The three-dimensional structure of human procarboxypeptidase A2. Deciphering the basis of the inhibition, activation and intrinsic activity of the zymogen

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Abstract
The three‐dimensional structure of human procarboxypeptidase A2 has been determined using X‐ray crystallography at 1.8 Å resolution. This is the first detailed structural report of a human pancreatic carboxypeptidase and of its zymogen. Human procarboxypeptidase A2 is formed by a pro‐segment of 96 residues, which inhibits the enzyme, and a carboxypeptidase moiety of 305 residues. The pro‐enzyme maintains the general fold when compared with other non‐human counterparts. The globular part of the pro‐segment docks into the enzyme moiety and shields the S2‐S4 substrate binding sites, promoting inhibition. Interestingly, important differences are found in the pro‐segment which allow the identification of the structural determinants of the diverse activation behaviours of procarboxypeptidases A1, B and A2, particularly of the latter. The benzylsuccinic inhibitor is able to diffuse into the active site of procarboxypeptidase A2 in the crystals. The structure of the zymogen‐inhibitor complex has been solved at 2.2 Å resolution. The inhibitor enters the active site through a channel formed at the interface between the pro‐segment and the enzyme regions and interacts with important elements of the active site. The derived structural features explain the intrinsic activity of A1/A2 pro‐enzymes for small substrates.