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Blomberg MRA, Ädelroth P. Reduction of molecular oxygen in flavodiiron proteins - Catalytic mechanism and comparison to heme-copper oxidases. J Inorg Biochem 2024; 255:112534. [PMID: 38552360 DOI: 10.1016/j.jinorgbio.2024.112534] [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: 12/27/2023] [Revised: 03/15/2024] [Accepted: 03/18/2024] [Indexed: 04/16/2024]
Abstract
The family of flavodiiron proteins (FDPs) plays an important role in the scavenging and detoxification of both molecular oxygen and nitric oxide. Using electrons from a flavin mononucleotide cofactor molecular oxygen is reduced to water and nitric oxide is reduced to nitrous oxide and water. While the mechanism for NO reduction in FDPs has been studied extensively, there is very little information available about O2 reduction. Here we use hybrid density functional theory (DFT) to study the mechanism for O2 reduction in FDPs. An important finding is that a proton coupled reduction is needed after the O2 molecule has bound to the diferrous diiron active site and before the OO bond can be cleaved. This is in contrast to the mechanism for NO reduction, where both NN bond formation and NO bond cleavage occurs from the same starting structure without any further reduction, according to both experimental and computational results. This computational result for the O2 reduction mechanism should be possible to evaluate experimentally. Another difference between the two substrates is that the actual OO bond cleavage barrier is low, and not involved in rate-limiting the reduction process, while the barrier connected with bond cleavage/formation in the NO reduction process is of similar height as the rate-limiting steps. We suggest that these results may be part of the explanation for the generally higher activity for O2 reduction as compared to NO reduction in most FDPs. Comparisons are also made to the O2 reduction reaction in the family of heme‑copper oxidases.
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Affiliation(s)
- Margareta R A Blomberg
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
| | - Pia Ädelroth
- Department of Biochemistry and Biophysics, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
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Sztachova T, Tomkova A, Cizmar E, Jancura D, Fabian M. Radical in the Peroxide-Produced F-Type Ferryl Form of Bovine Cytochrome c Oxidase. Int J Mol Sci 2022; 23:ijms232012580. [PMID: 36293434 PMCID: PMC9604133 DOI: 10.3390/ijms232012580] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2022] [Revised: 10/10/2022] [Accepted: 10/18/2022] [Indexed: 11/16/2022] Open
Abstract
The reduction of O2 in respiratory cytochrome c oxidases (CcO) is associated with the generation of the transmembrane proton gradient by two mechanisms. In one of them, the proton pumping, two different types of the ferryl intermediates of the catalytic heme a3-CuB center P and F forms, participate. Equivalent ferryl states can be also formed by the reaction of the oxidized CcO (O) with H2O2. Interestingly, in acidic solutions a single molecule of H2O2 can generate from the O an additional F-type ferryl form (F•) that should contain, in contrast to the catalytic F intermediate, a free radical at the heme a3-CuB center. In this work, the formation and the endogenous decay of both the ferryl iron of heme a3 and the radical in F• intermediate were examined by the combination of four experimental approaches, isothermal titration calorimetry, electron paramagnetic resonance, and electronic absorption spectroscopy together with the reduction of this form by the defined number of electrons. The results are consistent with the generation of radicals in F• form. However, the radical at the catalytic center is more rapidly quenched than the accompanying ferryl state of heme a3, very likely by the intrinsic oxidation of the enzyme itself.
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Affiliation(s)
- Tereza Sztachova
- Department of Biophysics, Faculty of Science, University of P. J. Safarik, Jesenna 5, 041 54 Kosice, Slovakia
| | - Adriana Tomkova
- Department of Biophysics, Faculty of Science, University of P. J. Safarik, Jesenna 5, 041 54 Kosice, Slovakia
| | - Erik Cizmar
- Department of Condensed Matter Physics, Faculty of Science, University of P. J. Safarik, Park Angelinum 9, 040 01 Kosice, Slovakia
| | - Daniel Jancura
- Department of Biophysics, Faculty of Science, University of P. J. Safarik, Jesenna 5, 041 54 Kosice, Slovakia
- Correspondence: (D.J.); (M.F.)
| | - Marian Fabian
- Center for Interdisciplinary Biosciences, Technology and Innovation Park, University of P. J. Safarik, Jesenna 5, 041 54 Kosice, Slovakia
- Correspondence: (D.J.); (M.F.)
