BIOTRANSFORMATIONS IN THE PEPTIDE AND CARBOHYDRATE FIELDS

Proteases and glycosidases are classes of hydrolytic enzymes that are used in biotransformations to catalyse the synthesis of peptides, and of glycosides and oligosaccharides, respectively, by a reversal of their normal mode of action. The practical application of both the methods shares several common features, namely the shift of rate-determining step through the use of activated substrates (aminoacyl and glycosyl donors respectively), the need to reduce or eliminate competing hydrolysis in the aminoacyl and glycosyl transfer steps, and the need to minimise hydrolytic cleavage of the products. Control of these factors is illustrated by a method for peptide synthesis using a-chymotrypsin suspended in dichloromethane containing low concentrations of water and a method for disaccharide synthesis using an N-acetylgalactosaminidase from Aspergillus oryzae. Molecular modelling of the oxyanion intermediate has been found to explain a number of features of reactions catalysed by alpha-chymotrypsin and to predict others that were the subject of experimental test. The concept of ''anomeric control'' has been used to modulate the regioselectivity of N-acetylgalactosaminyl transfer catalysed by the N-acetylgalactosaminidase