1
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Lewis NM, Kisgeropoulos EC, Lubner CE, Fixen KR. Characterization of ferredoxins involved in electron transfer pathways for nitrogen fixation implicates differences in electronic structure in tuning 2[4Fe4S] Fd activity. J Inorg Biochem 2024; 254:112521. [PMID: 38471286 DOI: 10.1016/j.jinorgbio.2024.112521] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2023] [Revised: 03/03/2024] [Accepted: 03/05/2024] [Indexed: 03/14/2024]
Abstract
Ferredoxins (Fds) are small proteins which shuttle electrons to pathways like biological nitrogen fixation. Physical properties tune the reactivity of Fds with different pathways, but knowledge on how these properties can be manipulated to engineer new electron transfer pathways is lacking. Recently, we showed that an evolved strain of Rhodopseudomonas palustris uses a new electron transfer pathway for nitrogen fixation. This pathway involves a variant of the primary Fd of nitrogen fixation in R. palustris, Fer1, in which threonine at position 11 is substituted for isoleucine (Fer1T11I). To understand why this substitution in Fer1 enables more efficient electron transfer, we used in vivo and in vitro methods to characterize Fer1 and Fer1T11I. Electrochemical characterization revealed both Fer1 and Fer1T11I have similar redox transitions (-480 mV and - 550 mV), indicating the reduction potential was unaffected despite the proximity of T11 to an iron‑sulfur (FeS) cluster of Fer1. Additionally, disruption of hydrogen bonding around an FeS cluster in Fer1 by substituting threonine with alanine (T11A) or valine (T11V) did not increase nitrogenase activity, indicating that disruption of hydrogen bonding does not explain the difference in activity observed for Fer1T11I. Electron paramagnetic resonance spectroscopy studies revealed key differences in the electronic structure of Fer1 and Fer1T11I, which indicate changes to the high spin states and/or spin-spin coupling between the FeS clusters of Fer1. Our data implicates these electronic structure differences in facilitating electron flow and sets a foundation for further investigations to understand the connection between these properties and intermolecular electron transfer.
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Affiliation(s)
- Nathan M Lewis
- Department of Plant and Microbial Biology and the Biotechnology Institute, University of Minnesota, Minneapolis, MN, United States of America
| | | | - Carolyn E Lubner
- National Renewable Energy Laboratory, Golden, CO, United States of America.
| | - Kathryn R Fixen
- Department of Plant and Microbial Biology and the Biotechnology Institute, University of Minnesota, Minneapolis, MN, United States of America.
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2
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Kisgeropoulos E, Bharadwaj VS, Ledinina A, Lubner CE, Mulder DW, Smolinski SL, Boehm M, Gutekunst K, King PW, Svedruzic D. Structural and biophysical properties of a [4Fe4S] ferredoxin-like protein from Synechocystis sp. PCC 6803 with a unique two domain structure. J Inorg Biochem 2024; 251:112428. [PMID: 38008043 DOI: 10.1016/j.jinorgbio.2023.112428] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2023] [Revised: 10/23/2023] [Accepted: 11/11/2023] [Indexed: 11/28/2023]
Abstract
Electron carrier proteins (ECPs), binding iron-sulfur clusters, are vital components within the intricate network of metabolic and photosynthetic reactions. They play a crucial role in the distribution of reducing equivalents. In Synechocystis sp. PCC 6803, the ECP network includes at least nine ferredoxins. Previous research, including global expression analyses and protein binding studies, has offered initial insights into the functional roles of individual ferredoxins within this network. This study primarily focuses on Ferredoxin 9 (slr2059). Through sequence analysis and computational modeling, Ferredoxin 9 emerges as a unique ECP with a distinctive two-domain architecture. It consists of a C-terminal iron‑sulfur binding domain and an N-terminal domain with homology to Nil-domain proteins, connected by a structurally rigid 4-amino acid linker. Notably, in contrast to canonical [2Fe2S] ferredoxins exemplified by PetF (ssl0020), which feature highly acidic surfaces facilitating electron transfer with photosystem I reaction centers, models of Ferredoxin 9 reveal a more neutral to basic protein surface. Using a combination of electron paramagnetic resonance spectroscopy and square-wave voltammetry on heterologously produced Ferredoxin 9, this study demonstrates that the protein coordinates 2×[4Fe4S]2+/1+ redox-active and magnetically interacting clusters, with measured redox potentials of -420 ± 9 mV and - 516 ± 10 mV vs SHE. A more in-depth analysis of Fdx9's unique structure and protein sequence suggests that this type of Nil-2[4Fe4S] multi-domain ferredoxin is well conserved in cyanobacteria, bearing structural similarities to proteins involved in homocysteine synthesis in methanogens.
