding a V(max) for the Fe protein specific activity of about 3200 nmol of H(2) min(-1) mg(-1) Fe protein, significantly higher than for reactions conducted with DT as reductant. Lag phase experiments (Hageman, R. V., and Burris, R. H. (1978) Proc. Natl. Acad. Sci. U. S. A. 75, 2699-2702) were carried out at MoFe:Fe protein ratios of 100 and 300 using both DT and Ti(III). A lag phase was observed for DT but, with Ti(III) product formation, began immediately and remained linear for over 30 min. Activity measurements using Av-Cp heterologous crosses were examined using both DT and Ti(III) as reductants to compare the reactivity of the [Fe(4)S(4)](2+)/[Fe(4)S(4)](1+) and [Fe(4)S(4)](2+)/[Fe(4)S(4)](0) redox couples and both were inactive. The results are discussed in terms of the Fe protein transferring two electrons per MoFe protein encounter using the [Fe(4)S(4)](2+)/[Fe(4)S(4)](0) redox couple with Ti(III) as reductant.
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Affiliation(s)
- A C Nyborg
- Undergraduate Research Program and Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84604, USA
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59
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Lei S, Pulakat L, Suh M, Gavini N. Identification of a second site compensatory mutation in the Fe-protein that allows diazotrophic growth of Azotobacter vinelandii UW97. FEBS Lett 2000; 478:192-6. [PMID: 10922495 DOI: 10.1016/s0014-5793(00)01847-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Azotobacter vinelandii UW97 is defective in nitrogen fixation due to a replacement of serine at position 44 by phenylalanine in the Fe-protein [Pulakat, L., Hausman, B.S., Lei, S. and Gavini, N. (1996) J. Biol. Chem. 271, 1884-1889]. Serine residue 44 is located in a conserved domain that links the nucleotide binding site and the MoFe-protein docking surface of the Fe-protein. Therefore, it is possible that the loss of function by A. vinelandii UW97-Fe-protein may be caused by global conformational disruption or disruption of the conformational change upon MgATP binding. To determine whether it is possible to generate a functional nitrogenase complex via a compensating second site mutation(s) in the Fe-protein, we have attempted to isolate genetic revertants of A. vinelandii UW97 that can grow on nitrogen-free medium. One such revertant, designated A vinelandii BG9, encoded a Fe-protein that retained the Ser44Phe mutation and also had a second mutation that caused the replacement of a lysine at position 170 by glutamic acid. Lysine 170 is highly conserved and is located in a conserved region of the Fe-protein. This region is implicated in stabilizing the MgATP-induced conformation of the Fe-protein and in docking to the MoFe-protein. Further complementation analysis showed that the Fe-protein mutant that retained serine 44 but contained the substitution of lysine at position 170 by glutamic acid was also non-functional. Thus, neither Ser44Phe nor Lys170Glu mutants of Fe-protein were functional; however, the Fe-protein in A. vinelandii BG9 that contained both substitutions could support diazotrophic growth on the strain.
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Affiliation(s)
- S Lei
- Department of Biological Sciences, Bowling Green State University, 43403, Bowling Green, OH, USA
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60
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Ribbe MW, Bursey EH, Burgess BK. Identification of an Fe protein residue (Glu146) of Azotobacter vinelandii nitrogenase that is specifically involved in FeMo cofactor insertion. J Biol Chem 2000; 275:17631-8. [PMID: 10837496 DOI: 10.1074/jbc.275.23.17631] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Fe protein of nitrogenase has three separate functions. Much is known about the regions of the protein that are critical to its function as an electron donor to the MoFe protein, but almost nothing is known about the regions of the protein that are critical to its functions in either FeMo cofactor biosynthesis or FeMo cofactor insertion. Using computer modeling and information obtained from Fe protein mutants that were made decades ago by chemical mutagenesis, we targeted a surface residue Glu(146) as potentially being involved in FeMo cofactor biosynthesis and/or insertion. The Azotobacter vinelandii strain expressing an E146D Fe protein variant grows at approximately 50% of the wild type rate. The purified E146D Fe protein is fully functional as an electron donor to the MoFe protein, but the MoFe protein synthesized by that strain is partially ( approximately 50%) FeMo cofactor-deficient. The E146D Fe protein is fully functional in an in vitro FeMo cofactor biosynthesis assay, and the strain expressing this protein accumulates "free" FeMo cofactor. Assays that compared the ability of wild type and E146D Fe proteins to participate in FeMo cofactor insertion demonstrate, however, that the mutant is severely altered in this last reaction. This is the first known mutation that only influences the insertion reaction.
