DISCUSSIONThe activation of trypsin

DISCUSSION

The activation of trypsinogen is a key step in the activation of other digestive hydrolases within the lumen of the gut, and efficient catalysis of this reaction depends on enteropeptidase. Almost all vertebrate trypsinogens are activated by proteolytic cleavage of a Lys–Ile bond in an amino-terminal peptide that contains the sequence Asp-Asp-Asp-Asp-Lys-Ile (2). Molecular modeling of enteropeptidase suggests that specific basic residues on the surface of the catalytic subunit (light chain) interact directly with the acidic residues of trypsinogen activation peptides (25). Such interactions may account for the recognition of small peptide substrates, but probably are not sufficient to explain the recognition of trypsinogen. The isolated light chain has been prepared by partial reduction of purified bovine enteropeptidase (9) or by expression of recombinant light chain (10). Both preparations had normal activity toward small synthetic peptides, but had dramatically reduced activity toward
trypsinogen. Therefore, the recognition of small substrates requires only the light chain, whereas efficient cleavage of trypsinogen may also depend on the heavy chain. Similar selective defects in trypsinogen recognition were produced in two-chain enteropeptidase by heating (6, 11) or by acetylation (12). This behavior suggests that the catalytic center and at least one secondary substrate-binding site (exosite) cooperate to recognize trypsinogen, and exosites sensitive to these treatments
could be located on the heavy chain or the light chain.