Synthetic application of bacterial gamma-glutamyltransferases (GGTs)

γ-Glutamyl dipeptides are compounds characterized by an amide bond involving the amino group of one amino acid and the γ-carboxyl group of a glutamic acid residues. They show interesting properties with respect to their parent amino acids. For example, the bitterness of aromatic and branched-chain amino acids used in oral dietary supplements is alleviated or even abolished upon γ-glutamylation, as does the unpleasant smell of seleno amino acids, the source of the micronutrient selenium. γ-Glutamyl derivatives of S-substituted cysteines are naturally occurring flavor enhancers found in garlic and onion.1 Although their possible applications render the γ-glutamyl derivatives economically interesting compounds, their supply remains a problem. Isolation from natural sources, if any, is laborious and low-yielding, as their content may vary with cultivation and storage.2 Chemical synthesis is not economical, due to the need of protection/deprotection steps. A viable alternative could then rely on an enzymatic approach taking advantage by the use of a γ-glutamyltransferase. γ-Glutamyltransferases (GGTs, EC 2.3.2.2) are widespread, conserved enzymes found in bacteria, plants and animals.3 They catalyze the transfer of a γ-glutamyl moiety from a donor compound, usually glutathione, to an acceptor amino acid through a γ-glutamyl-enzyme intermediate involving a catalytically active threonine residue. As a first approach, a commercially available, crude γ-glutamyltransferase of animal origin was used in our laboratories for the synthesis of some naturally occurring derivatives found in garlic.4 Then, our research group turned attention to bacterial enzymes, especially from GRAS (Generally Referred as Safe) microorganisms. The GGT from B. subtilis seemed to be well suited for our purposes and a detailed study of this enzyme was since then undertaken.5 Recent findings obtained about the enzymatic activity of B. subtilis GGT and related to the peculiar architecture of its active site will be presented, in relation to its application as a biocatalyst for the synthesis of γ-glutamyl derivatives of economical interest. This work is supported by Fondazione Cariplo (TailGluTran Project, 2016-0741)