APPLICATION OF GLUTATHIONE SYNTHETASE ON PEPTIDE SYNTHESIS

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We have been studying the crystal structure of glutathione synthetase (GSHase) -substrates complex. These studies showed that the binding site of ATP is between two anti-parallel beta-sheets and that some residues interact with the bound ATP. However, the binding site of gamma-Glu- Cys is not clear because gamma-Glu-Cys gave poor electron density. We assumed some residues in the putative binding site of gamma-Glu-Cys and investigated their properties on the catalysis. Some mutant GSHases in which the residues around proposed binding site of gamma-Glu-Cys are replaced by site- directed mutagenesis are prepared and investigated in their properties. The analysis of the mutant GSHases shows that gamma-Glu-Cys was binding between Arg86 and Arg210 and that the thiol group of gamma-Glu-Cys was recognized by Thr288.The binding site of ATP was confirmed by the technique of affinity labeling. The crystallography of the labeled GSHase showed that the binding site of the modifier (adenosine-5'-tetraphospho-5'-pyridoxal) is the same as that of ATP.Crystallography of glutathione synthetase showed that a loop from Ile226 to Gly242 is above the binding sites and gave no electron density because of its flexibility. The position of the loop suggested that the loop have some roles on the catalysis. The analysis of the loop indicated that the loop moved over the active site to protect a labile intermediate from hydrolysis, controlled the recognition of glycine, and accelelared the catalysis by aligning the substrates in proper position.These results suggest that the mutation of residues around the binding sites and of the loop give new enzymes which catalyze synthesis of a novel peptide.