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[edit]

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[edit]

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

Formation of citrate
Isomerization of citrate
First oxidation and decarboxylation
Second oxidation and decarboxylation
Substrate-level phosphorylation
Dehydration
Hydration
Regeneration of oxaloacetate

Regulation[edit]

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[edit]

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

References[edit]

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