1
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Fatima S, Olshansky L. Conformational control over proton-coupled electron transfer in metalloenzymes. Nat Rev Chem 2024:10.1038/s41570-024-00646-7. [PMID: 39223400 DOI: 10.1038/s41570-024-00646-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/29/2024] [Indexed: 09/04/2024]
Abstract
From the reduction of dinitrogen to the oxidation of water, the chemical transformations catalysed by metalloenzymes underlie global geochemical and biochemical cycles. These reactions represent some of the most kinetically and thermodynamically challenging processes known and require the complex choreography of the fundamental building blocks of nature, electrons and protons, to be carried out with utmost precision and accuracy. The rate-determining step of catalysis in many metalloenzymes consists of a protein structural rearrangement, suggesting that nature has evolved to leverage macroscopic changes in protein molecular structure to control subatomic changes in metallocofactor electronic structure. The proton-coupled electron transfer mechanisms operative in nitrogenase, photosystem II and ribonucleotide reductase exemplify this interplay between molecular and electronic structural control. We present the culmination of decades of study on each of these systems and clarify what is known regarding the interplay between structural changes and functional outcomes in these metalloenzyme linchpins.
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Affiliation(s)
- Saman Fatima
- Department of Chemistry, College of Liberal Arts and Sciences, University of Illinois Urbana-Champaign, Urbana, IL, USA
| | - Lisa Olshansky
- Department of Chemistry, College of Liberal Arts and Sciences, University of Illinois Urbana-Champaign, Urbana, IL, USA.
- Center for Biophysics and Quantitative Biology, University of Illinois Urbana-Champaign, Urbana, IL, USA.
- Materials Research Laboratory, The Grainger College of Engineering, University of Illinois Urbana-Champaign, Urbana, IL, USA.
- The Beckman Institute for Advanced Science and Technology, University of Illinois Urbana-Champaign, Urbana, IL, USA.
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2
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Zhu Q, Soudackov AV, Tommos C, Hammes-Schiffer S. Proton-Coupled Electron Transfer upon Oxidation of Tyrosine in a De Novo Protein: Analysis of Proton Acceptor Candidates. Biochemistry 2024; 63:1999-2008. [PMID: 39024184 PMCID: PMC11402026 DOI: 10.1021/acs.biochem.4c00211] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/20/2024]
Abstract
Redox-active residues, such as tyrosine and tryptophan, play important roles in a wide range of biological processes. The α3Y de novo protein, which is composed of three α helices and a tyrosine residue Y32, provides a platform for investigating the redox properties of tyrosine in a well-defined protein environment. Herein, the proton-coupled electron transfer (PCET) reaction that occurs upon oxidation of tyrosine in this model protein by a ruthenium photosensitizer is studied by using a vibronically nonadiabatic PCET theory that includes hydrogen tunneling and excited vibronic states. The input quantities to the analytical nonadiabatic rate constant expression, such as the diabatic proton potential energy curves and associated proton vibrational wave functions, reorganization energy, and proton donor-acceptor distribution functions, are obtained from density functional theory calculations on model systems and molecular dynamics simulations of the solvated α3Y protein. Two possible proton acceptors, namely, water or a glutamate residue in the protein scaffold, are explored. The PCET rate constant is greater when glutamate is the proton acceptor, mainly due to the more favorable driving force and shorter equilibrium proton donor-acceptor distance, although contributions from excited vibronic states mitigate these effects. Nevertheless, water could be the dominant proton acceptor if its equilibrium constant associated with hydrogen bond formation is significantly greater than that for glutamate. Although these calculations do not definitively identify the proton acceptor for this PCET reaction, they elucidate the conditions under which each proton acceptor can be favored. These insights have implications for tyrosine-based PCET in a wide variety of biochemical processes.
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Affiliation(s)
- Qiwen Zhu
- Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
| | - Alexander V Soudackov
- Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
| | - Cecilia Tommos
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843, United States
| | - Sharon Hammes-Schiffer
- Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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3
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Nilsen-Moe A, Reinhardt CR, Huang P, Agarwala H, Lopes R, Lasagna M, Glover S, Hammes-Schiffer S, Tommos C, Hammarström L. Switching the proton-coupled electron transfer mechanism for non-canonical tyrosine residues in a de novo protein. Chem Sci 2024; 15:3957-3970. [PMID: 38487244 PMCID: PMC10935721 DOI: 10.1039/d3sc05450k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2023] [Accepted: 01/23/2024] [Indexed: 03/17/2024] Open
Abstract
The proton-coupled electron transfer (PCET) reactions of tyrosine (Y) are instrumental to many redox reactions in nature. This study investigates how the local environment and the thermodynamic properties of Y influence its PCET characteristics. Herein, 2- and 4-mercaptophenol (MP) are placed in the well-folded α3C protein (forming 2MP-α3C and 4MP-α3C) and oxidized by external light-generated [Ru(L)3]3+ complexes. The resulting neutral radicals are long-lived (>100 s) with distinct optical and EPR spectra. Calculated spin-density distributions are similar to canonical Y˙ and display very little spin on the S-S bridge that ligates the MPs to C32 inside the protein. With 2MP-α3C and 4MP-α3C we probe how proton transfer (PT) affects the PCET rate constants and mechanisms by varying the degree of solvent exposure or the potential to form an internal hydrogen bond. Solution NMR ensemble structures confirmed our intended design by displaying a major difference in the phenol OH solvent accessible surface area (≤∼2% for 2MP and 30-40% for 4MP). Additionally, 2MP-C32 is within hydrogen bonding distance to a nearby glutamate (average O-O distance is 3.2 ± 0.5 Å), which is suggested also by quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations. Neither increased exposure of the phenol OH to solvent (buffered water), nor the internal hydrogen bond, was found to significantly affect the PCET rates. However, the lower phenol pKa values associated with the MP-α3C proteins compared to α3Y provided a sufficient change in PT driving force to alter the PCET mechanism. The PCET mechanism for 2MP-α3C and 4MP-α3C with moderately strong oxidants was predominantly step-wise PTET for pH values, but changed to concerted PCET at neutral pH values and below when a stronger oxidant was used, as found previously for α3Y. This shows how the balance of ET and PT driving forces is critical for controlling PCET mechanisms. The presented results improve our general understanding of amino-acid based PCET in enzymes.
