How Amylases Achieve Their Perfect Stereoselectivity.

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The enzyme-substrate complex structure of α-amylase is modeled by a 3-D computer-simulated docking, and the reactivity of α-and β-amylases is analyzed to elucidate the difference in their stereoselectivity. The mechanism by which α-amylase perfectly produces only α-anomers in hydrolysisis explained. It is revealed that Asp206 is at the bottom of the enzyme cleft to hold the carbonium cation intermediate after the cleavage of glucosidic linkage Cl-Oglyc-C4′, and that G1u230 and Asp297 are on the upper side to catch the water molecule . Therefore the water molecule (OH<SUP>-</SUP>) needed for the hydrolysis is allowed to come only from the top of the cleft to attack Cl<SUP>+</SUP> 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 Cl-Oglyc-C4′) points to the cleft top. For β-amylase, the 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 R-anomers . The significance of that directional change of glucosidic linkage introduced by the 180° rotation of the dihedral angle from its normal value is discussed.
日本応用糖質科学会の論文