Structural basis for the specific activation of human enteropeptidase.

Human cationic trypsinogen is activated by human enteropeptidase much more readily than bovine trypsinogen, the ratios kcat/Km being 330 and 11 mM-1S-1, respectively. Conversely, porcine enteropeptidase activates bovine trypsinogen much more rapidly (kcat/Km = 630 mM-1S-1) than human cationic trypsinogen (kcat/Km = 2.4 mM-1S-1). The primary structure of the activation region of human cationic trypsinogen has been investigated in an attempt to elucidate the basis for these findings. The sequence of the first 12 residues at the NH2-terminus of human cationic trypsinogen has been shown to be Asp-Lys-Ile-Val-Gly-Gly-Tyr-Asn-Cys-Glu-Glu-Asn. Furthermore, the activation peptide derived from human cationic trypsinogen has been isolated and shown to be the dipeptide Asp-Lys. This result is in contrast to the Val-(Asp)4-Lys activation peptide from bovine trypsinogen and demonstrates that human cationic trypsinogen does not contain the (Asp)4 sequence present in many other mammalian trypsinogens. It is proposed that the high degree of specificity for activation of human cationic trypsinogen by human enteropeptidase is due to the preferential recognition of the novel activation peptide sequence in the human zymogen. Thus, these two functionally related proteins, cationic trypsinogen and enteropeptidase, may have evolved in a parallel manner in the human lineage.