Structure-function relationships in glucosidase I: amino acids involved in binding the substrate to the enzyme

As the enzyme that initiates the maturation phase of the oligosaccharide moiety of N-linked glycoproteins, glucosidase I controls the flux of carbohydrate during the biosynthesis of these proteins. In a previous study to elucidate the structure-function relationships, we reported the presence of a cysteine residue at or near the active site of the enzyme from the bovine mammary gland (Pukazhenthi,BS., Muniappa,N. and Vijay,I.K., 1993, J. Biol. Chem, 268, 6445–6452). We have now extended this approach to identify the participation of an arginine and a tryptophan residue in the enzyme that may play an important role in binding the substrate. The data have been combined with the results of the previous study and the cDNA-derived sequence to propose a ERHLDLRCW motif in the active site of the enzyme in the rat mammary gland that is involved in binding the incipient glycoprotein substrate for processing.