1
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Ekanger LA, Shah RK, Porowski ME, Ziolkowski Z, Calello A. Spectroscopic, electrochemical, and kinetic trends in Fe(III)-thiolate disproportionation near physiologic pH. J Biol Inorg Chem 2024; 29:291-301. [PMID: 38722396 PMCID: PMC11111527 DOI: 10.1007/s00775-024-02051-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2023] [Accepted: 02/01/2024] [Indexed: 05/24/2024]
Abstract
In addition to its primary oxygen-atom-transfer function, cysteamine dioxygenase (ADO) exhibits a relatively understudied anaerobic disproportionation reaction (ADO-Fe(III)-SR → ADO-Fe(II) + ½ RSSR) with its native substrates. Inspired by ADO disproportionation reactivity, we employ [Fe(tacn)Cl3] (tacn = 1,4,7-triazacyclononane) as a precursor for generating Fe(III)-thiolate model complexes in buffered aqueous media. A series of Fe(III)-thiolate model complexes are generated in situ using aqueous [Fe(tacn)Cl3] and thiol-containing ligands cysteamine, penicillamine, mercaptopropionate, cysteine, cysteine methyl ester, N-acetylcysteine, and N-acetylcysteine methyl ester. We observe trends in UV-Vis and electron paramagnetic resonance (EPR) spectra, disproportionation rate constants, and cathodic peak potentials as a function of thiol ligand. These trends will be useful in rationalizing substrate-dependent Fe(III)-thiolate disproportionation reactions in metalloenzymes.
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Affiliation(s)
- Levi A Ekanger
- Department of Chemistry, The College of New Jersey, Ewing, NJ, 08628, USA.
| | - Ruhi K Shah
- Department of Chemistry, The College of New Jersey, Ewing, NJ, 08628, USA
| | - Matthew E Porowski
- Department of Chemistry, The College of New Jersey, Ewing, NJ, 08628, USA
| | - Zach Ziolkowski
- Department of Chemistry, The College of New Jersey, Ewing, NJ, 08628, USA
| | - Alana Calello
- Department of Chemistry, The College of New Jersey, Ewing, NJ, 08628, USA
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2
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Pierce BS, Schmittou AN, York NJ, Madigan RP, Nino PF, Foss FW, Lockart MM. Improved resolution of 3-mercaptopropionate dioxygenase active site provided by ENDOR spectroscopy offers insight into catalytic mechanism. J Biol Chem 2024; 300:105777. [PMID: 38395308 PMCID: PMC10966181 DOI: 10.1016/j.jbc.2024.105777] [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: 11/28/2023] [Revised: 02/13/2024] [Accepted: 02/15/2024] [Indexed: 02/25/2024] Open
Abstract
3-mercaptopropionate (3MPA) dioxygenase (MDO) is a mononuclear nonheme iron enzyme that catalyzes the O2-dependent oxidation of thiol-bearing substrates to yield the corresponding sulfinic acid. MDO is a member of the cysteine dioxygenase family of small molecule thiol dioxygenases and thus shares a conserved sequence of active site residues (Serine-155, Histidine-157, and Tyrosine-159), collectively referred to as the SHY-motif. It has been demonstrated that these amino acids directly interact with the mononuclear Fe-site, influencing steady-state catalysis, catalytic efficiency, O2-binding, and substrate coordination. However, the underlying mechanism by which this is accomplished is poorly understood. Here, pulsed electron paramagnetic resonance spectroscopy [1H Mims electron nuclear double resonance spectroscopy] is applied to validate density functional theory computational models for the MDO Fe-site simultaneously coordinated by substrate and nitric oxide (NO), (3MPA/NO)-MDO. The enhanced resolution provided by electron nuclear double resonance spectroscopy allows for direct observation of Fe-bound substrate conformations and H-bond donation from Tyr159 to the Fe-bound NO ligand. Further inclusion of SHY-motif residues within the validated model reveals a distinct channel restricting movement of the Fe-bound NO-ligand. It has been argued that the iron-nitrosyl emulates the structure of potential Fe(III)-superoxide intermediates within the MDO catalytic cycle. While the merit of this assumption remains unconfirmed, the model reported here offers a framework to evaluate oxygen binding at the substrate-bound Fe-site and possible reaction mechanisms. It also underscores the significance of hydrogen bonding interactions within the enzymatic active site.
