We have concentrated so far on the metabolism of carbohydrates, but both lipids and proteins may also act as energy sources. Both are converted by a series of reactions to an intermediate compound that can then enter the pathways of metabolism we have discussed above.
Lipids often form an important energy source for microorganisms; they are plentiful in nature, as they form the major component of cell membranes, and may also exist as cellular storage structures. Lipids are hydrolysed by a class of enzymes called lipases to their constituent parts; the fatty acids so produced enter the cyclic j-oxidation pathway. In this, fatty acids are joined to coenzyme A to form an acyl-CoA, and shortened by two carbons in a series of reactions (Figure 6.24). Molecules of NADH and FADH2 derived from j-oxidation can enter the electron transport chain to produce ATP. Acetyl-CoA, you will recall from earlier in this chapter, serves as the entry point into the TCA cycle. When you consider that a single turn of the TCA cycle gives rise to the production of 14 molecules of ATP, you can appreciate what a rich source of energy a 16- or 18-carbon fatty acid represents. The glycerol component of a lipid requires only slight modification in order to enter the glycolytic pathway as dihydroxyacetone phosphate (see 'Glycolysis' above).
Proteins are a less useful source of energy than lipids or carbohydrates, but may be utilised when these are in short supply. Like lipids, they are initially hydrolysed to their constituent 'building blocks', in this case, amino acids. These then undergo the loss of an amino group (deamination), resulting in a compound that is able to enter, either directly or indirectly, the TCA cycle.
Figure 6.24 j-oxidation of lipids. j-Oxidation comprises a series of four reactions, repeated for the removal of each two-carbon unit. Formation of the acyl CoA ester requires the expenditure of ATP, but there is a net gain in reducing power (NADH + FADH2), which can feed into the electron transport chain. The shortened acyl chain at the end of the process can re-enter the cycle and become further shortened, while acetyl CoA can enter directly into the TCA cycle (CoA-SH = coenzyme A)
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