1
|
Pellows LM, Willis MA, Ruzicka JL, Jagilinki BP, Mulder DW, Yang ZY, Seefeldt LC, King PW, Dukovic G, Peters JW. High Affinity Electrostatic Interactions Support the Formation of CdS Quantum Dot:Nitrogenase MoFe Protein Complexes. NANO LETTERS 2023; 23:10466-10472. [PMID: 37930772 DOI: 10.1021/acs.nanolett.3c03205] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2023]
Abstract
Nitrogenase MoFe protein can be coupled with CdS nanocrystals (NCs) to enable photocatalytic N2 reduction. The nature of interactions that support complex formation is of paramount importance in intermolecular electron transfer that supports catalysis. In this work we have employed microscale thermophoresis to examine binding interactions between 3-mercaptopropionate capped CdS quantum dots (QDs) and MoFe protein over a range of QD diameters (3.4-4.3 nm). The results indicate that the interactions are largely electrostatic, with the strength of interactions similar to that observed for the physiological electron donor. In addition, the strength of interactions is sensitive to the QD diameter, and the binding interactions are significantly stronger for QDs with smaller diameters. The ability to quantitatively assess NC protein interactions in biohybrid systems supports strategies for understanding properties and reaction parameters that are important for obtaining optimal rates of catalysis in biohybrid systems.
Collapse
Affiliation(s)
- Lauren M Pellows
- Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
| | - Mark A Willis
- Institute of Biological Chemistry, Washington State University, Pullman, Washington 99163, United States
| | - Jesse L Ruzicka
- Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
| | - Bhanu P Jagilinki
- Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States
| | - David W Mulder
- Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
| | - Zhi-Yong Yang
- Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
| | - Lance C Seefeldt
- Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
| | - Paul W King
- Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
| | - Gordana Dukovic
- Department of Chemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
- Materials Science and Engineering, University of Colorado Boulder, Boulder, Colorado 80303, United States
- Renewable and Sustainable Energy Institute (RASEI), University of Colorado Boulder, Boulder, Colorado 80303, United States
| | - John W Peters
- Institute of Biological Chemistry, Washington State University, Pullman, Washington 99163, United States
- Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States
| |
Collapse
|
2
|
Abstract
Biological nitrogen fixation, the conversion of dinitrogen (N2) into ammonia (NH3), stands as a particularly challenging chemical process. As the entry point into a bioavailable form of nitrogen, biological nitrogen fixation is a critical step in the global nitrogen cycle. In Nature, only one enzyme, nitrogenase, is competent in performing this reaction. Study of this complex metalloenzyme has revealed a potent substrate reduction system that utilizes some of the most sophisticated metalloclusters known. This chapter discusses the structure and function of nitrogenase, covers methods that have proven useful in the elucidation of enzyme properties, and provides an overview of the three known nitrogenase variants.
Collapse
|
3
|
Negative cooperativity in the nitrogenase Fe protein electron delivery cycle. Proc Natl Acad Sci U S A 2018; 113:E5783-E5791. [PMID: 27698129 DOI: 10.1073/pnas.1613089113] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Nitrogenase catalyzes the ATP-dependent reduction of dinitrogen (N2) to two ammonia (NH3) molecules through the participation of its two protein components, the MoFe and Fe proteins. Electron transfer (ET) from the Fe protein to the catalytic MoFe protein involves a series of synchronized events requiring the transient association of one Fe protein with each αβ half of the α2β2 MoFe protein. This process is referred to as the Fe protein cycle and includes binding of two ATP to an Fe protein, association of an Fe protein with the MoFe protein, ET from the Fe protein to the MoFe protein, hydrolysis of the two ATP to two ADP and two Pi for each ET, Pi release, and dissociation of oxidized Fe protein-(ADP)2 from the MoFe protein. Because the MoFe protein tetramer has two separate αβ active units, it participates in two distinct Fe protein cycles. Quantitative kinetic measurements of ET, ATP hydrolysis, and Pi release during the presteady-state phase of electron delivery demonstrate that the two halves of the ternary complex between the MoFe protein and two reduced Fe protein-(ATP)2 do not undergo the Fe protein cycle independently. Instead, the data are globally fit with a two-branch negative-cooperativity kinetic model in which ET in one-half of the complex partially suppresses this process in the other. A possible mechanism for communication between the two halves of the nitrogenase complex is suggested by normal-mode calculations showing correlated and anticorrelated motions between the two halves.
