Enzymes reversible covalent modification

REVERSIBLE COVALENT MODIFICATION REGULATES KEY MAMMALIAN ENZYMES... 

Note that in some cases one may follow the time course of covalent E-A formation by equilibrium binding methods (e.g., LC/MS, HPLC, NMR, radioligand incorporation, or spectroscopic methods) rather than by activity measurements. In these cases substrate should also be able to protect the enzyme from inactivation according to Equation (8.7). Likewise a reversible competitive inhibitor should protect the enzyme from covalent modification by a mechanism-based inactivator. In this case the terms. S and Ku in Equation (8.7) would be replaced by [7r] and K respectively, where these terms refer to the concentration and dissociation constant for the reversible inhibitor. 

Some Regulatory Enzymes Undergo Reversible Covalent Modification... 

Rapid alteration of the activities of enzymes is often accomplished by reversible covalent modification.39 47... 

Phosphorylation, Adenylylation, and Disulfide Reduction Lead to Reversible Covalent Modifications Allosteric Regulation Allows an Enzyme to Be... 

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-... 

Partial proteolysis, an irreversible process, is used to activate proteases and other digestive enzymes after their secretion and to switch on enzymes that cause blood coagulation. Common types of reversible covalent modification include phosphorylation, adenylyla-tion, and disulfide reduction. 

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... 

Other types of reversible covalent modification that are used to regulate the activity of certain enzymes include adenylylation (the transfer of adenylate from ATP) and ADP-ribosylation [the transfer of an adenosine diphosphate (ADP)-ribosyl moiety from NAD ]. 

There are two major ways of control. One mechanism involves reversible covalent modifications, such as phosphorylation dephosphorylation, the other requires conformational transitions by binding an allosteric ligand or regulator protein. It follows an example of regulation of an enzyme, of which the activity is subject to control by both mechanisms, then we compare the regulation of an enzyme with regulation of components of cellular signalling pathways, of which many have no enzymic activity. 

Posttranslational modifications are enzyme-catalyzed covalent modifications of a mature protein after it has been synthesized. Examples of posttranslational modifications are phosphorylation, glycosylation, sulfation, methylation and prenylation. Espedally those modifications that are reversible, such as phosphorylation by de-phosphorylation through the action of phosphatases are important in regulation. 

Reversible covalent modification. The catalytic properties of many enzymes are markedly altered by the covalent attachment of a modifying group, most commonly a phosphoryl group. ATP serves as the phosphoryl donor in these reactions, which are catalyzed by protein kinases. The removal of phosphoryl groups by hydrolysis is catalyzed by protein phosphatases. This chapter considers the structure, specificity, and control of protein kinase A (PKA), a ubiquitous eukaryotic enzyme that regulates diverse target proteins. 

Another noteworthy difference is that proteolytic activation, in contrast with allosteric control and reversible covalent modification, occurs just once in the life of an enzyme molecule. 

Covalent modification of proteins is a potent means of controlling the activity of enzymes and other proteins. Phosphorylation is the most common type of reversible covalent modification. Signals can be highly amplified by phosphorylation because a single kinase can act on many target molecules. The regulatory actions of protein kinases are reversed by protein phosphatases, which catalyze the hydrolysis of attached phosphoryl groups. 

The catalytic activity of enzymes is controlled in several ways. Reversible allosteric control is especially important. For example, the first reaction in many biosynthetic pathways is allosterically inhibited by the ultimate product of the pathway. The inhibition of aspartate trans carbamoyl as e by cytidine triphosphate (Section 10.1) is a well-understood example offeedback inhibition. This type of control can be almost instantaneous. Another recurring mechanism is reversible covalent modification. For example, glycogen phosphorylase, the enzyme catalyzing the breakdovm of glycogen, a storage form of sugar, is activated by phosphorylation of a particular serine residue when glucose is scarce (Section 21.2.1). 

The activity of glutamine synthetase is also controlled by reversible covalent modification —the attachment of an AMP unit by a phosphodiester bond to the hydroxyl group of a specific tyrosine residue in each subunit (Figure 24.26). This adenylylated enzyme is less active and more susceptible to cumulative feedback inhibition than is the deadenylylated form. The covalently attached AMP unit is removed from the adenylylated enzyme by phosphorolysis. The attachment of an AMP unit is the final step in an enzymatic cascade that is initiated several steps back by reactants and immediate products in glutamine synthesis. 

Rapid alteration of fhe activities of enzymes is offen accomplished by reversible covalent modification. ... 

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 ... 

We turn now to a different mechanism of enzyme regulation. Many enzymes acquire full enzymatic activity as they spontaneously fold into their characteristic three-dimensional forms. In contrast, other enzymes are synthesized as inactive precursors that are subsequently activated by cleavage of one or a few specific peptide bonds. The inactive precursor is called a zymogen (or a proenzyme). A energy source (ATP) is not needed for cleavage. Therefore, in contrast with reversible regulation by phosphorylation, even proteins located outside cells can be activated by this means. Another noteworthy difference is that proteolytic activation, in contrast with allosteric control and reversible covalent modification, occurs just once in the life of an enzyme molecule. 

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). 

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