Metabolic pathways

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 3.3.1.2). 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.

Metabolic pathways can be divided into three broad groups:

  • Catabolicpathways, involved in the breakdown of relatively large molecules and oxidation, ultimately to carbon dioxide and water. These are the main energy-yielding metabolic pathways.
  • Anabolic pathways, involved in the synthesis of compounds from simpler precursors. These are the main energy-requiring metabolic pathways. Many are reduction reactions, and many involve condensation reactions. Similar reactions are also involved in the metabolism of drugs and other foreign compounds, and hormones and neurotransmitters, to yield products that are excreted in the urine or bile.
  • Central pathways, involved in interconversions of substrates, that can be regarded as being both catabolic and anabolic. The principal such pathway is the citric acid cycle (section 5.4.4).

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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