What are the main products of the citric acid cycle?

The main products of the citric acid cycle are ATP, NADH, FADH2, and carbon dioxide.

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle, is a key metabolic pathway that unifies carbohydrate, fat, and protein metabolism. The cycle is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into ATP and carbon dioxide.

The cycle begins with the reaction between acetyl-CoA and the four-carbon oxaloacetate to form six-carbon citrate. This citrate is then decarboxylated and isomerised to form isocitrate. Isocitrate is then oxidatively decarboxylated by the enzyme isocitrate dehydrogenase to form the five-carbon α-ketoglutarate. This molecule is further decarboxylated to form the four-carbon succinyl-CoA. The conversion of succinyl-CoA to succinate is accompanied by the formation of ATP from ADP, in a process known as substrate-level phosphorylation.

The remaining steps of the cycle involve the regeneration of oxaloacetate so the cycle can continue. Succinate is converted to fumarate, then to malate, and finally to oxaloacetate. These reactions are accompanied by the reduction of NAD+ to NADH and FAD to FADH2.

In summary, for each acetyl-CoA molecule that enters the citric acid cycle, the cycle produces three molecules of NADH, one molecule of FADH2, one molecule of ATP (or GTP, depending on the cell type), and two molecules of carbon dioxide. The NADH and FADH2 produced are high-energy molecules that are used in the electron transport chain to produce ATP, the main energy currency of the cell. The carbon dioxide produced is a waste product and is excreted from the body.