Metabolic regulation covalent enzyme modification

Covalent modification is a major mechanism for the rapid and transient regulation of enzyme activity. Numerous enzymes of intermediary metabolism are affected by phosphorylation, either positively or negatively. Covalent phosphorylations can be reversed by a separate subclass of enzymes known as phosphatases. The aberrant phosphorylation of growth factor and hormone receptors, as well as of proteins that regulate cell division, often leads to unregulated cell growth or cancer. The usual sites for phosphate addition to proteins are the serine, threonine and tyrosine R-group hydroxyl residues. 

Enzyme activity can be regulated by covalent modification or by noncovalent (allosteric) modification. A few enzymes can undergo both forms of modification (e.g., glycogen phosphorylase and glutamine synthetase). Some covalent chemical modifications are phosphorylation and dephosphorylation, acetylation and deacetylation, adeny-lylation and deadenylylation, uridylylation and deuridyly-lation, and methylation and demethylation. In mammalian systems, phosphorylation and dephosphorylation are most commonly used as means of metabolic control. Phosphorylation is catalyzed by protein kinases and occurs at specific seryl (or threonyl) residues and occasionally at tyrosyl residues these amino acid residues are not usually part of the catalytic site of the enzyme. Dephosphorylation is accomplished by phosphoprotein phosphatases ... 

Metabolic pathways are regulated by rapid mechanisms affecting the activity of existing enzymes, eg, allosteric and covalent modification (often in response to hormone action) and slow mechanisms affecting the synthesis of enzymes. 

The principal enzymes controlling glycogen metabolism—glycogen phosphorylase and glycogen synthase— are regulated by allosteric mechanisms and covalent modifications due to reversible phosphorylation and... 

Ghanges in the availability of substrates are responsible for most changes in metabolism either directly or indirectly acting via changes in hormone secretion. Three mechanisms are responsible for regulating the activity of enzymes in carbohydrate metabolism (1) changes in the rate of enzyme synthesis, (2) covalent modification by reversible phosphorylation, and (3) allosteric effects. 

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

The role of GSH in cellular protection (see below) means that if depleted of GSH, the cell is more vulnerable to toxic compounds. However, GSH is compartmentalized, and this compartmentalization exerts an influence on the relationship between GSH depletion or oxidation and injury. The loss of reduced GSH from the cell leaves other thiol groups, such as those in critical proteins, vulnerable to attack with subsequent oxidation, cross-linking, and formation of mixed disulfides or covalent adducts. The sulfydryl groups of proteins seem to be the most susceptible nucleophilic targets for attack, as shown by studies with paracetamol (see chap. 7), and are often crucial to the function of enzymes. Consequently, modification of thiol groups of enzyme proteins, such as by mercury and other heavy metals, often leads to inhibition of the enzyme function. Such enzymes may have critical endogenous roles such as the regulation of ion concentrations, active transport, or mitochondrial metabolism. There is... 

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 regulation of the reaction velocity of enzymes is essential if an organ ism is to coordinate its numerous metabolic processes. The rates of most enzymes are responsive to changes in substrate concentration, because the intracellular level of many substrates is in the range of the Km. Thus, an increase in substrate concentration prompts an increase in reaction rate, which tends to return the concentration of substrate toward normal. In addition, some enzymes with specialized regulatory functions respond to allosteric effectors or covalent modification, or they show altered rates of enzyme synthesis when physiologic conditions are changed. 

Phosphorylation also can modify an enzyme s sensi-tivity to allosteric effectors. Phosphorylation of glycogen phosphorylase reduces its sensitivity to the allosteric activator adenosine monophosphate (AMP). Thus, a covalent modification triggered by an extracellular signal can override the influence of intracellular allosteric regulators. In other cases, variations in the concentrations of intracellular effectors can modify the response to the covalent modification, depending on the metabolic state of affairs in the cell. 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

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... 

Cells regulate their metabolic activities by controlling rates of enzyme synthesis and degradation and by adjusting the activities of specific enzymes. Enzyme activities vary in response to changes in pH, temperature, and the concentrations of substrates or products, but also can be controlled by covalent modifications of the protein or by interactions with activators or inhibitors. 

The most common way of regulating metabolic activity is by direct control of enzyme activity. Enzyme activities are usually regulated by noncovalent interaction with small-molecule regulatory factors (see chapter 9) or by a reversible covalent modification, such as phosphorylation or... 

Cohen, P., Control of Enzyme Activity, 2d ed. London and New York Chapman and Hall, 1983. Brief discussion of some types of regulation of activity of metabolic enzymes, emphasizing regulation by covalent modification of the enzymes. 

Understand enzyme regulation via cellular enzyme levels, compartmentation, metabolic pathway regulation, and covalent modifications. 

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