AP Syllabus focus:
‘G protein–coupled receptors are important eukaryotic receptors that activate intracellular signaling cascades when ligands bind.’
G protein–coupled receptors (GPCRs) are a major class of eukaryotic membrane receptors that translate an external ligand-binding event into intracellular activity. They illustrate how receptor shape changes can launch multi-step signaling inside cells.
Overview of G protein–coupled receptors (GPCRs)
GPCRs are plasma-membrane proteins that detect many types of extracellular signals (ligands) and convert them into intracellular signals by activating G proteins, which then regulate downstream targets.
Practice Questions
FAQ
Different GPCRs couple to different G proteins (and different effectors), so the same general mechanism can trigger distinct downstream targets.
Cell-specific expression patterns of G proteins and effectors further diversify outcomes.
Orthosteric ligands bind the primary (natural ligand) binding site.
Allosteric ligands bind elsewhere and change receptor behaviour (e.g., increasing or decreasing responsiveness) without necessarily activating the receptor alone.
Some receptors show basal activity due to spontaneous conformational shifts into an active-like state.
Cells can counterbalance this with regulatory proteins that stabilise inactive receptor states.
Internalisation can reduce signalling by removing receptors from the cell surface.
In some cases, internalised receptors continue signalling from intracellular compartments, changing the timing and location of downstream effects.
Some toxins chemically modify G protein subunits, locking them in active or inactive states.
This disrupts normal GDP/GTP cycling and leads to abnormally persistent or absent signalling.
