Oxalosuccinate. decarboxylation from isocitrate

The oxidative decarboxylation of isocitrate to a-kctoglutaratc, catalyzed by mitochondrial isocitrate dehydrogenase. The intermediate, oxalosuccinate, is not released from the enzyme. B represents a catalytic side chain from the enzyme. 

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. 

This is an enzyme of the citric acid cycle which should not he confused with isocitrate dehydrogenase E.C. 1.1.1.42, from which it can be distinguished by the latter s requirement for NADP and ability to decarboxylate oxalosuccinate. It may be assayed by u.v. spectroscopy [457] or colorimetrically [458—459]. 

The biological decarboxylation of a P-keto acid presents a slight difficulty. We recall that carboxyhc acids exist as carboxylate anions at pH 7. And, the decarboxylation of P-keto acids occurs from the carboxylic acid, not the anion. Why, then, is this decarboxylation a favorable process Decarboxylation of oxalosuccinate produces an enolate anion. An enolate anion has a piST of -20 and is very unstable at pH 7. A reaction that produces an unstable intermediate has a high activation energy and is slow. Thus, the enzyme-catalyzed decarboxylation of the anionic form of a P-keto acid has to stabilize the enolate anion intermediate. Isocitrate dehydrogenase requires Mg ions. An Mg ion forms a complex with the carbonyl group of the P-keto acid. This has two effects. 

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