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Affiliation(s)
- Effie Kisgeropoulos
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Vivek S Bharadwaj
- Renewable Resources and Enabling Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Anastasia Ledinina
- Department of Molecular and Structural Biochemistry, North Carolina State University, USA
| | - Carolyn E Lubner
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - David W Mulder
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Sharon L Smolinski
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Marko Boehm
- Department of Biology, Botanical Institute, Christian-Albrechts-University, Kiel, Germany; Department of Molecular Plant Physiology, Bioenergetics in Photoautotrophs, University of Kassel, Kassel, Germany
| | - Kirstin Gutekunst
- Department of Biology, Botanical Institute, Christian-Albrechts-University, Kiel, Germany; Department of Molecular Plant Physiology, Bioenergetics in Photoautotrophs, University of Kassel, Kassel, Germany
| | - Paul W King
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA
| | - Drazenka Svedruzic
- Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA.
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3
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Katsyv A, Essig M, Bedendi G, Sahin S, Milton RD, Müller V. Characterization of ferredoxins from the thermophilic, acetogenic bacterium Thermoanaerobacter kivui. FEBS J 2023; 290:4107-4125. [PMID: 37074156 DOI: 10.1111/febs.16801] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2023] [Revised: 03/21/2023] [Accepted: 04/19/2023] [Indexed: 04/20/2023]
Abstract
A major electron carrier involved in energy and carbon metabolism in the acetogenic model organism Thermoanaerobacter kivui is ferredoxin, an iron-sulfur-containing, electron-transferring protein. Here, we show that the genome of T. kivui encodes four putative ferredoxin-like proteins (TKV_c09620, TKV_c16450, TKV_c10420 and TKV_c19530). All four genes were cloned, a His-tag encoding sequence was added and the proteins were produced from a plasmid in T. kivui. The purified proteins had an absorption peak at 430 nm typical for ferredoxins. The determined iron-sulfur content is consistent with the presence of two predicted [4Fe4S] clusters in TKV_c09620 and TKV_c19530 or one predicted [4Fe4S] cluster in TKV_c16450 and TKV_c10420 respectively. The reduction potential (Em ) for TKV_c09620, TKV_c16450, TKV_c10420 and TKV_c19530 was determined to be -386 ± 4 mV, -386 ± 2 mV, -559 ± 10 mV and -557 ± 3 mV, respectively. TKV_c09620 and TKV_c16450 served as electron carriers for different oxidoreductases from T. kivui. Deletion of the ferredoxin genes led to only a slight reduction of growth on pyruvate or autotrophically on H2 + CO2 . Transcriptional analysis revealed that TKV_c09620 was upregulated in a ΔTKV_c16450 mutant and vice versa TKV_c16450 in a ΔTKV_c09620 mutant, indicating that TKV_c09620 and TKV_c16450 can replace each other. In sum, our data are consistent with the hypothesis that TKV_c09620 and TKV_c16450 are ferredoxins involved in autotrophic and heterotrophic metabolism of T. kivui.
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Affiliation(s)
- Alexander Katsyv
- Department of Molecular Microbiology & Bioenergetics, Institute of Molecular Biosciences, Johann Wolfgang Goethe University, Frankfurt am Main, Germany
| | - Melanie Essig
- Department of Molecular Microbiology & Bioenergetics, Institute of Molecular Biosciences, Johann Wolfgang Goethe University, Frankfurt am Main, Germany
| | - Giada Bedendi
- Department of Inorganic and Analytical Chemistry, University of Geneva, Geneva, Switzerland
| | - Selmihan Sahin
- Department of Inorganic and Analytical Chemistry, University of Geneva, Geneva, Switzerland
| | - Ross D Milton
- Department of Inorganic and Analytical Chemistry, University of Geneva, Geneva, Switzerland
| | - Volker Müller
- Department of Molecular Microbiology & Bioenergetics, Institute of Molecular Biosciences, Johann Wolfgang Goethe University, Frankfurt am Main, Germany
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4
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Lai TY, Chen C, Chu K, Chien S, Ong T, Chiang M. Biologically inspired
3Fe4S
cluster as structural mimics of
FeMoco
M‐cluster. J CHIN CHEM SOC-TAIP 2023. [DOI: 10.1002/jccs.202300062] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/03/2023]
Affiliation(s)
- Ting Yi Lai
- Institute of Chemistry Academia Sinica Taipei Taiwan
| | - Chang‐Ting Chen
- Institute of Chemistry Academia Sinica Taipei Taiwan
- Department of Chemistry National Taiwan University Taipei Taiwan
| | - Kai‐Ti Chu
- Institute of Chemistry Academia Sinica Taipei Taiwan
| | - Su‐Ying Chien
- Instrumentation Center National Taiwan University Taipei Taiwan
| | - Tiow‐Gan Ong
- Institute of Chemistry Academia Sinica Taipei Taiwan
- Department of Chemistry National Taiwan University Taipei Taiwan
| | - Ming‐Hsi Chiang
- Institute of Chemistry Academia Sinica Taipei Taiwan
- Department of Medicinal and Applied Chemistry Kaohsiung Medical University Kaohsiung Taiwan
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5