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Affiliation(s)
- Brad S Pierce
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA.
| | - Allison N Schmittou
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Nicholas J York
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Ryan P Madigan
- Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas, USA
| | - Paula F Nino
- Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas, USA
| | - Frank W Foss
- Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas, USA
| | - Molly M Lockart
- Department of Chemistry and Biochemistry, Samford University, Homewood, Alabama, USA.
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3
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York NJ, Lockart MM, Sardar S, Khadka N, Shi W, Stenkamp RE, Zhang J, Kiser PD, Pierce BS. Structure of 3-mercaptopropionic acid dioxygenase with a substrate analog reveals bidentate substrate binding at the iron center. J Biol Chem 2021; 296:100492. [PMID: 33662397 PMCID: PMC8050391 DOI: 10.1016/j.jbc.2021.100492] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Revised: 02/18/2021] [Accepted: 02/26/2021] [Indexed: 12/20/2022] Open
Abstract
Thiol dioxygenases are a subset of nonheme iron oxygenases that catalyze the formation of sulfinic acids from sulfhydryl-containing substrates and dioxygen. Among this class, cysteine dioxygenases (CDOs) and 3-mercaptopropionic acid dioxygenases (3MDOs) are the best characterized, and the mode of substrate binding for CDOs is well understood. However, the manner in which 3-mercaptopropionic acid (3MPA) coordinates to the nonheme iron site in 3MDO remains a matter of debate. A model for bidentate 3MPA coordination at the 3MDO Fe-site has been proposed on the basis of computational docking, whereas steady-state kinetics and EPR spectroscopic measurements suggest a thiolate-only coordination of the substrate. To address this gap in knowledge, we determined the structure of Azobacter vinelandii 3MDO (Av3MDO) in complex with the substrate analog and competitive inhibitor, 3-hydroxypropionic acid (3HPA). The structure together with DFT computational modeling demonstrates that 3HPA and 3MPA associate with iron as chelate complexes with the substrate-carboxylate group forming an additional interaction with Arg168 and the thiol bound at the same position as in CDO. A chloride ligand was bound to iron in the coordination site assigned as the O2-binding site. Supporting HYSCORE spectroscopic experiments were performed on the (3MPA/NO)-bound Av3MDO iron nitrosyl (S = 3/2) site. In combination with spectroscopic simulations and optimized DFT models, this work provides an experimentally verified model of the Av3MDO enzyme-substrate complex, effectively resolving a debate in the literature regarding the preferred substrate-binding denticity. These results elegantly explain the observed 3MDO substrate specificity, but leave unanswered questions regarding the mechanism of substrate-gated reactivity with dioxygen.
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Affiliation(s)
- Nicholas J York
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Molly M Lockart
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA
| | - Sinjinee Sardar
- Department of Chemistry & Biochemistry, The University of Texas at Arlington, Arlington, Texas, USA
| | - Nimesh Khadka
- Department of Pharmacology, Case Western Reserve University, Cleveland, Ohio, USA
| | - Wuxian Shi
- National Synchrotron Light Source-II, Brookhaven National Laboratory, Upton, New York, USA
| | - Ronald E Stenkamp
- Departments of Biological Structure and Biochemistry, University of Washington, Seattle, Washington, USA
| | - Jianye Zhang
- Department of Ophthalmology, School of Medicine, University of California, Irvine, Irvine, California, USA
| | - Philip D Kiser
- Department of Ophthalmology, School of Medicine, University of California, Irvine, Irvine, California, USA; Department of Physiology & Biophysics, School of Medicine, University of California, Irvine, Irvine, California, USA; Research Service, VA Long Beach Healthcare System, Long Beach, California, USA.
| | - Brad S Pierce
- Department of Chemistry & Biochemistry, University of Alabama, Tuscaloosa, Alabama, USA.