Collapse
|
4
|
Abstract
Although the nitrogen-fixing enzyme nitrogenase critically requires both a reductase component (Fe protein) and a catalytic component, considerably more work has focused on the latter species. Properties of the catalytic component, which contains two highly complex metallocofactors and catalyzes the reduction of N2 into ammonia, understandably making it the “star” of nitrogenase. However, as its obligate redox partner, the Fe protein is a workhorse with multiple supporting roles in both cofactor maturation and catalysis. In particular, the nitrogenase Fe protein utilizes nucleotide binding and hydrolysis in concert with electron transfer to accomplish several tasks of critical importance. Aside from the ATP-coupled transfer of electrons to the catalytic component during substrate reduction, the Fe protein also functions in a maturase and insertase capacity to facilitate the biosynthesis of the two-catalytic component metallocofactors: fusion of the [Fe8S7] P-cluster and insertion of Mo and homocitrate to form the matured [(homocitrate)MoFe7S9C] M-cluster. These and key structural-functional relationships of the indispensable Fe protein and its complex with the catalytic component will be covered in this review.
Collapse
|
5
|
Pence N, Tokmina-Lukaszewska M, Yang ZY, Ledbetter RN, Seefeldt LC, Bothner B, Peters JW. Unraveling the interactions of the physiological reductant flavodoxin with the different conformations of the Fe protein in the nitrogenase cycle. J Biol Chem 2017; 292:15661-15669. [PMID: 28784660 DOI: 10.1074/jbc.m117.801548] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2017] [Revised: 08/02/2017] [Indexed: 01/21/2023] Open
Abstract
Nitrogenase reduces dinitrogen (N2) to ammonia in biological nitrogen fixation. The nitrogenase Fe protein cycle involves a transient association between the reduced, MgATP-bound Fe protein and the MoFe protein and includes electron transfer, ATP hydrolysis, release of Pi, and dissociation of the oxidized, MgADP-bound Fe protein from the MoFe protein. The cycle is completed by reduction of oxidized Fe protein and nucleotide exchange. Recently, a kinetic study of the nitrogenase Fe protein cycle involving the physiological reductant flavodoxin reported a major revision of the rate-limiting step from MoFe protein and Fe protein dissociation to release of Pi Because the Fe protein cannot interact with flavodoxin and the MoFe protein simultaneously, knowledge of the interactions between flavodoxin and the different nucleotide states of the Fe protein is critically important for understanding the Fe protein cycle. Here we used time-resolved limited proteolysis and chemical cross-linking to examine nucleotide-induced structural changes in the Fe protein and their effects on interactions with flavodoxin. Differences in proteolytic cleavage patterns and chemical cross-linking patterns were consistent with known nucleotide-induced structural differences in the Fe protein and indicated that MgATP-bound Fe protein resembles the structure of the Fe protein in the stabilized nitrogenase complex structures. Docking models and cross-linking patterns between the Fe protein and flavodoxin revealed that the MgADP-bound state of the Fe protein has the most complementary docking interface with flavodoxin compared with the MgATP-bound state. Together, these findings provide new insights into the control mechanisms in protein-protein interactions during the Fe protein cycle.
Collapse
Affiliation(s)
- Natasha Pence
- From the Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164.,the Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, and
| | | | - Zhi-Yong Yang
- the Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
| | - Rhesa N Ledbetter
- the Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
| | - Lance C Seefeldt
- the Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
| | - Brian Bothner
- the Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, and
| | - John W Peters
- From the Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164, .,the Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, and
| |
Collapse
|
6
|
Hoffman BM, Lukoyanov D, Yang ZY, Dean DR, Seefeldt LC. Mechanism of nitrogen fixation by nitrogenase: the next stage. Chem Rev 2014; 114:4041-62. [PMID: 24467365 PMCID: PMC4012840 DOI: 10.1021/cr400641x] [Citation(s) in RCA: 979] [Impact Index Per Article: 97.9] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- Brian M Hoffman
- Department of Chemistry and Biochemistry, Utah State University , 0300 Old Main Hill, Logan, Utah 84322, United States
| | | | | | | | | |
Collapse
|
7
|
Abstract
Nitrogenase is the enzyme responsible for biological reduction of dinitrogen (N(2)) to ammonia, a form usable for life. Playing a central role in the global biogeochemical nitrogen cycle, this enzyme has been the focus of intensive research for over 60 years. This chapter provides an overview of the features of nitrogenase as a background to the subsequent chapters of this volume that detail the many methods that have been applied in an attempt to gain a deeper understanding of this complex enzyme.
