Structural characterization of porcine enteropeptidase

Enteropeptidase (EC 3.4.21.9) is a key enzyme in the intestinal digestion cascade responsible for the conversion of trypsinogen to trypsin, which then activates various pancreatic zymogens. In order to structurally characterize the enzyme, we purified the enzyme from porcine duodenal mucosa and showed that it consists of three polypeptide chains, which we named 'mini' chain (M chain), light chain (L chain), and heavy chain (H chain) in order of increasing molecular size. Based on their NH₂-terminal sequences, a cDNA clone for porcine enteropeptidase was isolated and analyzed. The clone was 3597 base pairs long, which encoded 1034 amino acid residues of a single- chain precursor form of enteropeptidase. The precursor contained an additional NH₂-terminal 51-residue sequence including a putative internal signal sequence, followed by the M chain (66 residues), the H chain (682 residues), and the L chain (235 residues) in that order. The H chain had regions partially homologous in sequence with low density lipoprotein receptor and complement components. On the other hand, the L chain was highly homologous with the catalytic domains of trypsin-like serine proteinases. The structural model of the L chain suggests that the sequence, Arg⁸⁸⁵-Arg- Arg-Lys⁸⁸⁸, is probably involved in the unique substrate specificity of the enzyme, preferring acidic amino acid residues at the P₂-P₅ sites.