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4
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Flückger S, Igareta NV, Seebeck FP. Convergent Evolution of Fungal Cysteine Dioxygenases. Chembiochem 2020; 21:3082-3086. [PMID: 32543095 DOI: 10.1002/cbic.202000317] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2020] [Revised: 06/11/2020] [Indexed: 12/16/2022]
Abstract
Cupin-type cysteine dioxygenases (CDOs) are non-heme iron enzymes that occur in animals, plants, bacteria and in filamentous fungi. In this report, we show that agaricomycetes contain an entirely unrelated type of CDO that emerged by convergent evolution from enzymes involved in the biosynthesis of ergothioneine. The activity of this CDO type is dependent on the ergothioneine precursor N-α-trimethylhistidine. The metabolic link between ergothioneine production and cysteine oxidation suggests that the two processes might be part of the same chemical response in fungi, for example against oxidative stress.
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Affiliation(s)
- Sebastian Flückger
- Department for Chemistry, University of Basel, Mattenstrasse 24a, 4002, Basel, Switzerland
| | - Nico V Igareta
- Department for Chemistry, University of Basel, Mattenstrasse 24a, 4002, Basel, Switzerland
| | - Florian P Seebeck
- Department for Chemistry, University of Basel, Mattenstrasse 24a, 4002, Basel, Switzerland
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5
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Sardar S, Weitz A, Hendrich MP, Pierce BS. Outer-Sphere Tyrosine 159 within the 3-Mercaptopropionic Acid Dioxygenase S-H-Y Motif Gates Substrate-Coordination Denticity at the Non-Heme Iron Active Site. Biochemistry 2019; 58:5135-5150. [PMID: 31750652 PMCID: PMC10071547 DOI: 10.1021/acs.biochem.9b00674] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Thiol dioxygenases are non-heme mononuclear iron enzymes that catalyze the O2-dependent oxidation of free thiols (-SH) to produce the corresponding sulfinic acid (-SO2-). Regardless of the phylogenic domain, the active site for this enzyme class is typically comprised of two major features: (1) a mononuclear ferrous iron coordinated by three protein-derived histidines and (2) a conserved sequence of outer Fe-coordination-sphere amino acids (Ser-His-Tyr) spatially adjacent to the iron site (∼3 Å). Here, we utilize a promiscuous 3-mercaptopropionic acid dioxygenase cloned from Azotobacter vinelandii (Av MDO) to explore the function of the conserved S-H-Y motif. This enzyme exhibits activity with 3-mercaptopropionic acid (3mpa), l-cysteine (cys), as well as several other thiol-bearing substrates, thus making it an ideal system to study the influence of residues within the highly conserved S-H-Y motif (H157 and Y159) on substrate specificity and reactivity. The pKa values for these residues were determined by pH-dependent steady-state kinetics, and their assignments verified by comparison to H157N and Y159F variants. Complementary electron paramagnetic resonance and Mössbauer studies demonstrate a network of hydrogen bonds connecting H157-Y159 and Fe-bound ligands within the enzymatic Fe site. Crucially, these experiments suggest that the hydroxyl group of Y159 hydrogen bonds to Fe-bound NO and, by extension, Fe-bound oxygen during native catalysis. This interaction alters both the NO binding affinity and rhombicity of the 3mpa-bound iron-nitrosyl site. In addition, Fe coordination of cys is switched from thiolate only to bidentate (thiolate/amine) for the Y159F variant, indicating that perturbations within the S-H-Y proton relay network also influence cys Fe binding denticity.
