Because enzymes are responsible for controlling so many reactions within the body, enzymes, in turn, are subject to numerous control mechanisms, including sensitivity to temperature, pH, substrate concentration, and, especially, chemical agents that can block or disable enzymes' active sites. Competitive inhibitors are chemical agents that resemble the substrate sufficiently to bind with the enzyme's active site, thereby preventing the substrate from doing so. Noncompetitive inhibitors are chemical agents that bind away from an enzyme's active site and decrease the rate at which the enzyme catalyzes reactions. With regard to allosteric enzymes, or enzymes that can exist in either an active or inactive state, negative modulators (which are noncompetitive inhibitors) stabilize such enzymes in their inactive state.
