Citric Acid Cycle


The citric acid cycle is a metabolic pathway common to aerobic cells by which carbohydrates, fats and amino acids are oxidized to carbon dioxide and water. Enzymes catalyse the condensation of two‐ and four‐carbon compounds to citric acid, and then enzymatic oxidations and decarboxylations regenerate the four‐carbon starting compound plus CO2 and H2O. The energy available from the downhill flow of electrons to oxygen is used indirectly for the synthesis of ATP, the energy currency of the cell.

Keywords: respiration; adenosine triphosphate; áketoglutarate; citrate synthase

Figure 1.

Overview of the citric acid cycle.

Figure 2.

Individual steps of the citric acid cycle: (1) citrate synthase; (2) aconitase; (3) isocitrate dehydrogenase; (4) α‐ketoglutarate dehydrogenase; (5) succinyl–CoA thiokinase; (6) succinate dehydrogenase; (7) fumarase; (8) malate dehydrogenase.


Further Reading

Gest H (1987) Evolutionary roots of the citric acid cycle in prokaryotes. Biochemical Society Symposium 54: 3–16.

Hansford RG (1991) Dehydrogenase activation by Ca2+ in cells and tissues. Journal of Bioenergetics and Biomembranes 23: 823–854.

Huynen MA, Dandekar T and Bork P (1999) Variation and evolution of the citric acid cycle: a genomic perspective. Trends in Microbiology 7: 281–291.

Kay J and Weitman PDJ (eds) (1987) Kreb's Citric Acid Cycle – Half Century and Still Turning. Biochemical Society Symposia, vol. 54. London: Portland Press.

Lowenstein JM (ed.) (1969) Citric acid cycle: control and compartmentation. New York: Marcel Dekker.

Romano AH and Conway T (1996) Evolution of carbohydrate metabolic pathways. Research in Microbiology 147: 448–455.

Wan B, Doumen C, Duszynski J et al. (1993) The effects of cardiac work on the electrical potential gradient across the mitochondrial membrane in perfused rat hearts. American Journal of Physiology 265: H453–H460.

Williamson JR and Cooper RH (1980) Regulation of citric acid cycle in mammalian systems. FEBS Letters117(supplement): K73–K85.

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LaNoue, Kathryn F(Apr 2001) Citric Acid Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001369]