Oxidative Decarboxylation of Isocitrate to a-Ketoglutarate

Oxidative decarboxylation of isocitrate to a-ketoglutarate. A P-ketoacid intermediate is formed in both reactions. See question... 

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-... 

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. 

In the conversion of isocitrate to a-ketoglutarate, oxidation and decarboxylation steps occur. Figure 13.9 shows the oxidation step first. Can you suggest a reason why the oxidation step is first ... 

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. 

In oxidative decarboxylation, the molecule that is oxidized loses a carboxyl group as carbon dioxide. Examples of oxidative decarboxylation include the conversion of pyruvate to acetyl-CoA, isocitrate to a-ketoglutarate, and a-ketoglutarate to succinyl-CoA. 

The chief source of glutamic add is a-ketoglutaric add produced in the Krebs cycle. This add can itself only arise in the oxidative decarboxylation of isocitric acid, for the decarboxylation of a-ketoglutarate to give succinate is irreversible. This is the sole pathway for the formation of glutamic add from carbohydrate. 

As mentioned in the introductory part, stereochemical course of the conversion of isocitric acid to a-ketoglutaric acid in TCA cycle is completely enantiose-lective although the reaction does not form an asymmetric carbon in the usual metabolic path. If such type of oxidative decarboxylation can be applied to synthetic compounds, it is expected that an entirely new type of asymmetric biotransformation will be developed. 

Isocitrate Dehydrogenase Catalyzes the First Oxidation in the TCA Cycle a-Ketoglutarate Dehydrogenase Catalyzes the Decarboxylation of a-Ketoglutarate to Succinyl-CoA... 

The cycle starts with the condensation of oxaloacetate (C4) and acetyl CoA (C2) to give citrate (Cg), which is isomerized to isocitrate (Cg). Oxidative decarboxylation of this intermediate gives a-ketoglutarate (C5). The second molecule of... 

Although citrate has been excluded as the primary condensation product of pyruvate and oxalacetate, no direct evidence bearing upon the nature of this product has as yet been obtained. The participation of cfs-aconitic and isocitric acids is speculative. Nor is there any evidence supporting the hypothesis that pyruvate and oxalacetate condense to form a hypothetical intermediate oxalcitraconic acid which can be oxidatively decarboxylated to citric acid. Since citrate, aconitate and isocitrate are in equilibrium with each other, the participation of the last two substances as intermediates of carbohydrate oxidation would, on the surface, appear to be doubtful. Krebs, however, believes that the conversion of cis-aconitate to a-ketoglutarate occurs so rapidly in liver that equilibrium with citrate is not attained. 

D-isocitrate is an intermediate product of the citrate cycle. It is formed as a by-product of the fermentative production of citric acid. The enzymatic assay of isocitrate is based on the oxidative decarboxylation to a-ketoglutarate in the presence of isocitrate dehydrogenase (EC 1.1.1.42) ... 

Acetyl-GoA condenses with oxaloacetate to give citrate, a six-carbon compound. Gitrate isomerizes to isocitrate, which then undergoes an oxidative decarboxylation to a-ketoglutarate, a five-carbon compound. This then undergoes another oxidative decarboxylation producing succinyl-GoA, a four-carbon compound. The two decarboxylation steps also produce NADH. Succinyl-GoA is converted to succinate with the concomitant production of GTP. Succinate is oxidized to fumarate, and FADHg is produced. Fumarate is converted to malate, which is then oxidized to oxaloacetate while another NADH is produced. 

The oxidation of a jS-hydroxyacid with decarboxylation of the j8-ketonic acid formed has already been described above in the case of the passage of isocitric acid into oxalosuccinic acid and then to a-ketoglutaric acid during the tricarboxylic acid cycle. 

In 1948 Ochoa demonstrated the existence of an enzyme, isocitric dehydrogenase, which catalyzed the oxidation of isocitric acid and required NADP. He was, however, unable to demonstrate the formation of the expected product oxalosuccinic acid (Fig. 1). The existence of this acid as an intermediate in the Krebs cycle had been postulated by Carl Martius. Ochoa was able to prepare the compound by chemical synthesis and showed that cell extracts catalyzed the decarboxylation of this very unstable /S-keto acid to a-ketoglutaric acid. Similar results were simultaneously obtained by Lynen in Germany. 

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. 

The first oxidative conversion of the TCA cycle is catalyzed by isocitrate dehydrogenase. This conversion takes place in two steps oxidation of the secondary alcohol to a ketone (oxalosuccinate), followed by a j8 decarboxylation to produce a-ketoglutarate (fig. 13.9). 

It is important to note that animals are unable to effect the net synthesis of glucose from fatty acids. Specifically, acetyl CoA cannot be converted into pyruvate or oxaloacetate in animals. The two carbon atoms of the acetyl group of acetyl CoA enter the citric acid cycle, but two carbon atoms leave the cycle in the decarboxylations catalyzed by isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. Consequently, oxaloacetate is regenerated, but it is not formed de novo when the acetyl unit of acetyl CoA is oxidized by the citric acid cycle. In contrast, plants have two additional enzymes enabling them to convert the carbon atoms of acetyl CoA into oxaloacetate (Section 17.4.). 

COs to form oxalacetate which under anaerobic conditions is reduced to malate. The malate in turn may be converted to fumarate and succinate (Fig, 5). The last step in this series of reactions is blocked by malonate. The second pathway involves the aerobic condensation of pyruvate and oxalacetate followed by oxidation of the condensation product to form -ketoglutarate and succinate. Wood has proposed that the first condensation product of the aerobic tricarboxylic cycle is cfs-aconitic acid which is then converted to succinate by way of isocitric, oxalosuccinic, and a-ketoglutaric acids. The a-ketoglutarate is decarboxylated and oxidized to succinic acid. Isotopic a-ketoglutarate containing isotopic carbon only in the carboxyl group located a to the carbonyl would be expected to yield non-isotopic succinate after decarboxylation. This accounts for the absence of isotopic carbon in succinate isolated from malonate-poisoned liver after incubation with pyruvate and isotopic bicarbonate. 

---