Amino acid sequence of an active fragment of potato proteinase inhibitor IIb.

The amino acid sequence of an active fragment of potato proteinase inhibitor IIb was determined by the Edman degradation procedure and the carboxypeptidase technique. Analyses were carried out on peptides derived from the reduced and carboxymethylated active fragment by digestion with trypsin and chymotrypsin. The active fragment consisted of a single polypeptide chain of 40 amino acid residues including 6 half-cystine residues. Degradation of the intact active fragment by trypsin and subtilisin at pH 6.3--6.4 yielded 3 cystine-containing peptides, and sequence analyses of these peptides revealed that the three disulfide linkages are located between Cys(2) and Cys(16), Cys(6) and Cys(28), and Cys(12) and Cys(38). The reactive site peptide bond of inhibitor IIb, a Lys-Ser bond, is located between positions 26 and 27. The overall sequence of the active fragment is compared with that of an active fragment of inhibitor IIa and the "structure-specificity" relationships of both are discussed. Correlation of the fragments to a naturally-occurring low molecular weight inhibitor is also discussed.