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Affiliation(s)
- Astrid Nilsen-Moe
- Department of Chemistry, Ångström Laboratory, Uppsala University Box 523 75120 Uppsala Sweden
| | - Clorice R Reinhardt
- Department of Molecular Biophysics and Biochemistry, Yale University New Haven CT 06520 USA
| | - Ping Huang
- Department of Chemistry, Ångström Laboratory, Uppsala University Box 523 75120 Uppsala Sweden
| | - Hemlata Agarwala
- Technical University Munich, Campus Straubing for Biotechnology and Sustainability Uferstraße 53 94315 Straubing Germany
| | - Rosana Lopes
- Department of Biochemistry and Biophysics, Texas A&M University College Station TX 77843 USA
| | - Mauricio Lasagna
- Department of Biochemistry and Biophysics, Texas A&M University College Station TX 77843 USA
| | - Starla Glover
- Department of Chemistry, Ångström Laboratory, Uppsala University Box 523 75120 Uppsala Sweden
| | | | - Cecilia Tommos
- Department of Biochemistry and Biophysics, Texas A&M University College Station TX 77843 USA
| | - Leif Hammarström
- Department of Chemistry, Ångström Laboratory, Uppsala University Box 523 75120 Uppsala Sweden
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4
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Abstract
Some oxidoreductase enzymes use redox-active tyrosine, tryptophan, cysteine, and/or glycine residues as one-electron, high-potential redox (radical) cofactors. Amino-acid radical cofactors typically perform one of four tasks-they work in concert with a metallocofactor to carry out a multielectron redox process, serve as storage sites for oxidizing equivalents, activate the substrate molecules, or move oxidizing equivalents over long distances. It is challenging to experimentally resolve the thermodynamic and kinetic redox properties of a single-amino-acid residue. The inherently reactive and highly oxidizing properties of amino-acid radicals increase the experimental barriers further still. This review describes a family of stable and well-structured model proteins that was made specifically to study tyrosine and tryptophan oxidation-reduction. The so-called α3X model protein system was combined with very-high-potential protein film voltammetry, transient absorption spectroscopy, and theoretical methods to gain a comprehensive description of the thermodynamic and kinetic properties of protein tyrosine and tryptophan radicals.
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Affiliation(s)
- Cecilia Tommos
- Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas, USA;
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5
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Nilsen-Moe A, Reinhardt CR, Glover SD, Liang L, Hammes-Schiffer S, Hammarström L, Tommos C. Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein: Mechanisms and Primary Proton Acceptor. J Am Chem Soc 2020; 142:11550-11559. [PMID: 32479070 PMCID: PMC7315633 DOI: 10.1021/jacs.0c04655] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
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Proton-coupled
electron transfer (PCET) from tyrosine produces
a neutral tyrosyl radical (Y•) that is vital to
many catalytic redox reactions. To better understand how the protein
environment influences the PCET properties of tyrosine, we have studied
the radical formation behavior of Y32 in the α3Y model protein. The previously solved α3Y solution NMR structure shows that Y32 is sequestered
∼7.7 ± 0.3 Å below the protein surface without any
primary proton acceptors nearby. Here we present transient absorption
kinetic data and molecular dynamics (MD) simulations to resolve the
PCET mechanism associated with Y32 oxidation. Y32• was generated in a bimolecular reaction with
[Ru(bpy)3]3+ formed by flash photolysis. At
pH > 8, the rate constant of Y32• formation
(kPCET) increases by one order of magnitude
per pH unit, corresponding to a proton-first mechanism via tyrosinate
(PTET). At lower pH < 7.5, the pH dependence is weak and shows
a previously measured KIE ≈ 2.5, which best fits a concerted
mechanism. kPCET is independent of phosphate
buffer concentration at pH 6.5. This provides clear evidence that
phosphate buffer is not the primary proton acceptor. MD simulations
show that one to two water molecules can enter the hydrophobic cavity
of α3Y and hydrogen bond to Y32, as well
as the possibility of hydrogen-bonding interactions between Y32 and E13, through structural fluctuations that
reorient surrounding side chains. Our results illustrate how protein
conformational motions can influence the redox reactivity of a tyrosine
residue and how PCET mechanisms can be tuned by changing the pH even
when the PCET occurs within the interior of a protein.
