Porcine pepsin zymogen

Pepsin (EC 3.4.23.1) is a typical aspartic proteinase produced in the gastric mucosa of vertebrates as a zymogen form [10], This enzyme has been extensively characterized, and its three-dimensional structure has been determined at high resolution. Porcine pepsin, in particular, has been studied as model to analyze the structure-function relationship of the aspartic proteinases. Although the aspartic proteinases including mammalian and fungal enzymes are quite similar in their three-dimentional structures, there are drastic differences in the catalytic properties, especially in substrate specificities. 

Having considered these difficulties, we have concluded that for the present purpose it is most rational to number the residues from the NH2-terminus of the longest known polypeptide chain of the gastric zymogens (prochymosin) and then continue by counting the longest completely sequenced chain of the active enzymes (porcine pepsin). 

By incubation of porcine, bovine, canine, or chicken pepsinogens and calf prochymosin with pepstatin at pH 2.5, the first active protein generated on activation is trapped in an inactive complex. The first activation peptide liberated from porcine pepsinogen has been identified as residues 1-16 whereas that from prochymosin is derived from residues 1-27. This suggests that pepsin and chymosin are not formed by one-step conversions from their zymogens, but by (different) sequential, activation mechanisms. 

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