Isocitrate dehydrogenase a-Ketoglutarate

Balance Sheet for the Citric Acid Cycle The citric acid cycle has eight enzymes citrate synthase, aconitase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase. 

Four of the Krebs cycle reactions are considered irreversible citrate synthase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and succinyl-CoA synthase. In two of these, COz is evolved. In one reaction, succinyl-CoA synthase, a substrate-level phosphorylation takes place, in which a high-energy compound, GTP, is generated. Note that in three of the reactions NADH is... 

Niacin is utilized in the synthesis of the nicotinamide portion of NAD, which is used in the isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and malate dehydrogenase reactions. 

Isocitrate Dehydrogenase a Ketoglutarate Dehydrogenase Complex Succinyl-CoA Synthetase Succinate Dehydrogenase... 

Isocitrate dehydrogenase a-Ketoglutarate dehydrogenase Succinyl-CoA synthetase Succinate dehydrogenase Fumarase... 

Isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and succinyl-CoA synthetase. 

Isocitric Dehydrogenase a-Ketoglutarate Decarboxylase Succinic Dehydrogenase Fumarase... 

The reductive citric acid cycle is generally considered to be the most energy-efficient CO2 fixation pathway ( 0.6 mol ATP/mol CO2 for pyruvate) [20, 30, 31]. This is reflected by the activity of the pathway s key carboxylases, isocitrate dehydrogenase, a-ketoglutarate ferredoxin oxidoreductase, and pyruvate ferre-doxin oxidoreductase. All three enzymes couple their carboxylation reaction to a subsequent reduction step. Whereas isocitrate dehydrogenase is an NAD(P)H-dependent enzyme, the latter two enzymes use ferredoxin as a reductant (Scheme 9.3) [32-34]. 

AceCyl-CoA + oxaloacetate + HgO. CoASH + citrate 2. Citrate. isocitrate 3. Isocitrate + NAD. a-ketoglntarate + NADH + CO, + 4. a-Ketoglntarate + CoASH + NAD. snccinyl-CoA + NADH + CO, + H Citrate synthase Aconitase Isocitrate dehydrogenase u-Ketoglutarate dehydrogenase complex... 

Isocitrate dehydrogenase catalyzes the NAD-dependent reduction of isocitrate to a-ketoglutarate. The dimeric enzyme is regulated via phosphorylation. Phosphorylation on SerllS leads to a complete inactivation of the enzyme. 

Oxidation of isocitrate to a-ketoglutarate (catalyzed by isocitrate dehydrogenase the reaction requires NAD+). 

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 ... 

Aconitase catalyzes the isomerization of citrate to isodtrate, isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to a-ketoglutarate, and a-ketoglutarate dehydrogenase catalyzes the oxidative decarboxylation of a-keto-glutarate to succinyl-CoA. Succinyl-CoA and the remaining intermediates are the 4-carbon intermediates of the Krebs cycle. Succinyl thiokinase catalyzes the release of coenzyme A from succinyl-CoA and the production of GTP. Succinate dehydro-... 

Immediate decarboxylation of oxalosuccinate results in the formation of a-keto-glu-tarate, an a-keto acid. There are two forms of isocitrate dehydrogenase in mammals. The NAD+-requiring isozyme is found only within mitochondria. The other isozyme, which requires NADP+, is found in both the mitochondrial matrix and the cytoplasm. In some circumstances the latter enzyme is used within both compartments to generate NADPH, which is required in biosynthetic processes. Note that the NADH produced in the conversion of isocitrate to a-ketoglutarate is the first link between the citric acid cycle and the ETC and oxidative phosphorylation. 

Oxidation and decarboxylation of isocitrate to a-ketoglutarate. Isocitrate dehydrogenase is also an allosteric enzyme however, the enz)une is controlled by the positive allosteric effector, ADP. ADP is a signal that the levels of ATP must be low, and therefore the rate of the citric acid cycle should be increased. Interestingly, isocitrate dehydrogenase is also inhibited by high levels of NADH and ATP. 

Two turns of the TCA cycle, with NADH produced at the pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketoglutarate, and malate dehydrogenase steps and FADH2 produced at the succinate dehydrogenase step and GTP (equivalent to ATP) produced at the succinyl-coenzyme A synthetase step. 

---