Peptide zymogen-catalyzed

Procarboxypeptidase A is activated by the removal of a peptide of some 64 residues from the N-terminus by trypsin.153 This zymogen has significant catalytic activity. As well as catalyzing the hydrolysis of small esters and peptides, procarboxypeptidase removes the C-terminal leucine from lysozyme only seven times more slowly than does carboxypeptidase. Also, the zymogen hydrolyzes Bz-Gly-L-Phe with kcsA = 3 s-1 and KM = 2.7 mM, compared with values of 120 s 1 and 1.9 mM for the reaction of the enzyme.154 In contrast to the situation in chymotrypsinogen, the binding site clearly pre-exists in procarboxypeptidase, and the catalytic apparatus must be nearly complete. 

Release of the zymogens into the lumen of the small intestine results in their exposure to a new environment. This environment is not acidic (unlike that of the stomach), and it contains enterokinase (also called enteropeptidase), a protease of the small intestine. Enterokinase is constitutively present (always present) in the small intestine. Studies with pig intestines revealed that enterokinase is present only in the duodenum, not the jejimum or ileum, and is bound to the outside of the enterocyte. Enterokinase catalyzes the cleavage of one specific peptide bond in tiypsinogen, resulting in its conversion to bypsin. The point of action is between one residue of lysine and one of isoleucine, as shown in Figure 2.4. 

The mechanism of action of a-chymotrypsin is probably understood in more detail than any other enzyme at the present time. Its physiological function is to catalyze the hydrolysis of peptide bonds of protein foods in the mammalian gut. It is secreted in the pancreas as an inactive zymogen precursor, chymotrypsinogen, having a single polypeptide chain of245 amino... 

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