citric acid cycle

citric acid cycle (Krebs' cycle, tricarboxylic acid cycle) The cyclic series of reactions which represent the principal means by which most living cells provide hydrogen and electrons for the generation of ATP via the electron-transport chain and oxidative phosphorylation. The hydrogen is taken up by NAD (nicotinamide adenine dinucleotide) or FAD (flavin adenine dinucleotide) and passed into the electron-transport chain. The reactions result in oxidative decarboxylation. The sequence is initiated when acetyl groups, fed in by acetyl coenzyme A, are dehydrogenated (i.e. oxidized), to release 4 pairs of hydrogen atoms, and decarboxylated to form 2 molecules of carbon dioxide. This yields oxaloacetic acid and then citric acid. The citric acid passes through a series of reactions wherein oxaloacetic acid is reformed, and two moles of CO₂ and water and one mole of ATP are synthesized. Since two molecules of acetyl coenzyme A are formed for each glucose molecule that is oxidized the cycle rotates twice, forming two moles of ATP.

citric-acid cycle See Krebs cycle.

citric-acid cycle (Krebs's cycle, tricarboxylic-acid cycle) A cyclic series of reactions which take place in mitochondria, in the presence of oxygen. The cycle represents the principal means by which most living cells provide electrons for the generation of ATP via the electron-transport chain and oxidative phosphorylation. The sequence is initiated by the condensation of acetyl coenzyme A with oxaloacetic acid to form citric acid. This then passes through a series of reactions wherein oxaloacetic acid is reformed, and 2 moles of CO₂ and water and 12 moles of ATP are synthesized.