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The action of the enzyme catalase is obvious when using hydrogen peroxide to cleanse a wound. Injured cells release catalase, and when hydrogen peroxide contacts them, bubbles of oxygen are set free. The resulting foam removes debris from inaccessible parts of the wound.

During an enzyme-catalyzed reaction, regions of the enzyme molecule called active sites temporarily combine with portions of the substrate, forming an enzymesubstrate complex. This interaction strains chemical bonds in the substrate in a way that makes a particular chemical reaction more likely to occur. When it does, the enzyme is released in its original form, able to bind another substrate molecule (fig. 4.4).

Enzyme catalysis can be summarized as follows:



Substrate + Enzyme ^ substrate ^ (changed + complex substrate)

Enzyme (unchanged)

The speed of an enzyme-catalyzed reaction depends partly on the number of enzyme and substrate molecules in the cell. The reaction occurs more rapidly if the concentration of the enzyme or the concentration of the substrate increases. Also, the efficiency of different kinds of enzymes varies greatly. Thus, some enzymes can process only a few substrate molecules per second, whereas others can handle thousands or nearly a million substrate molecules per second.

Cellular metabolism includes hundreds of different chemical reactions, each controlled by a specific kind of enzyme. Often sequences of enzyme-controlled reactions, called metabolic pathways, lead to synthesis or breakdown of particular biochemicals (fig. 4.5). Thus, hundreds of different kinds of enzymes are present in every cell.

Enzyme names are often derived from the names of their substrates, with the suffix -ase added. For example,

Shier-Butler-Lewis: I I. Levels of Organization I 4. Cellular Metabolism I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Substrate molecules

Product molecule

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