The amino acid sequence of rhizopuspepsin, an aspartic proteinase from Rhizopus chinensis.

Abstract

The complete amino acid sequence of rhizopuspepsin, an aspartic proteinase from the fungus Rhizopus chinensis was determined by conventional protein sequencing, using peptide fragments obtained mainly by several enzymatic cleavages of the reduced and carboxymethylated (RCm-) protein. The RCm-protein was first cleaved by trypsin, and the resulting peptides were purified and their amino acid sequences determined extensively. These tryptic peptides were aligned by the aid of overlapping peptides isolated from a tryptic and a chymotryptic digest of the citraconylated RCm-protein and the RCm-protein, respectively. The amino acid sequence thus deduced was further confirmed by isolation and sequence determination of peptides obtained by digestion of the RCm-protein with Staphylococcus aureus V8 protease. The location of the disulfide bonds was determined by isolation and analysis of cystine-containing peptides from a chymotryptic digest of intact rhizopuspepsin. These results showed that the protein is composed of a single polypeptide chain of 325 amino acid residues cross-linked by two disulfide bonds, and shows overall homology with other aspartic proteinases, including 36% identity with penicillopepsin and 38% identity with porcine pepsin.