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Prioretti L, D’Ermo G, Infossi P, Kpebe A, Lebrun R, Bauzan M, Lojou E, Guigliarelli B, Giudici-Orticoni MT, Guiral M. Carbon Fixation in the Chemolithoautotrophic Bacterium Aquifex aeolicus Involves Two Low-Potential Ferredoxins as Partners of the PFOR and OGOR Enzymes. Life (Basel) 2023; 13:life13030627. [PMID: 36983784 PMCID: PMC10052474 DOI: 10.3390/life13030627] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2023] [Revised: 02/21/2023] [Accepted: 02/22/2023] [Indexed: 02/27/2023] Open
Abstract
Aquifex aeolicus is a microaerophilic hydrogen- and sulfur -oxidizing bacterium that assimilates CO2 via the reverse tricarboxylic acid cycle (rTCA). Key enzymes of this pathway are pyruvate:ferredoxin oxidoreductase (PFOR) and 2-oxoglutarate:ferredoxin oxidoreductase (OGOR), which are responsible, respectively, for the reductive carboxylation of acetyl-CoA to pyruvate and of succinyl-CoA to 2-oxoglutarate, two energetically unfavorable reactions that require a strong reduction potential. We have confirmed, by biochemistry and proteomics, that A. aeolicus possesses a pentameric version of these enzyme complexes ((αβγδε)2) and that they are highly abundant in the cell. In addition, we have purified and characterized, from the soluble fraction of A. aeolicus, two low redox potential and oxygen-stable [4Fe-4S] ferredoxins (Fd6 and Fd7, E0 = −440 and −460 mV, respectively) and shown that they can physically interact and exchange electrons with both PFOR and OGOR, suggesting that they could be the physiological electron donors of the system in vivo. Shotgun proteomics indicated that all the enzymes assumed to be involved in the rTCA cycle are produced in the A. aeolicus cells. A number of additional enzymes, previously suggested to be part of a putative partial Wood-Ljungdahl pathway used for the synthesis of serine and glycine from CO2 were identified by mass spectrometry, but their abundance in the cell seems to be much lower than that of the rTCA cycle. Their possible involvement in carbon assimilation is discussed.
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Affiliation(s)
- Laura Prioretti
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Giulia D’Ermo
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Pascale Infossi
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Arlette Kpebe
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Régine Lebrun
- CNRS, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Marielle Bauzan
- CNRS, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Elisabeth Lojou
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | - Bruno Guigliarelli
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
| | | | - Marianne Guiral
- CNRS, Bioénergétique et Ingénierie des Protéines, Aix Marseille Université, IMM, 13009 Marseille, France
- Correspondence:
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6
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Evolutionary Aspects of the Oxido-Reductive Network of Methylglyoxal. J Mol Evol 2021; 89:618-638. [PMID: 34718825 DOI: 10.1007/s00239-021-10031-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2021] [Accepted: 10/08/2021] [Indexed: 10/19/2022]
Abstract
In the chemoautotrophic theory for the origin of life, offered as an alternative to broth theory, the archaic reductive citric acid cycle operating without enzymes is in the center. The non-enzymatic (methyl)glyoxalase pathway has been suggested to be the anaplerotic route for the reductive citric acid cycle. In the recent years, much has been learned about methylglyoxal, but its importance in the metabolic machinery is still uncovered. If methylglyoxal had been essential participant of the early stage of evolution, then it is a legitimate question whether it might have played a role in the early oxido-reduction network, too. Therefore, an oxido-reduction network of methylglyoxal that might have functioned under ancient circumstances without enzymes was constructed and analyzed by virtue of group contribution method. Taking methylglyoxal as input material, it turned out that the evolutionary value of reactions and biomolecules were not similar. Glycerol, glycerate, and tartonate, the output components, were conserved to different degrees. Although the tartonate route was similarly favorable from energetic point of view, its intermediates are almost not present in extant biochemistry. The presence of two carboxyl or aldehyde groups, or their combination in tricarbons of the constructed network seemed disadvantageous for selection, and the inductive effect, resulting in an asymmetry in electron cloud of chemicals, might have been important. The evolutionary role for cysteine, H2S, and formaldehyde in the emergence of high-energy bonds in the form of thioesters and in Fe-S cluster formation as well as in imidazole synthesis was shown to bridge the gap between prebiotic chemistry and contemporary biochemistry. Overall, the ideas developed here represent an approach fitting to chemoautotrophic origin of life and implying to the role of methylglyoxal in triose formation. The proposed network is expected to have an impact upon how one may think of prebiological chemical processes on methylglyoxal, too. Finally, along the evolutionary time line, the network functioning without enzymes is situated between the formation of simple organic compounds and primeval cells, being closer to the former and well preceding the last common metabolic ancestor developed after primitive cells emerged.
