Dicarboxylic acid cycle

Dicarboxylic acid cycle discovered by Szent-Gyorgi. Krebs and Johnson. The tricarboxylic (citric) acid cycle. 

Some bacteria use a "dicarboxylic acid cycle" to oxidize glyoxylate OHC-COO to C02. The regenerating substrate for this cycle is acetyl-CoA. It is synthesized from glyoxylate by a complex pathway that begins with conversion of two molecules of glyoxylate to tartronic semialdehyde ... 

Figure 17-6 The dicarboxylic acid cycle for oxidation of glyoxylate to carbon dioxide. The pathway for synthesis of the regenerating substrate Carbohydrate synthesis...

The S5mthetic pathway for the regenerating substrate of e dicarboxylic acid cycle is quite complex. Two molecules of glyoxylate imdergo a condensation with decarboxylation by glyoxylate carboligase ... 

Metabolic cycle a catalytic series of reactions in which the product of one bimolecular reaction is regenerated A-tB->->—>C + A. Thus A acts catalyti-cally, is required only in small amounts, and can be considered as a carrier of B.The catalytic function of A and other members of the M.c. insure economic conversion of B into C. B is the substrate of the M.c. and C is the product. If intermediates are withdrawn from the M.c., e.g. for biosynthesis, the stationary concentrations of the M.c. intermediates must be maintained by synthesis. Replenishment of depleted M.c. intermediates is called anaplerosis. Only one anaplerotic reaction is necessary, since the resulting intermediate is in equilibrium with all other members of the cycle. Anaplerosis may be served by a single reaction, which converts a common metabolite into an intermediate of the M.c. (e.g. pyruvate to oxalo-acetate in the tricarboxylic acid cycle), or it may involve a metabolic sequence of reactions, i. e. an anaplerotic sequence (e.g. the glycerate pathway which provides phosphoenofpyruvate for anaplerosis of the dicarboxylic acid cycle). 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

The wide occurrence in different types of microorganisms of a tricarboxylic acid cycle, either identical with or very similar to that in animal tissues, cannot be doubted. However, the evidence for the claim that the cycle is the terminal pathway of oxidation is, in many instances, incomplete on quantitative grounds. In many organisms another terminal oxidation mechanism seems to play a major role. Its nature is unknown in the case of yeast. It may be a dicarboxylic acid cycle in certain bacteria. 

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