Fungal Proteinase A , aspartic proteinase superfamily.
Fungal Proteinase A, a proteolytic enzyme distributed among a variety of organisms, is a member of the aspartic proteinase superfamily. In Saccharomyces cerevisiae, targeted to the vacuole as a zymogen, activation of proteinases A at acidic pH can occur by two different pathways: a one-step process to release mature proteinase A, involving the intervention of proteinase B, or a step-wise pathway via the auto-activation product known as pseudo-proteinase A. Once active, S. cerevisiae proteinase A is essential to the activities of other yeast vacuolar hydrolases, including proteinase B and carboxypeptidase Y. The mature enzyme is bilobal, with each lobe providing one of the two catalytically essential aspartic acid residues in the active site. The crystal structure of free proteinase A shows that flap loop is atypically pointing directly into the S(1) pocket of the enzyme. Proteinase A preferentially hydrolyzes hydrophobic residues such as Phe, Leu or Glu at the P1 position and Phe, Ile, Leu or Ala at P1'. Moreover, the enzyme is inhibited by IA3, a natural and highly specific inhibitor produced by S. cerevisiae. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).