Mesotrypsin Has Evolved Four Unique Residues to Cleave Trypsin Inhibitors as Substrates.

Human mesotrypsin is highly homologous to other mammalian trypsins, and yet it is functionally unique in possessing resistance to inhibition by canonical serine protease inhibitors and in cleaving these inhibitors as preferred substrates. Arg-193 and Ser-39 have been identified as contributors to the inhibitor resistance and cleavage capability of mesotrypsin, but it is not known whether these residues fully account for the unusual properties of mesotrypsin. Here, we use human cationic trypsin as a template for engineering a gain of catalytic function, assessing mutants containing mesotrypsin-like mutations for resistance to inhibition by bovine pancreatic trypsin inhibitor (BPTI) and amyloid precursor protein Kunitz protease inhibitor (APPI), and for the ability to hydrolyze these inhibitors as substrates.