Citrate Cycle


          



It's the final common pathway for the oxidation of fuel molecules. In eukaryotics, the reactions of the cycle occur inside mitochondria, in contrast with those of glycolysis, wich occur in the cytosol.

The citric acid cycle is an ingenious series of reactions that oxidizes the acetyl group pf acetyl-CoA to two molecules of CO2, in a manner that conserves the liberated free energy for utilization in ATP generation. This many steps degradation avoids waste or excess heat production. The most important thing to note in this cycle, is that acetyl-CoA is not an intermediate, but just an enter point, a fuel. Just the molecules synthesized on the cycle and constantly regenerated as it goes on are its intermediates.

Actually, the acetyl carbons that enter in the cycle are not the same that leave it as CO2 A few laps are necessary to release these carbons.

Besides the energy production (specially in the four reaction that conserve energy as NADH and FADH2, since the ATP production is too small - only one at each cycle), the cycle also provides intermediates for biosynthesis. The most important are:

-Citrate in the synthesis of fatty acids and steroids:

-2-oxoglutarate in the synthesis of amino acids and nitrogen bases;

-Succynil-CoA in the synthesis of porphyrins:

-Oxaloacetate in the synthesis of aspartate, asparagine and pyrimidines; and in the gluconeogensis.

The citric acid cycle is therefore amphibolic (both anabolic and catabolic).

These citric acid intermediates must be replenished if any are used for biosynthesis. Reactions that replenish them are called anaplerotic reactions. This allows many molecules to be converted to citrate cycle intermediates, contributing in the energy production instead of being degraded to CO2 themselves.

References: (1), (2), (3)

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