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Affiliation(s)
- Sinjinee Sardar
- Department of Chemistry and Biochemistry , The University of Texas at Arlington , 700 Planetarium Place , Arlington , Texas 76019 , United States
| | - Andrew Weitz
- Department of Chemistry , Carnegie Mellon University , 4400 Fifth Avenue , Pittsburgh , Pennsylvania 15213 , United States
| | - Michael P Hendrich
- Department of Chemistry , Carnegie Mellon University , 4400 Fifth Avenue , Pittsburgh , Pennsylvania 15213 , United States
| | - Brad S Pierce
- Department of Chemistry and Biochemistry , University of Alabama , 250 Hackberry Lane , Tuscaloosa , Alabama 35487 , United States
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6
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Morrow WP, Sardar S, Thapa P, Hossain MS, Foss FW, Pierce BS. Thiol dioxygenase turnover yields benzothiazole products from 2-mercaptoaniline and O 2-dependent oxidation of primary alcohols. Arch Biochem Biophys 2017; 631:66-74. [PMID: 28826737 PMCID: PMC5616182 DOI: 10.1016/j.abb.2017.08.013] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2017] [Revised: 08/15/2017] [Accepted: 08/16/2017] [Indexed: 10/19/2022]
Abstract
Thiol dioxygenases are non-heme mononuclear iron enzymes that catalyze the O2-dependent oxidation of free thiols (-SH) to produce the corresponding sulfinic acid (-SO2-). Previous chemical rescue studies identified a putative FeIII-O2- intermediate that precedes substrate oxidation in Mus musculus cysteine dioxygenase (Mm CDO). Given that a similar reactive intermediate has been identified in the extradiol dioxygenase 2, 3-HCPD, it is conceivable that these enzymes share other mechanistic features with regard to substrate oxidation. To explore this possibility, enzymatic reactions with Mm CDO (as well as the bacterial 3-mercaptopropionic acid dioxygenase, Av MDO) were performed using a substrate analogue (2-mercaptoaniline, 2ma). This aromatic thiol closely approximates the catecholic substrate of homoprotocatechuate of 2, 3-HPCD while maintaining the 2-carbon thiol-amine separation preferred by Mm CDO. Remarkably, both enzymes exhibit 2ma-gated O2-consumption; however, none of the expected products for thiol dioxygenase or intra/extradiol dioxygenase reactions were observed. Instead, benzothiazoles are produced by the condensation of 2ma with aldehydes formed by an off-pathway oxidation of primary alcohols added to aqueous reactions to solubilize the substrate. The observed oxidation of 1º-alcohols in 2ma-reactions is consistent with the formation of a high-valent intermediate similar to what has been reported for cytochrome P450 and mononuclear iron model complexes.
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Affiliation(s)
- William P Morrow
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States
| | - Sinjinee Sardar
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States
| | - Pawan Thapa
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States
| | - Mohammad S Hossain
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States
| | - Frank W Foss
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States
| | - Brad S Pierce
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, United States.
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7
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Driggers CM, Kean KM, Hirschberger LL, Cooley RB, Stipanuk MH, Karplus PA. Structure-Based Insights into the Role of the Cys-Tyr Crosslink and Inhibitor Recognition by Mammalian Cysteine Dioxygenase. J Mol Biol 2016; 428:3999-4012. [PMID: 27477048 DOI: 10.1016/j.jmb.2016.07.012] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2016] [Revised: 07/15/2016] [Accepted: 07/18/2016] [Indexed: 10/21/2022]
Abstract
In mammals, the non-heme iron enzyme cysteine dioxygenase (CDO) helps regulate Cys levels through converting Cys to Cys sulfinic acid. Its activity is in part modulated by the formation of a Cys93-Tyr157 crosslink that increases its catalytic efficiency over 10-fold. Here, 21 high-resolution mammalian CDO structures are used to gain insight into how the Cys-Tyr crosslink promotes activity and how select competitive inhibitors bind. Crystal structures of crosslink-deficient C93A and Y157F variants reveal similar ~1.0-Å shifts in the side chain of residue 157, and both variant structures have a new chloride ion coordinating the active site iron. Cys binding is also different from wild-type CDO, and no Cys-persulfenate forms in the C93A or Y157F active sites at pH6.2 or 8.0. We conclude that the crosslink enhances activity by positioning the Tyr157 hydroxyl to enable proper Cys binding, proper oxygen binding, and optimal chemistry. In addition, structures are presented for homocysteine (Hcy), D-Cys, thiosulfate, and azide bound as competitive inhibitors. The observed binding modes of Hcy and D-Cys clarify why they are not substrates, and the binding of azide shows that in contrast to what has been proposed, it does not bind in these crystals as a superoxide mimic.
