Towards the design of improved g-glutamyltransferase as biocatalyst for nutraceuticals and food additives

Gamma-Glutamyl peptides are compounds deriving from the acylation of an amino acid by the gamma-carboxyl carbon of L-glutamic acid. Gamma-Glutamylation of amino acids affects some chemical-physical as well as organoleptic properties of the parent molecule. Some naturally occurring gamma-glutamyl derivatives show pharmacological activity or represent taste-active compounds with flavor-enhancing capacities. Enzymatic approaches based on the use of -glutamyltransferases (GGT, E.C. 2.3.2.2) for the preparation of gamma-glutamyl derivatives have been proposed over time. They circumvent the drawbacks of chemical synthesis, which is cumbersome and uneconomical due to the requirement for multiple protection-deprotection steps on the interfering functionalities of the reacting molecules. Bacterial GGTs appears to be convenient biocatalysts, able to use the cheap and readily available glutamine as the gamma-glutamyl donor compound We report preliminary results obtained with two bacterial GGTs with different architectures of the active site for the synthesis of gamma-glutamyl derivatives of S-substituted cysteines, which are naturally occurring flavor enhancers found in plant of the genus Allium. The effects of pH, temperature and substrates molar ratio were examined in reactions catalyzed by both enzymes and will be discussed in the light of the structural differences between the two biocatalysts. This work represents a first, unprecedented study in which two bacterial GGTs of different origins are directly compared and in which the attitude of the enzymes to function as biocatalysts is related to their structural characteristics. This approach could be useful for the design of mutant enzymes better suited for preparative purposes.