Tricarboxylic acid cycle malate production

The aspartate entering the cycle is produced by reaction of glutamate with ox-alacetate, the former being produced from a-ketoglutarate plus ammonia released by deamination of an amino acid. The oxalacetate is derived from the fumarate released in the production of arginine from ai inosuccinate, which enters the tricarboxylic acid cycle and is converted to malate and then oxalacetate. We then have a second associated cycle linking the luea and the tricarboxylic acid cycles, which may be visualised as shown in Fig. 9.13. 

Catabolism of amino acids, except for leucine and lysine, results in synthesis of tricarboxylic acid cycle intermediates or the production of pyruvate (see p. 210). Conversion of the latter into glucose then takes place as shown in Fig. 9.20. The tricarboxylic acid cycle intermediates enter the cycle and are converted into malate. 

Malic acid is mainly produced as an acidulant and taste enhancer in the beverage and food industries. The most preferred metabolic pathway for malate production starts from glucose and proceeds with the carboxylation of pyruvate, followed by the reduction of oxaloacetate to malate. These pathways have been identified in bacterial, yeast, and fungal species (Werpy et al., 2004). In the microalgae reduction of oxaloacetate to malate by NADP malate dehydrogenase (Ouyang et al., 2013 Kuo et al., 2013), the condensation of oxaloacetate and acetyl-coenzyme A (acetyl-CoA) to citric acid is followed by oxidation steps of the tricarboxylic acid (TC A) cycle or glyoxyl-ate shunt to malate (Steinhauser et al., 2012 Pearce et al., 1969 Woodward and Merrett, 1975). 

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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