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Affiliation(s)
- Astrid Nilsen-Moe
- Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, Uppsala 75120, Sweden
| | - Clorice R Reinhardt
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, United States
| | - Starla D Glover
- Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, Uppsala 75120, Sweden
| | - Li Liang
- Departments of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States
| | | | - Leif Hammarström
- Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, Uppsala 75120, Sweden
| | - Cecilia Tommos
- Departments of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States.,Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843-2128, United States
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6
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Yoon JH, Kulesha AV, Lengyel-Zhand Z, Volkov AN, Rempillo JJ, D'Souza A, Costeas C, Chester C, Caselle ER, Makhlynets OV. Uno Ferro, a de novo Designed Protein, Binds Transition Metals with High Affinity and Stabilizes Semiquinone Radical Anion. Chemistry 2019; 25:15252-15256. [PMID: 31509280 DOI: 10.1002/chem.201904020] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2019] [Indexed: 11/07/2022]
Abstract
Metalloenzymes often utilize radicals in order to facilitate chemical reactions. Recently, DeGrado and co-workers have discovered that model proteins can efficiently stabilize semiquinone radical anion produced by oxidation of 3,5-di-tert-butylcatechol (DTBC) in the presence of two zinc ions. Here, we show that the number and the nature of metal ions have relatively minor effect on semiquinone stabilization in model proteins, with a single metal ion being sufficient for radical stabilization. The radical is stabilized by both metal ion, hydrophobic sequestration, and interactions with the hydrophilic residues in the protein interior resulting in a remarkable, nearly 500 mV change in the redox potential of the SQ. - /catechol couple compared to bulk aqueous solution. Moreover, we have created 4G-UFsc, a single metal ion-binding protein with pm affinity for zinc that is higher than any other reported model systems and is on par with many natural zinc-containing proteins. We expect that the robust and easy-to-modify DFsc/UFsc family of proteins will become a versatile tool for mechanistic model studies of metalloenzymes.
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Affiliation(s)
- Jennifer H Yoon
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Alona V Kulesha
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Zsofia Lengyel-Zhand
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Alexander N Volkov
- VIB Centre for Structural Biology, Vlaams Instituut voor Biotechnologie (VIB), Pleinlaan 2, Brussels, 1050, Belgium.,Jean Jeener NMR Centre, Vrije Universiteit Brussel (VUB), Pleinlaan 2, Brussels, 1050, Belgium
| | - Joel J Rempillo
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Areetha D'Souza
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Christos Costeas
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Cara Chester
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Elizabeth R Caselle
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
| | - Olga V Makhlynets
- Department of Chemistry, Syracuse University, 111 College Place, Syracuse, NY, 13244, USA
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7
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Glover SD, Tyburski R, Liang L, Tommos C, Hammarström L. Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W 32• and Y 32• Generated Inside the Structurally Characterized α 3W and α 3Y Proteins. J Am Chem Soc 2017; 140:185-192. [PMID: 29190082 DOI: 10.1021/jacs.7b08032] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°') with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y ( Ravichandran , K. R. ; Zong , A. B. ; Taguchi , A. T. ; Nocera , D. G. ; Stubbe , J. ; Tommos , C. J. Am. Chem. Soc. 2017 , 139 , 2994 - 3004 ). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°'(W32(N•+/NH)) of 1293 mV, and a E°'(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32• is presented, and analysis of decay kinetics show that W32• persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.
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Affiliation(s)
- Starla D Glover
- Department of Biochemistry and Biophysics, University of Pennsylvania Perelman School of Medicine , Philadelphia, Pennsylvania 19104, United States.,Department of Chemistry, Ångström Laboratory, Uppsala University , Box 523, SE-75120 Uppsala, Sweden
| | - Robin Tyburski
- Department of Chemistry, Ångström Laboratory, Uppsala University , Box 523, SE-75120 Uppsala, Sweden
| | - Li Liang
- Department of Biochemistry and Biophysics, University of Pennsylvania Perelman School of Medicine , Philadelphia, Pennsylvania 19104, United States
| | - Cecilia Tommos
- Department of Biochemistry and Biophysics, University of Pennsylvania Perelman School of Medicine , Philadelphia, Pennsylvania 19104, United States
| | - Leif Hammarström
- Department of Chemistry, Ångström Laboratory, Uppsala University , Box 523, SE-75120 Uppsala, Sweden
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8
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Ravichandran KR, Zong AB, Taguchi AT, Nocera DG, Stubbe J, Tommos C. Formal Reduction Potentials of Difluorotyrosine and Trifluorotyrosine Protein Residues: Defining the Thermodynamics of Multistep Radical Transfer. J Am Chem Soc 2017; 139:2994-3004. [PMID: 28171730 DOI: 10.1021/jacs.6b11011] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Redox-active tyrosines (Ys) play essential roles in enzymes involved in primary metabolism including energy transduction and deoxynucleotide production catalyzed by ribonucleotide reductases (RNRs). Thermodynamic characterization of Ys in solution and in proteins remains a challenge due to the high reduction potentials involved and the reactive nature of the radical state. The structurally characterized α3Y model protein has allowed the first determination of formal reduction potentials (E°') for a Y residing within a protein (Berry, B. W.; Martı́nez-Rivera, M. C.; Tommos, C. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9739-9743). Using Schultz's technology, a series of fluorotyrosines (FnY, n = 2 or 3) was site-specifically incorporated into α3Y. The global protein properties of the resulting α3(3,5)F2Y, α3(2,3,5)F3Y, α3(2,3)F2Y and α3(2,3,6)F3Y variants are essentially identical to those of α3Y. A protein film square-wave voltammetry approach was developed to successfully obtain reversible voltammograms and E°'s of the very high-potential α3FnY proteins. E°'(pH 5.5; α3FnY(O•/OH)) spans a range of 1040 ± 3 mV to 1200 ± 3 mV versus the normal hydrogen electrode. This is comparable to the potentials of the most oxidizing redox cofactors in nature. The FnY analogues, and the ability to site-specifically incorporate them into any protein of interest, provide new tools for mechanistic studies on redox-active Ys in proteins and on functional and aberrant hole-transfer reactions in metallo-enzymes. The former application is illustrated here by using the determined α3FnY ΔE°'s to model the thermodynamics of radical-transfer reactions in FnY-RNRs and to experimentally test and support the key prediction made.
