Citric acid cycle
The Citric acid cycle, also known as the Krebs cycle or the Tricarboxylic acid cycle, takes place in the mitochondrial matrix. It can be divided into 3 steps:
- The acetyl CoA combines with a 4C compound, oxaloacetate, to form a 6C compound, citrate.
- The citrate is decarboxylated (carbon dioxide removed) and dehydrogenated (oxidised by the removal of hydrogen) in a series of steps. At 2 steps, carbon dioxide is removed and given off as a waste gas. At 4 places, pairs of hydrogen atoms are removed and accepted by NAD and FAD which get reduced to NADH2 and FADH2 respectively.
- Oxaloacetate is regenerated to combine with the second acetyl CoA.
Thus, for each turn of the cycle, 2 carbon dioxide molecules are formed. One reduced FAD and 3 reduced NAD are also formed and 1 ATP molecule is generated.
For 1 mole of glucose, that is both moles of acetyl CoA, the Citric acid cycle yields:
- 8 pairs of hydrogen atoms,
- 2 molecules of ATP and
- 4 molecules of carbon dioxide.
The Citric acid cycle is regulated by several enzymes, in contrast to the preceeding process of glycolisis which is primarily regulated by PFK-1. The most important enzymes are those that catalyze reactions with highly negative delta G0. The 3 enzymes responsible are; citrate synthase, which produces citrate by synthesis of oxaloacetate and acetyl CoA. Isocitrate dehydrogenase, which is the oxidative decarboxylation of isocitrate. Finally, a-ketoglutarate dehydrogenase complex, which oxidativly carboxylates a-ketoglutarate to succinyl CoA.
- ↑ Clackamas Community College (2003) Citric Acid Cycle. Available at: dl.clackmas..edu/ch106-06/citric.htm (01/12/2011)
- ↑ Essential Biochemistry. Citric Acid Cycle. Available at: www.wiley.com/college/pratt/0471393878/student/animations/citric_acid_cycle/index.html (29/11/2011)
- ↑ Jeremy M.Berg, John L.tymoczko, Lubert Stryer (2007) Biochemistry, 7th Edition, England, FREEMAN. (30/11/2011)
- ↑ Ferrier D. Lippincott's illustrated reviews biochemistry, 6th Edition. Lippincott Williams and Wilkins, Baltimore.