Partial amino acid sequences around sulfhydryl groups of soybean beta-amylase

Sulfhydryl (SH) groups of soybean beta-amylase were modified with 5-(iodoaceto-amidoethyl)aminonaphthalene-1-sulfonate (IAEDANS) and the SH-containing peptides exhibiting fluorescence were purified after chymotryptic digestion of the modified enzyme. The sequence analysis of the peptides derived from the modification of all SH groups in the denatured enzyme revealed the existence of six SH groups, in contrast to five reported previously. One of them was found to have extremely low reactivity toward SH-reagents without reduction. In the native state, IAEDANS reacted with 2 mol of SH groups per mol of the enzyme (SH1 and SH2) accompanied with inactivation of the enzyme owing to the modification of SH2 located near the active site of this enzyme. The selective modification of SH2 with IAEDANS was attained after the blocking of SH1 with 5,5'-dithiobis-(2-nitrobenzoic acid). The amino acid sequences of the peptides containing SH1 and SH2 were determined to be Cys-Ala-Asn-Pro-Gln and His-Gln-Cys-Gly-Gly-Asn-Val-Gly-Asp-Ile-Val-Asn-Ile-Pro-Ile-Pro-Gln-Trp, respectively.