抄録
Soybean β-amylase is a monomeric enzyme consisting of a single polypeptide chain (molecular weight 57, 000). Zsozyme patterns of β-amylase in seeds are characteristics to the varieties of soybean and the related species. Five isozymes were purified and crystallized from commercial defatted soybean meal, and their chemical, physical, immunological and enzymatic properties were investigated comparatively. Large single crystals of the enzyme were analyzed by X-ray diffraction method with two isomorphous derivatives with K2PtCl4 and p-chloromercuribenzene sulfonate. By the selective modification of five SH groups in the molecule with organic mercu rials and alkylating reagents, the second reactive one was found to be related to the enzyme activity, but it participates neither to the catalytic nor substrate-binding action of the enzyme . The site of five SH groups on the polypeptide chain were determined by cleaving the chain by the cyanide method and by evaluating the molecular weights of the resulting peptide fragments by SDS-polyacrylamide gel electrophoresis. As to the action of β-amylase on the substrate, the substrate-binding affinities of five subsites of the soybean enzyme were determined, and the first site was found to be the strongest, which may determine the mode of action of β-amylase.
