The citric acid cycle, also known as the Krebs cycle or TCA cycle, is the central hub of cellular metabolism. It takes place in the mitochondrial matrix and completes the oxidation of carbohydrates, fats, and proteins by converting acetyl-CoA into carbon dioxide.
How the Citric Acid Cycle Works
- Entry: Citrate Formation
Acetyl-CoA (derived from pyruvate, fatty acids, or amino acids) combines with oxaloacetate, a four-carbon molecule, to form citrate, a six-carbon molecule. This reaction is catalyzed by citrate synthase and is essentially irreversible.
- Isomerization and First Oxidation
Citrate is isomerized to isocitrate via the intermediate cis-aconitate. Isocitrate is then oxidized by isocitrate dehydrogenase, releasing the first molecule of CO2 and producing NADH. The product is alpha-ketoglutarate, a five-carbon molecule.
- Second Oxidation
Alpha-ketoglutarate is oxidized by alpha-ketoglutarate dehydrogenase, releasing a second CO2 and producing NADH. This reaction is similar to the pyruvate dehydrogenase reaction and produces succinyl-CoA.
- Substrate-Level Phosphorylation
Succinyl-CoA is converted to succinate by succinyl-CoA synthetase. This reaction produces GTP (or ATP in some organisms) through substrate-level phosphorylation—the only direct ATP-producing step of the cycle.
- Regeneration of Oxaloacetate
Succinate is oxidized to fumarate by succinate dehydrogenase, producing FADH2. Fumarate is then hydrated to malate, and malate is oxidized to oxaloacetate by malate dehydrogenase, producing another NADH. The regenerated oxaloacetate is ready to combine with another acetyl-CoA.
- Net Yield per Turn
Each turn of the cycle produces:
- 3 NADH
- 1 FADH2
- 1 GTP (or ATP)
- 2 CO2
Practical Measurement of TCA Cycle Intermediates
TCA cycle intermediates can be quantified by liquid chromatography-mass spectrometry (LC-MS) to assess metabolic fluxes. Harvest cells (1–5 × 10⁶) or tissue (10–50 mg) by rapid quenching in ice-cold 80% methanol to stop metabolism. Add an internal standard mix containing ¹³C-labeled isotopologues of citrate, succinate, fumarate, malate, and alpha-ketoglutarate (1 nmol each). Extract metabolites by three freeze-thaw cycles in 80% methanol, centrifuge at 15,000 × g for 10 minutes at 4°C, and dry the supernatant under nitrogen gas. Reconstitute in 50 µL of water and inject 5 µL onto a C18 column (2.1 × 100 mm, 1.7 µm) coupled to a triple quadrupole MS operated in negative ion mode. Separate with a gradient of water (0.1% formic acid) and acetonitrile at 0.3 mL/min over 15 minutes. Monitor specific MRM transitions: citrate (191 → 111 m/z), succinate (117 → 73 m/z), fumarate (115 → 71 m/z), malate (133 → 115 m/z), and alpha-ketoglutarate (145 → 101 m/z). For flux analysis, incubate cells with [U-¹³C]glucose for 6 hours; the labeling pattern in TCA intermediates reveals the relative contributions of glucose vs. glutamine as carbon sources. Alternatively, use chemical inhibitors as metabolic probes — fluorocitrate (10–50 µM) inhibits aconitase, blocking the cycle at citrate and causing citrate accumulation measurable by LC-MS within 2 hours.
Real-World Application
In a study of ischemia-reperfusion injury in rat hearts, TCA cycle intermediates are measured by LC-MS. Compared to controls, ischemic hearts show a 60% reduction in succinate and a 3-fold increase in fumarate, indicating Complex II dysfunction. Upon reperfusion, a burst of succinate oxidation contributes to reactive oxygen species production. These findings link TCA cycle disruption to cardiac injury and suggest succinate as a therapeutic target.
