Citric Acid Cycle (TCA / Krebs Cycle)

1. 1

   Oxaloacetate + acetyl-CoA + H2O → citrate + CoA-SH

   Citrate synthase 2.3.3.1 ST-0269 Irreversible CoA-SH H2O
   In Oxaloacetate Acetyl-CoA H2O

   Out Citrate CoA-SH
   › Notes

   Cofactors: no redox cofactor; water hydrolyzes the thioester, and CoA-SH is released. This strongly exergonic, essentially irreversible step is a major control point and commits acetyl-CoA to citrate formation when oxaloacetate is available.

2. 2

   Citrate ⇌ cis-aconitate ⇌ isocitrate

   ↪ cis-aconitate → isocitrate

   Aconitase (aconitate hydratase) 4.2.1.3 ST-0270 Reversible Fe-S cluster
   In Citrate

   Out cis-Aconitate
   › Notes

   Required cofactor: a [4Fe–4S] iron-sulfur cluster that assists substrate binding and catalysis; it is not consumed. This reversible step relocates the hydroxyl group, converting a tertiary alcohol into the secondary alcohol needed for oxidation.

3. 3

   Isocitrate + NAD+ → oxalosuccinate + NADH + H+; oxalosuccinate → alpha-ketoglutarate + CO2

   ↪ oxalosuccinate + NADH + H+; oxalosuccinate → alpha-ketoglutarate + CO2

   Isocitrate dehydrogenase (mitochondrial NAD+-dependent IDH3) 1.1.1.41 ST-0271 Irreversible rate-limiting NAD+ NADH Mg2+ Mn2+
   In Isocitrate NAD+

   Out Oxalosuccinate NADH H+
   › Notes

   Required cofactors: NAD+ and Mg2+ or Mn2+. This is an essentially irreversible, rate-limiting regulatory step of the cycle; the CO2 released in the first turn is not necessarily derived from the newly introduced acetyl carbons.

4. 4

   Alpha-ketoglutarate + CoA-SH + NAD+ → succinyl-CoA + CO2 + NADH + H+

   Alpha-ketoglutarate dehydrogenase complex 1.2.4.2 ST-0272 Irreversible NAD+ NADH FAD CoA-SH TPP (B1) Lipoamide Mg2+
   In Alpha-ketoglutarate CoA-SH NAD+

   Out Succinyl-CoA CO2 NADH H+
   › Notes

   Required cofactors: TPP, lipoic acid/lipoamide, CoA-SH, FAD, NAD+, and Mg2+, with E1, E2, and E3 activities analogous to PDC. This is an essentially irreversible and important regulatory step; it is inhibited by NADH and succinyl-CoA.

5. 5

   Succinyl-CoA + GDP + Pi ⇌ succinate + GTP + CoA-SH

   Succinyl-CoA synthetase (succinate thiokinase) 6.2.1.4 / 6.2.1.5 ST-0273 Reversible GTP CoA-SH Mg2+ Pi
   In Succinyl-CoA GDP Pi

   Out Succinate GTP CoA-SH
   › Notes

   Required substrates/cofactors: GDP (or ADP in some tissue-specific isoenzymes), inorganic phosphate, and Mg2+. This reversible step produces the cycle’s direct high-energy phosphate: 1 GTP, readily converted to ATP by nucleoside diphosphate kinase.

6. 6

   Succinate + FAD ⇌ fumarate + FADH2 (enzyme-bound); ubiquinone is reduced to ubiquinol in the respiratory chain

   Succinate dehydrogenase (Complex II) 1.3.5.1 ST-0274 Reversible FAD FADH2 Fe-S cluster Heme/cytochrome Ubiquinone (CoQ)
   In Succinate FAD

   Out Fumarate FADH2 (enzyme-bound)
   › Notes

   Required cofactors: covalently bound FAD, iron-sulfur centers, and heme b; electrons pass from FADH2 through Fe–S centers to ubiquinone (CoQ). This reversible step does not pump protons and yields reducing equivalents at the CoQ pool rather than free NADH.

7. 7

   Fumarate + H2O ⇌ L-malate

   Fumarase (fumarate hydratase) 4.2.1.2 ST-0275 Reversible H2O
   In Fumarate H2O

   Out Malate
   › Notes

   Required cofactor: none beyond water. The reaction is reversible and forms only L-malate.

8. 8

   L-Malate + NAD+ ⇌ oxaloacetate + NADH + H+

   Malate dehydrogenase 1.1.1.37 ST-0276 Reversible NAD+ NADH
   In Malate NAD+

   Out Oxaloacetate NADH H+
   › Notes

   Required cofactor: NAD+. Although the standard free-energy change is unfavorable, the reaction proceeds in vivo because citrate synthase rapidly consumes oxaloacetate; it is reversible and regenerates the acceptor required for the next turn.

Showing all 8 steps.

Overview

The citric acid cycle (TCA cycle, Krebs cycle) oxidizes the acetyl group of acetyl-CoA to two molecules of CO2 while capturing reducing equivalents as NADH and FADH2 and generating one nucleoside triphosphate equivalent. It is amphibolic: beyond its catabolic role, cycle intermediates provide carbon for amino acids, heme, gluconeogenesis, fatty-acid synthesis, and other biosynthetic pathways. Continuous operation requires both acetyl-CoA entry and replenishment of withdrawn intermediates by anaplerotic reactions, especially pyruvate carboxylase–mediated oxaloacetate formation.

Cellular location

All eight TCA-cycle enzymes are located in the mitochondrial matrix except succinate dehydrogenase, which is embedded in the inner mitochondrial membrane as respiratory Complex II with its catalytic domain facing the matrix. The cycle is active in virtually all aerobic human tissues and is especially important in myocardium, brain, liver, renal cortex, and oxidative skeletal muscle. Mature erythrocytes lack mitochondria and cannot perform the TCA cycle.

Net energetics

One turn per acetyl-CoA produces 2 CO2, 3 NADH, 1 FADH2, and 1 GTP (approximately 1 ATP equivalent). With modern P/O ratios, these reducing equivalents yield about 7.5 ATP from NADH plus 1.5 ATP from FADH2; adding GTP gives approximately 10 ATP equivalents per acetyl-CoA. Per glucose, two turns yield 6 NADH, 2 FADH2, 2 GTP, and 4 CO2, or approximately 20 ATP equivalents from the cycle itself after oxidative phosphorylation. Combined with glycolysis and PDC, complete aerobic glucose oxidation is approximately 30–32 ATP per glucose; the historical 36–38 ATP accounting uses older, higher P/O assumptions.

Clinical significance

The TCA cycle is a central metabolic hub whose rate responds chiefly to ATP/ADP, NADH/NAD+, Ca2+, and substrate availability rather than to a single obligatory pacemaker. Because intermediates are continuously withdrawn for biosynthesis, anaplerosis is essential; pyruvate carboxylase, activated by acetyl-CoA, replenishes oxaloacetate. Defects in enzymes such as fumarase, succinate dehydrogenase, or isocitrate dehydrogenase can cause neurologic disease or, in particular genetic contexts, contribute to tumorigenesis through oncometabolite signaling. Inhibition of alpha-ketoglutarate dehydrogenase in thiamine deficiency further impairs oxidative metabolism and can exacerbate lactic acidosis.