Analysis of the active center of branching enzyme II from maize endosperm.

Analysis of the primary structure of mBEII, with those of other branching and amylolytic enzymes as reference, identifies four highly conserved regions which may be involved in substrate binding and in catalysis. When one of the amino acid residues corresponding to the putative catalytic sites of mBEII, i.e., Asp-386, Glu-441, and Asp-509, was replaced, activity disappeared. These putative catalytic residues are located in three different regions (regions 2-4) of the four highly conserved regions (regions 1-4) which exist in the primary structure of most starch hydrolases and related enzymes, including branching enzymes. Region 3, which contains Glu-441 as one of the putative catalytic residues, was located downstream of the carboxyl-terminal position previously reported. The importance of the carboxyl amino acid residues was also demonstrated by chemical modification of the branching enzyme protein using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide.