Biological functions are carried out by protein receptors, transporters, and enzymes. Receptors bind specific molecules to alter cellular function. Enzymes bind specific molecules to carry out a reaction (conversion of substrate to product). Transporters and channels carry specific molecules across cell membranes.
When the binding of one molecule to another causes a response within a cell, the molecule that binds is called a ligand and the molecule that initiates the response is the receptor. This binding of ligands to receptors is highly specific. A ligand is said to be high affinity if low concentrations are effective in causing a response. Higher concentrations of low affinity ligands are required to cause responses (activity).
Drugs act on specific receptors to alter biological functions. Agonists duplicate the activity of a natural ligand. Antagonists compete with the natural ligand for binding to the receptor. A molecule that stops an enzymatic reaction is an inhibitor.
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