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Affiliation(s)
| | - Allan B Zong
- Department of Biochemistry and Biophysics, University of Pennsylvania Perelman School of Medicine , Philadelphia, Pennsylvania 19104, United States
| | | | - Daniel G Nocera
- Department of Chemistry and Chemical Biology, Harvard University , 12 Oxford Street, Cambridge, Massachusetts 02138, United States
| | | | - Cecilia Tommos
- Department of Biochemistry and Biophysics, University of Pennsylvania Perelman School of Medicine , Philadelphia, Pennsylvania 19104, United States
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9
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Ulas G, Lemmin T, Wu Y, Gassner GT, DeGrado WF. Designed metalloprotein stabilizes a semiquinone radical. Nat Chem 2016; 8:354-9. [PMID: 27001731 PMCID: PMC4857601 DOI: 10.1038/nchem.2453] [Citation(s) in RCA: 66] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2015] [Accepted: 01/07/2016] [Indexed: 12/25/2022]
Abstract
Enzymes use binding energy to stabilize their substrates in high-energy states that are otherwise inaccessible at ambient temperature. Here we show that a de novo designed Zn(II) metalloprotein stabilizes a chemically reactive organic radical that is otherwise unstable in aqueous media. The protein binds tightly to and stabilizes the radical semiquinone form of 3,5-di-tert-butylcatechol. Solution NMR spectroscopy in conjunction with molecular dynamics simulations show that the substrate binds in the active site pocket where it is stabilized by metal-ligand interactions as well as by burial of its hydrophobic groups. Spectrochemical redox titrations show that the protein stabilized the semiquinone by reducing the electrochemical midpoint potential for its formation via the one-electron oxidation of the catechol by approximately 400 mV (9 kcal mol(-1)). Therefore, the inherent chemical properties of the radical were changed drastically by harnessing its binding energy to the metalloprotein. This model sets the basis for designed enzymes with radical cofactors to tackle challenging chemistry.
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Affiliation(s)
- Gözde Ulas
- Department of Pharmaceutical Chemistry, University of California – San Francisco, San Francisco, California 94158, USA
| | - Thomas Lemmin
- Department of Pharmaceutical Chemistry, University of California – San Francisco, San Francisco, California 94158, USA
| | - Yibing Wu
- Department of Pharmaceutical Chemistry, University of California – San Francisco, San Francisco, California 94158, USA
| | - George T. Gassner
- Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, California 94132, USA
| | - William F. DeGrado
- Department of Pharmaceutical Chemistry, University of California – San Francisco, San Francisco, California 94158, USA
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10
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Glover SD, Jorge C, Liang L, Valentine KG, Hammarström L, Tommos C. Photochemical tyrosine oxidation in the structurally well-defined α3Y protein: proton-coupled electron transfer and a long-lived tyrosine radical. J Am Chem Soc 2014; 136:14039-51. [PMID: 25121576 PMCID: PMC4195373 DOI: 10.1021/ja503348d] [Citation(s) in RCA: 64] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
![]()
Tyrosine oxidation–reduction involves proton-coupled electron
transfer (PCET) and a reactive radical state. These properties are
effectively controlled in enzymes that use tyrosine as a high-potential,
one-electron redox cofactor. The α3Y model protein
contains Y32, which can be reversibly oxidized and reduced in voltammetry
measurements. Structural and kinetic properties of α3Y are presented. A solution NMR structural analysis reveals that
Y32 is the most deeply buried residue in α3Y. Time-resolved
spectroscopy using a soluble flash-quench generated [Ru(2,2′-bipyridine)3]3+ oxidant provides high-quality Y32–O•
absorption spectra. The rate constant of Y32 oxidation (kPCET) is pH dependent: 1.4 × 104 M–1 s–1 (pH 5.5), 1.8 × 105 M–1 s–1 (pH 8.5), 5.4
× 103 M–1 s–1 (pD
5.5), and 4.0 × 104 M–1 s–1 (pD 8.5). kH/kD of Y32 oxidation is 2.5 ± 0.5 and 4.5 ± 0.9 at
pH(D) 5.5 and 8.5, respectively. These pH and isotope characteristics
suggest a concerted or stepwise, proton-first Y32 oxidation mechanism.
The photochemical yield of Y32–O• is 28–58% versus
the concentration of [Ru(2,2′-bipyridine)3]3+. Y32–O• decays slowly, t1/2 in the range of 2–10 s, at both pH 5.5 and 8.5,
via radical–radical dimerization as shown by second-order kinetics
and fluorescence data. The high stability of Y32–O•
is discussed relative to the structural properties of the Y32 site.
Finally, the static α3Y NMR structure cannot explain
(i) how the phenolic proton released upon oxidation is removed or
(ii) how two Y32–O• come together to form dityrosine.
