1gtu
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LIGAND-FREE HUMAN GLUTATHIONE S-TRANSFERASE M1A-1A
OverviewOverview
Domain interchange analyses and site-directed mutagenesis indicate that, the His107 residue of the human subunit hGSTM1 has a pronounced influence, on catalysis of nucleophilic aromatic substitution reactions, and a H107S, substitution accounts for the marked differences in the properties of the, homologous hGSTM1-1 (His107) and hGSTM4-4 (Ser107) glutathione, S-transferases. Reciprocal replacement of His107 and Ser107 in chimeric, enzymes results in reciprocal conversion of catalytic properties. With, 1-chloro-2, 4-dinitrobenzene as a substrate, the His107 residue primarily, influences the pH dependence of catalysis by lowering the apparent pKa of, kcat/Km from 7.8 for the Ser107-containing enzymes to 6.3 for the, His107-containing enzymes. There is a parallel shift in the pKa for, thiolate anion formation of enzyme-bound GSH. Y6F mutations have no effect, on the pKa for these enzymes. Crystal structures of hGSTM1a-1a indicate, that the imidazole ring of His107 is oriented toward the substrate binding, cleft approximately 6 A from the GSH thiol group. Thus, His107 has the, potential to act as a general base in proton transfer mediated through an, active site water molecule or directly following a modest conformational, change, to promote thiolate anion formation. All wild-type enzymes and, H107S chimera have nearly identical equilibrium constants for formation of, enzyme-GSH complexes (Kd values of 1-2 x 10(-)6 M); however, KmGSH and Ki, values for S-methylglutathione inhibition determined by steady-state, kinetics are nearly 100-fold higher. The functions of His107 of hGSTM1a-1a, are unexpected in view of a substantial body of previous evidence that, excluded participation of histidine residues in the catalytic mechanisms, of other glutathione S-transferases. Consequences of His107 involvement in, catalysis are also substrate-dependent; in contrast to, 1-chloro-2,4-dinitrobenzene, for the nucleophilic addition reaction of GSH, to ethacrynic acid, the H107S substitution has no effect on catalysis, presumably because product release is rate-limiting.
About this StructureAbout this Structure
1GTU is a Single protein structure of sequence from Homo sapiens. Active as Glutathione transferase, with EC number 2.5.1.18 Full crystallographic information is available from OCA.
ReferenceReference
Functions of His107 in the catalytic mechanism of human glutathione S-transferase hGSTM1a-1a., Patskovsky YV, Patskovska LN, Listowsky I, Biochemistry. 1999 Jan 26;38(4):1193-202. PMID:9930979
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