= Receptor
{disambiguate=biochemistry}
{wiki}
= Receptor
{synonym}
A biochemical <receptor> is a <protein> that recognizes a signal and couples its detection to a biological effect. Its binding partner is a <ligand>; recognition depends on compatible shape and chemical interactions at a binding site within the <protein structure>. Specificity is selective recognition, not a guarantee that only one <ligand> can bind. Binding also need not activate the <receptor>: a competing <ligand> may occupy it while preventing activation by another <ligand>. See https://www.ncbi.nlm.nih.gov/books/NBK26911/[Alberts et al., Molecular Biology of the Cell, 4th edition: protein binding and specificity].
Compare a <cell-surface receptor>, accessible to an extracellular <ligand>, with an <intracellular receptor>, whose <ligand> must reach the interior. A <cell-surface receptor> can detect a <hydrophilic> <ligand> without transporting it through the <cell membrane>. In contrast, some membrane-permeable signals reach <intracellular receptors> that regulate <gene expression>. Location and downstream coupling are separate distinctions: an <ionotropic receptor> directly gates an <ion channel>, whereas a <metabotropic receptor> works through intracellular <signal transduction>. See https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors[Clark, Douglas and Choi, Biology 2e, 9.1: receptor locations and mechanisms].
<Receptor binding occupancy> quantifies recognition separately from response. A <dissociation constant> measures binding in a specified equilibrium model; it does not by itself predict the cellular effect. This separation is useful when comparing signals that bind equally well but produce different responses, or when interpreting a binding experiment alongside a functional measurement.
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