Opening Image: Porcine pepsinogen, shown here in spacefill.
The N-terminal half of the propeptide is locked in an extended conformation that lies in a groove in the pepsin moiety. This groove leads to the much wider and deeper active site cleft. Here the C-terminal half of the propeptide becomes more globular, giving the propeptide a comma-like profile.between backbone trace and CPK spacefill. The propeptide comprises residues 1p-44p of the zymogen.
Initially some pepsinogen is activated slowly by H+. The pepsin formed can then quickly activate other pepsinogen molecules by cleaving the peptide bond between Leu-44p and Ile-1 (the N-terminal residue of pepsin).the cleaving site.