MUTATIONS TO ALTER ASPERGILLUS-AWAMORI GLUCOAMYLASE SELECTIVITY - I -TYR48PHE49-]TRP, TYR116-]TRP, TYR175-]PHE, ARG241-]LYS, SER411-]ALA AND SER411-]GLY

Glucoamylase mutations to reduce isomaltose formation from glucose con densation and thus increase glucose yield from starch hydrolysis were designed to produce minor changes in the active site at positions not totally conserved. Tyr175-->Phe and Ser411-->Gly glucoamylases had cat alytic efficiencies on DP 2-7 maltooligosaccharides like those of ,wil d-type glucoamylase, while the catalytic efficiencies of Tyr116-->Trp, Arg241-->Lys and Ser411-->Ala glucoamylases were reduced by about hal f and Tyr48Phe49-->Trp glucoamylase had little remaining activity. Tyr 175-->Phe, Ser411-->Ala and Ser411-->Gly glucoamylases had decreased r atios of the initial rate of isomaltose formation from glucose condens ation to that of glucose formation from maltodextrin hydrolysis at bot h 35 and 55 degrees C compared with wild-type glucoamylase. Arg241-->L ys glucoamylase had a very similar ratio, while Tyr116-->Trp glucoamyl ase had a higher ratio. The highest glucose yields from maltodextrin h ydrolysis were by the mutant glucoamylases having the lowest ratios of initial rates of isomaltose formation to glucose formation and this p redicted high glucose yields better than the ratio of catalytic effici ency for maltose hydrolysis to that for isomaltose hydrolysis.