A mutant γ-glutamyltransferase with improved transpeptidase activity

Despite their potential applicative interest as biologically active compounds and as flavor enhancers, γ-glutamyl derivatives are commercially underexploited compounds. This is mainly due to the difficulties connected with their supply at a reasonable cost. As a consequence, enzymatic approaches to their preparation, based on the use of γ-glutamyltransferases (GGTs), have been proposed1 to circumvent both the low-yielding extractive procedures from natural sources and the troublesome chemical synthesis, rendered uneconomical by the need of protection and deprotection steps. GGTs catalyze the transfer of a γ-glutamyl moiety from a donor substrate (e.g. glutathione or glutamine) to the primary amino group of an acceptor compound in a so-called transpeptidation reaction through the formation of a γ-glutamyl-enzyme intermediate. However, also the use of GGTs as biocatalysts is not free from drawbacks. In addition to the transpeptidase activity, GGTs show a non-negligible hydrolase activity towards both the donor substrate and the newly formed transpeptidation product, affording irreversibly glutamic acid.2 In our ongoing studies on bacterial GGTs, we found that the presence of the lid loop – a short amino acids sequence covering the active site in most of the known GGTs – not only affects substrate selection, but also modulates hydrolase/transpeptidase activities.3 Within the TailGluTran Project,4 aimed at the development of mutant GGTs with improved transpeptidase activity, is currently under investigation a mutant enzyme obtained by inserting the sequence of the lid loop on the structure of a GGT naturally lacking it. The mutant enzyme shows promising high transpeptidase activity with respect to wild type counterparts and represents a starting point for further modifications in the search of a suitable biocatalyst intended for preparative purposes.