Collapse
Affiliation(s)
- Zhi-Yong Yang
- Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA.
| | | | | |
Collapse
|
8
|
Wilson PE, Nyborg AC, Kenealey J, Lowery TJ, Crawford K, King CR, Engan AJ, Johnson JL, Watt GD. Evidence for a synergistic salt-protein interaction -- complex patterns of activation vs. inhibition of nitrogenase by salt. Biophys Chem 2006; 122:184-94. [PMID: 16603308 DOI: 10.1016/j.bpc.2006.03.007] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2005] [Revised: 03/16/2006] [Accepted: 03/19/2006] [Indexed: 11/26/2022]
Abstract
The molybdenum nitrogenase enzyme system, comprised of the MoFe protein and the Fe protein, catalyzes the reduction of atmospheric N(2) to NH(3). Interactions between these two proteins and between Fe protein and nucleotides (MgADP and MgATP) are crucial to catalysis. It is well established that salts are inhibitors of nitrogenase catalysis that target these interactions. However, the implications of salt effects are often overlooked. We have reexamined salt effects in light of a comprehensive framework for nitrogenase interactions to offer an in-depth analysis of the sources of salt inhibition and underlying apparent cooperativity. More importantly, we have identified patterns of salt activation of nitrogenase that correspond to at least two mechanisms. One of these mechanisms is that charge screening of MoFe protein-Fe protein interactions in the nitrogenase complex accelerates the rate of nitrogenase complex dissociation, which is the rate-limiting step of catalysis. This kind of salt activation operates under conditions of high catalytic activity and low salt concentrations that may resemble those found in vivo. While simple kinetic arguments are strong evidence for this kind of salt activation, further confirmation was sought by demonstrating that tight complexes that have previously displayed little or no activity due to the inability of Fe protein to dissociate from the complex are activated by the presence of salt. This occurs for the combination Azotobacter vinelandii MoFe protein with: (a) the L127Delta Fe protein; and (b) Clostridium pasteurianum Fe protein. The curvature of activation vs. salt implies a synergistic salt-protein interaction.
Collapse
Affiliation(s)
- Phillip E Wilson
- Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA
| | | | | | | | | | | | | | | | | |
Collapse
|
9
|
Sen S, Krishnakumar A, McClead J, Johnson MK, Seefeldt LC, Szilagyi RK, Peters JW. Insights into the role of nucleotide-dependent conformational change in nitrogenase catalysis: Structural characterization of the nitrogenase Fe protein Leu127 deletion variant with bound MgATP. J Inorg Biochem 2006; 100:1041-52. [PMID: 16616373 DOI: 10.1016/j.jinorgbio.2006.02.016] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2005] [Revised: 02/11/2006] [Accepted: 02/13/2006] [Indexed: 11/16/2022]
Abstract
In the present work, determination of the structure of the nitrogenase Leu 127 deletion variant Fe protein with MgATP bound is presented, along with density functional theory calculations, to provide insights into the roles of MgATP in the nitrogenase reaction mechanism. Comparison of the MgATP-bound structure of this Fe protein to the nucleotide-free form indicates that the binding of MgATP does not alter the overall structure of the variant significantly with only small differences in the conformation of amino acids in direct contact with the two bound MgATP molecules being seen. The earlier observation of splitting of the [4Fe-4S] cluster into two [2Fe-2S] clusters was observed to be unaltered upon binding MgATP. Density functional theory was used to probe the assignment of ligands to the two [2Fe-2S] rhombs. The Mg(2+) environment in the MgATP-bound structure of the Leu127 deletion Fe protein is similar to that observed for the Fe protein in the nitrogenase Fe protein: MoFe protein complex stabilized by MgADP and tetrafluoroaluminate suggesting that large scale conformational change implicated for the Fe protein may not be mediated by changes in the Mg(2+) coordination. The results presented here indicated that MgATP may enhance the stability of an open conformation and prohibit intersubunit interactions, which have been implicated in promoting nucleotide hydrolysis. This could be critical to the tight control of MgATP hydrolysis observed within the nitrogenase complex and may be important for maintaining unidirectional electron flow toward substrate reduction.
