Crystal structures of the starch-binding domain from Rhizopus oryzae glucoamylase reveal a polysaccharide-binding path
2008
GA (glucoamylase) hydrolyses starch and polysaccharides to β-D-glucose. Ro GA ( Rhizopus oryzae GA) consists of two functional domains, an N-terminal SBD (starch-binding domain) and a C-terminal catalytic domain, which are connected by an O-glycosylated linker. In the present study, the crystal structures of the SBD from Ro GA ( Ro GACBM21) and the complexes with β-cyclodextrin (SBD–βCD) and maltoheptaose (SBD–G7) were determined. Two carbohydrate binding sites, I (Trp 47 ) and II (Tyr 32 ), were resolved and their binding was co-operative. Besides the hydrophobic interaction, two unique polyN loops comprising consecutive asparagine residues also participate in the sugar binding. A conformational change in Tyr 32 was observed between unliganded and liganded SBDs. To elucidate the mechanism of polysaccharide binding, a number of mutants were constructed and characterized by a quantitative binding isotherm and Scatchard analysis. A possible binding path for long-chain polysaccharides in Ro GACBM21 was proposed.
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