The carboxyl-terminal sequence of porcine pepsin.

The sequence of 27 residues at the carboxyl end of the single polypeptide chain of porcine pepsin has been found to be: -Ile-Leu-Gly-Asp-Val-Phe-Ile-Arg-Gln-Tyr-Tyr-ThrVal-Phe-Asp-Arg-Ala-Asn-Asn-Lys-Val-Gly-Leu-Ala-ProVal-Ala. The peptides from which this sequence has been derived were isolated from tryptic and chymotryptic digests of pepsin and its reduced aminoethylated and trifluoracetylated derivatives. All 3 of the strongly basic residues in pepsin (1 lysine and 2 arginine residues) are thus found in this terminal segment of the chain. There is no half-cystine residue in this portion of the molecule, and hence the crosslinking of the enzyme by three -S-S bonds occurs within a highly acidic sequence of about 300 residues in which the only basic residue is that of the single histidine. The absence of positive charges in this sequence undoubtedly contributes to the acid stability and alkali lability of the enzyme.