Structure and Action of Glucoamylases

Two forms of glucoamylase (often incorrectly called amyloglucosidase) have been observed in the culture supernatants of Aspergillus niger, A. awamori and 

Three forms of glucoamylase occur in the culture supernatants of Rhizopus sp. They have molecular weights of 74000, 58000 and 61000, and have been designated GA-I, GA-II and GA-III.150 GA-I bound and hydrolyzed native starch granules, but GA-II and GA-III neither bound nor hydrolyzed native starch granules.151 The three glucoamy-lases had the same C-terminal end, but differed in their N-terminal ends.152 Two 

The active-sites of the glucoamylases are pockets that do not have a flexible, mobile loop that can act as a trap door to let the product (glucose) out. After the glycosidic bond is cleaved by glucoamylase, the remaining starch chain must dissociate and leave the active-site before glucose can leave. This explains why glucoamylase has such a low turnover number, and why it does not have a multiple attack mechanism. 

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