Primary structures of alpha- and beta-subunits of alpha-amylase inhibitors from seeds of three cultivars of Phaseolus beans.

The primary structures of three alpha-amylase inhibitors (TAI, DAI, and MAI-2) consisting of glycoprotein subunits alpha and beta from the respective seeds of three cultivars of Phaseolus beans, Toramame (Phaseolus vulgaris L.), Daifukumame (Phaseolus vulgaris L.), and Murasakihanamame (Phaseolus coccineus L.) were determined by sequencing the peptide fragments derived from their enzymatic digestions. Major sugar chains of the inhibitors were also assessed by analyzing glycopeptides in the enzymatic digests. The subunits, alpha and beta, were shown to be composed of 76 and 139 amino acid residues, respectively, in each inhibitor. The overall amino acid sequences of the inhibitors were slightly different from one another. Furthermore, the sequence of TAI was the same as that deduced from a cDNA clone encording alpha-amylase inhibitor-1 from the common bean (Phaseolus vulgaris L.). It was also revealed that there were two N-glycosylation sites in each alpha-subunit: PA-derivatives of the major N-glycans were estimated to be M6B at Asn(12) and M9A at Asn(65). Each beta-subunit of TAI and MAI-2 had two N-glycosylation sites, while the beta-subunit of DAI had only one site. The major N-glycans pyridylaminated were estimated to be M3X at Asn(63) in each beta-subunit and M3FX at Asn(83) in beta-subunits of TAI and MAI-2.