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Affiliation(s)
- Camden M Driggers
- Department of Biochemistry and Biophysics, 2011 Ag & Life Sciences Building, Oregon State University, Corvallis, OR 97331, USA
| | - Kelsey M Kean
- Department of Biochemistry and Biophysics, 2011 Ag & Life Sciences Building, Oregon State University, Corvallis, OR 97331, USA
| | - Lawrence L Hirschberger
- Department of Nutritional Sciences, 227 Savage Hall, Cornell University, Ithaca, NY 14853, USA
| | - Richard B Cooley
- Department of Biochemistry and Biophysics, 2011 Ag & Life Sciences Building, Oregon State University, Corvallis, OR 97331, USA
| | - Martha H Stipanuk
- Department of Nutritional Sciences, 227 Savage Hall, Cornell University, Ithaca, NY 14853, USA.
| | - P Andrew Karplus
- Department of Biochemistry and Biophysics, 2011 Ag & Life Sciences Building, Oregon State University, Corvallis, OR 97331, USA.
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8
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Crowell JK, Sardar S, Hossain MS, Foss FW, Pierce BS. Non-chemical proton-dependent steps prior to O2-activation limit Azotobacter vinelandii 3-mercaptopropionic acid dioxygenase (MDO) catalysis. Arch Biochem Biophys 2016; 604:86-94. [PMID: 27311613 DOI: 10.1016/j.abb.2016.06.009] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2016] [Revised: 06/09/2016] [Accepted: 06/10/2016] [Indexed: 11/26/2022]
Abstract
3-mercaptopropionate dioxygenase from Azotobacter vinelandii (Av MDO) is a non-heme mononuclear iron enzyme that catalyzes the O2-dependent oxidation of 3-mercaptopropionate (3mpa) to produce 3-sulfinopropionic acid (3spa). With one exception, the active site residues of MDO are identical to bacterial cysteine dioxygenase (CDO). Specifically, the CDO Arg-residue (R50) is replaced by Gln (Q67) in MDO. Despite this minor active site perturbation, substrate-specificity of Av MDO is more relaxed as compared to CDO. In order to investigate the relative timing of chemical and non-chemical events in Av MDO catalysis, the pH/D-dependence of steady-state kinetic parameters (kcat and kcat/KM) and viscosity effects are measured using two different substrates [3mpa and l-cysteine (cys)]. The pL-dependent activity of Av MDO in these reactions can be rationalized assuming a diprotic enzyme model in which three ionic forms of the enzyme are present [cationic, E((z+1)); neutral, E(z); and anionic, E((z-1))]. The activities observed for each substrate appear to be dominated by electrostatic interactions within the enzymatic active site. Given the similarity between MDO and the more extensively characterized mammalian CDO, a tentative model for the role of the conserved 'catalytic triad' is proposed.
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Affiliation(s)
- Joshua K Crowell
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, USA
| | - Sinjinee Sardar
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, USA
| | - Mohammad S Hossain
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, USA
| | - Frank W Foss
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, USA
| | - Brad S Pierce
- Department of Chemistry & Biochemistry, College of Science, The University of Texas at Arlington, Arlington, TX 76019, USA.
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9
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Goncharenko KV, Seebeck FP. Conversion of a non-heme iron-dependent sulfoxide synthase into a thiol dioxygenase by a single point mutation. Chem Commun (Camb) 2016; 52:1945-8. [DOI: 10.1039/c5cc07772a] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
EgtB from Mycobacterium thermoresistibile catalyzes O2-dependent sulfur–carbon bond formation between the side chains of Nα-trimethyl histidine and γ-glutamyl cysteine as a central step in ergothioneine biosynthesis.
