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Drugs generally work by interacting with receptors on the surface of cells or enzymes (which regulate the rate of chemical reactions) within cells. Receptor and enzyme molecules have a specific three-dimensional structure which allows only substances that fit precisely to attach to it. This is often referred to as a lock and key model.
Most drugs work because by binding to the target receptor site, they can either block the physiological function of the protein, or mimics it's effect. If a drug causes the protein receptor to respond in the same way as the naturally occurring substance, then the drug is referred to as an agonist. Examples of agonists are morphine, nicotine, phenylephrine, and isoproterenol. Antagonists are drugs that interact selectively with receptors but do not lead to an observed effect. Instead they reduce the action of an agonist at the receptor site involved. Receptor antagonists can be classified as reversible or irreversible. Reversible antagonists readily dissociate from their receptor. Irreversible antagonists form a stable chemical bond with their receptor (eg, in alkylation). Examples of antagonist drugs are: beta-blockers, such as propranolol.
Instead of receptors, some drugs target enzymes, which regulate the rate of chemical reactions. Drugs that target enzymes are classified as inhibitors or activators (inducers). Examples of drugs that target enzymes are: aspirin, cox-2 inhibitors and hiv protease inhibitors (see below).
Many drug companies will design structural variants for compounds that bind receptor sites in hope of making a compound that is more effective. Until recently design of new drugs was very difficult. Scientists had no way to know what the binding site of the protein looked like. Scientist now have a powerful new tool. Molecular modeling allows researchers to view the 3-D structure of proteins and their binding sites using data from X-ray crystallography and NMR spectroscopy . The synthesis of several recent drugs (including HIV Protease Inhibitors for treatment of AIDS) have been assisted using the 3-D structure of protein.