The TCA (tricarboxylic acid) cycle, also known as the Krebs cycle or citric acid cycle, is a metabolic pathway that occurs in the mitochondria of eukaryotic cells. It is responsible for the oxidation of acetyl-CoA, derived from carbohydrates, fats, and proteins, to produce energy in the form of ATP. The TCA cycle begins with a process called condensation.

Condensation

In this process, the acetyl group from acetyl-CoA is combined with oxaloacetate to form citrate, a six-carbon molecule. This reaction is catalyzed by the enzyme citrate synthase, and it requires the hydrolysis of one molecule of ATP.

Dehydrogenation

The next step in the TCA cycle is dehydrogenation, which involves the removal of hydrogen atoms from the citrate molecule. This reaction is catalyzed by the enzyme aconitase, and it produces a molecule called cis-aconitate. The cis-aconitate molecule is then converted to isocitrate by the enzyme aconitase.

Decarboxylation

The third step in the TCA cycle is decarboxylation, which involves the removal of a carbon dioxide molecule from isocitrate. This reaction is catalyzed by the enzyme isocitrate dehydrogenase, and it produces a molecule called alpha-ketoglutarate. During this reaction, two hydrogen atoms are also removed, which are transferred to NAD+ to produce NADH.

Further steps

The alpha-ketoglutarate molecule then undergoes another round of dehydrogenation and decarboxylation to produce the molecule succinyl-CoA. This reaction is catalyzed by the enzyme alpha-ketoglutarate dehydrogenase complex, and it produces a molecule of NADH. Succinyl-CoA then reacts with a molecule of GDP and inorganic phosphate to produce a molecule of GTP and succinate. This reaction is catalyzed by the enzyme succinyl-CoA synthetase.

The succinate molecule is then oxidized to produce fumarate, which is catalyzed by the enzyme succinate dehydrogenase. This reaction also produces a molecule of FADH2. Fumarate is then converted to malate by the enzyme fumarase. Finally, malate is oxidized to produce oxaloacetate, which is catalyzed by the enzyme malate dehydrogenase. This reaction also produces a molecule of NADH.

Conclusion

In conclusion, the TCA cycle starts with the process of condensation, in which the acetyl group from acetyl-CoA is combined with oxaloacetate to form citrate. The cycle then proceeds through a series of dehydrogenation, decarboxylation, and other reactions to produce ATP, NADH, and FADH2, which can be used for energy production.

The TCA cycle is a central pathway into which many metabolites feed. It consists of a number of reactions which generate NADH and FADH₂ which can in turn be used by the oxidative phosphorylation pathway to generate ATP. The TCA cycle occurs in the matrix of the mitochondria.