THE HYPERDIGESTION OF RAW STARCH BY A CARBOHYDRATE-RICH GLUCOAMYLASE FROM A PROTEASE-NEGATIVE AND GLYCOSIDASE-NEGATIVE MUTANT OF ASPERGILLUS-AWAMORI VAR KAWACHI F-2035

A protease- and glycosidase-negative mutant F-2035, of Aspergillus awamori var. kawachi has been isolated that produces a glucoamylase that contains a large carbohydrate moiety and has enhanced ability to digest raw starch (GA MU-H; MW, 110,000). This enzyme digested raw corn starch 2.5 times faster than did the parental glucoamylase I (GA I; MW, 90,000). When grown from an enriched seed culture, this mutant also produced a glucoamylase with less ability to digests raw starch (GA MU-L; MW, 110,000). Its activity was 25% of that of GA MU-H with raw starch. Both GA MU-H and GA MU-L proved to be identical to GA I in terms of adsorption to raw starch, molar activity against gelatinized starch, amino acid composition, and terminal amino acid sequence. The carbohydrate contents of GA I, GA MU-H, and GA MU-L were 17%, 33%, and 33% by weight, respectively. The carbohydrates of GA I and GA MU-H were mostly mannose, but that of GA MU-L was composed of mannose (71%) and glucose (26%). Partial removal of the carbohydrate from GA I and GA MU-H by Turbo glycosidase caused a parallel decrease in the ability to digest raw starch. Thus, the carbohydrate moiety of the glucoamylase molecule, in particular the mannose residues, appears to be important in the digestion of raw starch, and promote the hydration of micelles of raw starch but not the actual adsorption of the enzyme to raw starch.