Identification and Manipulation of Subsite Structure and Starch Granule Binding Site in Plant α-Amylase

Abstract

In germinating plant seeds, α-amylases degrade starch accumulated in seeds, and that requires two functions: catalysis itself and starch granule binding ability. All plant α-amylases belong to the α-amylase family and share the same catalytic machinery as other members, but are different in extended subsite structure accommodating the non-reducing end side of substrate even with high affinity, particularly in subsite -6, shown in α-amylases of kidney bean as well as barley. Barley α-amylase isozyme 1 (AMY1) mutants introduced site-directed mutagenesis along the predicted substrate binding site and the recent crystal structure solved in complex with a substrate occupying subsite -1 to -7 revealed that amino acid residues situated in a shallow cleft extending between domain A and B were involved in the subsite formation. Although plant α-amylases possess no additional starch-binding domain as seen in several α-amylases from microorganisms, plant α-amylases examined acted on starch granules. The residue corresponding to “sugar tongs” Tyr380_AMY1 was proven to be involved in starch granule binding in adzuki bean α-amylase.