Secreted aspartic endopeptidase that allows assimilation of proteinaceous substrates. The scissile peptide bond is attacked by a nucleophilic water molecule activated by two aspartic residues in the active site. Shows a broad primary substrate specificity. Favors hydrophobic residues at the P1 and P1' positions, but also accepts a lysine residue in the P1 position, leading to the activation of trypsinogen and chymotrypsinogen A (By similarity). Hydrolyzes myoglobin, hemoglobin and other natural proteins. Hydrolyzes equine myoglobin between positions 'Met-1' and 'Gly-2', 'Lys-43' and 'Phe-44', and 'Leu-70' and 'Thr-71' (PubMed:23044751). Hydrolysis of proteins with broad specificity. Generally favors hydrophobic residues in P1 and P1', but also accepts Lys in P1, which leads to activation of trypsinogen. Does not clot milk. Inhibited by the microbial peptide pepstatin A. kcat is 0.05 min(-1) with equine myoglobin as substrate and 0.0646 min(-1) with bovine hemoglobin as substrate. Optimum pH is 2-4. Stable from pH 2 to pH 6. Optimum temperature is 60 degrees Celsius. Monomer. Used in Japanese cuisine. During the fermentation process of Katsuobushi - dried, smoked and fermented meat of the skipjack tuna (bonito) Katsuwonus pelamis - aspartic protease plays a role in the decolorisation of Katsuobushi from a dark reddish-brown to pale pink by the hydrolysis of heme proteins. The change in color gives Katsuobushi a higher ranking and price. Belongs to the peptidase A1 family.