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Siletsky SA, Borisov VB. Proton Pumping and Non-Pumping Terminal Respiratory Oxidases: Active Sites Intermediates of These Molecular Machines and Their Derivatives. Int J Mol Sci 2021; 22:10852. [PMID: 34639193 PMCID: PMC8509429 DOI: 10.3390/ijms221910852] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2021] [Revised: 10/04/2021] [Accepted: 10/05/2021] [Indexed: 11/16/2022] Open
Abstract
Terminal respiratory oxidases are highly efficient molecular machines. These most important bioenergetic membrane enzymes transform the energy of chemical bonds released during the transfer of electrons along the respiratory chains of eukaryotes and prokaryotes from cytochromes or quinols to molecular oxygen into a transmembrane proton gradient. They participate in regulatory cascades and physiological anti-stress reactions in multicellular organisms. They also allow microorganisms to adapt to low-oxygen conditions, survive in chemically aggressive environments and acquire antibiotic resistance. To date, three-dimensional structures with atomic resolution of members of all major groups of terminal respiratory oxidases, heme-copper oxidases, and bd-type cytochromes, have been obtained. These groups of enzymes have different origins and a wide range of functional significance in cells. At the same time, all of them are united by a catalytic reaction of four-electron reduction in oxygen into water which proceeds without the formation and release of potentially dangerous ROS from active sites. The review analyzes recent structural and functional studies of oxygen reduction intermediates in the active sites of terminal respiratory oxidases, the features of catalytic cycles, and the properties of the active sites of these enzymes.
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Affiliation(s)
- Sergey A. Siletsky
- Department of Bioenergetics, Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
| | - Vitaliy B. Borisov
- Department of Molecular Energetics of Microorganisms, Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia;
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Blomberg MRA. Activation of O 2 and NO in heme-copper oxidases - mechanistic insights from computational modelling. Chem Soc Rev 2021; 49:7301-7330. [PMID: 33006348 DOI: 10.1039/d0cs00877j] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Heme-copper oxidases are transmembrane enzymes involved in aerobic and anaerobic respiration. The largest subgroup contains the cytochrome c oxidases (CcO), which reduce molecular oxygen to water. A significant part of the free energy released in this exergonic process is conserved as an electrochemical gradient across the membrane, via two processes, electrogenic chemistry and proton pumping. A deviant subgroup is the cytochrome c dependent NO reductases (cNOR), which reduce nitric oxide to nitrous oxide and water. This is also an exergonic reaction, but in this case none of the released free energy is conserved. Computational studies applying hybrid density functional theory to cluster models of the bimetallic active sites in the heme-copper oxidases are reviewed. To obtain a reliable description of the reaction mechanisms, energy profiles of the entire catalytic cycles, including the reduction steps have to be constructed. This requires a careful combination of computational results with certain experimental data. Computational studies have elucidated mechanistic details of the chemical parts of the reactions, involving cleavage and formation of covalent bonds, which have not been obtainable from pure experimental investigations. Important insights regarding the mechanisms of energy conservation have also been gained. The computational studies show that the reduction potentials of the active site cofactors in the CcOs are large enough to afford electrogenic chemistry and proton pumping, i.e. efficient energy conservation. These results solve a conflict between different types of experimental data. A mechanism for the proton pumping, involving a specific and crucial role for the active site tyrosine, conserved in all CcOs, is suggested. For the cNORs, the calculations show that the low reduction potentials of the active site cofactors are optimized for fast elimination of the toxic NO molecules. At the same time, the low reduction potentials lead to endergonic reduction steps with high barriers. To prevent even higher barriers, which would lead to a too slow reaction, when the electrochemical gradient across the membrane is present, the chemistry must occur in a non-electrogenic manner. This explains why there is no energy conservation in cNOR.
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Affiliation(s)
- Margareta R A Blomberg
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden.
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Blomberg MRA. The Redox-Active Tyrosine Is Essential for Proton Pumping in Cytochrome c Oxidase. Front Chem 2021; 9:640155. [PMID: 33937193 PMCID: PMC8079940 DOI: 10.3389/fchem.2021.640155] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2020] [Accepted: 02/05/2021] [Indexed: 11/13/2022] Open
Abstract
Cellular respiration involves electron transport via a number of enzyme complexes to the terminal Cytochrome c oxidase (CcO), in which molecular oxygen is reduced to water. The free energy released in the reduction process is used to establish a transmembrane electrochemical gradient, via two processes, both corresponding to charge transport across the membrane in which the enzymes are embedded. First, the reduction chemistry occurring in the active site of CcO is electrogenic, which means that the electrons and protons are delivered from opposite sides of the membrane. Second, the exergonic chemistry is coupled to translocation of protons across the entire membrane, referred to as proton pumping. In the largest subfamily of the CcO enzymes, the A-family, one proton is pumped for every electron needed for the chemistry, making the energy conservation particularly efficient. In the present study, hybrid density functional calculations are performed on a model of the A-family CcOs. The calculations show that the redox-active tyrosine, conserved in all types of CcOs, plays an essential role for the energy conservation. Based on the calculations a reaction mechanism is suggested involving a tyrosyl radical (possibly mixed with tyrosinate character) in all reduction steps. The result is that the free energy released in each reduction step is large enough to allow proton pumping in all reduction steps without prohibitively high barriers when the gradient is present. Furthermore, the unprotonated tyrosine provides a mechanism for coupling the uptake of two protons per electron in every reduction step, i.e. for a secure proton pumping.
