The reactions of the tricarboxylic acid (TCA cycle) mediate the complete oxidation of two carbon atoms from acetyl CoA, derived from catabolism of glucose and long chain fatty acids, to CO2, and the generation of NADH + H+ and FADH2. Carbon skeletons derived from catabolism of amino acids enter the TCA cycle as oxaloacetate, 2-oxoglutarate, and succinyl CoA. The TCA cycle thus plays a central role in the generation of energy by catabolism of energy-rich molecules from the diet, but also plays a critical role in the interconversions of metabolic intermediates needed to maintain pools of amino acids and other metabolites at physiological levels.