The citric acid cycle (also called the Krebs cycle or TCA cycle) is a cyclic pathway that completes the oxidation of acetyl groups derived from carbohydrates, fats, and many amino acids. In eukaryotes it operates primarily in the mitochondrial matrix (with one enzyme embedded in the inner mitochondrial membrane), while in most prokaryotes the analogous reactions occur in the cytosol and at the plasma membrane.
Core purpose and biochemical context
The central function of the citric acid cycle is the transfer of high-energy electrons from carbon fuels to the carriers NADH and FADH2, plus substrate-level formation of GTP (often readily converted to ATP). These reduced carriers supply electrons to oxidative phosphorylation, where a proton-motive force drives ATP synthesis. The pathway also serves as a metabolic hub: several intermediates are precursors for biosynthesis, and replenishment reactions maintain cycle flux when intermediates are withdrawn.
The citric acid cycle does not directly consume O2, yet sustained activity is strongly dependent on O2 in aerobic cells because NAD+ and FAD must be regenerated by the electron transport chain. Limited regeneration of NAD+ restricts the availability of oxidized carriers and slows the cycle.
Net reaction and energy yield per acetyl-CoA
One turn of the citric acid cycle oxidizes the two-carbon acetyl group of acetyl-CoA to carbon dioxide while regenerating oxaloacetate. A commonly used net stoichiometry per acetyl-CoA is:
Using widely adopted coupling ratios for oxidative phosphorylation (approximately \(2.5\) ATP per NADH and \(1.5\) ATP per FADH2), the ATP-equivalent yield per acetyl-CoA is:
The value above represents ATP equivalents attributable to the citric acid cycle products under typical textbook assumptions; actual yields vary with organism, shuttle systems, and membrane coupling efficiency.
Cycle map with intermediates, carbon counts, and carrier outputs
Intermediates and outputs in a structured summary
| Intermediate (sequence) | Carbon atoms | Primary transformation | Energy carrier output |
|---|---|---|---|
| Oxaloacetate → Citrate | 4C → 6C | Condensation with acetyl-CoA | None (CoA-SH released) |
| Citrate → Isocitrate | 6C → 6C | Isomerization via dehydration/rehydration | None |
| Isocitrate → α-Ketoglutarate | 6C → 5C | Oxidative decarboxylation | NADH + CO₂ |
| α-Ketoglutarate → Succinyl-CoA | 5C → 4C | Oxidative decarboxylation | NADH + CO₂ |
| Succinyl-CoA → Succinate | 4C → 4C | Thioester cleavage and substrate-level phosphorylation | GTP (ATP-equivalent) |
| Succinate → Fumarate | 4C → 4C | Dehydrogenation | FADH2 |
| Fumarate → Malate | 4C → 4C | Hydration | None |
| Malate → Oxaloacetate | 4C → 4C | Dehydrogenation regenerating oxaloacetate | NADH |
Regulation and integration with cellular metabolism
Control of citric acid cycle flux is concentrated at strongly exergonic, highly regulated reactions. High-energy status tends to restrain flux (high ratios of ATP/ADP, NADH/NAD+), while increased demand for ATP tends to accelerate flux (higher ADP and NAD+ availability through oxidative phosphorylation).
Major regulatory nodes
- Citrate synthase
- Regulation reflects substrate availability (oxaloacetate and acetyl-CoA) and feedback by downstream products; citrate accumulation often correlates with slowed flux.
- Isocitrate dehydrogenase
- A principal rate-influencing step in many tissues; activity is typically favored by ADP and restrained by ATP and NADH, aligning electron-carrier production with energy demand.
- α-Ketoglutarate dehydrogenase
- A multi-enzyme complex with regulation similar to other oxidative decarboxylation systems; NADH and succinyl-CoA commonly act as inhibitory signals.
Amphibolic role and replenishment
The citric acid cycle is amphibolic: it supports catabolism through oxidation of acetyl groups and supports anabolism by supplying intermediates (for example, citrate for cytosolic lipid synthesis, α-ketoglutarate and oxaloacetate for amino acid synthesis, and succinyl-CoA for heme synthesis). Anaplerotic reactions replenish intermediates when biosynthetic withdrawal would otherwise lower oxaloacetate and constrain cycle turnover.
Common conceptual pitfalls
The two CO₂ molecules released per turn originate from decarboxylations within the cycle, yet the carbon atoms that leave as CO₂ during a given turn are not necessarily the same two carbon atoms that entered as the acetyl group in that same turn; carbon scrambling within citrate and isocitrate delays the fate of the acetyl carbons across subsequent turns. Another frequent confusion is oxygen usage: O₂ is not a reactant of the citric acid cycle itself, but aerobic regeneration of NAD+ and FAD via the electron transport chain supports continuous flux.