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Affiliation(s)
- Margareta R A Blomberg
- Arrhenius Laboratory, Department of Organic Chemistry, Stockholm University, Stockholm, Sweden
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Mikulova L, Pechova I, Jancura D, Stupak M, Fabian M. Thermodynamics of the P-type Ferryl Form of Bovine Cytochrome c Oxidase. BIOCHEMISTRY (MOSCOW) 2021; 86:74-83. [DOI: 10.1134/s0006297921010077] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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Noodleman L, Han Du WG, McRee D, Chen Y, Goh T, Götz AW. Coupled transport of electrons and protons in a bacterial cytochrome c oxidase-DFT calculated properties compared to structures and spectroscopies. Phys Chem Chem Phys 2021; 22:26652-26668. [PMID: 33231596 DOI: 10.1039/d0cp04848h] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
After a general introduction to the features and mechanisms of cytochrome c oxidases (CcOs) in mitochondria and aerobic bacteria, we present DFT calculated physical and spectroscopic properties for the catalytic reaction cycle compared with experimental observations in bacterial ba3 type CcO, also with comparisons/contrasts to aa3 type CcOs. The Dinuclear Complex (DNC) is the active catalytic reaction center, containing a heme a3 Fe center and a near lying Cu center (called CuB) where by successive reduction and protonation, molecular O2 is transformed to two H2O molecules, and protons are pumped from an inner region across the membrane to an outer region by transit through the CcO integral membrane protein. Structures, energies and vibrational frequencies for Fe-O and O-O modes are calculated by DFT over the catalytic cycle. The calculated DFT frequencies in the DNC of CcO are compared with measured frequencies from Resonance Raman spectroscopy to clarify the composition, geometry, and electronic structures of different intermediates through the reaction cycle, and to trace reaction pathways. X-ray structures of the resting oxidized state are analyzed with reference to the known experimental reaction chemistry and using DFT calculated structures in fitting observed electron density maps. Our calculations lead to a new proposed reaction pathway for coupling the PR → F → OH (ferryl-oxo → ferric-hydroxo) pathway to proton pumping by a water shift mechanism. Through this arc of the catalytic cycle, major shifts in pKa's of the special tyrosine and a histidine near the upper water pool activate proton transfer. Additional mechanisms for proton pumping are explored, and the role of the CuB+ (cuprous state) in controlling access to the dinuclear reaction site is proposed.
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Affiliation(s)
- Louis Noodleman
- Department of Integrative Structural and Computational Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.
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Computing Proton-Coupled Redox Potentials of Fluorotyrosines in a Protein Environment. J Phys Chem B 2020; 125:128-136. [PMID: 33378205 DOI: 10.1021/acs.jpcb.0c09974] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The oxidation of tyrosine to form the neutral tyrosine radical via proton-coupled electron transfer is essential for a wide range of biological processes. The precise measurement of the proton-coupled redox potentials of tyrosine (Y) in complex protein environments is challenging mainly because of the highly oxidizing and reactive nature of the radical state. Herein, a computational strategy is presented for predicting proton-coupled redox potentials in a protein environment. In this strategy, both the reduced Y-OH and oxidized Y-O• forms of tyrosine are sampled with molecular dynamics using a molecular mechanical force field. For a large number of conformations, a quantum mechanical/molecular mechanical (QM/MM) electrostatic embedding scheme is used to compute the free-energy differences between the reduced and oxidized forms, including the zero-point energy and entropic contributions as well as the impact of the protein electrostatic environment. This strategy is applied to a series of fluorinated tyrosine derivatives embedded in a de novo α-helical protein denoted as α3Y. The force fields for both the reduced and oxidized forms of these noncanonical fluorinated tyrosine residues are parameterized for general use. The calculated relative proton-coupled redox potentials agree with experimentally measured values with a mean unsigned error of 24 mV. Analysis of the simulations illustrates that hydrogen-bonding interactions between tyrosine and water increase the redox potentials by ∼100-250 mV, with significant variations because of the fluctuating protein environment. This QM/MM approach enables the calculation of proton-coupled redox potentials of tyrosine and other residues such as tryptophan in a variety of protein systems.
