51
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Steinbach A, Fraas S, Harder J, Warkentin E, Kroneck PMH, Ermler U. Crystal structure of a ring-cleaving cyclohexane-1,2-dione hydrolase, a novel member of the thiamine diphosphate enzyme family. FEBS J 2012; 279:1209-19. [DOI: 10.1111/j.1742-4658.2012.08513.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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52
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Balakrishnan A, Gao Y, Moorjani P, Nemeria NS, Tittmann K, Jordan F. Bifunctionality of the thiamin diphosphate cofactor: assignment of tautomeric/ionization states of the 4'-aminopyrimidine ring when various intermediates occupy the active sites during the catalysis of yeast pyruvate decarboxylase. J Am Chem Soc 2012; 134:3873-85. [PMID: 22300533 PMCID: PMC3295232 DOI: 10.1021/ja211139c] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Thiamin diphosphate (ThDP) dependent enzymes perform crucial C-C bond forming and breaking reactions in sugar and amino acid metabolism and in biosynthetic pathways via a sequence of ThDP-bound covalent intermediates. A member of this superfamily, yeast pyruvate decarboxylase (YPDC) carries out the nonoxidative decarboxylation of pyruvate and is mechanistically a simpler ThDP enzyme. YPDC variants created by substitution at the active center (D28A, E51X, and E477Q) and on the substrate activation pathway (E91D and C221E) display varying activity, suggesting that they stabilize different covalent intermediates. To test the role of both rings of ThDP in YPDC catalysis (the 4'-aminopyrimidine as acid-base, and thiazolium as electrophilic covalent catalyst), we applied a combination of steady state and time-resolved circular dichroism experiments (assessing the state of ionization and tautomerization of enzyme-bound ThDP-related intermediates), and chemical quench of enzymatic reaction mixtures followed by NMR characterization of the ThDP-bound intermediates released from YPDC (assessing occupancy of active centers by these intermediates and rate-limiting steps). Results suggest the following: (1) Pyruvate and analogs induce active site asymmetry in YPDC and variants. (2) The rare 1',4'-iminopyrimidine ThDP tautomer participates in formation of ThDP-bound intermediates. (3) Propionylphosphinate also binds at the regulatory site and its binding is reflected by catalytic events at the active site 20 Å away. (4) YPDC stabilizes an electrostatic model for the 4'-aminopyrimidinium ionization state, an important contribution of the protein to catalysis. The combination of tools used provides time-resolved details about individual events during ThDP catalysis; the methods are transferable to other ThDP superfamily members.
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Affiliation(s)
| | - Yuhong Gao
- Department of Chemistry, Rutgers University, Newark, NJ 07102, USA
| | - Prerna Moorjani
- Department of Chemistry, Rutgers University, Newark, NJ 07102, USA
| | | | - Kai Tittmann
- Albrecht-von-Haller Institute & Göttingen Center for Molecular Biosciences, Georg-August University Göttingen, D-37077 Göttingen, Germany
| | - Frank Jordan
- Department of Chemistry, Rutgers University, Newark, NJ 07102, USA
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53
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Bortolini O, Fantin G, Fogagnolo M, Giovannini PP, Venturi V, Pacifico S, Massi A. α-Diketones as acyl anion equivalents: a non-enzymatic thiamine-promoted route to aldehyde–ketone coupling in PEG400 as recyclable medium. Tetrahedron 2011. [DOI: 10.1016/j.tet.2011.08.056] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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54
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Cyclohexane-1,2-dione hydrolase from denitrifying Azoarcus sp. strain 22Lin, a novel member of the thiamine diphosphate enzyme family. J Bacteriol 2011; 193:6760-9. [PMID: 21965568 DOI: 10.1128/jb.05348-11] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Alicyclic compounds with hydroxyl groups represent common structures in numerous natural compounds, such as terpenes and steroids. Their degradation by microorganisms in the absence of dioxygen may involve a C-C bond ring cleavage to form an aliphatic intermediate that can be further oxidized. The cyclohexane-1,2-dione hydrolase (CDH) (EC 3.7.1.11) from denitrifying Azoarcus sp. strain 22Lin, grown on cyclohexane-1,2-diol as a sole electron donor and carbon source, is the first thiamine diphosphate (ThDP)-dependent enzyme characterized to date that cleaves a cyclic aliphatic