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7
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Chen N, Rao G, Britt RD, Wang LP. Quantum Chemical Study of a Radical Relay Mechanism for the HydG-Catalyzed Synthesis of a Fe(II)(CO) 2(CN)cysteine Precursor to the H-Cluster of [FeFe] Hydrogenase. Biochemistry 2021; 60:3016-3026. [PMID: 34569243 DOI: 10.1021/acs.biochem.1c00379] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The [FeFe] hydrogenase catalyzes the redox interconversion of protons and H2 with a Fe-S "H-cluster" employing CO, CN, and azadithiolate ligands to two Fe centers. The biosynthesis of the H-cluster is a highly interesting reaction carried out by a set of Fe-S maturase enzymes called HydE, HydF, and HydG. HydG, a member of the radical S-adenosylmethionine (rSAM) family, converts tyrosine, cysteine, and Fe(II) into an organometallic Fe(II)(CO)2(CN)cysteine "synthon", which serves as the substrate for HydE. Although key aspects of the HydG mechanism have been experimentally determined via isotope-sensitive spectroscopic methods, other important mechanistic questions have eluded experimental determination. Here, we use computational quantum chemistry to refine the mechanism of the HydG catalytic reaction. We utilize quantum mechanics/molecular mechanics simulations to investigate the reactions at the canonical Fe-S cluster, where a radical cleavage of the tyrosine substrate takes place and proceeds through a relay of radical intermediates to form HCN and a COO•- radical anion. We then carry out a broken-symmetry density functional theory study of the reactions at the unusual five-iron auxiliary Fe-S cluster, where two equivalents of CN- and COOH• coordinate to the fifth "dangler iron" in a series of substitution and redox reactions that yield the synthon as the final product for further processing by HydE.
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Affiliation(s)
- Nanhao Chen
- Department of Chemistry, University of California Davis, Davis, California 95616, United States
| | - Guodong Rao
- Department of Chemistry, University of California Davis, Davis, California 95616, United States
| | - R David Britt
- Department of Chemistry, University of California Davis, Davis, California 95616, United States
| | - Lee-Ping Wang
- Department of Chemistry, University of California Davis, Davis, California 95616, United States
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8
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Di Rocco G, Battistuzzi G, Borsari M, Bortolotti CA, Ranieri A, Sola M. The enthalpic and entropic terms of the reduction potential of metalloproteins: Determinants and interplay. Coord Chem Rev 2021. [DOI: 10.1016/j.ccr.2021.214071] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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9
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Li B, Steindel P, Haddad N, Elliott SJ. Maximizing (Electro)catalytic CO 2 Reduction with a Ferredoxin-Based Reduction Potential Gradient. ACS Catal 2021. [DOI: 10.1021/acscatal.1c00092] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- Bin Li
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Phillip Steindel
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Narmien Haddad
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Sean J. Elliott
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
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10
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Zhu W, Walker LM, Tao L, Iavarone AT, Wei X, Britt RD, Elliott SJ, Klinman JP. Structural Properties and Catalytic Implications of the SPASM Domain Iron-Sulfur Clusters in Methylorubrum extorquens PqqE. J Am Chem Soc 2020; 142:12620-12634. [PMID: 32643933 DOI: 10.1021/jacs.0c02044] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Understanding the relationship between the metallocofactor and its protein environment is the key to uncovering the mechanism of metalloenzymes. PqqE, a radical S-adenosylmethionine enzyme in pyrroloquinoline quinone (PQQ) biosynthesis, contains three iron-sulfur cluster binding sites. Two auxiliary iron-sulfur cluster binding sites, designated as AuxI and AuxII, use distinctive ligands compared to other proteins in the family while their functions remain unclear. Here, we investigate the electronic properties of these iron-sulfur clusters and compare the catalytic efficiency of wild-type (WT) Methylorubrum extorquens AM1 PqqE to a range of mutated constructs. Using native mass spectrometry, protein film electrochemistry, and electron paramagnetic resonance spectroscopy, we confirm the previously proposed incorporation of a mixture of [2Fe-2S] and [4Fe-4S] clusters