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Affiliation(s)
- M W Ribbe
- Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697, USA
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61
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Liu A, Gräslund A. Electron paramagnetic resonance evidence for a novel interconversion of [3Fe-4S](+) and [4Fe-4S](+) clusters with endogenous iron and sulfide in anaerobic ribonucleotide reductase activase in vitro. J Biol Chem 2000; 275:12367-73. [PMID: 10777518 DOI: 10.1074/jbc.275.17.12367] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We report an EPR study of the iron-sulfur enzyme, anaerobic ribonucleotide reductase activase from Lactococcus lactis. The activase (nrdG gene) together with S-adenosyl-L-methionine (AdoMet) give rise to a glycyl radical in the NrdD component. A semi-reduced [4Fe-4S](+) cluster with an axially symmetric EPR signal was produced upon photochemical reduction of the activase. Air exposure of the reduced enzyme gave a [3Fe-4S](+) cluster. The Fe(3)S(4) cluster was convertible to the EPR-active [4Fe-4S](+) cluster by renewed treatment with reducing agents, demonstrating a reversible [3Fe-4S](+)- to-[4Fe-4S](+) cluster conversion without exogenous addition of iron or sulfide. Anaerobic reduction of the activase by a moderate concentration of dithionite also resulted in a semi-reduced [4Fe-4S](+) cluster. Prolonged reduction gave an EPR-silent fully reduced state, which was enzymatically inactive. Both reduced states gave the [3Fe-4S](+) EPR signal after air exposure. The iron-sulfur cluster interconversion was also studied in the presence of AdoMet. The EPR signal of semi-reduced activase-AdoMet had rhombic symmetry and was independent of which reductant was applied, whereas the EPR signal of the [3Fe-4S](+) cluster after air exposure was unchanged. The results indicate that an AdoMet-mediated [4Fe-4S](+) center is the native active species that induces the formation of a glycyl radical in the NrdD component.
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Affiliation(s)
- A Liu
- Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden
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62
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Belinsky MI. Exchange variation of zero-field splittings in [Fe4S4]+ clusters of ferredoxins with high-spin S=3/2 ground state. Chem Phys 2000. [DOI: 10.1016/s0301-0104(00)00047-1] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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63
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Jang SB, Seefeldt LC, Peters JW. Modulating the midpoint potential of the [4Fe-4S] cluster of the nitrogenase Fe protein. Biochemistry 2000; 39:641-8. [PMID: 10651628 DOI: 10.1021/bi991694v] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Protein-bound [FeS] clusters function widely in biological electron-transfer reactions, where their midpoint potentials control both the kinetics and thermodynamics of these reactions. The polarity of the protein environment around [FeS] clusters appears to contribute largely to modulating their midpoint potentials, with local protein dipoles and water dipoles largely defining the polarity. The function of the [4Fe-4S] cluster containing Fe protein in nitrogenase catalysis is, at least in part, to serve as the nucleotide-dependent electron donor to the MoFe protein which contains the sites for substrate binding and reduction. The ability of the Fe protein to function in this manner is dependent on its ability to adopt the appropriate conformation for productive interaction with the MoFe protein and on its ability to change redox potentials to provide the driving force required for electron transfer. Phenylalanine at position 135 is located near the [4Fe-4S] cluster of nitrogenase Fe protein and has been suggested by amino acid substitution studies to participate in defining both the midpoint potential and the nucleotide-induced changes in the [4Fe-4S] cluster. In the present study, the crystal structure of the Azotobacter vinelandii nitrogenase Fe protein variant having phenylalanine at position 135 substituted by tryptophan has been determined by X-ray diffraction methods and refined to 2.4 A resolution. A comparison of available Fe protein structures not only provides a structural basis for the more positive midpoint potential observed in the tryptophan substituted variant but also suggests a possible general mechanism by which the midpoint potential could be controlled by nucleotide interactions and nitrogenase complex formation.
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Affiliation(s)
- S B Jang
- Department of Chemistry, Utah State University, Logan, Utah 84322, USA
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64
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Erickson JA, Nyborg AC, Johnson JL, Truscott SM, Gunn A, Nordmeyer FR, Watt GD. Enhanced efficiency of ATP hydrolysis during nitrogenase catalysis utilizing reductants that form the all-ferrous redox state of the Fe protein. Biochemistry 1999; 38:14279-85. [PMID: 10572002 DOI: 10.1021/bi991389+] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The amount of MgATP hydrolyzed per pair of electrons transferred (ATP/2e) during nitrogenase catalysis (1.0 atm N(2), 30 degrees C) using titanium(III) citrate (Ti(III)) as reductant was measured and compared to the same reaction using dithionite (DT). ATP/2e values near 2.0 for Ti(III) and 5.0 for DT indicate that nitrogenase has a much lower ATP requirement using Ti(III) as reductant. Using reduced Azotobacter vinelandii flavoprotein (AvFlpH(2)), a possible in vivo nitrogenase reductant, ATP/2e values near 2.0 were also observed. When the reaction was conducted using Ti(III) under N(2), 5% CO in N(2), Ar, 5% CO in Ar, or acetylene, ATP/2e values near 2.0 were also observed. With Ti(III) as reductant, ATP/2e values near 2.0 were measured as a function of temperature, Fe:MoFe protein ratio, and MoFe:Fe protein ratio, in contrast to measured values of 4.0-25 when DT is used under the same conditions. Both Ti(III) and AvFlpH(2) are capable of forming the [Fe(4)S(4)](0) cluster state of the Fe protein whereas DT is not, suggesting that ATP/2e values near 2.0 arise from operation of the [Fe(4)S(4)](2+)/[Fe(4)S(4)](0) redox couple with hydrolysis of only 2 ATPs per pair of electrons transferred. Additional experiments showed that ATP/2e values near 2. 0 correlated with slower rates of product formation and that faster rates of product formation produced ATP/2e values near 5.0. ATP/2e values of 5.0 are consistent with the operation of the [Fe(4)S(4)](2+)/[Fe(4)S(4)](1+) redox couple while ATP/2e values of 2.0 could arise from operation of the [Fe(4)S(4)](2+)/[Fe(4)S(4)](0) redox couple. These results suggest that two distinct Fe protein redox couples may be functional during nitrogenase catalysis and that the efficiency of ATP utilization depends on which of these redox couples is dominant.
