Molecular and enzymatic properties of an aspartic proteinase from Rhizopus hangchow

An aspartic proteinase, rhizopuspepsin (EC 3.4.23.21), from Rhizopus hangchow was purified. The M(r) and isoelectric point were determined as ca 37,000 and 4.5, respectively. The first 19 amino acids in the N-terminal region were SGSGVVPMTDYEYDIEYYG. The contents of the alpha-helix, beta-structure and random coil were calculated to be ca 7.5, 88.9 and 2.7%, respectively. The enzyme can activate trypsinogen at pH 3.0. The activity was completely inactivated by pepstatin A. The specificity and mode of action of the enzyme were investigated with oxidized insulin B-chain at pH 3. The enzyme hydrolysed primarily two peptide bonds, the Leu15-Tyr16 bond and the Tyr16-Leu17 bond, while additional cleavage of the bonds, Ala14-Leu15 and Phe24-Phe25 was also noted.