Subunit III of ruminant procarboxypeptidase A‐S6 complexes and pancreatic proteases E a new family of pancreatic serine endopeptidases?

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Abstract
Subunit III (BSIII) of the bovine ternary complex of procarboxypeptidase A‐S6 (PCPA‐S6), a defective serine endopeptidase‐like protein, actively synthesized by the pancreas of some ruminant species, is highly homologous to human protease E (HPE). Both proteins possess the same atypical disulfide bridge in position 98–99b. They are structurally related to porcine elastase 1 and human elastase 2 (about 56% identity). However, in contrast to those two enzymes which have an overall positive net charge, BSIII and HPE are negatively charged. Three‐dimensional models of BSIII and HPE have been constructed from the crystallographic structure of porcine pancreatic elastase 1. The inhibitor‐binding site for TFAI in these three proteins seems to be very similar; the atypical disulfide bridge does not seem to be involved in this binding site. The specific structural features of BSIII and HPE strongly support the assumption that BSIII is a truncated protease E and that both proteins belong to a separate serine endopeptidase family.