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In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling (or "signal") molecule may attach. A molecule which binds to a receptor is called a "ligand," and may be a peptide (such as a neurotransmitter), a hormone, a pharmaceutical drug, or a toxin, and when such binding occurs, the receptor goes into a conformational change which ordinarily initiates a cellular response. However, some ligands merely block receptors without inducing any response (e.g. antagonists). Ligand-induced changes in receptors result in physiological changes which constitute the biological activity of the ligands.
The shapes and actions of receptors are studied by X-ray crystallography and computer modelling, which have advanced the understanding of drug action at the binding sites of receptors.
Depending on their functions and ligands, several types of receptors may be identified:
·Some receptor proteins are peripheral membrane proteins.
·Many hormone and neurotransmitter receptors are transmembrane proteins: transmembrane receptors are embedded in the phospholipid bilayer of cell membranes, that allow the activation of signal transduction pathways in response to the activation by the binding molecule, or ligand.
oMetabotropic receptors are coupled to G proteins and affect the cell indirectly through enzymes which control ion channels.
oIonotropic receptors contain a central pore which functions as a ligand-gated ion channel.
·Another major class of receptors are intracellular proteins such as those for steroid and intracrine peptide hormone receptors. These receptors often can enter the cell nucleus and modulate gene expression in response to the activation by the ligand.