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Factors Affecting Enzyme Activity

Sometimes it takes more than just the presence of the substrate and the enzyme to allow a reaction to take place. Many enzymes require the presence of a cofactor to function. Simple cofactors are usually ionic minerals.
For example, many enzymes require magnesium or zinc ions, for which they have binding sites, in order to act as catalysts. If the cofactor is not present, the enzyme will not work or will have reduced activity.
Sometimes the cofactor is a nonprotein organic molecule, called a coenzyme. The coenzyme plays a central role in the catalyzed reaction, often by accepting or providing electrons, and becoming reduced or oxidized, respectively, in the process. The vitamins of the B-complex group or their derivatives serve as coenzymes in the catabolic reactions that oxidize food molecules to release energy (see Chapter 4).
Environmental conditions also play a major role in determining an enzyme's effectiveness. In particular, temperature and pH influence enzyme activity enormously, and this is the major reason these parameters are so tightly controlled in humans. The temperature and pH of our bodies cannot vary considerably without dire consequences. Why are enzymes affected so profoundly by these environmental conditions?
Enzyme activity, in general, is reduced as the temperature drops and is increased as the temperature rises. If our body temperature becomes colder than normal, enzymes will function, but at slower and slower rates as the temperature falls. Ultimately, these rates will be too slow to sustain life. Raising the temperature will increase enzyme activity, to a certain point. If the temperature is too high, the interactions that maintain the shape of the enzyme will be disrupted, changing the overall shape of the protein and destroying, often irreversibly, the functionality of the active site. When an enzyme loses its activity due to disruption of its three-dimensional configuration, that enzyme has become denatured. Both low and high pH values can also denature enzymes by disrupting the interactions that hold them in their proper orientation, in this case by affecting the charges of various R groups and subsequently changing their affinities for each other. Every enzyme has an optimum pH and temperature at which it functions best; any deviation from these values will reduce its ability to catalyze reactions.

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