Citric acid cycle

The citric acid cycle is a sequence of reactions that oxidizes acetyl-CoA to carbon dioxide while producing NADH, FADH₂, and GTP or ATP.

Citrate synthase: An enzyme that combines acetyl-CoA and oxaloacetate to form citrate. This enzyme catalyzes the cycle’s entry step, committing acetyl-CoA to citrate formation.
Acetyl-CoA: A two-carbon acetyl-group carrier that connects the breakdown of fuels to many biosynthetic reactions. It supplies the carbon unit that enters the cycle by condensing with oxaloacetate.
Oxidative phosphorylation: ATP production driven by electron transfer through a membrane respiratory chain and the resulting proton gradient. NADH and FADH₂ from the cycle supply electrons for most ATP made during aerobic respiration.
Hans Krebs: A German-born British biochemist who identified the citric acid cycle and received the 1953 Nobel Prize in Physiology or Medicine. Krebs pieced together the cycle’s reactions through experiments on tissue respiration.
Anaplerotic reactions: Reactions that replenish intermediates withdrawn from a metabolic pathway. Biosynthesis drains cycle intermediates, so anaplerotic reactions restore their levels.
Isocitrate dehydrogenase: An enzyme that oxidizes isocitrate to α-ketoglutarate, releasing carbon dioxide and reducing NAD⁺ or NADP⁺. Its reaction is the first oxidative decarboxylation in the cycle.
NAD⁺: An electron-accepting coenzyme that is reduced to NADH in oxidation-reduction reactions. Several cycle reactions transfer electrons from carbon intermediates to NAD⁺.
Gluconeogenesis: The metabolic pathway that synthesizes glucose from non-carbohydrate precursors. Oxaloacetate and malate derived from cycle intermediates can supply carbon for glucose synthesis.
Albert Szent-Györgyi: A Hungarian biochemist whose work established key reactions involving fumarate and succinate in cellular respiration. His investigations supplied findings that preceded Krebs’s full cycle proposal.
Pyruvate carboxylase: A biotin-dependent enzyme that converts pyruvate and bicarbonate to oxaloacetate using ATP. It replenishes oxaloacetate when cycle carbon is diverted to biosynthesis.

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