Isocitrate tricarboxylic cycle

Aconitase, an unstable enzyme,4 is concerned with the reversible conversion of cis-aconitate to either citric acid or isocitric acid. It may be noted that the entire system of tricarboxylic cycle enzymes are present in the mitochondria separated from cells, and, furthermore, it has been found that the mitochondrial enzymes differ from the isolated enzymes in that the former require no addition of D.P.N. (co-enzyme I) or T.P.N. (co-enzyme II) for activity. Peters suggests that the citrate accumulation is caused by the competitive reaction of the fluorocitrate with aconitase required for the conversion of citrate to isocitrate. This interference with the tricarboxylic 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. 

A pathway by which acetate and acetoacetate are oxidized is indicated in Fig. 8. The similarity to the mechanism of oxidation of pyruvate is striking. According to this proposed scheme acetate and acetoacetate are converted to a hypothetical two-carbon intermediate which is capable of condensing with oxalacetate or one of the other four-carbon dicarboxylic acids to yield cis-aconitate or isocitrate. The rest of the cycle is identical with the carbohydrate tricarboxylic cycle. Reference to this cycle illustrates several points of interest. It makes clear the pathway by which labelled carbon in acetate or acetoacetate may be transformed into D-glucose or glycogen. Since oxalacetate labelled in either carboxyl position is generated in the cycle, carboxyl-labelled... 

Hansford, R.G. Johnson, R.N. (1975) J. Biol. Chem. 250, 8361-8375. The steady state concentrations of coenzyme-ASH and coenzyme A thioester, citrate, and isocitrate during tricarboxylate cycle oxidations in rabbit heart mitochondria. 

It is generally accepted that only NADPH would be able to supply the hydrogen necessary for lipogenesis in the condensation system (Lynen, 1961 Ball, 1966). Lowenstein, therefore, has suggested another pathway for the formation of NADPH in the cytoplasm (Lowenstein, 1961b). It is dependent on the capacity of the cytoplasm to accomplish the first reactions of the tricarboxylic cycle, i.e., the reactions that yield a-ketoglutaric acid. A cytoplasmic NADP-isocitrate dehydrogenase, in contrast to the mitochondrial enzyme, the coen-... 

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. 

Phosphorylation of an enzyme can affect catalysis in another way by altering substrate-binding affinity. For example, when isocitrate dehydrogenase (an enzyme of the citric acid cycle Chapter 16) is phospho-rylated, electrostatic repulsion by the phosphoryl group inhibits the binding of citrate (a tricarboxylic acid) at the active site. 

Fig. 5.22. Oxidation of acetyl-CoA via the tricarboxylic acid (TCA) cycle. Individual enzymes of the pathway are marked. 1, citrate synthase 2 and 3, cis-aconitate hydratase 4 and 3, isocitrate dehydrogenase 6, a-oxo glutarate dehydrogenas 7, succinate thiokinase 8, succinate...

Notice in the initial steps of the tricarboxylic acid cycle, citrate is converted to isocitrate, which is then oxidized. Why didn t nature just oxidize citrate and save an enzymatic step ... 

Much has been published on the controversial subject of the control of glycolysis. The following brief summary of some of the controls responsible for the Pasteur effect in yeasts is based mainly on a review by Sols and coworkers144 (see also, Fig. 7). (i) Isocitrate dehydrogenase (NAD ) (EC 1.1.1.41), one of the controlling enzymes of the tricarboxylic acid cycle (see Fig. 5), catalyzes the reaction... 

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. 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

Other C-0 lyase enzymes include aconitate hydratase or aconitase (E. C. 4.2.1.3), an enzyme that catalyzes two tricarboxylic acid cycle steps from isocitric acid to citrate (14)1141 or vice versa, via the intermediate cis-aconitate (13). Citrate dehydratase (E. C. 4.2.1.4) is only capable of converting citrate to cis-aconitate and does not act on isocitrate (15) 115l... 

The TCA cycle is frequently called the Krebs cycle because Sir Hans Krebs first formulated its reactions into a cycle. It is also called the "citric acid cycle" because citrate was one of the first compounds known to participate. The most common name for this pathway, the tricarboxylic acid or TCA cycle, denotes the involvement of the tricarboxylates citrate and isocitrate. 

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