Signal Transduction
The process by which a cell converts an extracellular signal — such as a peptide hormone binding to its receptor — into an intracellular response. Signal transduction pathways amplify the original signal through cascades of molecular interactions, explaining how small amounts of peptide hormones produce large biological effects.
Technical Context
Most peptide hormone receptors are GPCRs, which activate heterotrimeric G-proteins (Gαs, Gαi, Gαq subtypes). Gαs-coupled receptors (GLP-1R, GHRHR) activate adenylyl cyclase, increasing intracellular cAMP, which activates protein kinase A (PKA). Gαq-coupled receptors activate phospholipase C, generating IP3 (calcium release) and DAG (protein kinase C activation). Gαi-coupled receptors inhibit adenylyl cyclase, decreasing cAMP. Beyond G-protein signalling, GPCRs also signal through beta-arrestin pathways (biased agonism), which can produce different downstream effects than G-protein signalling. The concept of biased agonism — where a ligand preferentially activates one signalling pathway over another — is an emerging strategy in peptide drug design.