A simple reaction, such as the oxidation of ethanol (alcohol) to carbon dioxide and water, can proceed in a single step — for example, simply by setting fire to the alcohol in air. The reaction is exothermic, and the oxidation of ethanol to carbon dioxide and water yields an output of 29 kJ/g.
When alcohol is metabolized in the body, although the overall reaction is the same, it does not proceed in a single step, but as a series of linked reactions, each resulting in a small change in the substrate. As shown in Figure 2.17, the metabolic oxidation of ethanol involves 11 enzyme-catalysed reactions, as well as the mitochondrial electron transport chain (section 18.104.22.168). The energy yield is still 29 kJ/g, as the starting material (ethanol) and the end products (carbon dioxide and water) are the same, and hence the change in energy level is the same overall, regardless of the route taken. Such a sequence of linked enzyme-catalysed reactions is a metabolic pathway.
In some metabolic pathways all the enzymes are free in solution, and intermediate products are released from one enzyme, equilibrate with the pool of intermediate in the cell, and then bind to the next enzyme.
In some cases, two or more enzymes catalysing consecutive steps in a pathway may be physically adjacent, either bound to a membrane or in a multienzyme complex, so that the product of one enzyme is passed directly to the active site of the next, without equilibrating with the pool of intermediate in the cell.
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