These observations suggest that the dynamic properties of the protein
ensemble may play an essential role in controlling the PCET and radical
decay characteristics of α3Y.
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Affiliation(s)
- Starla D Glover
- Department of Chemistry, Ångström Laboratory, Uppsala University , Box 523, SE75120 Uppsala, Sweden
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11
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Ravichandran KR, Liang L, Stubbe J, Tommos C. Formal reduction potential of 3,5-difluorotyrosine in a structured protein: insight into multistep radical transfer. Biochemistry 2013; 52:8907-15. [PMID: 24228716 DOI: 10.1021/bi401494f] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
The reversible Y-O•/Y-OH redox properties of the α3Y model protein allow access to the electrochemical and thermodynamic properties of 3,5-difluorotyrosine. The unnatural amino acid has been incorporated at position 32, the dedicated radical site in α3Y, by in vivo nonsense codon suppression. Incorporation of 3,5-difluorotyrosine gives rise to very minor structural changes in the protein scaffold at pH values below the apparent pK (8.0±0.1) of the unnatural residue. Square-wave voltammetry on α3(3,5)F2Y provides an E°'(Y-O•/Y-OH) of 1026±4 mV versus the normal hydrogen electrode (pH 5.70±0.02) and shows that the fluoro substitutions lower the E°' by -30±3 mV. These results illustrate the utility of combining the optimized α3Y tyrosine radical system with in vivo nonsense codon suppression to obtain the formal reduction potential of an unnatural aromatic residue residing within a well-structured protein. It is further observed that the protein E°' values differ significantly from peak potentials derived from irreversible voltammograms of the corresponding aqueous species. This is notable because solution potentials have been the main thermodynamic data available for amino acid radicals. The findings in this paper are discussed relative to recent mechanistic studies of the multistep radical-transfer process in Escherichia coli ribonucleotide reductase site-specifically labeled with unnatural tyrosine residues.
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Affiliation(s)
- Kanchana R Ravichandran
- Department of Chemistry, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
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12
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Yosca TH, Rittle J, Krest CM, Onderko EL, Silakov A, Calixto JC, Behan RK, Green MT. Iron(IV)hydroxide pK(a) and the role of thiolate ligation in C-H bond activation by cytochrome P450. Science 2013; 342:825-9. [PMID: 24233717 PMCID: PMC4299822 DOI: 10.1126/science.1244373] [Citation(s) in RCA: 242] [Impact Index Per Article: 22.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Cytochrome P450 enzymes activate oxygen at heme iron centers to oxidize relatively inert substrate carbon-hydrogen bonds. Cysteine thiolate coordination to iron is posited to increase the pK(a) (where K(a) is the acid dissociation constant) of compound II, an iron(IV)hydroxide complex, correspondingly lowering the one-electron reduction potential of compound I, the active catalytic intermediate, and decreasing the driving force for deleterious auto-oxidation of tyrosine and tryptophan residues in the enzyme's framework. Here, we report on the preparation of an iron(IV)hydroxide complex in a P450 enzyme (CYP158) in ≥90% yield. Using rapid mixing technologies in conjunction with Mössbauer, ultraviolet/visible, and x-ray absorption spectroscopies, we determine a pK(a) value for this compound of 11.9. Marcus theory analysis indicates that this elevated pK(a) results in a >10,000-fold reduction in the rate constant for oxidations of the protein framework, making these processes noncompetitive with substrate oxidation.
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Affiliation(s)
- Timothy H Yosca
- Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA
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13
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Tommos C, Valentine KG, Martínez-Rivera MC, Liang L, Moorman VR. Reversible phenol oxidation and reduction in the structurally well-defined 2-Mercaptophenol-α₃C protein. Biochemistry 2013; 52:1409-18. [PMID: 23373469 DOI: 10.1021/bi301613p] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
2-Mercaptophenol-α₃C serves as a biomimetic model for enzymes that use tyrosine residues in redox catalysis and multistep electron transfer. This model protein was tailored for electrochemical studies of phenol oxidation and reduction with specific emphasis on the redox-driven protonic reactions occurring at the phenol oxygen. This protein contains a covalently modified 2-mercaptophenol-cysteine residue. The radical site and the phenol compound were specifically chosen to bury the phenol OH group inside the protein. A solution nuclear magnetic resonance structural analysis (i) demonstrates that the synthetic 2-mercaptophenol-α₃C model protein behaves structurally as a natural protein, (ii) confirms the design of the radical site, (iii) reveals that the ligated phenol forms an interhelical hydrogen bond to glutamate 13 (phenol oxygen-carboxyl oxygen distance of 3.2 ± 0.5 Å), and (iv) suggests a proton-transfer pathway from the buried phenol OH (average solvent accessible surface area of 3 ± 5%) via glutamate 13 (average solvent accessible surface area of the carboxyl oxygens of 37 ± 18%) to the bulk solvent. A square-wave voltammetry analysis of 2-mercaptophenol-α₃C further demonstrates that (v) the phenol oxidation-reduction cycle is reversible, (vi) formal phenol reduction potentials can be obtained, and (vii) the phenol-O(•) state is long-lived with an estimated lifetime of ≥180 millisecond. These properties make 2-mercaptophenol-α₃C a unique system for characterizing phenol-based proton-coupled electron transfer in a low-dielectric and structured protein environment.
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Affiliation(s)
- Cecilia Tommos
- Graduate Group in Biochemistry and Molecular Biophysics and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States.
