Cysteine protease. Displays a high level of diversity in substrate specificity at the P1-P1' cleavage site. A hydrophilic P1 residue is preferred, with Gln or Arg strongly preferred. Favors an Ile/Leu residue at the P2 position of substrates, with an overall higher preference for Leu. The optimal tripeptide for cleavage is Pro-Leu-Gln, with cleavage occurring after the Gln residue. Another optimal tripeptide is Val-Leu-Arg, which may imply that a hydrophobic residue at the P3 position of substrates is preferred. Hydrolysis of proteins with broad specificity for peptide bonds. Best reported small molecule substrate Bz-Phe-Val-Arg-|-NHMec, but broader specificity than fruit bromelain. Strongly inhibited by chicken egg-white cystatin (Probable). Inhibited by iodoacetamide and the active-site-directed inhibitor trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane (E-64) (PubMed:31306685). kcat is 13.9 sec(-1) for Z-Phe-Val-Arg-AMC. kcat is 1.58 sec(-1) for Z-Phe-Arg-AMC. kcat is 0.16 sec(-1) for Z-Arg-Arg-AMC. kcat is 9.80 sec(-1) for Pro-Leu-Gln. kcat is 7.13 sec(-1) for Val-Leu-Arg. kcat is 5.62 sec(-1) for Pro-Leu-Arg. kcat is 4.96 sec(-1) for Ala-Leu-Arg. kcat is 6.29 sec(-1) for Val-Leu-Lys. kcat is 6.08 sec(-1) for Pro-Leu-Lys. kcat is 3.58 sec(-1) for Ala-Leu-Lys. kcat is 0.36 sec(-1) for Pro-Leu-Asn. Stem (at protein level). Belongs to the peptidase C1 family.