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:

1. The acetyl CoA combines with a 4C compound, oxaloacetate, to form a 6C compound, citrate.
2. 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.
3. 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 pairs of hydrogen atoms will be channelled on the inner membrane of the mitochondria to be used in oxidative phosphorylation to provide energy to make ATP^[1][2][3].

The Citric acid cycle is regulated by several enzymes, in contrast to the preceding process of glycolysis 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, alpha-ketoglutarate dehydrogenase complex, which oxidatively carboxylates alpha-ketoglutarate to succinyl CoA^[4].

Reference

1. ↑ Clackamas Community College (2003) Citric Acid Cycle. Available at: dl.clackmas..edu/ch106-06/citric.htm (01/12/2011)
2. ↑ Essential Biochemistry. Citric Acid Cycle. Available at: www.wiley.com/college/pratt/0471393878/student/animations/citric_acid_cycle/index.html (29/11/2011)
3. ↑ Jeremy M.Berg, John L.tymoczko, Lubert Stryer (2007) Biochemistry, 7th Edition, England, FREEMAN. (30/11/2011)
4. ↑ Ferrier D. Lippincott's illustrated reviews biochemistry, 6th Edition. Lippincott Williams and Wilkins, Baltimore.