Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The Citric Acid Cycle
General Characteristics of the Cycle

Thus, we have completed the description of one turn of The Citric Acid Cycle. A single two-carbon acetyl group enters the cycle by combining with oxaloacetate. Two carbon atoms are released as carbon dioxide by the end of the cycle. At the end of the cycle, one molecule of oxaloacetate is regenerated. Four pairs of hydrogen atoms are removed from four cycle intermediates via enzymatic dehydrogenation reactions. Of these, three pairs are used to reduce three molecules of NAD⁺ to NADH, and one pair reduces the FAD of succinate dehydrogenase to FADH₂. Four pairs of electrons from these hydrogen atoms are transferred to the Electron Transport Chain and ultimately reduce two molecules of O₂ to form four molecules of H₂O. It is worth noting that the two carbon atoms emerging as CO₂ are not the exact same atoms that entered the cycle as the acetyl group. For the carbon atoms introduced via the acetyl group to finally be released as CO₂, additional turns of the cycle are required, as illustrated in Figs. 16-12 and 16-13.

As a byproduct of the cycle, one molecule of ATP is produced, synthesized from ADP and phosphate via GTP, which is generated by the succinyl-CoA synthetase reaction. In the following chapter, we will explore how four pairs of electrons, released from the four pairs of hydrogen atoms during dehydrogenation reactions, trans

fer through The electron transport chain to molecular oxygen, yielding H₂O. Although only a single ATP molecule is formed per turn of the Krebs cycle, the four dehydrogenation reactions encompassed by the cycle generate a powerful flux of high-energy electrons; these electr