Nadh ch

cooh acetate

9 enzyme-catalysed reactions (the citric acid cycle)

mitochondrial electron transport chain

Figure 2.17 The oxidation of ethanol. The box shows the rapid non-enzymic reaction when ethanol is burned in air; metabolic oxidation of ethanol involves 11 separate enzyme-catalysed steps, as well as the mitochondrial electron transport chain.

shown in Figure 2.18, in which an intermediate may proceed down one branch or another. The fate of an intermediate at a branch point will depend on the relative activities of the two enzymes that are competing for the same substrate. As discussed above (section 2.3.3), if the enzymes catalysing the reactions from D ^ P and from D ^ X have different values of Km, then it is possible to predict which branch will predominate at any given intracellular concentration of D.

Enzymes catalysing reactions at branch points are usually subject to regulation (section 10.1), so as to direct substrates through one branch or the other, depending on the body's requirements at the time.

Figure 2.18 Linear and branched metabolic pathways.

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