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Purification and cloning of a gamma-glutamyl transpeptidase from onion (Allium cepa)
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c-Glutamyl transpeptidase (E.C.; GGT) catalyses hydrolysis of c-glutamyl linkages in c-glutamyl peptides and transfer of the c-glutamyl group to amino acids and peptides. Although plant c-glutamyl peptide metabolism is important in biosynthesis andmetabolism of secondary products and xenobiotics, plant GGTs are poorly aracterised. We purified a membrane-associated GGT from sprouting onion bulbs that catalyses transpeptidation of methionine by the synthetic substrate c-glutamyl-p-nitroanilide(GGPNA) and obtained N-terminal peptide sequence. We also cloned the full-length coding region of an onion GGT by homology with the Arabidopsis enzyme and confirmed that this shared the same N-terminal sequence. Enzyme kinetic studies show that the enzyme has high affinity for glutathione and glutathione conjugates, and that affinity for S-substituted glutathione analogs decreases as the substituted chain length increases. The major onion c-glutamyl peptide, c-glutamyl trans-S-1-propenyl cysteine sulfoxide (GGPrCSO) exhibited uncompetitive inhibition of transpeptidation by GGPNA. This suggests that GGPrCSO is a poor glutamyl donor and therefore unlikely to be an in vivo substrate for peptidase activity by this enzyme.
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 Record created 2010-03-10, last modified 2019-06-12

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