Isocitrate dehydrogenase reaction

FIGURE 20.10 (a) The isocitrate dehydrogenase reaction, (b) The active site of isocitrate dehydrogenase. Isocitrate is shown in green, NADP is shown in gold, with Ca" in red. 

This is the isocitrate dehydrogenase reaction of the TCA cycle. Writing the two half-cell reactions, we have... 

Isocitrate Dehydrogenase Reaction What type of chemical reaction is involved in the conversion of isocitrate to a-ketoglutarate Name and describe the role of any cofactors. What other reaction(s) of the citric acid cycle are of this same type ... 

The second regulatory site is the isocitrate dehydrogenase reaction. In this case, ADP and NAD+ are allosteric activators of the enzyme. We have called attention to the recurring pattern in which ATP and NADH inhibit enzymes of the pathway, and ADP and NAD+ activate these enzymes. 

For example, in isocitrate dehydrogenase reaction, we shall see the formation of an enzyme-isocitrate-NADH complex because the extra hydrogen is not a problem (Grissom Cleland, 1988). When the common group is larger, such as a phosphate group or bigger, one usually will not see the EAP complex, or the dissociation constants for this complex are elevated. Thus, in the hexokinase reaction... 

Figure 29.1 shows that the high ratio of NADH NAD in the mitochondrion favours reduction of oxaloacetate to malate in the malate dehydrogenase reaction. It also restricts oxidation in the a-ketoglutarate dehydrogenase and isocitrate dehydrogenase reactions. The result is that Krebs cycle is inhibited. 

The anion (oxalosuccinate) is an intermediate in the isocitrate dehydrogenase reaction of the Tricarboxylic add cycle (see). Isodtrate dehydrogenase catalyses both the oxidation of isodtrate to oxalosuccinate, and its decarboxylation to 2-oxoglutarate. 

FIGURE 12.5 Intermediates of cycle reactions, (a) Citrate synthase reaction, (b) Aconitate hydratase reaction, (c) Isocitrate dehydrogenase reaction... 

Step 3 of Figure 29.12 Oxidation and Decarboxylation (2K,3S)-lsocitrate, a secondary alcohol, is oxidized by NAD+ in step 3 to give the ketone oxalosuccinate, which loses C02 to givea-ketoglutarate. Catalyzed by isocitrate dehydrogenase, the decarboxylation is a typical reaction of a /3-keto acid, just like that in the acetoacetic ester synthesis (Section 22.7). The enzyme requires a divalent cation as cofactor, presumably to polarize the ketone carbonyl group. 

Figure 22-4. Sequence of reactions in the oxidation of unsaturated fatty acids, eg, linoleic acid. A -c/s-fatty acids or fatty acids forming A -c/s-enoyl-CoA enter the pathway at the position shown. NADPH for the dienoyl-CoA reductase step is supplied by intramitochondrial sources such as glutamate dehydrogenase, isocitrate dehydrogenase,and NAD(P)H transhydrogenase.

Colman, R.F. (1969) The role of sulfhydryl groups in the catalytic function of isocitrate dehydrogenase. I. Reaction with 5,5 -dithiobis(2-nitrobenzoic acid). Biochemistry 8, 888. 

Some of the results obtained by differential centrifugation showed enzyme distribution between different cell fractions which were difficult to interpret. Enzymes like carbamoyl phosphate synthase or isocitrate dehydrogenase were found both in mitochondria and in the soluble fraction of the cell. This led to detailed kinetic studies with purified enzymes which indicated there might be populations of enzymes with slightly different properties (isozymes) catalyzing similar reactions in different compartments or in different cell types. Variations in kinetic behavior appeared to tailor the enzyme appropriately to the particular compartment or cell where the reaction took place. 

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. 

Citrate synthase, isocitrate dehydrogenase and oxogluta-rate dehydrogenase are key enzymes regulating the flux through the cycle all three catalyse non-equilibrium reactions (Chapter 3). 

The second function, and the one pertinent to this section, is the decarboxylation of oxalosuccinic acid to 2-oxoglutaric acid. This is simply a biochemical example of the ready decarboxylation of a P-ketoacid, involving an intramolecular hydrogen-bonded system. This reaction could occur chemically without an enzyme, but it is known that isocitric acid, the product of the dehydrogenation, is still bound to the enzyme isocitrate dehydrogenase when decarboxylation occurs. 

The first step is carboxylation of acetyl CoA to malonyl CoA. This reaction is catalyzed by acetyl-CoA carboxylase [5], which is the key enzyme in fatty acid biosynthesis. Synthesis into fatty acids is carried out by fatty acid synthase [6]. This multifunctional enzyme (see p. 168) starts with one molecule of ace-tyl-CoA and elongates it by adding malonyl groups in seven reaction cycles until palmi-tate is reached. One CO2 molecule is released in each reaction cycle. The fatty acid therefore grows by two carbon units each time. NADPH+H is used as the reducing agent and is derived either from the pentose phosphate pathway (see p. 152) or from isocitrate dehydrogenase and malic enzyme reactions. 

The NADPH level is clearly important for phase 1 reactions, yet many biochemical processes, such as fatty acid biosynthesis, use this coenzyme. It is derived from either the pentose phosphate shunt or isocitrate dehydrogenase. Consequently, the overall metabolic... 

There are two different forms of isocitrate dehydrogenase in all cells, one requiring NAD+ as electron acceptor and the other requiring NADP+. The overall reactions are otherwise identical. In eukaryotic cells, the NAD-dependent enzyme occurs in the mitochondrial matrix and serves in the citric acid cycle. The main function of the NADP-dependent enzyme, found in both the... 

MECHANISM FIGURE 16-11 Isocitrate dehydrogenase. In this reaction, the substrate, isocitrate, loses one carbon by oxidative decarboxylation. In step (T), isocitrate binds to the enzyme and is oxidized by hydride transfer to NAD+ or NADP+, depending on the isocitrate dehydrogenase isozyme. (See Fig. 14-12 for more information on hydride transfer reactions involving NAD+ and NADP+.) The resulting... 

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