Enzyme regulation catalytic activity

Receptor-effector mechanisms include (1) enzymes with catalytic activities, (2) ion channels that gate the transmembrane flux of ions (ionotropic receptors), (3) G protein-coupled receptors that activate intracellular messengers (metabotropic receptors), and (4) cytosolic receptors that regulate gene transcription. Cytosolic receptors are a specific mechanism of many steroid and thyroid hormones. The ionotropic and metabotropic receptors are discussed in relevance to specific neurotransmitters in chapter 2. 

Allosteric enzymes constitute an important class of enzymes whose catalytic activity can be regulated. These enzymes, which do not conform to Michaelis-Menton kinetics, have multiple active sites. These active sites display cooperativity, as evidenced by a sigmoidal depen-dence of reaction velocity on substrate concentration. 

Regulation of ribonucleotide reductase is complex. The binding of dATP (deoxyadenosine triphosphate) to a regulatory site on the enzyme decreases catalytic activity. The binding of deoxyribonucleoside triphosphates to several other enzyme sites alters substrate specificity so that there are differential increases in the concentrations of each of the deoxyribonucleotides. This latter process balances the production of the 2 -deoxyribonucleotides required for cellular processes, especially that of DNA synthesis. 

Protein kinase A provides a means for hormones to control metabolic pathways. Adrenaline and many other hormones increase the intracellular concentration of the allosteric regulator 3, 5 -cychc AMP (cAMP), which is referred to as a hormonal second messenger (Fig. 9.10). cAMP binds to regulatory subunits of protein kinase A, which dissociate and release the activated catalytic subunits (Fig. 9.11). Dissociation of inhibitory regulatory subunits is a common theme in enzyme regulation. The active catalytic subunits phosphorylate glycogen phosphorylase and other enzymes at serine residues. 

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. 

Protein phosphorylation-dephosphorylation is a highly versatile and selective process. Not all proteins are subject to phosphorylation, and of the many hydroxyl groups on a protein s surface, only one or a small subset are targeted. While the most common enzyme function affected is the protein s catalytic efficiency, phosphorylation can also alter the affinity for substrates, location within the cell, or responsiveness to regulation by allosteric ligands. Phosphorylation can increase an enzyme s catalytic efficiency, converting it to its active form in one protein, while phosphorylation of another converts it into an intrinsically inefficient, or inactive, form (Table 9—1). 

Enzymes require an optimum pH at which their catalytic activity is maximal. The pH-activity profiles of enzymes indicate the pH at which the catalytic sites are in their necessary state of ionization. The optimal pH of an enzyme may be different from that of its normal environment. The action of enzymes in cells may be regulated by variation in the pH of the surrounding medium. 

The regulation of phosphorylation of tyrosine hydroxylase is affected by stimuli that increase Ca2+ or cAMP concentrations in neurons, including nerve impulse conduction and certain neurotransmitters in well-defined regions of the nervous system, in the adrenal medulla and in cultured pheochromocytoma cells. In addition, tyrosine hydroxylase phosphorylation is stimulated by nerve growth factor in certain cell types, possibly via the activation of ERKs. These changes in the phosphorylation of tyrosine hydroxylase have been shown to correlate with changes in the catalytic activity of the enzyme and in the rate of catecholamine biosynthesis. 

Several key questions remain with regard to the regulation of tyrosine hydroxylase by phosphorylation. What is the precise effect of the phosphorylation of each of these serine residues on the catalytic activity of the enzyme How does the phosphorylation of multiple residues affect enzyme activity Does the phosphorylation of one residue affect the ability of the others to be phosphorylated Tyrosine hydroxylase provides a striking example as to how multiple intracellular messengers and protein kinases converge functionally through the phosphorylation of a single substrate protein. Phosphorylation of tyrosine hydroxylase by cAMP-dependent and Ca2+-dependent protein kinases and by MAPK cascades... 

The catalytic activity of cPLA2 is stimulated by phosphorylation catalyzed by the mitogen-activated protein kinase (MAPK) at Ser505. This modification stimulates enzyme activity only, indicating that translocation and phosphorylation are independent mechanisms of cPLA2 regulation [21]. 

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