compound. The degradation of cyclohexane-1,2-dione (CDO) to 6-oxohexanoate comprises the cleavage of a C-C bond adjacent to a carbonyl group, a typical feature of reactions catalyzed by ThDP-dependent enzymes. In the subsequent NAD(+)-dependent reaction, 6-oxohexanoate is oxidized to adipate. CDH has been purified to homogeneity by the criteria of gel electrophoresis (a single band at ∼59 kDa; calculated molecular mass, 64.5 kDa); in solution, the enzyme is a homodimer (∼105 kDa; gel filtration). As isolated, CDH contains 0.8 ± 0.05 ThDP, 1.0 ± 0.02 Mg(2+), and 1.0 ± 0.015 flavin adenine dinucleotide (FAD) per monomer as a second organic cofactor, the role of which remains unclear. Strong reductants, Ti(III)-citrate, Na(+)-dithionite, and the photochemical 5-deazaflavin/oxalate system, led to a partial reduction of the FAD chromophore. The cleavage product of CDO, 6-oxohexanoate, was also a substrate; the corresponding cyclic 1,3- and 1,4-diones did not react with CDH, nor did the cis- and trans-cyclohexane diols. The enzymes acetohydroxyacid synthase (AHAS) from Saccharomyces cerevisiae, pyruvate oxidase (POX) from Lactobacillus plantarum, benzoylformate decarboxylase from Pseudomonas putida, and pyruvate decarboxylase from Zymomonas mobilis were identified as the closest relatives of CDH by comparative amino acid sequence analysis, and a ThDP binding motif and a 2-fold Rossmann fold for FAD binding could be localized at the C-terminal end and central region of CDH, respectively. A first mechanism for the ring cleavage of CDO is presented, and it is suggested that the FAD cofactor in CDH is an evolutionary relict.
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55
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Shim DJ, Nemeria NS, Balakrishnan A, Patel H, Song J, Wang J, Jordan F, Farinas ET. Assignment of function to histidines 260 and 298 by engineering the E1 component of the Escherichia coli 2-oxoglutarate dehydrogenase complex; substitutions that lead to acceptance of substrates lacking the 5-carboxyl group. Biochemistry 2011; 50:7705-9. [PMID: 21809826 DOI: 10.1021/bi200936n] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The first component (E1o) of the Escherichia coli 2-oxoglutarate dehydrogenase complex (OGDHc) was engineered to accept substrates lacking the 5-carboxylate group by subjecting H260 and H298 to saturation mutagenesis. Apparently, H260 is required for substrate recognition, but H298 could be replaced with hydrophobic residues of similar molecular volume. To interrogate whether the second component would allow synthesis of acyl-coenzyme A derivatives, hybrid complexes consisting of recombinant components of OGDHc (o) and pyruvate dehydrogenase (p) enzymes were constructed, suggesting that a different component is the "gatekeeper" for specificity for these two multienzyme complexes in bacteria, E1p for pyruvate but E2o for 2-oxoglutarate.
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Affiliation(s)
- Da Jeong Shim
- Department of Chemistry and Environmental Science, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102, United States
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56
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Assary RS, Broadbelt LJ. Computational screening of novel thiamine-catalyzed decarboxylation reactions of 2-keto acids. Bioprocess Biosyst Eng 2011; 34:375-88. [PMID: 21061135 DOI: 10.1007/s00449-010-0481-z] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2010] [Accepted: 10/18/2010] [Indexed: 01/02/2023]
Abstract
A molecular modeling strategy to screen the capacity of known enzymes to catalyze the reactions of non-native substrates is presented. The binding of pyruvic acid and non-native ketoacids in the active site of pyruvate ferredoxin oxidoreductase was examined using docking analysis, and our results suggest that enzyme-non-native ketoacid-bound species are feasible. Quantum mechanics/molecular mechanics methods were then used to study the geometry of the covalent intermediate formed from the enzyme and the various ketoacids. Finally, quantum mechanical methods were used to study the decarboxylation reaction of 2-keto acids at the mechanistic level. This hierarchical screening ranked the substrates from those that cannot be accommodated by the enzyme (phenyl pyruvate) to those whose conversion rate would most closely approach that of the native substrate (2-ketobutanoic acid and 2-ketovaleric acid). Most notably, our investigation suggests that novel pathways generated using generalized enzyme actions may be screened using the hierarchical approach employed here.
