pH-Dependent hydrolase, glutaminase, transpeptidase and autotranspeptidase activities of Bacillus subtilis γ-glutamyltransferase

γ-​Glutamyltransferases (γ-​GTs) are heterodimeric enzymes that catalyze the transfer of a γ-​glutamyl group from a donor species to an acceptor mol. in a transpeptidation reaction through the formation of an intermediate γ-​glutamyl enzyme. In the search for a γ-​GT from a generally recognized as safe microorganism suitable for the prodn. of γ-​glutamyl derivs. with flavor-​enhancing properties intended for human use, the authors cloned and overexpressed γ-​GT from Bacillus subtilis. Here, the authors report the behavior of B. subtilis γ-​GT in reactions involving glutamine as the donor compd. and various acceptor amino acids. The common thread emerging from these results is a strong dependence of the hydrolase, transpeptidase, and autotranspeptidase activities of B. subtilis γ-​GT on pH, also in relation to the pKa of the acceptor amino acids. Glutamine, commonly referred to as a poor acceptor mol., underwent rapid autotranspeptidation at elevated pH, affording oligomeric species, in which up to 4 γ-​glutamyl moieties were linked to a single glutamine. Moreover, it was found that L-​glutamine was also recognized both as a donor and as an acceptor substrate. These results prove that the B. subtilis γ-​GT-​catalyzed transpeptidation reaction is feasible, and the obsd. activities of γ-​GT from B. subtilis could be interpreted in relation to the known ability of the enzyme to process the polymeric material, γ-​polyglutamic acid.