Receptors

Receptors are molecular structures that detect and respond to signals from the external environment or from within an organism. In most cells, receptors are proteins or protein complexes that bind specific ligands with high specificity. Upon ligand binding, receptors undergo conformational changes that initiate a cascade of intracellular events, translating a signal at the cell surface into a functional response. Receptors are broadly categorized as cell-surface receptors, which detect hydrophilic ligands outside the cell, and intracellular receptors, which bind lipophilic ligands that cross the membrane.

Cell-surface receptors include G protein-coupled receptors (GPCRs), receptor tyrosine kinases, ligand-gated ion channels, and enzyme-linked receptors. Activation of these receptors can involve second messengers such as cAMP, Ca2+, IP3, and DAG, as well as direct phosphorylation events. Intracellular receptors, such as steroid hormone receptors, function as ligand-activated transcription factors that regulate gene expression.

Ligand binding is characterized by affinity and specificity; saturation and dose-response relationships describe receptor activation. Receptors can be modulated by allosteric effectors and by upregulation or downregulation in response to prolonged stimuli. Drugs may act as agonists to activate receptors, antagonists to block them, or allosteric modulators to alter receptor activity.

Receptors play essential roles in physiology, including nervous, endocrine, and immune systems, and are common drug targets. Variations in receptor structure or expression can influence sensitivity to signals and contribute to disease. Understanding receptor function is central to pharmacology, neuroscience, and cell biology.