Collapse
Affiliation(s)
- Sanchayita Sen
- Department of Chemistry and Biochemistry, Montana State University, 108 Gaines Hall, Bozeman, MT 59717, USA
| | | | | | | | | | | | | |
Collapse
|
10
|
Wilson PE, Bunker J, Lowery TJ, Watt GD. Reduction of nitrogenase Fe protein from Azotobacter vinelandii by dithionite: quantitative and qualitative effects of nucleotides, temperature, pH and reaction buffer. Biophys Chem 2005; 109:305-24. [PMID: 15110948 DOI: 10.1016/j.bpc.2003.12.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2003] [Revised: 12/03/2003] [Accepted: 12/08/2003] [Indexed: 11/27/2022]
Abstract
Oxidized Fe protein from Azotobacter vinelandii (Av2(0)) was reduced by dithionite (DT) in the absence and presence of nucleotides, over the temperature range 10-40 degrees C, over the pH range 7-8, and in various buffers--inorganic phosphate, TES, HEPES, and Tris. The reduction of each species of Fe protein--Av2(0), Av2(0)(MgATP)2, and Av2(0)(MgADP)2--was resolved into at least three exponential phases, with relative amplitudes of each phase varying over the range of experimental conditions, suggesting a dynamic population shift of kinetically distinct species. The rapid phase of Av2(0) reduction predominated at low temperature and pH, and in Tris buffer; rapid Av2(0)(MgATP)2 reduction was favored at high temperature and pH, and in phosphate buffer; and Av2(0)(MgADP)2 reduction was favored under more physiologically relevant conditions of 20 degrees C, pH 7.5, and in phosphate buffer. The rates of reduction of Fe protein species did not change with buffer, but temperature and pH do have an effect on the rates. With the appropriate constants, an empirically derived equation estimates the rate of Fe protein reduction at any temperature and pH within the limits 10-40 degrees C and pH 7-8, for a given species of Fe protein, and a given phase of the reaction. At 23.0 degrees C and pH 7.4, the rate of the dominant phase of Av2(0) reduction is 1.9 x 10(8) M(-1) s(-1). Under the same conditions, the rates of the two dominant phases of Av2(0)(MgATP)2 reduction are 1.2 x 10(6) and 1.5 x10 (5) M(-1) s(-1); and the rate of the dominant phase of Av2(0)(MgADP)2 reduction is 3.5 x 10(6) in M(-1) s(-1). Thermodynamic activation parameters for each phase of reduction were calculated. No breaks in the Arrhenius plots for any Fe protein species were observed.
Collapse
Affiliation(s)
- P E Wilson
- Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84604, USA
| | | | | | | |
Collapse
|
11
|
Rees DC, Akif Tezcan F, Haynes CA, Walton MY, Andrade S, Einsle O, Howard JB. Structural basis of biological nitrogen fixation. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2005; 363:971-84; discussion 1035-40. [PMID: 15901546 DOI: 10.1098/rsta.2004.1539] [Citation(s) in RCA: 194] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
Biological nitrogen fixation is mediated by the nitrogenase enzyme system that catalyses the ATP dependent reduction of atmospheric dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the MoFe-protein with the FeMo-cofactor that provides the active site for substrate reduction, and the Fe-protein that couples ATP hydrolysis to electron transfer. An overview of the nitrogenase system is presented that emphasizes the structural organization of the proteins and associated metalloclusters that have the remarkable ability to catalyse nitrogen fixation under ambient conditions. Although the mechanism of ammonia formation by nitrogenase remains enigmatic, mechanistic inferences motivated by recent developments in the areas of nitrogenase biochemistry, spectroscopy, model chemistry and computational studies are discussed within this structural framework.
Collapse
Affiliation(s)
- Douglas C Rees
- Division of Chemistry and Chemical Engineering, 114-96, California Institute of Technology, Pasadena, CA 91125, USA.
