receptorsespecially

Receptors are macromolecules, usually proteins, that detect chemical signals and translate them into cellular responses. They are essential for communication within and between cells, mediating responses to hormones, neurotransmitters, growth factors, and sensory stimuli. Binding of a ligand to a receptor initiates a conformational change that sets off intracellular signaling cascades.

Classification and structure: The main classes are cell-surface receptors, which reside on the plasma membrane and bind hydrophilic ligands, and intracellular receptors, which reside inside the cell and bind lipophilic ligands that cross the membrane. Cell-surface receptors include ligand-gated ion channels, G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and cytokine receptors. Intracellular receptors include nuclear receptors that regulate gene expression.

Ligand binding and pharmacology: Ligands can act as agonists, antagonists, partial agonists, or inverse agonists. Receptors show specificity and affinity, and binding triggers conformational changes that initiate signaling. Receptor activity can be modulated by regulatory processes such as upregulation or downregulation, desensitization, and tachyphylaxis, affecting responsiveness to subsequent signals.

Signaling and outcomes: Activated receptors transduce signals via second messengers (such as cyclic AMP, calcium, IP3, and DAG), phosphorylation cascades, and transcriptional programs. The resulting responses can be rapid, such as changes in ion flux, or longer-term, including altered gene expression and metabolism.

Clinical relevance: Receptors are major targets in medicine. Drugs commonly act by activating or inhibiting receptors, with GPCRs and receptor tyrosine kinases representing prominent drug targets. Receptor dysfunction or dysregulation underlies many diseases, making understanding receptor biology central to pharmacology and therapeutics.