Covalent modifications to regulate enzyme activity

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

See also a-Helix, / -Sheet, Factors Determining Secondary and Tertiary Structure, Thermodynamics of Protein Folding, Dynamics of Protein Folding, Covalent Modifications to Regulate Enzyme Activity (from Chapter 11). 

Other regulatory enzymes are modulated by covalent modification of a specific functional group necessary for activity. The phosphorylation of specific amino acid residues is a particularly common way to regulate enzyme activity. 

FIGURE 6-31 Regulation of glycogen phosphorylase activity by covalent modification. In the more active form of the enzyme, phosphorylase a, specific Ser residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active phosphorylase b by enzymatic loss of these phosphoryl groups, promoted by phosphorylase phosphatase. Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase kinase. 

Other groups that can be attached covalently to enzymes include fatty acids, isoprenoid alcohols such as far-nesol, and carbohydrates. Although such modifications are widespread, our understanding of how cells use them to regulate enzymatic activities is still fragmentary. 

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 ]. 

Short-term hormonal regulation of ACC is achieved by covalent modifications of the enzyme by phosphorylation or dephosphorylation, which either increase or decrease its activity. These changes in enzyme activity are observed within minutes of exposure to hormones and thus are not likely due to changes in the amount of enzyme (Kim, 1983). Lee and Kim (1979) reported that incubation of rat adipocytes with epinephrine doubled the incorporation of 32P into ACC within 30 minutes and reduced enzyme activity by 61%. Witters et al. (1979) established a similar relationship between phosphorylation and inactivation of rat hepatocyte ACC following glucagon treament. 

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. 

N is often limiting in the marine environment. Further, many enzymes are sensitive to cellular substrate concentrations rather than extracellular concentrations and it is difficult to measure the relevant intracellular metabohte pools. In vitro assays may affect the conformation of enzymes and the degree to which they are modified. For example, allosteric effects (see Section 1.3.3) may be modified under in vitro conditions. Many enzymes undergo posttranslational regulation wherein enzyme activity is affected by binding of activator/inactivator proteins and covalent modification of the enzyme (e.g., adenylylation, phosphorylation or carbamylation) (Ottaway, 1988). When there is posttranslational modification of enzymes, enzyme activity measured in assays may be unrelated to in vivo activity (see Section 2.2.1) and there are few ways to determine the extent of enzyme modification in nature. 

The body regulates enzyme activity in many ways. It changes the amount of enzyme protein present by changing the rate of synthesis or degradation. In other cases, a covalent modification of the enzyme protein will cause the activity to increase or decrease. Alternatively, other molecules may bind reversibly to the enzyme to change its activity. We will discuss examples of each type of enzyme regulation as they occur in the body. 

Enzymes function in assembly line-like fashion to catalyze the thousands of reactions occurring in cells each second. Coordinating and regulating enzymatic activities is essential for efficient functioning of cells. Several control mechanisms that do not involve covalent modification of the enzymes are possible ... 

However, in most cases, covalent modifications can be either activation or deactivation and are normally energy-dependent and reversible, but catalyzed by separate enzymes. These include phosphorylation/dephosphorylation, adenylylation (nucleotidyla-tion)/deadenylylation and ADP-ribosylation (Stadtman and Chock, 1978). This cyclic interconversion of key enzymes between covalently modified and unmodified forms is a mechanism of singular importance in cellular regulation. The interconversion of an enzyme between the active and inactive forms involving separate modification and demodification (i.e. different converter enzymes) is a dynamic process that may lead to a steady state in a cascade system. The covalent interconversion of regulatory enzymes is characterized by ... 

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... 

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

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. 

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