the appropriate site for mutation for several glucosidase enzymes thus in agrobacterium u03b2 glucosidase the mutant enzyme of the invention is prepared by replacing the glutamic acid at position 358 with another amino acid for example alanine mutant u03b1 amylase human or porcine in accordance with the invention has the aspartic acid at position 197 replaced with another amino acid for example alanine while in yeast u03b1 glucosidase the aspartic acid at position 216 is replaced once the site for mutation has been identified a mutant gene is prepared using site directed mutagenesis to arrive at the desired result in general this involves the construction of a plasmid containing the coding sequence for the wild type gene and isolation of single stranded dna copies are then made of the isolated plasmid dna using a template dependant dna polymerase and a primer which overlaps the site of the desired mutation and which differs from the wild type sequence in the manner necessary to yield the desired mutation the mutated plasmid is then transformed into a host organism e g e coli transformants are initially selected using a marker contained within the plasmid and then further selected by sequencing of the expressed glycosidase enzyme to confirm the nature of the mutation mutant enzymes according to the invention may be purified from the growth medium of the host organism by column chromatography for example on deae cellulose if desired high levels of purity are not required for use in catalyzing oligosaccharide synthesis however provided that impurities with wild type glycosidase activity must be substantially absent the enzymes of the invention are used to couple u03b1 modified glycosyl donors with glycoside.