Regulation of Enzyme Activities

Ca2+-pumps. After entering the cell, Ca2+ is reversibly complexed to specific Ca2+-binding proteins that fiilfil multiple functions, including Ca2+-buffering and transport, activation of enzymes, regulation of contraction,... 

A wide variety of compounds and treatments have been reported to activate microsomal glucose-6-phosphatase phosphotransferase. These are described briefly below the possible relevance of the action of a number of these factors in the biological regulation of activities of the enzyme has been considered in Section II,C,3. 

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. 

Ghorobekova (1987) showed the inhibitory effect of humic matter on protease activity. Inhibition kinetics are of mixed order, and humic acids can be used as a regulator of activity and biosynthesis of proteolytic enzymes. 

Ghorobekova, C. (1987). Humic acids as regulators of activity and biosynthesis of proteolytic enzymes. FECS Int. 3rd Conf. Chem. Biotechnol. Biol. Act. Natl. Prod., 1985 5, 427 130. 

Protein Encoded by genes, proteins are composed of amino acids arranged in precise sequences and joined by peptide linkages. Proteins can serve as enzymes, regulators of gene activity, transporters, hormones, or other catalytic and structural elements. 

At least two additional enzymes regulate the activity of the complex. 

Enzyme Changes in Zinc Deficiency. Since zinc is required for many enzymes, it is reasonable to speculate that the level of zinc in cells controls the physiological processes through the formation and/or regulation of activity of zinc-dependent enzymes. Until 1965, there was no evidence in the literature to support this concept. During the past decade it has been shown that the activity of various zinc-dependent enzymes was reduced in the testes, bones, esophagus, and kidneys of zinc-deficient rats in comparison with their pair-fed controls (90,91,100,101), These results correlated with the decreased zinc content in the above tissues... 

Our hypothesis regarding the mechanism for the pH-controlled regulation of LCF activity is that the histidine residues make contacts that stabilize the N-terminal domain and the helix-loop-helix. Disruption of such contacts by protonation in the wild-type, or in his to ala mutants at pH 8, causes the N-terminal domain and the helix-loop-helix to move and open the catalytic active site. This appears to be altogether novel in enzyme chemistry as a mechanism for regulation of activity. 

Many reactions in the body are broken down into steps, each of which is catalyzed by a specific enzyme. Regulation of such a metabolic pathway is achieved by feedback inhibition of the committing enzyme by the product of the pathway. Inhibition is by binding of the inhibitor to a site on the enzyme different from the active site. Typically, such enzymes obey cooperative substrate binding and the inhibition is referred to as allosteric inhibition. 

Molecular studies on various GDH s are now proceeding rapidly since elucidation of the primary structures of some of these enzymes. It is essential to note, however, that these enzymes vary not only in coenzyme specificity but also in other properties, e.g., induction and repression of synthesis by metabolites, regulation of activity by purine nucleoside di-and triphosphates, e.g., ADP, GDP, ATP, GTP, and other ligands, and in molecular properties. The successful isolation from some species of modified forms of the enzyme produced by mutant strains, particularly of Neurospora, now permits identification of residues important for maintenance of normal activity. 

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. 

Winter H, Huber SC. Regulation of sucrose metabolism in higher plants localization and regulation of activity of key enzymes. Crit Rev Biochem Mol 2000 35 253-289. 

Normally a fraction of the cellular enzyme complement is sufficient to maintain proper function and the regulation of activity is a function of substrate level. In the case of those subjects with partial deficiency of HPRT, it is possible to demonstrate the superiority of the intact cell assay as a measure of the true in vivo situation. In addition, these studies demonstrate the advantage of using an intact cell over the crude lysate in looking for an index of actual enzyme performance in tissues in general. 

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