Harpers

THE CITRIC ACID CYCLE: THE CATABOLISM OF ACETYL-CoA / 135OxaloacetatePyruvateAcetyl-CoACitricacidcyclePYRUVATEDEHYDROGENASECO 2 CO 2MITOCHONDRIALMEMBRANECitrateGlucoseFattyacidsAcetyl-CoACitrateOxaloacetateATP-CITRATELYASEFigure 16–5. Participation of the citric acid cycle infatty acid synthesis from glucose. See also Figure 21–5.mately, on the rate of utilization of ATP in chemicaland physical work. In addition, individual enzymes ofthe cycle are regulated. The most likely sites for regulationare the nonequilibrium reactions catalyzed bypyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase,and α-ketoglutarate dehydrogenase. Thedehydrogenases are activated by Ca 2+ , which increasesin concentration during muscular contraction and secretion,when there is increased energy demand. In atissue such as brain, which is largely dependent on carbohydrateto supply acetyl-CoA, control of the citricacid cycle may occur at pyruvate dehydrogenase. Severalenzymes are responsive to the energy status, asshown by the [ATP]/[ADP] and [NADH]/[NAD + ] ratios.Thus, there is allosteric inhibition of citrate synthaseby ATP and long-chain fatty acyl-CoA. Allostericactivation of mitochondrial NAD-dependent isocitratedehydrogenase by ADP is counteracted by ATP andNADH. The α-ketoglutarate dehydrogenase complex isregulated in the same way as is pyruvate dehydrogenase(Figure 17–6). Succinate dehydrogenase is inhibited byoxaloacetate, and the availability of oxaloacetate, ascontrolled by malate dehydrogenase, depends on the[NADH]/[NAD + ] ratio. Since the K m for oxaloacetateof citrate synthase is of the same order of magnitude asthe intramitochondrial concentration, it is likely thatthe concentration of oxaloacetate controls the rate ofcitrate formation. Which of these mechanisms are importantin vivo has still to be resolved.SUMMARY• The citric acid cycle is the final pathway for the oxidationof carbohydrate, lipid, and protein whosecommon end-metabolite, acetyl-CoA, reacts with oxaloacetateto form citrate. By a series of dehydrogenationsand decarboxylations, citrate is degraded,releasing reduced coenzymes and 2CO 2 and regeneratingoxaloacetate.• The reduced coenzymes are oxidized by the respiratorychain linked to formation of ATP. Thus, thecycle is the major route for the generation of ATPand is located in the matrix of mitochondria adjacentto the enzymes of the respiratory chain and oxidativephosphorylation.• The citric acid cycle is amphibolic, since in additionto oxidation it is important in the provision of carbonskeletons for gluconeogenesis, fatty acid synthesis,and interconversion of amino acids.REFERENCESBaldwin JE, Krebs HA: The evolution of metabolic cycles. Nature1981;291:381.Goodwin TW (editor): The Metabolic Roles of Citrate. AcademicPress, 1968.Greville GD: Vol 1, p 297, in: Carbohydrate Metabolism and ItsDisorders. Dickens F, Randle PJ, Whelan WJ (editors). AcademicPress, 1968.Kay J, Weitzman PDJ (editors): Krebs’ Citric Acid Cycle—Half aCentury and Still Turning. Biochemical Society, London,1987.Srere PA: The enzymology of the formation and breakdown of citrate.Adv Enzymol 1975;43:57.Tyler DD: The Mitochondrion in Health and Disease. VCH Publishers,1992.