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Blomberg MRA. Role of the Two Metals in the Active Sites of Heme Copper Oxidases-A Study of NO Reduction in cbb3 Cytochrome c Oxidase. Inorg Chem 2020; 59:11542-11553. [PMID: 32799475 DOI: 10.1021/acs.inorgchem.0c01351] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The superfamily of heme copper oxidases reduces molecular oxygen or nitric oxide, and the active sites comprise a high-spin heme group (a3 or b3) and a non-heme metal (CuB or FeB). The cbb3 C family of cytochrome c oxidases, with the high-spin heme b3 and CuB in the active site, is a subfamily of the heme copper oxidases that can reduce both molecular oxygen, which is the main substrate, and nitric oxide. The mechanism for NO reduction in cbb3 oxidase is studied here using hybrid density functional theory and compared to other cytochrome c oxidases (A and B families), with a high-spin heme a3 and CuB in the active site, and to cytochrome c dependent NO reductase, with a high-spin heme b3 and a non-heme FeB in the active site. It is found that the reaction mechanism and the detailed reaction energetics of the cbb3 oxidases are not similar to those of cytochrome c dependent NO reductase, which has the same type of high-spin heme group but a different non-heme metal. This is in contrast to earlier expectations. Instead, the NO reduction mechanism in cbb3 oxidases is very similar to that in the other cytochrome c oxidases, with the same non-heme metal, CuB, and is independent of the type of high-spin heme group. The conclusion is that the type of non-heme metal (CuB or FeB) in the active site of the heme copper oxidases is more important for the reaction mechanisms than the type of high-spin heme, at least for the NO reduction reaction. The reason is that the proton-coupled reduction potentials of the active site cofactors determine the energetics for the NO reduction reaction, and they depend to a larger extent on the non-heme metal. Observed differences in NO reduction reactivity among the various cytochrome c oxidases may be explained by differences outside the BNC, affecting the rate of proton transfer, rather than in the BNC itself.
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The structure of the oxidized state of cytochrome c oxidase - experiments and theory compared. J Inorg Biochem 2020; 206:111020. [PMID: 32062501 DOI: 10.1016/j.jinorgbio.2020.111020] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2019] [Revised: 01/31/2020] [Accepted: 02/05/2020] [Indexed: 11/22/2022]
Abstract
Cytochrome c oxidase (CcO), the terminal enzyme in the respiratory chain, reduces molecular oxygen to water. Experimental data on the midpoint potentials of the heme iron/copper active site cofactors do not match the overall reaction energetics, and are also in conflict with the observed efficiency of energy conservation in CcO. Therefore it has been postulated that the ferric/cupric intermediate (the oxidized state) exists in two forms. One form, labelled OH, is presumably involved during catalytic turnover, and should have a high CuB midpoint potential due to a metastable high energy structure. When no more electrons are supplied, the OH state supposedly relaxes to the resting form, labelled O, with a lower energy and a lower midpoint potential. It has been suggested that there is a pure geometrical difference between the OH and O states, obtained by moving a water molecule inside the active site. It is shown here that the difference between the two forms of the oxidized state must be of a more chemical nature. The reason is that all types of geometrically relaxed structures of the oxidized intermediate have similar energies, all with a high proton coupled reduction potential in accordance with the postulated OH state. One hypothesized chemical modification of the OH state is the transfer of an extra proton, possibly internal, into the active site. Such a protonated state has several properties that agree with experimental data on the relaxed oxidized state, including a decreased midpoint potential.
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Blomberg MRA. The mechanism for oxygen reduction in the C family cbb 3 cytochrome c oxidases - Implications for the proton pumping stoichiometry. J Inorg Biochem 2019; 203:110866. [PMID: 31706225 DOI: 10.1016/j.jinorgbio.2019.110866] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2019] [Revised: 09/12/2019] [Accepted: 09/15/2019] [Indexed: 11/16/2022]
Abstract
Cytochrome c oxidases (CcOs) couple the exergonic reduction of molecular oxygen to proton pumping across the membrane in which they are embedded, thereby conserving a significant part of the free energy. The A family CcOs are known to pump four protons per oxygen molecule, while there is no consensus regarding the proton pumping stoichiometry for the C family cbb3 oxidases. Hybrid density functional theory is used here to investigate the catalytic mechanism for oxygen reduction in cbb3 oxidases. A surprising result is that the barrier for O O bond cleavage at the mixed valence reduction level seems to be too high compared to the overall reaction rate of the enzyme. It is therefore suggested that the O O bond is cleaved only after the first proton coupled reduction step, and that this reduction step most likely is not coupled to proton pumping. Furthermore, since the cbb3 oxidases have only one proton channel leading to the active site, it is proposed that the activated EH intermediate, suggested to be responsible for proton pumping in one of the reduction steps in the A family, cannot be involved in the catalytic cycle for cbb3, which results in the lack of proton pumping also in the E to R reduction step. In summary, the calculations indicate that only two protons are pumped per oxygen molecule in cbb3 oxidases. However, more experimental information on this divergent enzyme is needed, e.g. whether the flow of electrons resembles that in the other more well-studied CcO families.
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Affiliation(s)
- Margareta R A Blomberg
- Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
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