Cell Biology/Energy supply/Krebs cycle

The Krebs Cycle is also known as the Citric Acid Cycle or Tricarboxylic Acid (TCA) cycle and occurs within the Matrix of the Mitocondria.

Acetyl-Coenzyme A produced from the Link Reaction is synthesised into Citric Acid by the action of citrate synthase and its combination with Oxaloacetate. The citric acid, known as Citrate in its ionised form is then converted into Isocitrate by aconitase. This is then converted to Alpha-Ketoglutarate by citrate dehydrogenase in an oxidative decarboxylation reaction, in which one molecule of CO2 is given off and one molecule of NAD+ is oxidised to NADH. Another oxidative deecarboxylation converts the alpha-ketoglutarate to Succinyl Coenzyme A with the addition of CoA-SH and again the loss of a CO2 and the oxidation of NAD+ is seen. Succinyl CoA is then converted into Succinate by the action of Succinyl CoA Synthetase, the addition of water, the loss of CoA-SH and the substrate level phosphorylation of one molecule of ADP to ATP. Succinate is then converted to Fumarate by Succinate Dehydrogenase, a dehydrogenation reaction which results in the reduction of FADH+ to FADH2. Fumarate is, in turn, hydrated by the addition of one water molecule by Fumarase to Malate which is finally converted back to Oxaloacetate by Malate Dehydrogenase in a dehydrogenation reaction that oxidises one NAD+ to NADH. The cycle can then begin again.

There are many products of, and substrates for the Krebs cycle: for instance, amino acids can be metabolised to or catabolised from Acetyl CoA, Alpha-Ketoglutarate, Succinyl CoA, Malate and Oxaloacetate, depending which specific amino acid. Pyruvate can be re-synthesised by Pyruvate carboxylase from Oxaloacetate, which can either replenish the Krebs Cycle or make glucose by Gluconeogenesis. Citrate can be used for fatty acid (fat) or cholesterol synthesis. Succinyl CoA can be used to make Haem for Red Blood Cells.

NADH and FADH2 are then used in the electron transport chain in the inner mitochondrial membrane.

Net gain for the Krebs Cycle: 2 Carbon Dioxide, 1 ATP, 3 NADH, and 1 FADH2 per molecule of Acetyl CoA