Glutamyl Transferases in Plants

They are widely distributed in animals and microorganisms and their properties have been investigated in detail (220, 221, 253). y-Glutamyl transferases have also been identified in a number of plants (329), and an enzyme from Phaseolus vulgaris has been most intensively studied (73, 74, 333)  

The enzyme was also able to catalyze the following reaction  

It was subsequently shown, using a more purified enzyme preparation, that the enzyme had no transpeptidase activity below pH 7.5, whereas the hydrolytic reaction showed optimum activity at both pH 9.5 and 6.5. The amino group of the acceptor molecule must presumably be in the non-protonated form during the transpeptidation reaction. From these data it was concluded that the normal function of the enzyme was to catalyze hydrolysis (75). Despite this, it has repeatedly been suggested that y-glutamyltranspeptidase is responsible for the biosynthesis of many y-glutamyl derivatives 330, 347, 364). 

Transpeptidase activity was further studied in extracts of root and sprouting bulbs of Allium cepa. The pH optimum for the hydrolytic activity was 9.0, and it was again concluded that hydrolytic activity was adequate to explain the hydrolysis of y-glutamyl peptides during sprouting of onions. It was confirmed that no enzymatic activity was present in dormant bulbs 8). Subsequently a partially purified enzyme was obtained from sprouted onion bulbs and was shown to have both hydrolytic activity and transpeptidase activity with an optimum pH of 9, although the transpeptidation action was presumably the essential one. The possibility that the crude onion extract also contained a genuine 

In some cases y-glutamyl transpeptidase has been found together with y-glutamyl derivatives in Basidiomycetes. Thus an enzyme catalyzing the following reaction 

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