The amino-terminal sequence of the catalytic subunit of bovine enterokinase

Bovine enterokinase (Enteropeptidase) is a serine protease and functions as the physiological activator of trypsinogen. The Enzyme has a heavy chain (115 kD) covalently linked to a LIGHT or catalytic subunit (35 kD). The amino acid composition showed that the LIGHT chain has nine half-cystine residues (four as intramolecular disulfides) and that one half-cystine was in a disulfide link between the LIGHT and heavy subunits. The amino-terminal 27 residues of the S-vinylpyridyl derivative of the LIGHT chain were determined by gas-phase Edman degradation. The sequence has homologies with other serine proteases containing one or two chains. The homologies suggest that the catalytic subunit has the same three-dimensional structure and, therefore, the same mechanism of enzymatic action as pancreatic chymotrypsin, trypsin, and Elastase. The presence of the conserved amino-terminal activation peptide sequence (IVGG) shows that enterokinase must have a zymogen precursor and that the two-chain Enzyme arises from limited proteolysis during posttranslational processing.