Protein enzyme-catalyzed hydrolytic modifications

Table IV. In Vitro Enzyme-Catalyzed Hydrolytic Modifications of Food Proteins...

For the purpose of this chapter, enzyme-catalyzed modifications of proteins will be divided into two groups hydrolytic and nonhydrolytic reactions. Generally speaking, post-translational reactions occurring in vivo are catalyzed by highly specific enzymes under rather restricted conditions in contrast with in vitro modifications which are carried out under less specific conditions. 

The hydrolytic modifications of proteins catalyzed by enzymes include both generalized reactions, where a relatively large number of peptide bonds are split, and limited reactions where hydrolysis of one or only a few bonds are necessary in order to achieve the desired product (see Table III). Examples of both types of reactions will be presented below. 

In mammalian cells, the two most common forms of covalent modification are partial proteolysis and ph osphorylation. Because cells lack the ability to reunite the two portions of a protein produced by hydrolysis of a peptide bond, proteolysis constitutes an irreversible modification. By contrast, phosphorylation is a reversible modification process. The phosphorylation of proteins on seryl, threonyl, or tyrosyl residues, catalyzed by protein kinases, is thermodynamically spontaneous. Equally spontaneous is the hydrolytic removal of these phosphoryl groups by enzymes called protein phosphatases. 

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