Kreb's Cycle

The cyclic mechanism by which urea is synthesized. The cyclic metabolic mechanism by which the complete oxidation of the acetyl moiety of acetyl-coenzyme A is effected.

The citric acid cycle is an elegant sequence that cells use to convert carbon atoms into carbon dioxide. Acetyl-CoA adds two carbon atoms, and then two carbon atoms are lost as carbon dioxide. In the process, the cell makes more NADH and other related molecules, which can eventually yield 12 more ATP molecules per acetyl-CoA molecule.

To begin the citric acid cycle, acetyl-CoA combines with a 4-carbon compound (oxaloacetic acid, or oxaloacetate) to form a 6-carbon compound (citric acid, or citrate). The CoA molecule is released. Recall that the portion of acetyl-CoA that interests us, the acetic acid, had two carbon atoms. The basic function now of the citric acid cycle is to take this 6-carbon citrate molecule and turn it back into a 4-carbon oxaloacetate molecule. In the process, two carbon dioxide molecules and lost. More important, however, the process also produces potential ATP in the form of NADH and other compounds.

Overall, the citric acid cycle begins and ends with the same compound, oxaloacetate.

References

Guyton, Arthur C., Textbook of Medical Physiology, eighth edition, Harcourt Brace Publishing, Philadelphia, PA, pages 20, 748. 1991

Taylor, E.J., Dorland's Illustrated Medical Dictionary 27th Edition, W.B. Saunders, Co. Publishing, Philadelphia, page 177. 1988.

Wardlaw, Gordon M., Paul M. Insel, Perspectives in Nutrition, Times Mirror/Mosby College Publishing, St. Louis, MO. p 196. 1990.