Active site
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The active site of an enzyme is the binding site where catalysis occurs. The structure and chemical properties of the active site allow the recognition and binding of the substrate.
The active site in many enzymes can be inhibited or suppressed by the presence of another molecule. There are three primary modes of inhibition. In competitive inhibition, the active site itself is blocked when a molecule chemically similar to the substrate binds to the active site but cannot be processed by the enzyme. In noncompetitive inhibition, the inhibitor binds to the enzyme at another site, the allosteric site, and this causes a structural change in the enzyme such that the active site is rendered useless. Uncompetitive inhibition, is similar to noncompetitive inhibition except that the inhibitor can only bind the enzyme-substrate complex rather than the free enzyme.
Models
There are several models of how enzymes work. The lock-and-key model and the induced fit model whereby the active site binds and encloses the substrate molecule. Often enzymes bond to their substrate by Van der Waals forces or hydrogen bonds between the R' groups in the amino acid monomers.