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Affiliation(s)
- Rajeev S Assary
- Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA
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57
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Vyazmensky M, Steinmetz A, Meyer D, Golbik R, Barak Z, Tittmann K, Chipman DM. Significant Catalytic Roles for Glu47 and Gln 110 in All Four of the C−C Bond-Making and -Breaking Steps of the Reactions of Acetohydroxyacid Synthase II. Biochemistry 2011; 50:3250-60. [DOI: 10.1021/bi102051h] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Maria Vyazmensky
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
| | - Andrea Steinmetz
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Danilo Meyer
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Ralph Golbik
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Ze'ev Barak
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
| | - Kai Tittmann
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - David M. Chipman
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
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58
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Meyer D, Neumann P, Parthier C, Friedemann R, Nemeria N, Jordan F, Tittmann K. Double duty for a conserved glutamate in pyruvate decarboxylase: evidence of the participation in stereoelectronically controlled decarboxylation and in protonation of the nascent carbanion/enamine intermediate . Biochemistry 2010; 49:8197-212. [PMID: 20715795 DOI: 10.1021/bi100828r] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Pyruvate decarboxylase (PDC) catalyzes the nonoxidative decarboxylation of pyruvate into acetaldehyde and carbon dioxide and requires thiamin diphosphate (ThDP) and a divalent cation as cofactors. Recent studies have permitted the assignment of functional roles of active site residues; however, the underlying reaction mechanisms of elementary steps have remained hypothetical. Here, a kinetic and thermodynamic single-step analysis in conjunction with X-ray crystallographic studies of PDC from Zymomonas mobilis implicates active site residue Glu473 (located on the re-face of the ThDP thiazolium nucleus) in facilitating both decarboxylation of 2-lactyl-ThDP and protonation of the 2-hydroxyethyl-ThDP carbanion/enamine intermediate. Variants carrying either an isofunctional (Glu473Asp) or isosteric (Glu473Gln) substitution exhibit a residual catalytic activity of less than 0.1% but accumulate different intermediates at the steady state. Whereas the predecarboxylation intermediate 2-lactyl-ThDP is accumulated in Glu473Asp because of a 3000-fold slower decarboxylation compared to that of the wild-type enzyme, Glu473Gln is not impaired in decarboxylation but generates a long-lived 2-hydroxyethyl-ThDP carbanion/enamine postdecarboxylation intermediate. CD spectroscopic analysis of the protonic and tautomeric equilibria of the cocatalytic aminopyrimidine part of ThDP indicates that an acidic residue is required at position 473 for proper substrate binding. Wild-type PDC and the Glu473Asp variant bind the substrate analogue acetylphosphinate with the same affinity, implying a similar stabilization of the predecarboxylation intermediate analogue on the enzyme, whereas Glu473Gln fails to bind the analogue. The X-ray crystallographic structure of 2-lactyl-ThDP trapped in the decarboxylation-deficient variant Glu473Asp reveals a common stereochemistry of the intermediate C2α stereocenter; however, the scissile C2α-C(carboxylate) bond deviates by ∼25-30° from the perpendicular "maximum overlap" orientation relative to the thiazolium ring plane as commonly observed in ThDP enzymes. Because a reactant-state stabilization of the predecarboxylation intermediate can be excluded to account for the slower decarboxylation, the data suggest a strong stereoelectronic effect for the transition state of decarboxylation as supported by additional DFT studies on models. To the best of our knowledge, this is a very rare example in which the magnitude of a stereoelectronic effect could be experimentally estimated for an enzymatic system. Given that variant Glu473Gln is not decarboxylation-deficient, electrostatic stress can be excluded as a driving force for decarboxylation. The apparent dual function of Glu473 further suggests that decarboxylation and protonation of the incipient carbanion are committed and presumably proceed in the same transition state.
