INTRODUCTORY METABOLISM MODULE

CITRIC ACID CYCLE AND ELECTRON TRANSPORT CHAIN
Use these links to navigate to selected parts of this section of the Introductory Metabolism module.

Introduction
Acetyl CoA may be formed from glucose metabolism, fatty acid metabolism or amino acid metabolism.
Irrespective of the source, the bulk of the acetyl CoA will be further oxidised by the pathways described in this section.
These events all occur within the mitochondria of cells.

Citric acid cycle
The first reaction in this cycle is the splitting of the coenzyme A group from acetyl CoA. This splitting releases bond energy which powers the next reaction which is the coupling of the acetyl group to a compound called oxaloacetate. The product of this reaction is citrate.

 
Citrate then undergoes a number of conversions which collectively make up the reactions of the citric acid cycle. The final product of these conversions is oxaloacetate, permitting the intake of another molecule of acetyl CoA so the cycle can repeat itself. The acetyl group of the incoming acetyl CoA is completely oxidised to two molecules of carbon dioxide (CO2).
One reaction produces a molecule of GTP, which is converted to ATP. This is the only direct production of ATP by the citric acid cycle.

Oxidation reactions
Four of the reactions within this cycle are oxidation reactions. In three, NAD acts as the electron acceptor and in the fourth, FAD is the electron acceptor. As the supply of these oxidised cofactors is limited, their reduced forms NADH and FADH must be re-oxidised. This is one of the functions of a pathway coupled to the citric acid cycle and known as the electron transport chain.

Electron transport chain

The re-oxidation of NADH to NAD or FADH to FAD begins a series of oxidation/reduction reactions resulting in two products ;
  • bond energy released by the reactions is harnessed to the reaction by which ATP is synthesised

ADP + Pi ¾¾¾¾¾¾® ATP

Three ATP are synthesised for each NADH entering the reaction pathway, while each FADH results in the synthesis of two ATP molecules. (Some texts give figures of 2.5 and 1.5 respectively.)
  • water
The final electron acceptor of the electron transport chain is oxygen. This reaction reduces oxygen to water.

This completes this section on the citric acid cycle and the electron transport chain.

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