Journal
PHYTOCHEMISTRY
Volume 66, Issue 5, Pages 515-522Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.phytochem.2005.01.017
Keywords
Allium cepa; Alliaceae; onion; enzymology; cloning; gamma-glutamyl pqptides; gamma-glutamyl transpeptidase; glutathione
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gamma-Glutamyl transpepticlase (E.C. 2.3.2.2; GGT) catalyses hydrolysis of gamma-glutamyl linkages in gamma-glutamyl peptides and transfer of the gamma-glutamyl group to amino acids and peptides. Although plant gamma-glutamyl peptide metabolism is important in biosynthesis and metabolism of secondary products and xenobiotics, plant GGTs are poorly characterised. We purified a membrane-associated GGT from sprouting onion bulbs that catalyses transpepticlation of methionine by the synthetic substrate gamma-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 gamma-glutamyl peptide, 7-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. (c) 2005 Elsevier Ltd. All rights reserved.
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