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Affiliation(s)
- Danilo Meyer
- Albrecht-von-Haller-Institute for Plant Sciences and Göttingen Center for Molecular Biosciences, Georg-August-University Göttingen, Ernst-Caspari-Haus, Justus-von-Liebig-Weg 11, D-37077 Göttingen, Germany
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59
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Steinmetz A, Vyazmensky M, Meyer D, Barak Z, Golbik R, Chipman DM, Tittmann K. Valine 375 and Phenylalanine 109 Confer Affinity and Specificity for Pyruvate as Donor Substrate in Acetohydroxy Acid Synthase Isozyme II from Escherichia coli. Biochemistry 2010; 49:5188-99. [DOI: 10.1021/bi100555q] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Andrea Steinmetz
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
| | - Maria Vyazmensky
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Danilo Meyer
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
| | - Ze′ev Barak
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Ralph Golbik
- Institute for Biochemistry and Biotechnology, Martin-Luther University Halle-Wittenberg, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - David M. Chipman
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Kai Tittmann
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
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60
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Pei XY, Erixon KM, Luisi BF, Leeper FJ. Structural insights into the prereaction state of pyruvate decarboxylase from Zymomonas mobilis . Biochemistry 2010; 49:1727-36. [PMID: 20099870 PMCID: PMC2855724 DOI: 10.1021/bi901864j] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Pyruvate decarboxylase (PDC) uses thiamine diphosphate as an essential cofactor to catalyze the formation of acetaldehyde on the pathway of ethanol synthesis. Here we report the crystallographic image of a prereaction intermediate of a bacterial pyruvate decarboxylase prepared by cocrystallizing the enzyme with pyruvate and a stable analogue of the cofactor's activated ylid form. A second crystal structure of PDC in complex with fluoride shows that the ion organizes a water molecule that occludes the pyruvate binding site, accounting for the inhibitory effect of the halide. Also reported is a structure of the cofactor-free apo form, which when compared to the structure of the holo form indicates how thiamine diphosphate organizes the active site pocket of pyruvate decarboxylase to support catalysis. Guided by the structural and enzymatic data, we propose roles for several key residues in the catalytic mechanism.
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Affiliation(s)
- Xue-Yuan Pei
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QA, UK
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61
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Nemeria NS, Arjunan P, Chandrasekhar K, Mossad M, Tittmann K, Furey W, Jordan F. Communication between thiamin cofactors in the Escherichia coli pyruvate dehydrogenase complex E1 component active centers: evidence for a "direct pathway" between the 4'-aminopyrimidine N1' atoms. J Biol Chem 2010; 285:11197-209. [PMID: 20106967 DOI: 10.1074/jbc.m109.069179] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Kinetic, spectroscopic, and structural analysis tested the hypothesis that a chain of residues connecting the 4'-aminopyrimidine N1' atoms of thiamin diphosphates (ThDPs) in the two active centers of the Escherichia coli pyruvate dehydrogenase complex E1 component provides a signal transduction pathway. Substitution of the three acidic residues (Glu(571), Glu(235), and Glu(237)) and Arg(606) resulted in impaired binding of the second ThDP, once the first active center was filled, suggesting a pathway for communication between the two ThDPs. 1) Steady-state kinetic and fluorescence quenching studies revealed that upon E571A, E235A, E237A, and R606A substitutions, ThDP binding in the second active center was affected. 2) Analysis of the kinetics of thiazolium C2 hydrogen/deuterium exchange of enzyme-bound ThDP suggests half-of-the-sites reactivity for the E1 component, with fast (activated site) and slow exchanging sites (dormant site). The E235A and E571A variants gave no evidence for the slow exchanging site, indicating that only one of two active sites is filled with ThDP. 3) Titration of the E235A and E237A variants with methyl acetylphosphonate monitored by circular dichroism suggested that only half of the active sites were filled with a covalent predecarboxylation intermediate analog. 4) Crystal structures of E235A and E571A in complex with ThDP revealed the structural basis for the spectroscopic and kinetic observations and showed that either substitution affects cofactor binding, despite the fact that Glu(235) makes no direct contact with the cofactor. The role of the conserved Glu(571) residue in both catalysis and cofactor orientation is revealed by the combined results for the first time.
