Krebs Cycle or Citric Acid Cycle or Tricarboxylic Acid Cycle
Krebs cycle or citric acid cycle or TCA cycle is a series of biochemical reactions that takes place in mitochondria (in eukaryotes) or cytoplasm (in prokaryotes) and is used to release stored energy derived from carbohydrates, fats and proteins in the body. It is used by organisms to generate energy for various metabolic processes.
Under acrobic conditions, pyruvate is oxidised to CO2 and H2O through a series of biochemical reactions, collectively called as Citric acid cycle (Tricarboxylic acid cycle) or more commonly as Kreb's cycle. It is the most important metabolic pathway related with energy production in animal systems. All the enzymes, catalyzing various reactions in the pathway, are localized in mitochondria. Citric acid cycle is also known as a common metabolic pathway because the intermediates of this cycle are also synthesized from compounds other than carbohydrates, like fats and proteins.
The reactions and the intermediates formed in the cycle are described below-
Pyruvate to Acetyl-CoA
Pyruvate is oxidatively decarboxylated to acetyl-CoA for its entry into citric acid cycle. This reaction is catalysed by a group of enzymes called pyruvate dehydrogenase complex which includes three different enzymes. It also utilizes thiamine pyrophosphate (TPP), lipoic acid, CoA, FAD and NAD+ as coenzymes.
Oxidative decarboxylation of Pyruvate
1. Pyruvate dehydrogenase
2 & 3. Dihydrolipoyl transacetylase
4. Dihydrolipoyl dehydrogenase
Pyruvate is decarboxylated with the help of TPP to form acetaldehyde-TPP in the presence of pyruvate dehydrogenase. Oxidized lipoate then reacts with acetaldehyde-TPP and converts it to acetyl lipoate which in turn accepts CoA and forms acetyl-CoA. Lipoic acid is released in the reduced form. Both these reactions are catalysed by dihydrolipoyl transacetylase. Reduced lipoate is reoxidized by dihydrolipoyl dehydrogenase and FAD.
Oxaloacetate + acetyl-CoA to Citrate
Acetyl-CoA combines wth oxaloacetate to form citrate. A molecule of water is consumed and CoA is released. This reaction is catalysed by a condensing enzyme citrate synthetase. It is an irreversible reaction.
Citrate to isocitrate
The conversion of citrate to isocitrate is catalysed by aconitase and takes place in two steps. In the first step citrate is dehydrated to cis-aconitate which is then rehydrated to form isocitrate. The reaction is inhibited in the presence of fluoroacetate.
1. Pyruvate dehydrogenase complex 2. Citrate synthetase 3. Aconitase 4. Isocitrate dehydrogenase 5. Isocitrate dehydrogenase 6. α-ketoglutarate dehydrgenase complex 7. Succinic thiokinase 8. Succinate dehydrogenase 9. Fumarase
10. Malate dehydrogenase
Isocitrate to Oxalosuccinate
Isocitrate undergoes dehydrogenation to form oxalosuccinate. The reaction is catalysed by isocitrate dehydrogenase and NAD+ is reduced to NADH + H+
Oxalosuccinate to α-Ketoglutarate
Oxalosuccinate is decarboxylated to α-ketoglutarate. This reaction is also catalysed by isocitrate dehydrogenase in the presence of manganese ions.
α-Ketoglutarate to Succinyl-CoA
α-ketoglutarate is oxidatively decarboxylated to succinyl-CoA. The mechanism of the reaction is similar to the oxidative decarboxylation of pyruvate. The reaction is irreversible and catalysed by α-ketoglutarate dehydrogenase in the presence of TPP, lipoate, FAD and NAD+. It is inhibited by arsenite.
Succinyl-CoA to Succinate
Succinyl-CoA conversion to succinate is accompanied by formation of a high energy phosphate bond (GTP) in the presence of GDP and inorganic phosphate. GTP then transfers the high energy bond to form ATP. This type of phosphorylation is called substrate level phosphorylation. This reaction is catalysed by succinyl thiolase.
Succinate to fumarate
Succinate undergoes dehydrogenation to form fumarate in the presence of succinate dehydrogenase and FAD which is converted to FADH.
Fumarate to malate
Fumarate is hydrated to form malate in the presence of fumarase. Although no coenzyme is required in this reaction, phosphate is believed to have activation effect.
Malate to oxaloacetate
The cycle is thus completed with the conversion of malate to oxaloacetate. It is a dehydrogenation reaction catalysed by malate dehydrogenase in the presence of NAD+ which is subsequently reduced.
It should thus be noted that in the oxidation of pyruvate through the above process, 4 molecules of NADH and 1 molecule of FADH are formed per cycle. Those coenzymes are reoxidized through the respiratory chain enzymes of mitochondria yielding 3ATP molecules per molecule of NADH and 2ATP molecules per FADH molecule.
Energetics of Citric Acid Cycle
Total number of ATP formed during the oxidation of pyruvate through citric acid cycle.
| Reaction | ATP formed through oxidative phosphorylation | ATP formed by substrate level posphorylation |
|---|---|---|
| Puruvate → acetyl-CoA | 3 | — |
| Isocitrate → oxalosuccinate | 3 | — |
| α-ketoglutarate → succinyl-CoA | 3 | — |
| Succinyl-CoA → succinate | — | 1 |
| Succinate → fumarate | 2 | — |
| Malate → oxaloacetate | 3 | — |
| 14 | 1 | |
| Total | 15 ATP molecules |
As one mole of glucose gives two moles of pyruvate
total number of ATP formed in citric acid cycle = 15 x 2 = 30
Total number of ATP formed in aerobic breackdown of one mole of glucose = 30 (from CAC) + 8 (from glycolysis) = 38 ATP
