Chapter 15: Receptors, Hormones & Cell Signaling

A major focus is placed on the quantitative aspects of receptor-ligand interactions, utilizing the dissociation constant Kd to measure affinity and explaining how fractional occupancy relates to physiological sensitivity,. The summary elucidates the critical roles of molecular switches, specifically phosphorylation by protein kinases and phosphatases, and the cycling of GTP-binding proteins (G proteins) between active GTP-bound and inactive GDP-bound states,. Significant attention is given to G Protein-Coupled Receptors (GPCRs), the largest family of cell-surface receptors, which share a seven-transmembrane helix structure and activate heterotrimeric G proteins to regulate downstream effectors,. The chapter details two primary GPCR signaling cascades: the modulation of adenylyl cyclase to regulate cyclic AMP (cAMP) levels and Protein Kinase A (PKA) activity—illustrated by glycogen metabolism and CREB-mediated gene transcription—and the phosphoinositide pathway where Phospholipase C (PLC) cleaves PIP2 into the second messengers DAG and IP3,. The latter pathway triggers the release of calcium ions from the endoplasmic reticulum, which then bind to calmodulin or activate Protein Kinase C (PKC) to control diverse functions like muscle contraction and secretion,. Furthermore, the text explores the specialized phototransduction pathway in the eye, where the receptor rhodopsin uses the chromophore 11-cis-retinal to sense light, activating transducin and cGMP phosphodiesterase to hyperpolarize rod cells,. Finally, the summary covers the essential mechanisms of signal termination and adaptation, including the role of G-protein coupled receptor kinases (GRKs) and arrestins in receptor desensitization and endocytosis, ensuring cells can adjust their sensitivity to varying stimulation levels.