at the AuxI site and are able to assign redox potentials to each of the three iron-sulfur clusters. Significantly, a conservative mutation at AuxI, C268H, shown to selectively incorporate a [4Fe-4S] cluster, catalyzes an enhancement of uncoupled S-adenosylmethionine cleavage relative to WT, together with the elimination of detectable peptide cross-linked product. While a [4Fe-4S] cluster can be tolerated at the AuxI site, the aggregate findings suggest a functional [2Fe-2S] configuration within the AuxI site. PqqE variants with nondestructive ligand replacements at AuxII also show that the reduction potential at this site can be manipulated by changing the electronegativity of the unique aspartate ligand. A number of novel mechanistic features are proposed based on the kinetic and spectroscopic data. Additionally, bioinformatic analyses suggest that the unique ligand environment of PqqE may be relevant to its role in PQQ biosynthesis within an oxygen-dependent biosynthetic pathway.
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Affiliation(s)
- Wen Zhu
- California Institute for Quantitative Biosciences, University of California-Berkeley, Berkeley, California 94720, United States
| | - Lindsey M Walker
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Lizhi Tao
- Department of Chemistry, University of California-Davis, Davis, California 95616, United States
| | - Anthony T Iavarone
- California Institute for Quantitative Biosciences, University of California-Berkeley, Berkeley, California 94720, United States
| | - Xuetong Wei
- Department of Molecular and Cell Biology, University of California-Berkeley, Berkeley, California 94720, United States
| | - R David Britt
- Department of Chemistry, University of California-Davis, Davis, California 95616, United States
| | - Sean J Elliott
- Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
| | - Judith P Klinman
- California Institute for Quantitative Biosciences, University of California-Berkeley, Berkeley, California 94720, United States.,Department of Molecular and Cell Biology, University of California-Berkeley, Berkeley, California 94720, United States.,Department of Chemistry, University of California-Berkeley, Berkeley, California 94720, United States
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11
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Blaszczyk AJ, Knox HL, Booker SJ. Understanding the role of electron donors in the reaction catalyzed by Tsrm, a cobalamin-dependent radical S-adenosylmethionine methylase. J Biol Inorg Chem 2019; 24:831-839. [PMID: 31350635 DOI: 10.1007/s00775-019-01689-8] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2019] [Accepted: 07/11/2019] [Indexed: 02/06/2023]
Abstract
The cobalamin-dependent radical S-adenosylmethionine (SAM) enzyme TsrM catalyzes the methylation of C2 of L-tryptophan to form 2-methyltryptophan during the biosynthesis of thiostrepton A. Although TsrM is a member of the radical SAM superfamily, unlike all other annotated members, it does not catalyze a reductive cleavage of SAM to a 5'-deoxyadenosyl 5'-radical intermediate. In fact, it has been proposed that TsrM catalyzes its reaction through two polar nucleophilic displacements, with its cobalamin cofactor cycling directly between methylcobalamin (MeCbl) and cob(I)alamin. Nevertheless, the enzyme has been stated to require the action of a reductant, which can be satisfied by dithiothreitol. By contrast, all other annotated RS enzymes require a reductant that exhibits a much lower reduction potential, which is necessary for the reductive cleavage of SAM. Herein, we show that TsrM can catalyze multiple turnovers in the absence of any reducing agent, but only when it is pre-loaded with MeCbl. When hydroxocobalamin (OHCbl) or cob(II)alamin is bound to TsrM, a reductant is required to convert it to cob(I)alamin, which can acquire a methyl group directly from SAM. Our studies suggest that TsrM uses an external reductant to prime its reaction by converting bound OHCbl or cob(II)alamin to MeCbl, and to regenerate the MeCbl form of the cofactor upon adventitious oxidation of the cob(I)alamin intermediate to cob(II)alamin.
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Affiliation(s)
- Anthony J Blaszczyk
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, 16802, USA
| | - Hayley L Knox
- Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA
| | - Squire J Booker
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA, 16802, USA. .,Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802, USA. .,The Howard Hughes Medical Institute, The Pennsylvania State University, University Park, PA, 16802, USA.
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