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14
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Reversible voltammograms and a Pourbaix diagram for a protein tyrosine radical. Proc Natl Acad Sci U S A 2012; 109:9739-43. [PMID: 22675121 DOI: 10.1073/pnas.1112057109] [Citation(s) in RCA: 54] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Reversible voltammograms and a voltammetry half-wave potential versus solution pH diagram are described for a protein tyrosine radical. This work required a de novo designed tyrosine-radical protein displaying a unique combination of structural and electrochemical properties. The α(3)Y protein is structurally stable across a broad pH range. The redox-active tyrosine Y32 resides in a desolvated and well-structured environment. Y32 gives rise to reversible square-wave and differential pulse voltammograms at alkaline pH. The formal potential of the Y32-O(•)/Y32-OH redox couple is determined to 918 ± 2 mV versus the normal hydrogen electrode at pH 8.40 ± 0.01. The observation that Y32 gives rise to fully reversible voltammograms translates into an estimated lifetime of ≥30 ms for the Y32-O(•) state. This illustrates the range of tyrosine-radical stabilization that a structured protein can offer. Y32 gives rise to quasireversible square-wave and differential pulse voltammograms at acidic pH. These voltammograms represent the Y32 species at the upper edge of the quasirevesible range. The square-wave net potential closely approximates the formal potential of the Y32-O(•)/Y32-OH redox couple to 1,070 ± 1 mV versus the normal hydrogen electrode at pH 5.52 ± 0.01. The differential pulse voltammetry half-wave potential of the Y32-O(•)/Y32-OH redox pair is measured between pH 4.7 and 9.0. These results are described and analyzed.
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15
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Warren JJ, Winkler JR, Gray HB. Redox properties of tyrosine and related molecules. FEBS Lett 2012; 586:596-602. [PMID: 22210190 PMCID: PMC3298607 DOI: 10.1016/j.febslet.2011.12.014] [Citation(s) in RCA: 78] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/10/2011] [Revised: 12/04/2011] [Accepted: 12/12/2011] [Indexed: 11/25/2022]
Abstract
Redox reactions of tyrosine play key roles in many biological processes, including water oxidation and DNA synthesis. We first review the redox properties of tyrosine (and other phenols) in small molecules and related polypeptides, then report work on (H20)/(Y48)-modified Pseudomonas aeruginosa azurin. The crystal structure of this protein (1.18Å resolution) shows that H20 is strongly hydrogen bonded to Y48 (2.7-2.8Å tyrosine-O to histidine-N distance). A firm conclusion is that proper tuning of the tyrosine potential by a proton-accepting base is critical for biological redox functions.
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Affiliation(s)
- Jeffrey J. Warren
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
| | - Jay R. Winkler
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
| | - Harry B. Gray
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125
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16
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Martínez-Rivera MC, Berry BW, Valentine KG, Westerlund K, Hay S, Tommos C. Electrochemical and structural properties of a protein system designed to generate tyrosine Pourbaix diagrams. J Am Chem Soc 2011; 133:17786-95. [PMID: 22011192 DOI: 10.1021/ja206876h] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
This report describes a model protein specifically tailored to electrochemically study the reduction potential of protein tyrosine radicals as a function of pH. The model system is based on the 67-residue α(3)Y three-helix bundle. α(3)Y contains a single buried tyrosine at position 32 and displays structural properties inherent to a protein. The present report presents differential pulse voltammograms obtained from α(3)Y at both acidic (pH 5.6) and alkaline (pH 8.3) conditions. The observed Faradaic response is uniquely associated with Y32, as shown by site-directed mutagenesis. This is the first time voltammetry is successfully applied to detect a redox-active tyrosine residing in a structured protein environment. Tyrosine is a proton-coupled electron-transfer cofactor making voltammetry-based pH titrations a central experimental approach. A second set of experiments was performed to demonstrate that pH-dependent studies can be conducted on the redox-active tyrosine without introducing large-scale structural changes in the protein scaffold. α(3)Y was re-engineered with the specific aim to place the imidazole group of a histidine close to the Y32 phenol ring. α(3)Y-K29H and α(3)Y-K36H each contain a histidine residue whose protonation perturbs the fluorescence of Y32. We show that these variants are stable and well-folded proteins whose helical content, tertiary structure, solution aggregation state, and solvent-sequestered position of Y32 remain pH insensitive across a range of at least 3-4 pH units. These results confirm that the local environment of Y32 can be altered and the resulting radical site studied by voltammetry over a broad pH range without interference from long-range structural effects.
