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How alpha and beta-amylases achieve their perfect stereoselectivity

How alpha and beta-amylases achieve their perfect stereoselectivity

Research Communications in Biochemistry & Cell & Molecular Biology 2(1-2): 69-89

The enzyme-substrate complex structure of a-amylase is modeled by 3-D computer-simulated docking and the reactivity of alpha- and beta-amylases is analyzed to elucidate the difference in their stereoselectivity. The mechanism by which alpha-amylase perfectly produces only alpha-anomers in hydrolysis, is explained. It is revealed that Asp206 is located at the bottom of the enzyme cleft to hold the carbonium cation intermediate after the cleavage of glucosidic linkage C1-Oglyc-C4', and that Glu230 and Asp297 are located on the upper side to catch the water molecule. Therefore, the water molecule (OH-) needed for the hydrolysis is allowed to come only from the top of the cleft to attack C1+ of the intermediate. Consequently, this attack occurs from the same side that the leaving Oglyc-C4' was on, because the direction of the original glucosidic linkage (as indicated by the V-shape of the glucosidic linkage C1-Oglyc-C4') points to the cleft top. For beta-amylase, enzyme-substrate crystal structure indicates that the original glucosidic linkage to be hydrolyzed points toward the cleft bottom (the side opposite that from which the water molecule attacks), resulting only in beta-anomers. The significance of that directional change introduced by the 180degree rotation of the dihedral angle from its normal value is discussed.

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