Enzymes and Metabolism

Enzymes are proteins that function as biological catalysts.

They permit biochemical reactions to occur rapidly at normal body temperatures. Enzymes were initially given somewhat arbitrary names, still with us, such as pepsin

To appreciate the effect of an enzyme, think of what happens when paper burns. Paper is composed mainly of glucose (in the form of cellulose). The burning of glucose can be represented by the equation

Paper does not spontaneously burst into flame, because few of its molecules have enough kinetic energy to react. Lighting the paper with a match, however, raises the kinetic energy enough to initiate combustion (rapid oxidation). The energy needed to get the reaction started, supplied by the match, is called activation energy (fig. 2.26a).

In the body, we carry out the same reaction and oxidize glucose to water and carbon dioxide to extract its energy. We could not tolerate the heat of combustion in our bodies, however, so we must oxidize glucose in a more controlled way at a biologically feasible and safe temperature. Enzymes make this happen by lowering the activation energy—that is, by reducing the barrier to glucose oxidation (fig. 2.26b)—and by releasing the energy in small steps rather than a single burst of heat.

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

Chapter 2 The Chemistry of Life 83

Reaction occurring without a catalyst

Reaction occurring with a catalyst y rg er n e e e y rg er n e e e

Enzymes The Body And Energy

Energy level-of products


Energy level-of products

Effects Enzymes Activation Energy

Figure 2.26 Effect of an Enzyme on Activation Energy. (a) Without catalysts, some chemical reactions proceed slowly because of the high activation energy needed to get molecules to react. (b) A catalyst facilitates molecular interaction, thus lowering the activation energy and making the reaction proceed more rapidly.

Does an enzyme release more energy from its substrate than an uncatalyzed reaction would release?

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  • Joseph
    Does an enzyme release more energy from its substrate than an uncatalyzed reaction would release?
    6 years ago

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