| | | | | | | | | | | | | |
Collapse
|
12
|
Maria Christofi A, Garratt PJ, Hogarth G, Ibbett AJ, Ng YF, Steed JW. Molecular design using electrostatic interactions. Part 4: Synthesis and properties of flexible tetrapodand tetracations derived from naphthalene. Role of structured water in the electrostatic binding of polyanion guests: a model for interactions in biological systems. Tetrahedron 2002. [DOI: 10.1016/s0040-4020(02)00397-6] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
|
13
|
Kurnikov IV, Charnley AK, Beratan DN. From ATP to Electron Transfer: Electrostatics and Free-Energy Transduction in Nitrogenase. J Phys Chem B 2001. [DOI: 10.1021/jp002540o] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- I. V. Kurnikov
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
| | - A. K. Charnley
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
| | - D. N. Beratan
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
| |
Collapse
|
14
|
Huang HQ, Lin QM, Zhai WJ, Chen CH. Effect of redox mediators on nitrogenase and hydrogenase activities in Azotobacter vinelandii. JOURNAL OF PROTEIN CHEMISTRY 2000; 19:671-8. [PMID: 11307951 DOI: 10.1023/a:1007100319108] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
In bioelectrochemical studies, redox mediators such as methylene blue, natural red, and thionine are used to studying the redox characteristics of enzymes in the living cell. Here we show that nitrogenase activity in Azotobacter vinelandii is completely inhibited by oxidized methylene blue (MBo) when the concentration of this mediator in the medium is increased up to 72 microM. This activity in A. vinelandii is somewhat inhibited by a coenzyme, ascorbic acid (AA). However, the nitrogenase activity within the A. vinelandii cell is unchanged even for a high concentration of oxidized natural red (NRo) alone. Interestingly, these mediators and AA do not have the capacity to inhibit the H2 uptake activity of the hydrogenase in A. vinelandii. Average active rates of 66 nM H2 evolved/mg cell protein/min from the nitrogenase and 160 nM H2-uptake/mg cell protein/min from the hydrogenase in A. vinelandii are found in aid of the activities of the enzymes for H2 evolution and for H2 uptake are compared. The activities of both enzymes in A. vinelandii are strongly inhibited by thionine having high oxidative potential. Mechanisms of various mediators acting in vivo for both enzymes in A. vinelandii are discussed.
Collapse
Affiliation(s)
- H Q Huang
- School of Life Sciences, Xiamen University, China.
| | | | | | | |
Collapse
|
15
|
Chan JM, Wu W, Dean DR, Seefeldt LC. Construction and characterization of a heterodimeric iron protein: defining roles for adenosine triphosphate in nitrogenase catalysis. Biochemistry 2000; 39:7221-8. [PMID: 10852721 DOI: 10.1021/bi000219q] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
One molecule of MgATP binds to each subunit of the homodimeric Fe protein component of nitrogenase. Both MgATP molecules are hydrolyzed to MgADP and P(i) in reactions coupled to the transfer of one electron into the MoFe protein component. As an approach to assess the contributions of individual ATP binding sites, a heterodimeric Fe protein was produced that has an Asn substituted for residue 39 in the ATP binding domain in one subunit, while the normal Asp(39) residue within the other subunit remains unchanged. Separation of the heterodimeric Fe protein from a mixed population with homodimeric Fe proteins contained in crude extracts was accomplished by construction of a seven His tag on one subunit and a differential immobilized-metal-affinity chromatography technique. Three forms of the Fe protein (wild-type homodimeric Fe protein [Asp(39)/Asp(39)], altered homodimeric Fe protein [Asn(39)/Asn(39)], and heterodimeric Fe protein [Asp(39)/Asn(39)]) were compared on the basis of the biochemical and biophysical changes elicited by nucleotide binding. Among those features examined were the MgATP- and MgADP-induced protein conformational changes that are manifested by the susceptibility of the [4Fe-4S] cluster to chelation and by alterations in the electron paramagnetic resonance, circular dichroism, and midpoint potential of the [4Fe-4S] cluster. The results indicate that changes in the [4Fe-4S] cluster caused by nucleotide binding are the result of additive conformational changes contributed by the individual subunits. The [Asp(39)/Asn(39)] Fe protein did not support substrate reduction activity but did hydrolyze MgATP and showed MgATP-dependent primary electron transfer to the MoFe protein. These results support a model where each MgATP site contributes to the rate acceleration of primary electron transfer, but both MgATP sites must be functioning properly for substrate reduction. Like the altered homodimeric [Asn(39)/Asn(39)] Fe protein, the heterodimeric [Asp(39)/Asn(39)] Fe protein was found to form a high affinity complex with the MoFe protein, revealing that alteration on one subunit is sufficient to create a tight complex.
Collapse
Affiliation(s)
- J M Chan
- Department of Chemistry and Biochemistry, Utah State University, Logan 84322, USA
| | | | | | | |
Collapse
|