Redirected from Krebs cycle
The citric acid cycle forms part of carbohydrate catabolism, protein catabolism and fat catabolism. All these three processes produce acetyl-CoA, a two-carbon acetyl group bound to coenzyme A. Acetyl-CoA is the main input to the citric acid cycle.
|I. Citrate||1. Aconitase||Dehydration||H2O|
|II. cis-Aconitate||2. Aconitase||Hydration||H2O|
|III. Isocitrate||3. Isocitrate Dehydrogenase||Oxidation||NAD+||NADH+H+|
|IV. Oxalosuccinate||4. Isocitrate Dehydrogenase||Decarboxylation|
|V. α-Ketoglutarate||5. α-Ketoglutarate Dehydrogenase||Oxidative Decarboxylation||NAD+
|VI. Succinyl-CoA||6. Succinyl-CoA Synthetase||Hydrolysis||GDP
|VII. Succinate||7. Succinate Dehydrogenase||Oxidation||FAD+||FADH2|
|VIII. Fumarate||8. Fumerase||Addition (H2O)||H2O|
|IX. L-Malate||9. Malate Dehydrogenase||Oxidation||NAD+||NADH+H+|
|X. Oxaloacetate||10. Citrate Synthetase||Condensation|
The sum of all reactions in the citric acid cycle is :
Two carbons are oxidized to CO2, and the energy from these reactions are stored in ATP (ATP is the "universal energy currency" of the cell), NADH and FADH2. NADH and FADH2 are coenzymes[?] (Coenzymes are molecules that enable or enhance enzymes.) that store energy and can release it when needed.
Figure 2 : Schematic drawing of metabolic pathways associated with the citric acid cycle.
The citric acid cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon-molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle.
In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. The glycerol is then converted into glyceraldehyde-3-phosphate, and enters glycolysis and eventually the ctric acid cycle. Fatty acids are broken down through a process known as beta oxidation[?] which results in acetyl-CoA to be used in the citric acid cycle.
The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD+, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does.
The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the citric acid cycle and oxidative phosphorylation equals 38 ATP.