Citric acid cycle

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide and ATP. The cycle is a key component of the metabolic pathway by which all aerobic organisms generate energy.

Overview

The citric acid cycle takes place in the mitochondria of eukaryotic cells and in the cytoplasm of prokaryotic cells. It is a central part of cellular respiration, which also includes glycolysis and the electron transport chain.

Steps of the Citric Acid Cycle

The cycle consists of eight main steps:

1. **Citrate Synthase**: Acetyl-CoA combines with oxaloacetate to form citrate.
2. **Aconitase**: Citrate is converted into isocitrate.
3. **Isocitrate Dehydrogenase**: Isocitrate is oxidized to alpha-ketoglutarate, producing NADH and carbon dioxide.
4. **Alpha-Ketoglutarate Dehydrogenase**: Alpha-ketoglutarate is converted to succinyl-CoA, producing NADH and carbon dioxide.
5. **Succinyl-CoA Synthetase**: Succinyl-CoA is converted to succinate, producing GTP (or ATP).
6. **Succinate Dehydrogenase**: Succinate is oxidized to fumarate, producing FADH2.
7. **Fumarase**: Fumarate is converted to malate.
8. **Malate Dehydrogenase**: Malate is oxidized to oxaloacetate, producing NADH.

Importance

The citric acid cycle is crucial for the production of energy in the form of ATP. It also provides intermediates for the synthesis of various compounds, including amino acids, nucleotides, and other essential molecules.

Regulation

The cycle is tightly regulated by the availability of substrates and feedback inhibition by its products. Key regulatory enzymes include citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.

Related Pathways

The citric acid cycle is interconnected with several other metabolic pathways, including:

• Glycolysis
• Gluconeogenesis
• Fatty acid oxidation
• Amino acid metabolism

History

The cycle was first identified by Hans Adolf Krebs in 1937, for which he was awarded the Nobel Prize in Physiology or Medicine in 1953.

See Also

• Cellular respiration
• Oxidative phosphorylation
• Metabolism
• Mitochondrion