Abstract
X-ray crystal structure of a catalytic-site mutant EQ208 [Glu2O8→Gln] of α-amylase from Bacillus subtilis complexed with maltopentaose (G5) has been determined at 2.5 A resolution. The conformation around the third α- (1, 4) -glucosidic bond makes a sharp turn, allowing the substrate to fit into the L-shaped cleft of the active site. The amide nitrogen of G1n208 forms a hydrogen bond with the glucosidic oxygen in the scissile bond. The carboxyl group of Asp 176 has non-bonded contacts to the anomeric carbon atom and to the endocyclic oxygen atom. Thus, G1u208 may act as a proton donor and Asp176 may stabilize the oxocarbonium ion at the catalysis.
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