Citric Acid Cycle

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(1) A cycle of reactions catalyzed by enzymes in which pyruvate derived from nutrients and converted to Acetyl Coenzyme A is completely oxidized and broken down into carbon dioxide and water to produce high-energy phosphate compounds, which are the source of cellular energy.

(2) One of the major metabolic pathways of cellular respiration, and involves a cyclic series of enzymatic reactions by which pyruvate converted into Acetyl Coenzyme A is completely oxidized to CO2 and hydrogen is removed from the carbon molecules, transferring the hydrogen atoms and electrons to electron-carrier molecules (e.g. NADH and FADH2) as well as the metabolic energy to high energy bonds (e.g. ATP).


The citric acid cycle is an alternative name for Krebs cycle, which was first described by Hans Adolf Krebs in 1937.

It is called citric acid cycle because the citric acid is both the first product and the final reactant of this metabolic pathway. It involves eight step processes in which the Acetyl Coenzyme A is converted to Citrate, Isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.

In the end of the cycle, for every one pyruvate molecule a total of 3 NADH, 1 FADH2, and 1 GTP are produced.

Also called: tricarboxylic acid cycle, Krebs cycle.
See also: glycolysis, oxidative phosphorylation, cellular respiration.