Activation of zymogens

Several enzymes are synthesized as larger inactive precursor forms called proenzymes or zymogens. Activation of zymogens involves irreversible hydrolysis of one or more peptide bonds. 

Chymotrypsinogen and chymotrypsin Activation of zymogen —> Tyr A-spectrum autolysis or urea treatment of enzyme —> Try A-speotrum Chervenka (1959)... 

Beyond translation, proteins and peptides may be further modified by metabolic events within the cell. For instance, the hydroxyproline and hydroxylysine in collagen are formed from proltne and lysine that are hydroxyl-ated while they are part of the precursor protein molecule. A protein may also be post-translationally modified by cleavage of select bonds and/or by addition or subtraction of various kinds of groups. Insulin, for instance, is formed cleavage of a larger protnsultn molecule. Similar events occur in the activation of zymogens. 

The activation of zymogens involves the cleavage of one or more peptide bonds. In the case of pepsinogen, when the catalytic site is exposed by lowering the pH, it hydrolyzes the peptide bond between the percursor and pepsin moities. Note that this activation is autocatalytic. Therefore, the time required for activation of half the pepsinogen molecules is independent of the total number of the molecules present. 

Activation of zymogens and the production of new enzymes through induction... 

Exposure of active center by removal of peptides. Activation of zymogen by limited proteolysis... 

In limited P. only certain peptide bonds of a protein are hydrolysed this results in the production of biologically active (e.g. enzymes or hormones) or inactive (e.g. para-x-casein) proteins or peptides. Limited P. occurs in digestion, blood coagulation and milk clotting it is responsible for the activation of zymogens and for the release of certain peptide hormones, e.g. insulin, angiotensin, vasopressin, oxytocin and various kinins (see table). 

A. Meister, Biochemistry of Amino Acids, 2nd ed., 2 Vols., Academic Press, New York, 1965. H. Neurath, The activation of zymogens. Advances in Protein Chem., 12, 319-368 (1957). 

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