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10
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Pierce BS, Subedi BP, Sardar S, Crowell JK. The "Gln-Type" Thiol Dioxygenase from Azotobacter vinelandii is a 3-Mercaptopropionic Acid Dioxygenase. Biochemistry 2015; 54:7477-90. [PMID: 26624219 DOI: 10.1021/acs.biochem.5b00636] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the O2-dependent oxidation of l-cysteine to produce cysteinesulfinic acid. Bacterial CDOs have been subdivided as either "Arg-type" or "Gln-type" on the basis of the identity of conserved active site residues. To date, "Gln-type" enzymes remain largely uncharacterized. It was recently noted that the "Gln-type" enzymes are more homologous with another thiol dioxygenase [3-mercaptopropionate dioxygenase (MDO)] identified in Variovorax paradoxus, suggesting that enzymes of the "Gln-type" subclass are in fact MDOs. In this work, a putative "Gln-type" thiol dioxygenase from Azotobacter vinelandii (Av) was purified to homogeneity and characterized. Steady-state assays were performed using three substrates [3-mercaptopropionic acid (3mpa), l-cysteine (cys), and cysteamine (ca)]. Despite comparable maximal velocities, the "Gln-type" Av enzyme exhibited a specificity for 3mpa (kcat/KM = 72000 M(-1) s(-1)) nearly 2 orders of magnitude greater than those for cys (110 M(-1) s(-1)) and ca (11 M(-1) s(-1)). Supporting X-band electron paramagnetic resonance (EPR) studies were performed using nitric oxide (NO) as a surrogate for O2 binding to confirm obligate-ordered addition of substrate prior to NO. Stoichimetric addition of NO to solutions of 3mpa-bound enzyme quantitatively yields an iron-nitrosyl species (Av ES-NO) with EPR features consistent with a mononuclear (S = (3)/2) {FeNO}(7) site. Conversely, two distinct substrate-bound conformations were observed in Av ES-NO samples prepared with cys and ca, suggesting heterogeneous binding within the enzymatic active site. Analytical EPR simulations are provided to establish the relative binding affinity for each substrate (3map > cys > ca). Both kinetic and spectroscopic results presented here are consistent with 3mpa being the preferred substrate for this enzyme.
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Affiliation(s)
- Brad S Pierce
- Department of Chemistry and Biochemistry, College of Sciences, The University of Texas at Arlington , Arlington, Texas 76019, United States
| | - Bishnu P Subedi
- Department of Chemistry and Biochemistry, College of Sciences, The University of Texas at Arlington , Arlington, Texas 76019, United States
| | - Sinjinee Sardar
- Department of Chemistry and Biochemistry, College of Sciences, The University of Texas at Arlington , Arlington, Texas 76019, United States
| | - Joshua K Crowell
- Department of Chemistry and Biochemistry, College of Sciences, The University of Texas at Arlington , Arlington, Texas 76019, United States
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11
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Blaesi EJ, Fox BG, Brunold TC. Spectroscopic and Computational Investigation of the H155A Variant of Cysteine Dioxygenase: Geometric and Electronic Consequences of a Third-Sphere Amino Acid Substitution. Biochemistry 2015; 54:2874-84. [PMID: 25897562 DOI: 10.1021/acs.biochem.5b00171] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Cysteine dioxygenase (CDO) is a mononuclear, non-heme iron(II)-dependent enzyme that utilizes molecular oxygen to catalyze the oxidation of l-cysteine (Cys) to cysteinesulfinic acid. Although the kinetic consequences of various outer-sphere amino acid substitutions have previously been assessed, the effects of these substitutions on the geometric and electronic structures of the active site remained largely unexplored. In this work, we have performed a spectroscopic and computational characterization of the H155A CDO variant, which was previously shown to display a rate of Cys oxidation ∼100-fold decreased relative to that of wild-type (WT) CDO. Magnetic circular dichroism and electron paramagnetic resonance spectroscopic data indicate that the His155 → Ala substitution has a significant effect on the electronic structure of the Cys-bound Fe(II)CDO active site. An analysis of these data within the framework of density functional theory calculations reveals that Cys-bound H155A Fe(II)CDO possesses a six-coordinate Fe(II) center, differing from the analogous WT CDO species in the presence of an additional water ligand. The enhanced affinity of the Cys-bound Fe(II) center for a sixth ligand in the H155A CDO variant likely stems from the increased level of conformational freedom of the cysteine-tyrosine cross-link in the absence of the H155 imidazole ring. Notably, the nitrosyl adduct of Cys-bound Fe(II)CDO [which mimics the (O2/Cys)-CDO intermediate] is essentially unaffected by the H155A substitution, suggesting that the primary role played by the H155 side chain in CDO catalysis is to discourage the binding of a water molecule to the Cys-bound Fe(II)CDO active site.
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Affiliation(s)
- Elizabeth J Blaesi
- †Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
| | - Brian G Fox
- ‡Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
| | - Thomas C Brunold
- †Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
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