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Affiliation(s)
- Melissa C Martínez-Rivera
- Graduate Group in Biochemistry & Molecular Biophysics and Department of Biochemistry & Biophysics, 905 Stellar-Chance Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States
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17
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Alfonso-Prieto M, Oberhofer H, Klein ML, Rovira C, Blumberger J. Proton Transfer Drives Protein Radical Formation in Helicobacter pylori Catalase but Not in Penicillium vitale Catalase. J Am Chem Soc 2011; 133:4285-98. [DOI: 10.1021/ja1110706] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- M. Alfonso-Prieto
- Computer Simulation & Modeling Laboratory, Parc Científic de Barcelona, Baldiri Reixac 4, 08028 Barcelona, Spain
- Institute for Computational Molecular Science, Temple University, 1900 North 12th Street, Philadelphia, Pennsylvania 19122, United States
| | - H. Oberhofer
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
| | - M. L. Klein
- Institute for Computational Molecular Science, Temple University, 1900 North 12th Street, Philadelphia, Pennsylvania 19122, United States
| | - C. Rovira
- Computer Simulation & Modeling Laboratory, Parc Científic de Barcelona, Baldiri Reixac 4, 08028 Barcelona, Spain
- Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, 08028 Barcelona, Spain
- Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain
| | - J. Blumberger
- Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
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18
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Lagzi I, Kowalczyk B, Wang D, Grzybowski BA. Nanoparticle Oscillations and Fronts. Angew Chem Int Ed Engl 2010. [DOI: 10.1002/ange.201004231] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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19
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Lagzi I, Kowalczyk B, Wang D, Grzybowski BA. Nanoparticle Oscillations and Fronts. Angew Chem Int Ed Engl 2010; 49:8616-9. [DOI: 10.1002/anie.201004231] [Citation(s) in RCA: 107] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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20
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Yokoyama K, Uhlin U, Stubbe J. Site-specific incorporation of 3-nitrotyrosine as a probe of pKa perturbation of redox-active tyrosines in ribonucleotide reductase. J Am Chem Soc 2010; 132:8385-97. [PMID: 20518462 DOI: 10.1021/ja101097p] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
E. coli ribonucleotide reductase catalyzes the reduction of nucleoside 5'-diphosphates into 2'-deoxynucleotides and is composed of two subunits: alpha2 and beta2. During turnover, a stable tyrosyl radical (Y*) at Y(122)-beta2 reversibly oxidizes C(439) in the active site of alpha2. This radical propagation step is proposed to occur over 35 A, to use specific redox-active tyrosines (Y(122) and Y(356) in beta2, Y(731) and Y(730) in alpha2), and to involve proton-coupled electron transfer (PCET). 3-Nitrotyrosine (NO(2)Y, pK(a) 7.1) has been incorporated in place of Y(122), Y(731), and Y(730) to probe how the protein environment perturbs each pK(a) in the presence of the second subunit, substrate (S), and allosteric effector (E). The activity of each mutant is <4 x 10(-3) that of the wild-type (wt) subunit. The [NO(2)Y(730)]-alpha2 and [NO(2)Y(731)]-alpha2 each exhibit a pK(a) of 7.8-8.0 with E and E/beta2. The pK(a) of [NO(2)Y(730)]-alpha2 is elevated to 8.2-8.3 in the S/E/beta2 complex, whereas no further perturbation is observed for [NO(2)Y(731)]-alpha2. Mutations in pathway residues adjacent to the NO(2)Y that disrupt H-bonding minimally perturb its pK(a). The pK(a) of NO(2)Y(122)-beta2 alone or with alpha2/S/E is >9.6. X-ray crystal structures have been obtained for all [NO(2)Y]-alpha2 mutants (2.1-3.1 A resolution), which show minimal structural perturbation compared to wt-alpha2. Together with the pK(a) of the previously reported NO(2)Y(356)-beta2 (7.5 in the alpha2/S/E complex; Yee, C. et al. Biochemistry 2003, 42, 14541-14552), these studies provide a picture of the protein environment of the ground state at each Y in the PCET pathway, and are the starting point for understanding differences in PCET mechanisms at each residue in the pathway.
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Affiliation(s)
- Kenichi Yokoyama
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA
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Hay S, Scrutton NS. H-transfers in Photosystem II: what can we learn from recent lessons in the enzyme community? PHOTOSYNTHESIS RESEARCH 2008; 98:169-177. [PMID: 18766465 DOI: 10.1007/s11120-008-9326-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2008] [Accepted: 06/28/2008] [Indexed: 05/26/2023]
Abstract
Over the last 10 years, studies of enzyme systems have demonstrated that, in many cases, H-transfers occur by a quantum mechanical tunneling mechanism analogous to long-range electron transfer. H-transfer reactions can be described by an extension of Marcus theory and, by substituting hydrogen with deuterium (or even tritium), it is possible to explore this theory in new ways by employing kinetic isotope effects. Because hydrogen has a relatively short deBroglie wavelength, H-transfers are controlled by the width of the reaction barrier. By coupling protein dynamics to the reaction coordinate, enzymes have the potential ability to facilitate more efficient H-tunneling by modulating barrier properties. In this review, we describe recent advances in both experimental and theoretical studies of enzymatic H-transfer, in particular the role of protein dynamics or promoting motions. We then discuss possible consequences with regard to tyrosine oxidation/reduction kinetics in Photosystem II.
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Affiliation(s)
- Sam Hay
- Manchester Interdisciplinary Biocentre and Faculty of Life Sciences, University of Manchester, Manchester, UK.
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22
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Westerlund K, Moran SD, Privett HK, Hay S, Jarvet J, Gibney BR, Tommos C. Making a single-chain four-helix bundle for redox chemistry studies. Protein Eng Des Sel 2008; 21:645-52. [PMID: 18755707 DOI: 10.1093/protein/gzn043] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
The construction and characteristics of the stable and well-structured alpha(4)W protein are described. The 117-residue, single-chain protein has a molecular weight of 13.1 kDa and is designed to fold into a four-helix bundle. Experimental characterization of the expressed and purified protein shows a 69.8 +/- 0.8% helical content over a 5.5-10.0 pH range. The protein is thermostable with a T(M) > 355 K and has a free energy of unfolding as measured by chemical denaturation of -4.7 kcal mol(-1) at 25 degrees C and neutral pH. One-dimensional (1D) proton and 2D (15)N-HSQC spectra show narrow, well-dispersed spectral lines consistent with a uniquely structured alpha-helical protein. Analytical ultracentrifugation and NMR data show that the protein is monomeric over a broad protein concentration range. The 324 nm emission maximum of the unique Trp-106 is consistent with a sequestered position of the aromatic residue. Additionally, differential pulse voltammetry characterization indicates an elevated peak potential for Trp-106 when the protein is folded (pH range 7.0-8.5) relative to partly unfolded (pH range 11.4-13.2). The oxidation of Trp-106 is coupled to proton release as shown by a 53 +/- 3 mV/pH unit dependence of the peak potential over the 7.0-8.5 pH range.