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Affiliation(s)
- Natalia S Nemeria
- Department of Chemistry, Rutgers University, Newark, New Jersey 07102, USA
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62
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Song J, Park YH, Nemeria NS, Kale S, Kakalis L, Jordan F. Nuclear magnetic resonance evidence for the role of the flexible regions of the E1 component of the pyruvate dehydrogenase complex from gram-negative bacteria. J Biol Chem 2009; 285:4680-94. [PMID: 19996100 DOI: 10.1074/jbc.m109.082842] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Most bacterial pyruvate dehydrogenase complexes from either gram-positive or gram-negative bacteria have E1 components with an alpha(2) homodimeric quaternary structure. In a sequel to our previous publications, we present the first NMR study on the flexible regions of the E1 component from Escherichia coli and its biological relevance. We report sequence-specific NMR assignments for 6 residues in the N-terminal 1-55 region and for a glycine in each of the two mobile active center loops of the E1 component, a 200-kDa homodimer. This was accomplished by using site-specific substitutions and appropriate labeling patterns along with a peptide with the sequence corresponding to the N-terminal 1-35 amino acids of the E1 component. To study the functions of these mobile regions, we also examined the spectra in the presence of (a) a reaction intermediate analog known to affect the mobility of the active center loops, (b) an E2 component construct consisting of a lipoyl domain and peripheral subunit binding domain, and (c) a peptide corresponding to the amino acid sequence of the E2 peripheral subunit binding domain. Deductions from the NMR studies are in excellent agreement with our functional finding, providing a clear indication that the N-terminal region of the E1 interacts with the E2 peripheral subunit binding domain and that this interaction precedes reductive acetylation. The results provide the first structural support to the notion that the N-terminal region of the E1 component of this entire class of bacterial pyruvate dehydrogenase complexes is responsible for binding the E2 component.
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Affiliation(s)
- Jaeyoung Song
- Department of Chemistry, Rutgers, the State University of New Jersey, Newark, New Jersey 07102, USA
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63
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Jordan F, Nemeria NS. Experimental observation of thiamin diphosphate-bound intermediates on enzymes and mechanistic information derived from these observations. Bioorg Chem 2005; 33:190-215. [PMID: 15888311 PMCID: PMC4189838 DOI: 10.1016/j.bioorg.2005.02.001] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2004] [Revised: 02/08/2005] [Accepted: 02/10/2005] [Indexed: 11/27/2022]
Abstract
Thiamin diphosphate (ThDP), the vitamin B1 coenzyme, is an excellent representative of coenzymes, which carry out electrophilic catalysis by forming a covalent complex with their substrates. The function of ThDP is to greatly increase the acidity of two carbon acids by stabilizing their conjugate bases, the ylide/C2-carbanion of the thiazolium ring and the C2alpha-carbanion (or enamine) once the substrate binds to ThDP. In recent years, several ThDP-bound intermediates on such pathways have been characterized by both solution and solid-state (X-ray) methods. Prominent among these advances are X-ray crystallographic results identifying both oxidative and non-oxidative intermediates, rapid chemical quench followed by NMR detection of a several intermediates which are stable under acidic conditions, and circular dichroism detection of the 1',4'-imino tautomer of ThDP in some of the intermediates. Some of these methods also enable the investigator to determine the rate-limiting step in the complex series of steps.
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Affiliation(s)
- Frank Jordan
- Department of Chemistry, Rutgers University, Newark, NJ 07102, USA
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