The Activation of Zymogens

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. 

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). 

Trypsin is responsible for the activation of chymotrypsin, as well as for a range of other digestive enzymes synthesized in the pancreas. Trypsin itself is formed from its zymogen via digestion by the enzyme enteropeptidase. Enteropeptidase is secreted from intestinal cells and cleaves trypsinogen to trypsin as soon it travels from the pancreas to the intestine. 

The activation of pepsin from its zymogen pepsinogen occurs by a different mechanism. In this case, the pH of the environment plays a decisive role. In the strongly acidic milieu of the stomach cleavage of a 44 amino acid peptide occurs from the inactive precursor pepsinogen. The activation is intramolecular and depends on the pH of the solution. 

Nerve growth factor is commonly found in snake venoms. This is a protease with specificity for lysyl and arginyl bonds, which contains one Zn2+ per molecule. The function of the zinc is to prevent the activation of NGF, presumably through control of the structure. Removal of Zn11 from NGF with EDTA leads to activation of the zymogen. 

Interactions between serine proteases are common, and substrates of serine proteases are usually other serine proteases that are activated from an inactive precursor [66]. The involvement of serine proteases in cascade pathways is well documented. One important example is the blood coagulation cascade. Blood clots are formed by a series of zymogen activations. In this enzymatic cascade, the activated form of one factor catalyzes the activation of the next factor. Very small amounts of the initial factors are sufficient to trigger the cascade because of the catalytic nature of the process. These numerous steps yield a large amplification, thus ensuring a rapid and amplified response to trauma. A similar mechanism is involved in the dissolution of blood clots. A third important example of the coordinated action of serine proteases is the intestinal digestive enzymes. The apoptosis pathway is another important example of coordinated action of other types of proteases. 

Khan AR, James MN. Molecular mechanisms for the conversion of zymogens to active proteolytic enzymes. Protein Sci 1998 7 815-836. 

Figure 4. The role of enterokinase in the activation of trypsinogen and subsequent activation of other protease zymogens by trypsin, a two-stage...

Proteolytic conversion of inactive zymogens to active enzymes was noted in Chapter 5. Other examples of posttranslational proteolytic cleavages include the conversions of proalbumin to albumin (Chapter 7), of preprocollagen to collagen (Chapter 8), and of preproinsulin to insulin (Chapter 16) and the activation of the compounds of the blood-clotting cascade (Chapter 7). 

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