Metabolic Pathways

A metabolic pathway is a chain of reactions with each step usually catalyzed by a different enzyme. A simple metabolic pathway can be symbolized a P 7 A ^ B ^ C ^ D

where A is the initial reactant, B and C are intermediates, and D is the end product. The Greek letters above the reaction arrows represent enzymes that catalyze each step of the reaction. A is the substrate for enzyme a, B is the substrate for enzyme P, and C for enzyme 7. Such a pathway

28nicotinamide adenine dinucleotide

Glycolysis Aerobic respiration

Glycolysis Aerobic respiration

Figure 2.28 The Action of a Coenzyme. A coenzyme such as NAD acts as a shuttle that picks up electrons from one metabolic pathway (in this case, glycolysis) and delivers them to another pathway (in this case, aerobic respiration).

can be turned on or off by altering the conformation of any of these enzymes, thereby activating or deactivating them. This can be done by such means as the binding or dissociation of a cofactor, or by an end product of the pathway binding to an enzyme at an earlier step (product D binding to enzyme a and shutting off the reaction chain at that step, for example). In these and other ways, cells are able to turn on metabolic pathways when their end products are needed and shut them down when the end products are not needed.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

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