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of the TCA cycle does not have sufficient energy to reduce NAD+ or FAD but does have sufficient energy to phosphorylate GDP to GTP. One acetyl-CoA molecule reduces one FAD and three NAD+ molecules.

      The stoichiometry is precise, such that both the carbons of the acetyl group are oxidised (though as shown by radiolabelling experiments, not the actual two carbon atoms that entered the cycle), hence all their useful energy is extracted, resulting in two carbon dioxide molecules as ‘waste’ products. The NADH and FADH2 will subsequently be re-oxidised by passing on the electron(s) they carry down the redox proteins of the electron transport chain, regenerating NAD+ and FAD in the process for further electron carriage; by coupling this sequential oxidation-reduction to phosphorylation of ADP to ATP (‘oxidative phosphorylation’), a common energy carrier for diverse cellular functions (ATP) is synthesised (see below).

      1.3.1.5 Electron transport chain

      The final stage of energy production is the synthesis of ATP by phosphorylation of ADP, coupled to the re-oxidation of NADH and FADH2 to permit further electron carriage to occur (oxidative phosphorylation). The energy is now carried in the phosphoanhydride bonds of the phosphate groups of ATP; these are commonly called ‘high energybonds (see Box 1.6).

      The term ‘high-energy bond’ is not strictly accurate. They are not ‘special’ bonds, but rather they release their free energy when hydrolysed. They may be denoted ∼, and it can be seen that ADP can act as a source of energy when its terminal phosphate group is hydrolysed to AMP.

       AMP: Ad-P

       ADP: Ad-P∼P

       ATP: Ad-P∼P∼P

      ADP has a special role in bioenergetics because beside acting as a (limited) energy source in its own right, its availability is one factor that regulates the rate of ATP synthesis.

      A close structural analogue of adenosine triphosphate (ATP) is guanosine triphosphate (GTP) which also carries energy. The presence of these two forms of energy carriage likely represents a form of metabolic compartmentation, separating pathways by their molecular preferences. GTP also has a function in regulation, especially in signal transduction involving G-proteins.

      In the remainder of this chapter, we will outline the major metabolic pathways of ‘energy metabolism’ that will be considered further in this book, signposting the reader to where these are discussed in more detail.

      1.3.2 Carbohydrate metabolism

      1.3.2.1 Pathways of glucose metabolism

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