Affinity labeling and mutagenesis studies have demonstrated that the conserved tyrosine Y190 of the acetylcholine receptor (AChR) alpha-subunit is a key determinant of the agonist binding site. Here we describe the binding and gating kinetics of embryonic mouse AChRs with mutations at Y190. In Y190F the dissociation constant for ACh binding to closed channels was reduced approximately 35-fold at the first binding site and only approximately 2-fold at the second site. At both binding sites the association and dissociation rate constants were decreased by the mutation. Compared with wildtype AChRs, doubly-liganded alpha Y190F receptors open 400 times more slowly but close only 2 times more rapidly. Considering the overall activation reaction (vacant-closed to fully occupied-open), there is an increase of approximately 6.4 kcal/mol caused by the Y-to-F mutation, of which at least 2.1 and 0.3 kcal/mol comes from altered agonist binding to the first and second binding sites, respectively. The closing rate constant of alpha Y190F receptors was the same with ACh, carbamoylcholine, or tetramethylammonium as the agonist. This rate constant was approximately 3 times faster in ACh-activated S, W, and T mutants. The equilibrium dissociation constant for channel block by ACh was approximately 2-fold lower in alpha Y190F receptors compared with in wildtype receptors, suggesting that there are changes in the pore region of the receptor as a consequence of the mutation. The activation reaction is discussed with regard to energy provided by agonist-receptor binding contacts, and by the intrinsic folding energy of the receptor.
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