There are substrates that block active sites from bonding to a substrate. These substances are called inhibitors. There are two kinds of inhibitors: competitive and noncompetitive. A competitive inhibitor is a substance that binds to the active site of an enzyme, thus preventing the active site from binding with the substrate. For example, sulfa drugs contain the chemical sulfanilamid. Sulfa drugs inhibit microbial growth by fitting into the active site of an enzyme required in the conversion of paraaminobenzoic acid (PABA) into the B vitamin folic acid. Folic acid is needed for DNA synthesis in bacteria and thus prevents bacteria from growing. A noncompetitive inhibitor binds to another site on the enzyme called the allosteric site and in doing so alters the shape of the active site of the enzyme. The shape of the active site no longer complements the corresponding site on the substrate and therefore no binding occurs. Noncompetitive inhibitors do not bind to active sites.
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