Glucoamylase active site structure

Figure 2-29 Structure of the a,a barrel of a fungal enzyme glucoamylase. (A) side view (stereoscopic) (B) top view. The active site, which cleaves glucose units from the ends of starch chains, is in the depression in the center of the barrel. Here it is occupied by an inhibitor. See Aleshin et al.192 Courtesy of Alexander Aleshin.

Figure 7.17 Structures of active-site directed amylase inhibitors. The K values are for glucoamylase (GA), porcine pancreatic oamylase (PPA) and cyclomaltodextrin glucanosyltransferase (CGTase).

Little is known of the structure, the nature of the active site, or the physical properties of the glucoamylases. The purified enzyme firom A. niger has been reported to contain at least two isozymes, which are readily separated by electrophoresis, - and gommc-amylase has been separated into two components, one of which has an optimal pH of 4, and the other, of about 6.5. 

Recent crystal structure studies confirm the presence of two fixed and opposed active-site carboxyl groups in nearly all glycosidases, including /3-galactosidase 77 cellulases 4 of families 6,7,9,10, and 45 56 80-88 xylanases 89-93 alpha-amylases 94-100 beta-amylase 101-102 and glucoamylase.1031048 Hev-amine, a chitinase from Hevea braziliensis. is exceptional in lacking a cat-... 

A partially purified glucoamylase from A. niger has been characterized by biochemical, physicochemical, and optical methods. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate indicated that the enzyme consists of two principal components (mol. wts. 6.30 x 10 and 5.75 X 10 ). Small proportions of dissociated and aggregated species are also present, but the size of the monomer (3.0 x 10 ) was deduced from sedimentation studies, etc. Chemical modification of the enzyme indicated that tyrosyl residues are located at the active site. The tertiary structure of the molecule contains 15—25% of a-helix, as well as /9-structure and disordered segments. 

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