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Affiliation(s)
- J A Erickson
- Undergraduate Research Program, Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84604, USA
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65
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Chan JM, Christiansen J, Dean DR, Seefeldt LC. Spectroscopic evidence for changes in the redox state of the nitrogenase P-cluster during turnover. Biochemistry 1999; 38:5779-85. [PMID: 10231529 DOI: 10.1021/bi982866b] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Biological nitrogen fixation catalyzed by nitrogenase requires the participation of two component proteins called the Fe protein and the MoFe protein. Each alphabeta catalytic unit of the MoFe protein contains an [8Fe-7S] cluster and a [7Fe-9S-Mo-homocitrate] cluster, respectively designated the P-cluster and FeMo-cofactor. FeMo-cofactor is known to provide the site of substrate reduction whereas the P-cluster has been suggested to function in nitrogenase catalysis by providing an intermediate electron-transfer site. In the present work, evidence is presented for redox changes of the P-cluster during the nitrogenase catalytic cycle from examination of an altered MoFe protein that has the beta-subunit serine-188 residue substituted by cysteine. This residue was targeted for substitution because it provides a reversible redox-dependent ligand to one of the P-cluster Fe atoms. The altered beta-188(Cys) MoFe protein was found to reduce protons, acetylene, and nitrogen at rates approximately 30% of that supported by the wild-type MoFe protein. In the dithionite-reduced state, the beta-188(Cys) MoFe protein exhibited unusual electron paramagnetic resonance (EPR) signals arising from a mixed spin state system (S = 5/2, 1/2) that integrated to 0.6 spin/alphabeta-unit. These EPR signals were assigned to the P-cluster because they were also present in an apo-form of the beta-188(Cys) MoFe protein that does not contain FeMo-cofactor. Mediated voltammetry was used to show that the intensity of the EPR signals was maximal near -475 mV at pH 8.0 and that the P-cluster could be reversibly oxidized or reduced with concomitant loss in intensity of the EPR signals. A midpoint potential (Em) of -390 mV was approximated for the oxidized/resting state couple at pH 8.0, which was observed to be pH dependent. Finally, the EPR signals exhibited by the beta-188(Cys) MoFe protein greatly diminished in intensity under nitrogenase turnover conditions and reappeared to the original intensity when the MoFe protein returned to the resting state.
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Affiliation(s)
- J M Chan
- Department of Chemistry and Biochemistry, Utah State University, Logan 84322, USA
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66
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Abstract
In the past five to ten years, it has become increasingly apparent that the function of Fe-S clusters is not limited to electron transfer, a function implicit in their discovery. We now know that the vulnerability of these structures to oxidative destruction is used by nature in sensing O2, iron, and possibly also nitric oxide. Changes in the oxidation state of Fe-S clusters can also serve as a reversible switch.
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Affiliation(s)
- H Beinert
- Institute for Enzyme Research, Graduate School, Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin, 1710 University Avenue, Madison, WI 53705, USA.
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67
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Yoo SJ, Angove HC, Burgess BK, Hendrich MP, Münck E. Mössbauer and Integer-Spin EPR Studies and Spin-Coupling Analysis of the [4Fe-4S]0 Cluster of the Fe Protein from Azotobacter vinelandii Nitrogenase. J Am Chem Soc 1999. [DOI: 10.1021/ja9837405] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Sun Jae Yoo
- Contribution from the Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Molecular Biology and Biochemistry, University of California Irvine, Irvine, California 92697-3900
| | - Hayley C. Angove
- Contribution from the Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Molecular Biology and Biochemistry, University of California Irvine, Irvine, California 92697-3900
| | - Barbara K. Burgess
- Contribution from the Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Molecular Biology and Biochemistry, University of California Irvine, Irvine, California 92697-3900
| | - Michael P. Hendrich
- Contribution from the Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Molecular Biology and Biochemistry, University of California Irvine, Irvine, California 92697-3900
| | - Eckard Münck
- Contribution from the Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and Department of Molecular Biology and Biochemistry, University of California Irvine, Irvine, California 92697-3900
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