Tricarboxylic acid cycle isocitrate production

The consequent interpretation, accepted by Krebs in his review of the tricarboxylic acid cycle in 1943, was therefore that citric acid could not be an intermediate on the main path of the cycle, and that the product of the condensation between oxaloacetate and acetyl CoA would have to be isocitrate, which is asymmetric. This view prevailed between 1941 and 1948 when Ogston made the important suggestion that the embarrassment of the asymmetric treatment of citrate could be avoided if the acid was metabolized asymmetrically by the relevant enzymes, citrate synthase and aconitase. If the substrate was in contact with its enzyme at three or more positions a chiral center could be introduced. 

The activity of an enzyme may be inhibited by the presence of a toxic metabolite. Sodium fluoroacetate, known as rat poison 1080, is extremely toxic to animals. The toxic action, however, is not due to sodium fluoroacetate itself but to a metabolic conversion product, flu-orocitrate, formed through a reaction commonly known as "lethal synthesis," as shown in Figure 5.3. The resulting fluorocitrate is toxic because it is inhibitory to aconitase, the enzyme responsible for the conversion of citrate into czs-aconitate and then into isocitrate in the tricarboxylic acid cycle. Inhibition of aconitase results in citrate accumulation. The cycle stops for lack of metabolites, leading to disruption of energy metabolism. 

Fe-IRE-BP also acts as an iron sulphide-containing enzyme cytosolic aconitase (c-aconitase), which converts citrate to isocitrate via a d -aconitrate intermediate in a reversible reaction. Although m-aconitase is a mitochondrial enzyme mediating entry of glycolysis products into the tricarboxylic acid (TCA) cycle, there is no obvious physiological role for c-aconitase (reviewed by O Halloran 1993). 

Tricarboxylic acid, known as aconitic acid (prop-1-ene-1,2,3-tricarboxylic acid, also known as achilleic or citridinic acid), occurs in two geometric isomers, the (Z)- and the (E)-isomers (8-64). (Z)-Aconitate is widespread as an intermediate produced in the isomerisation of citrate to D-isocitrate (catalysed by aconitase) in the citric acid cycle. About 5% aconitic acid is found in molasses from cane sugar production, where the (E)-isomer prevails, as it is formed by isomerisation of (Z)-aconitic acid at elevated temperatures and low pH. The amount of (Z)-aconitic acid in the growing cane is low, because it is used in the citric acid cycle and not stored in the plant. Decarboxylation of aconitic acid at elevated temperatures yields itaconic acid. 

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. 

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