Metabolic Pathways

The many thousands of different types of enzymatic reactions within a cell do not occur independently of each other. They are, rather, all linked together by intricate webs of interrelationships, the total pattern of which constitutes cellular metabolism. A sequence of enzymatic reactions that begins with an initial substrate, progresses through a number of intermediates, and ends with a final product is known as a metabolic pathway.

The enzymes in a metabolic pathway cooperate in a manner analogous to workers on an assembly line, where each contributes a small part to the final product. In this process, the

■ Figure 4.7 The general pattern of a metabolic pathway. In metabolic pathways, the product of one enzyme becomes the substrate of the next.

■ Figure 4.8 A branched metabolic pathway. Two or more different enzymes can work on the same substrate at the branch point of the pathway, catalyzing two or more different reactions, product of one enzyme in the line becomes the substrate of the next enzyme, and so on (fig. 4.7).

Few metabolic pathways are completely linear. Most are branched so that one intermediate at the branch point can serve as a substrate for two different enzymes. Two different products can thus be formed that serve as intermediates of two pathways (fig. 4.8).

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