Regulable enzymes

Metabolic regulation is achieved via regulating enzyme activity in three prominent ways allosteric regulation, covalent modi-... 

Citrate synthase is the first step in this metabolic pathway, and as stated the reaction has a large negative AG°. As might be expected, it is a highly regulated enzyme. NADH, a product of the TCA cycle, is an allosteric inhibitor of citrate synthase, as is succinyl-CoA, the product of the fifth step in the cycle (and an acetyl-CoA analog). 

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate (Eigure 23.7), like all phosphate ester hydrolyses, is a thermodynamically favorable (exergonic) reaction under standard-state conditions (AG° = —16.7 kj/mol). Under physiological conditions in the liver, the reaction is also exergonic (AG = —8.6 kJ/mol). Fructose-1,6-bisphosphatase is an allosterically regulated enzyme. Citrate stimulates bisphosphatase activity, hut fructose-2,6-bisphosphate is a potent allosteric inhibitor. / MP also inhibits the bisphosphatase the inhibition by / MP is enhanced by fructose-2,6-bisphosphate. 

A. Scliellenbergei, G. Hiibner, fill. Symp. Mol. Cell Regul. Enzyme Act.. Part 1, Martin-Luther-Univ., Hallc-Wittcnbcrg 1984. 

Figure 9-5. Multiple feedback inhibition in a branched biosynthetic pathway. Superimposed on simple feedback loops (dashed, curved arrows) are multiple feedback loops (solid, curved arrows) that regulate enzymes common to biosynthesis of several end products.

To refer to the kinetics of allosteric inhibition as competitive or noncompetitive with substrate carries misleading mechanistic implications. We refer instead to two classes of regulated enzymes K-series and V-se-ries enzymes. For K-series allosteric enzymes, the substrate saturation kinetics are competitive in the sense that is raised without an effect on V. For V-series allosteric enzymes, the allosteric inhibitor lowers... 

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

Synthetic nonhydrolyzable analogs of nucleoside triphosphates (Figure 33-13) allow investigators to distinguish the effects of nucleotides due to phosphoryl transfer from effects mediated by occupancy of allosteric nucleotide-binding sites on regulated enzymes. 

The last phase of the excitotoxic cascade involves the activation of various biochemical pathways, among which phospholipases, proteases (in particular cal-pain), kinases and calmodulin-regulated enzymes such as nitric oxide synthase (NOS) play a prominent role. 

Names of reactants and products for each regulated enzyme and each enzyme making or using ATP equivalents... 

A major way to control enzyme activity is the reversible phosphorylation of serine or threonine residues. It s such a major way to regulate enzymes that you will spend much of your time trying to remember whether or not enzyme X is activated or inactivated by phosphorylation, and you will invariably forget one or two of them on the exam. By remembering a couple of generalities,2 you can actually figure out a lot of the effects of phosphorylation on specific enzymes without really memorizing them. 

Allosteric regulatory molecules are small molecular weight compounds which may be coenzymes (NAD+, ATP, etc.) or intermediate substrates, possibly generated by enzymes found within the same pathway as the regulated enzyme. Alternatively, the allosteric modulator may be generated within another, perhaps complementary, pathway. For example, a regulator may be stimulatory for a catabolic route and at the same time inhibitory for the opposing anabolic pathway. 

Phosphorylation of HI during mitosis is catalyzed by the cyclin B-Cdc2 kinase, a tightly regulated enzyme [25,26]. In Tetrahymena mitotically dividing nuclei, cyclic-AMP dependent kinase (PKA) or PKA-like kinase phosphorylates HI [27,28]. Protein phosphatase 1 dephosphorylates the phosphorylated HI [29]. 

Why did nature use an Fe-S cluster to catalyze this reaction, when an enzyme such as fumarase can catalyze the same type of chemistry in the absence of any metals or other cofactors One speculation would be that since aconitase must catalyze both hydrations and dehydrations, and bind substrate in two orientations, Fe in the comer of a cubane cluster may provide the proper coordination geometry and electronics to do all of these reactions. Another possibility is that the cluster interconversion is utilized in vivo to regulate enzyme activity, and thus, help control cellular levels of citrate. A third, but less likely, explanation is that during evolution an ancestral Fe-S protein, whose primary function was electron transfer, gained the ability to catalyze the aconitase reaction through random mutation. 

Amplification Enzymatic groups such as protein kinase C, calmodulin, and calpaine-regulating enzymes (e.g., phospholipase) are activated. These changes give way to the long-term activation of neuronal circuits, which increase excitotoxicity. 

Another example of a Ca regulated enzyme is protein kinase C (see 7.3). 

Another mode of regulating enzymic activity is through the formation of multienzyme complexes.82 These complexes are aggregates of enzymes that catalyze two or more steps in a metabolic sequence. The chemical... 

The final mode of regulating enzymic activity to be discussed is the coupling of an enzyme with a membrane. Several different types of regulation are possible (1) Specific interactions between the protein and phospholipid may be required (2) a general requirement for a hydrophobic type of environment may exist (3) the enzyme can be immobilized by a membrane and can be localized in a particular place where it is needed (4) the function of the enzyme can be coupled with another membrane function, such as transport (this coupling may require a closed membranous structure) and (5) the enzymic activity can be modulated by interaction of the enzyme with other membrane proteins (e.g., by coupling to other enzymes or to receptors). A few examples illustrating these possibilities are now considered. 

Protein kinases can be classified according to the amino acid residue that is phosphorylated in the cellular process. Consequently, there are tyrosine-specific kinases and serine/threonine kinases. Tyrosine kinases are a family of tightly regulated enzymes, and the aberrant activation of various members of this family is one of the hallmarks of cancer. Tyrosine phosphorylation has been linked to multiple cell growth and differentiation pathways. Imatinib mesylate (1) is a tyrosine kinase inhibitor (TKI). An important characteristic of imatinib mesylate (1) is that it is an ATP-competitive inhibitor. It binds at the ATP binding site and blocks ATP binding thereby inhibiting kinase activities. 

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. 

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