Abstract
β- Amylase [EC 3.2.1.2 a-1, 4-glucan maltohydrolase] is an exo-acting amylase, cleaving α-1, 4 linkages sequentially from the non-reducing end of starch, producing β-maltose and β-limit dextrins as the end products, ?A-Amylases are widely distributed in higher plants and some bacteria. Sweet potato β-amylase was first crystallized by Balls et al. in 1948. Some physicochemical properties, substrate specificity, and action mechanism have been reported. Sweet potato β-amylase is a homotetrameric enzyme with MW of 215; 000, judging from the experimental results. The complete amino acid sequence was established by the strategy of protein chemistry. The subunit of the enzyme consisted of 496 amino acid residues giving Mr of 55, 707. The native enzyme was affinity labeled with 2, 3-epoxypropyl-α-D-glucopyranoside (α-EPG). The α-EPG was incorporated stoichiometrically (one mol α-EPG/mol subunit) into the enzyme, resulting in the complete loss of the enzymatic activity. The Glu-187 affinity labeled with α-EPG was identified by isolating and sequencing the 14C-α-EPG-peptide. Sequence homology of sweet potato enzyme with those from soybean and barley was 64 and 53%, respectively. Ba polymyxa and B. circulans enzymes were 80% homologous with each other. It is noteworthy that there are at least six highly conserved sequences throughout plants and bacterial β-amylases and that the active site Glu-187 in sweet potato enzyme is the invariant residue in one of these conserved regions. These results suggest that glutamate residue corresponding to the Glu-187 in sweet potato β-amylase is the active site of other β-amylases from different origins.
