Isocitrate dehydrogenase inhibition

Isocitrate dehydrogenase inhibited by NADH (causing the citric acid cyde to stop when the ETC stops in the anaerobic cell). 

Direct kinetic assays are the only valid methods for the measurement of activators and inhibitors and calibration plots of the percentage activation or inhibition by known amounts of the substance can be made. Examples of inhibition assays include the quantitation of organophosphorus pesticides using the inhibition of cholinesterase (EC 3.1.1.7) while manganese can be measured in amounts as low as 1 X 10-12 mol using its activating effect on isocitrate dehydrogenase (EC 1.1.1.41). 

Many examples of product inhibition are to found. Some dehydrogenases are inhibited by NADH (a co-product of the reaction), e.g. PDH and isocitrate dehydrogenase (ICD), which are involved with the glycolysis and the TCA cycle are two such examples. Hexokinase isoenzymes in muscle (but not liver) and citrate synthase are inhibited by their products, glucose-6-phosphate and citrate respectively offering a very immediate fine tuning of reaction rate to match cellular requirements and possibly allowing their substrates to be used in alternative pathways. 

Isocitrate dehydrogenase, the major control enzyme, is inhibited by NADH and activated... 

Biochemical Effects Several enzymes that use nicotinamide cofactors were found to ye inhibited by PAN (at 125 ppm for 1 min) in in vitro studies. These enzymes were most susceptible in the absence of substrates. In some cases, an enzyme was protected by the nicotinamide cofactor (e.g., G-6-PD plus NADP), and in other cases, by the cosubstrate (e.g., isocitrate dehydrogenase plus isocitrate). Precisely the same protection could be obtained when compounds that react with sulfhydryl compounds (e.g., p-mercuricbenzoate) were used instead of PAN. Thus, the evidence indicated that PAN reacted with sulfhydryl groups. 

The most important factor in the regulation of the cycle is the NADH/NAD ratio. In addition to pyruvate dehydrogenase (PDH) and oxoglu-tarate dehydrogenase (ODH see p. 134), citrate synthase and isodtrate dehydrogenase are also inhibited by NAD deficiency or an excess of NADH+HT With the exception of isocitrate dehydrogenase, these enzymes are also subject to product inhibition by acetyl-CoA, suc-cinyl-CoA, or citrate. 

Isocitrate dehydrogenase is allosterically inhibited by NADH, an indicator of the availability of high levels of energy. 

Ikeda, T.P. Houtz, E. LaPorte, D.C. Isocitrate dehydrogenase kinase/phosphatase identification of mutations which selectively inhibit phosphatase activity. J. BacterioL, 174, 1414-1416 (1992)... 

Phosphorylation of an enzyme can affect catalysis in another way by altering substrate-binding affinity. For example, when isocitrate dehydrogenase (an enzyme of the citric acid cycle Chapter 16) is phospho-rylated, electrostatic repulsion by the phosphoryl group inhibits the binding of citrate (a tricarboxylic acid) at the active site. 

Some bacteria, including E. coli, have the full complement of enzymes for the glyoxylate and citric acid cycles in the cytosol and can therefore grow on acetate as their sole source of carbon and energy. The phosphoprotein phosphatase that activates isocitrate dehydrogenase is stimulated by intermediates of the citric acid cycle and glycolysis and by indicators of reduced cellular energy supply (Fig. 16-23). The same metabolites inhibit the protein kinase activity of the bifunctional polypeptide. Thus, the accumulation of intermediates of... 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

Isocitrate dehydrogenase catalyzes the irreversible oxidative decar boxylation of isocitrate, yielding the first of three NADH molecules produced by the cycle, and the first release of C02 (see Figure 9.5). This is one of the rate-limiting steps of the TCA cycle. The enzyme is allosterically activated by ADP (a low-energy signal) and Ca, and is inhibited by ATP and NADH, whose levels are elevated when the cell has abundant energy stores. 

Isocitrate is oxidized and decarboxylated by isocitrate dehydrogenase to a-ketoglutarate, producing C02and NADH. The enzyme is inhibited by ATP and NADH, and activated by ADP and Ca++... 

The effect of small-molecule modifiers on isocitrate dehydrogenase is appropriate in that a high-energy charge favors inhibition of isocitrate dehydrogenase and thus favors an accumulation of mitochondrial citrate. This leads to an increased flow of citrate from the mitochondrion to the cytosol where the citrate can exert its multiple positive effects on biosynthesis and its negative effects on glycolysis. 

Isocitrate dehydrogenase is allosterically stimulated by ADP, which enhances the enzyme s affinity for substrates. The binding of isocitrate, NAD+, Mg2+, and ADP is mutually cooperative. In contrast, NADH inhibits iso-citrate dehydrogenase by directly displacing NAD+. ATP, too, is inhibitory. It is important to note that several steps in the cycle require NAD+ or FAD, which are abundant only when the energy charge is low. 

Which of the following molecules is capable of inhibiting pyruvate dehydrogenase, citrate s)mthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase ... 

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