The fuels we take in in the diet are mainly fats, proteins, and carbohydrates. At the very center of metabolism is a cycle of reactions that takes place in the mitochondrial matrix. The cycle is named after its discoverer, Hans Krebs, and is also known as the tricarboxylic acid (TCA) cycle or the citric acid cycle. The foods we eat are converted to the two-carbon unit acetate, CH3COO-. The acetate is not free but is carried by a coenzyme called coenzyme A. Acetate bound to coenzyme A—acetyl-CoA for short—is then fed into the Krebs cycle and may be completely oxidized to carbon dioxide and water. In the process the energy currency NADH is produced. The Krebs cycle is central to carbohydrate, fat, and amino acid metabolism.
We shall first describe the Krebs cycle and then look at the other pathways that interact with it. The reactions are shown in Figure 13.2.
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