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Control of Enzyme Activity

While ultimately the control of enzymes is accomplished genetically (see Chapters 5 and 6), several other processes affect when and how efficiently an enzyme works, so the cell carries out the desired reactions at the right times and rates. A classic example of an enzyme control mechanism is called feedback inhibition. Many enzymes, in addition to their active sites, contain binding sites for other molecules. These are referred to as allosteric sites, and enzymes that contain them are allosteric enzymes. If the allosteric site has an affinity for the product of the enzyme-catalyzed reaction, a feedback loop will automatically control how much of the product is produced. If product concentrations are low, the cell requires the reaction to proceed; little of the product exists to bind to the allosteric site, and the enzyme functions normally. As the reaction progresses, and enough of the product is made, it binds to the allosteric site. When this happens, the overall shape of the enzyme changes so that the active site is either hidden or disrupted, and the reaction will take place at a reduced rate or not at all. When product levels drop again, the product dissociates from the allosteric site, exposing the active site and allowing the reaction to resume. This process acts like a “thermostat”, and is a common cellular strategy for regulating enzyme activity.
Sometimes molecules are present that act to inhibit an enzyme’s function, and these are appropriately called enzyme inhibitors. Competitive inhibitors resemble the substrate, and thus compete with it for binding to the active site. Non-competitive inhibitors bind to an allosteric site, causing the enzyme’s shape to change and affecting its function. Therefore, competitive inhibitors are less effective as the substrate concentration rises, while non-competitive inhibitors work regardless of the substrate concentration.

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