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Affiliation(s)
- Kristina Westerlund
- Department of Biochemistry and Biophysics, University of Pennsylvania, 905 Stellar-Chance Laboratories, Philadelphia, PA 19104-6059, USA
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23
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Brenner S, Hay S, Munro AW, Scrutton NS. Inter-flavin electron transfer in cytochrome P450 reductase - effects of solvent and pH identify hidden complexity in mechanism. FEBS J 2008; 275:4540-57. [PMID: 18681889 DOI: 10.1111/j.1742-4658.2008.06597.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
This study on human cytochrome P450 reductase (CPR) presents a comprehensive analysis of the thermodynamic and kinetic effects of pH and solvent on two- and four-electron reduction in this diflavin enzyme. pH-dependent redox potentiometry revealed that the thermodynamic equilibrium between various two-electron reduced enzyme species (FMNH*,FADH*; FMN,FADH2; FMNH2,FAD) is independent of pH. No shift from the blue, neutral di-semiquinone (FMNH*,FADH*) towards the red, anionic species is observed upon increasing the pH from 6.5 to 8.5. Spectrophotometric analysis of events following the mixing of oxidized CPR and NADPH (1 to 1) in a stopped-flow instrument demonstrates that the establishment of this thermodynamic equilibrium becomes a very slow process at elevated pH, indicative of a pH-gating mechanism. The final level of blue di-semiquinone formation is found to be pH independent. Stopped-flow experiments using excess NADPH over CPR provide evidence that both pH and solvent significantly influence the kinetic exposure of the blue di-semiquinone intermediate, yet the observed rate constants are essentially pH independent. Thus, the kinetic pH-gating mechanism under stoichiometric conditions is of no significant kinetic relevance for four-electron reduction, but rather modulates the observed semiquinone absorbance at 600 nm in a pH-dependent manner. The use of proton inventory experiments and primary kinetic isotope effects are described as kinetic tools to disentangle the intricate pH-dependent kinetic mechanism in CPR. Our analysis of the pH and isotope dependence in human CPR reveals previously hidden complexity in the mechanism of electron transfer in this complex flavoprotein.
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Affiliation(s)
- Sibylle Brenner
- Manchester Interdisciplinary Biocentre and Faculty of Life Sciences, University of Manchester, UK
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24
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Wydrzynski T, Hillier W, Conlan B. Engineering model proteins for Photosystem II function. PHOTOSYNTHESIS RESEARCH 2007; 94:225-233. [PMID: 17955341 DOI: 10.1007/s11120-007-9271-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2007] [Accepted: 10/04/2007] [Indexed: 05/25/2023]
Abstract
Our knowledge of Photosystem II and the molecular mechanism of oxygen production are rapidly advancing. The time is now ripe to exploit this knowledge and use it as a blueprint for the development of light-driven catalysts, ultimately for the splitting of water into O2 and H2. In this article, we outline the background and our approach to this technological application through the reverse engineering of Photosystem II into model proteins.
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Affiliation(s)
- Tom Wydrzynski
- Research School of Biological Sciences, Australian National University, Canberra, ACT, 0200, Australia.
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25
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Machczynski MC, Kuhl KP, McGuirl MA. Modulation of the electrochemical behavior of tyrosyl radicals by the electrode surface. Anal Biochem 2007; 362:89-97. [PMID: 17254538 DOI: 10.1016/j.ab.2006.11.040] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2006] [Revised: 11/22/2006] [Accepted: 11/30/2006] [Indexed: 11/20/2022]
Abstract
The ability to adsorb proteins and enzymes on electrode surfaces enhances opportunities for studying enzyme activity and redox-based catalysis. Proteins may be bound in a chosen orientation on surfaces so that specific sites within them may be preferentially studied, but to date no systematic study of a redox moiety from solvent to electrode surface to the protein milieu has been performed. We report the redox and ionization behavior of tyrosine-cysteine, using the cysteine residue to form covalent linkages with Au and self-assembled-monolayer (SAM)-modified Au surfaces and using the tyrosine for redox activity. In addition, the same redox fragment incorporated into a protein bound to a SAM is examined. We find that directly binding the dipeptide to Au causes the greatest change in properties, while binding it to the SAM causes a slight perturbation in redox potential and a significant perturbation in pK(a). When azurin with a surface-exposed tyrosine is bound to a SAM-modified electrode, the redox potential and pK(a) of the tyrosine are nearly unperturbed from the values found for the dipeptide free in solution. Finally, quantification of the voltammetric signal indicates that tyrosine oxidation in the protein triggers the additional oxidation of another nearby amino acid.
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Affiliation(s)
- Michael C Machczynski
- Division of Biological Sciences and Biomolecular Structure and Dynamics Program, The University of Montana, Missoula, MT 59812, USA
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