Amino acid sequence of yeast proteinase B inhibitor 1 comparison with inhibitor 2

The amino acid sequence of proteinase B inhibitor 1 (I B1) from bakers' yeast has been established by automated Edman degradation up to position 42. A comparison with the sequence of proteinase B inhibitor 2 (I B2) revealed two differences: LEU-32 and GLU-34 in I B2 are replaced by VAL-32 and LYS-34 in I B1. Identity of the COOH-terminal region of I B1 with that of I B2 was proved by degradation with the carboxypeptidases A and Y. Furthermore, a chymotryptic peptide was isolated from each of the 74 residues containing inhibitors. The two fragments, ranging from position 42 to the COOH termini of the inhibitors, were found to be identical with respect to electrophoretical mobility, end groups, amino acid composition and peptide pattern after tryptic digestion. It is concluded, that the two inhibitor sequences are identical beyond position 42. I B1 and I B2 are isoinhibitors, because they are coded by different genes.