Tricarboxylic add cycle

Glycolysis, the pentose phosphate pathway and the tricarboxylic add cycle... 

Aconitic add an unsaturated tricarboxylic acid, usually occurring in the cis form (m.p. 130°C), but sometimes in the irons (m.p. 194-195 °C). Free A. a. was first discovered in aconite, Aconitum napellus. The anionic form of cis-A. a. (propene-cis-l,23-trioic acid) is important as an intermediate in the isomerization of citrate to isocitrate in the Tricarboxylic add cycle (see). 

Dicarboxylic add cyda a cyclic pathway (Fig.) which enables the utilization of glyoxylic acid or one of its precursors (e.g. glycolic acid) as a carbohydrate source for the growth of microorganisms. The D.c.a. includes some of the reactions of the tricarboxylic add cycle. It also involves malate synthase (EC 4.1.3.2 see Glyoxylate cycle). If the concentration of D.c. a. interm iates is too severely reduced by diversion to synthetic pathways, they can be replenished from the Glycerate pathway (see). 

The anion (oxalosuccinate) is an intermediate in the isocitrate dehydrogenase reaction of the Tricarboxylic add cycle (see). Isodtrate dehydrogenase catalyses both the oxidation of isodtrate to oxalosuccinate, and its decarboxylation to 2-oxoglutarate. 

Thiamin pyrophosphate (or thiamin diphosphate) is a coenzyme involved in (1) the oxidative decarboxylation of pyruvate to acetyl coenzyme A (enzyme pyruvate dehydrogenase), (2) the oxidative decarboxylation of a-ketoglutarate to succinyl coenzyme A (a-ketoglutarate dehydrogenase) in the tricarboxylic add cycle, (3) the pentose phosphate pathway (transketolase) and (4) the synthesis of branched-chain amino acids such as valine (branched-chain ketoacid dehydrogenase) in bacteria, yeasts and plants. 

Bubber, P., Ke, Z.J., and Gibson G.E., 2004. Tricarboxylic add cycle enzymes following thiamine deficiency. Neurochemistry International. 45 1021-1028. 

Aspartic add, by transamination, gives oxaloacetic add which then also enters the tricarboxylic add cycle. 

Figure 56 shows the interrelations between glycolysis and the hoco semonophosphate shunt at the level of G—6—and F—6— P, between glycolysis, the fatty add cyde and the tricarboxylic acid cycle at the level of acetyl-CoA, between e tricarboxylic add cycle and the sucdnate-glycine cycle at the levd of sucdnyUCoA. 

The fumaric acid passes into the tricarboxylic add cycle or in the presence of fumarase it is transformed to malic add which also enters the Krebs cycle. 

In its overall effect the new cycle is a reversal of the oxidative tricarboxylic add cycle, but endergonic in nature and hence includes two reactions (a-ketoglutarate synthase and citrate lyase) that can by-pass the irreversible steps of the KLrebs cycle. Photosynthetic bacteria also contain pyruvate synthase that brings about the direct synthesis of pyruvate from acetyl-CoA, COg and reduced ferredoxin, as well as PEP synthase that catalyzes the direct synthesis of PEP from pyruvate and ATP. 

During the EBPR anaerobic phase, carbon substrates such as acetic and propionic acids are taken up to biosynthesize PHV. The energy and reduction equivalent required for PHA biosynthesis are provided by the degradation of intracellular saved polyphosphate (poly-P) and glycogen as well as substrate degradation in the tricarboxylic add cycle. The bulk phosphorus concentration in the anaerobic period is, therefore, increased with time. In the... 

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