Covalent modification of enzymes

Covalent modification of enzymes (molecular weight of several hundreds or thousands) by the incorporation of inorganic phosphate in the form of P03 (formula weight = 85), seems to represent a small chemical change in the enzyme yet is an important control mechanism of enzyme activity. Explain how phosphorylation can exert its controlling effect on the activity of the enzyme. 

Metabolism is tightly regulated by a number of mechanisms feedback inhibition, compartmentalization, covalent modification of enzymes (e.g., phosphorylation), and hormone action, among others. 

In Chapters 13 through 22 we have discussed metabolism at the level of the individual cell, emphasizing central pathways common to almost all cells, prokaryotic and eukaryotic. We have seen how metabolic processes within cells are regulated at the level of individual enzyme reactions, by substrate availability, by allosteric mechanisms, and by phosphorylation or other covalent modifications of enzymes. 

The flow of intermediates through metabolic pathways is controlled by 1bir mechanisms 1) the availability of substrates 2) allosteric activation and inhibition of enzymes 3) covalent modification of enzymes and 4) induction-repression of enzyme synthesis. This scheme may at first seem unnecessarily redundant however, each mechanism operates on a different timescale (Figure 24.1), and allows the body to adapt to a wde variety of physiologic situations. In the fed state, these regulatory mechanisms ensure that available nutrients are captured as glycogen, triacylglycerol, and protein. 

Covalent modifications of enzymes allow a cell to regulate its metabolic activities more rapidly and in much more intricate ways than is possible by changing the absolute concentrations of the same enzymes. They still do not provide truly instantaneous responses to changes in conditions, however, because each modification requires the action of... 

There are two general types of covalent modification of enzymes that regulate their activity. These are the irreversible activation of inactive enzyme precursors, the zymogens, and the reversible interconversion of active and inactive forms of an enzyme. 

Figure 5.15 Role of cAMP in covalent modification of enzyme activity.

Enzyme inhibitors are broadly classified as irreversible and reversible. Inhibitors of the first class usually cause an inactivating, covalent modification of enzyme structure. Cyanide is a classic example of an irreversible enzyme inhibitor. The kinetic effect of irreversible inhibitors is to decrease the concentration of active enzyme. Irreversible inhibitors are usually considered to be poisons and are generally unsuitable for therapeutic purposes. 

Some enzymes are regulated by the reversible interconversion between their active and inactive forms. Several covalent modifications of enzyme structure cause these changes in function. Many such enzymes have specific residues that may be phosphorylated and dephosphorylated. For example, glycogen phosphorylase... 

The reversible covalent modification of enzymes is important in control of metabolism, cell growth and division, response to hormones, and other processes. Examples of the types of reversible side-chain modifications found in cells include ... 

Some effects of insulin occur within seconds or minutes, including the activation of glucose and ion transport systems, the covalent modification of enzymes (i.e., phosphorylation or dephosphorylation), and some effects on gene transcription (i.e., inhibition of the phosphoenolpyruvate carboxykinase gene). Effects on protein synthesis and gene transcription require hours, while those on cell proliferation and differentiation may take days. 

Shulman, R. G., and D. L. Rothman. Enzymatic Phosphorylation of Muscle Glycogen Synthase A Mechanism for Maintenance of Metabolic Homeostasis. Proc. Nat. Acad. Sci. 93, 7491-7495 (1996). [An in-depth article about metabolic flux and covalent modification of enzymes.]... 

Finally, molecular conversion, namely, reversible covalent modification of enzymes, is another method of enzyme control. The best example of this is enzyme phosphorylation and dephosphorylation that occur in the control of glycogen synthesis and degradation (Chap. 11). 

Catalytic activity The regulation of enzyme activities is achieved in two modes, namely switching on-and-off and tuning up-and-down. Covalent modifications of enzymes effectively switches their activities on or off, whereas noncovalent effectors tune enzyme activities up or down by affecting their kinetic parameters. 

The overall control of lipogenesis has been reviewed by Saggerson (1980). So far as saturated fatty acid synthesis is concerned the activity of both acetyl-CoA carboxylase and fatty acid synthetase can be altered in various ways (Section 11.1.1). Short term or acute control involves metabolic or allosteric regulation and the covalent modification of enzymes. Long-term control involves alterations in the amounts of enzyme protein (Wakil et al, 1983). 

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