Enzyme regulation covalent modification

Two mechanisms that are commonly employed in altering enzyme activity are covalent modification and allosteric regulation. Covalent modification is an enzymatically catalyzed reaction that involves the reversible formation of a covalent bond between a small molecule and a specific amino acid side chain(s) on an enzyme that affects its activity. Allosteric regulation of an enzyme s activity involves noncovalent binding of a small molecule at a site other than the active site that alters the enzyme s activity. Unlike the limited examples of covalent modification that have been discovered (see Table 15-1), a wide variety of small molecules have been found to regulate the activity of particular enzymes allosterically. 

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. 

See also Covalent Modifications to Regulate Enzyme Activity, Covalent Modification of Proteins... 

Covalent modHication of enzymes, enzyme modulation, enzyme interconversion Oligomeric (i.e. multichain) enzymes may exist in two or more forms, which are interconvertible by enzyme-catalysed covalent modifications, and which differ in their catalytic properties, e.g. activity, substrate affinity and dependence on effectors. Usually the difference in activity is such that one form is active and the other inactive. The activities of the conversion enzymes are in turn regulated by other enzymes, metabolites and/ or effectors. Covalent modifications are therefore important in physiological regulation, in addition to Allostery (see). Whereas allostery provides fine adjustment of metablic rates, C provides an on/off switching of cellular functions, which is very sensitive to environmental influences. 

FIGURE 15.2 Enzymes regulated by covalent modification are called interconvertible enzymes. The enzymes protein kinase and protein phosphatase, in the example shown here) catalyzing the conversion of the interconvertible enzyme between its two forms are called converter enzymes. In this example, the free enzyme form is catalytically active, whereas the phosphoryl-enzyme form represents an inactive state. The —OH on the interconvertible enzyme represents an —OH group on a specific amino acid side chain in the protein (for example, a particular Ser residue) capable of accepting the phosphoryl group. 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

REVERSIBLE COVALENT MODIFICATION REGULATES KEY MAMMALIAN ENZYMES... 

Figure 9-7. Covalent modification of a regulated enzyme by phosphorylation-dephosphorylation of a seryl residue.

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. 

One of the most-studied covalent modifications is the acetylation of the lysine residues of histone tails. The acetylation state of lysines of nucleosomal histones modulates chromatin structure and regulates gene transcriptional activity. The balance of lysine acetylation is controlled by the antagonistic action of two enzyme families histone deacetylases (HDACs) and histone acetyltransferases (HATs). In humans there are essentially three main HDAC subclasses [6]. 

Allosteric regulators bind to the target enzyme in a non-covalent manner. An entirely different enzyme control mechanism is covalent modification. Here, the conformation of the enzyme protein, and thereby its activity, is changed by the... 

The change in the conformation of the control enzyme brought about by covalent modification alters the activity of the control enzyme and so regulates substrate flux through that step. This fact underlines the importance of the three-dimensional structure of an enzyme. The inclusion of phosphates may bring about quite a small architectural change to the protein structure but it is sufficient to affect substrate binding and therefore enzyme activity. 

Control of pymvate dehydrogenase activity is via covalent modification a specific kinase causes inactivation of the PDH by phosphorylation of three serine residues located in the pyruvate decarboxylase/dehydrogenase component whilst a phosphatase activates PDH by removing the phosphates. The kinase and phosphatase enzymes are non-covalently associated with the transacetylase unit of the complex. Here again we have an example of simultaneous but opposite control of enzyme activity, that is, reciprocal regulation. 

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 topic of interconversion cycles in providing inhibition or activation of a target enzyme, the activity of which regulates the flux through a pathway, is discussed above. In brief, an enzyme exists in two forms, conventionally designated a and b, one being a covalent modification of the other. This is brought about, for example, by phosphorylation with ATP, so that one form is a phos-... 

Figure 20.31 The principle of interconversion cycles in regulation of protein activity or changes in protein concentration as exemplified by translation/proteolysis or protein kinase/protein phosphatase. They result in very marked relative changes in regulator concentration or enzyme activity. The significance of the relative changes (or sensitivity in regulation) is discussed in Chapter 3. The principle of regulation by covalent modihcation is also described in Chapter 3. The modifications in cyclin concentration are achieved via translation and proteolysis, which, in effect, is an interconversion cycle. For the enzyme, they are achieved via phosphorylation and dephosphorylation reactions. In both cases, the relative change in concentration/activity by the covalent modification is enormous. This ensures, for example, that a sufficient increase in cyclin can occur so that an inactive cell cycle kinase can be converted to an active cell cycle kinase, or that a cell cycle kinase can be completely inactivated. Appreciation of the common principles in biochemistry helps in the understanding of what otherwise can appear to be complex phenomena.

---