Tricarboxylic acid cycle

Tricarboxylic acid cycle (TCA cycle), also known as the Krebs cycle or the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate (ATP). In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically.

Overview

The TCA cycle is the second stage of cellular respiration, the three stages being: glycolysis, TCA cycle, and oxidative phosphorylation. The cycle includes eight major steps. The cycle was first elucidated by scientist Hans Adolf Krebs (1900–1981).

Steps of the TCA cycle

The TCA cycle consists of eight steps, each catalyzed by a specific enzyme.

1. Formation of citrate
2. Isomerization of citrate
3. First oxidation and decarboxylation
4. Second oxidation and decarboxylation
5. Substrate-level phosphorylation
6. Dehydration
7. Hydration
8. Regeneration of oxaloacetate

Regulation

The TCA cycle is regulated mainly by product inhibition and substrate availability. The enzymes of the TCA cycle are regulated by the energy status of the cell. The key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

Clinical significance

Defects in TCA cycle enzymes can lead to various disorders, such as Leigh's syndrome, Kearns-Sayre syndrome, and MELAS syndrome.

See also

• Glycolysis
• Oxidative phosphorylation
• Pyruvate decarboxylation
• Electron transport chain

References

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Glucuronate metabolism

Pentose interconversion

Inositol metabolism

Cellulose and sucrose
metabolism

Starch and glycogen
metabolism

Other sugar
metabolism

Pentose phosphate pathway

Glycolysis and Gluconeogenesis

Amino sugars metabolism

Small amino acid synthesis

Branched amino acid
synthesis

Purine biosynthesis

Histidine metabolism

Aromatic amino
acid synthesis

Pyruvate
decarboxylation

Fermentation

Fatty acid
metabolism

Urea cycle

Aspartate amino acid
group synthesis

Porphyrins and
corrinoids
metabolism

Citric acid cycle

Glutamate amino
acid group
synthesis

Pyrimidine biosynthesis

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