Stopped flow kinetics of carbamylcholine binding to membrane bound acetylcholine receptor

Conformational changes upon binding of carbamylcholine to acetylcholine receptor-enriched membrane fragments have been observed by stopped-flow methods using the fluorescent probe ethidium bromide. A model consistent with both equilibrium and kinetic experiments is proposed in which the receptor binds two molecules of carbamylcholine with high affinity in a non-cooperative manner followed by binding of a third and possibly a fourth molecule with increasingly lower affinity. The receptor ligand precomplexes isomerize to different non-interconvertible complexes depending on the number of ligands bound. This kinetic model fits the data for carbamylcholine interactions with receptor prepared initially either in a low or high affinity form for ligands.