Functional differences between neurotransmitter binding sites of muscle acetylcholine receptors.

ABSTRACT: A muscle acetylcholine receptor (AChR) has two neurotransmitter binding sites located in the extracellular domain, at ?? and either ?? (adult) or ?? (fetal) subunit interfaces. We used single-channel electrophysiology to measure the effects of mutations of five conserved aromatic residues at each site with regard to their contribution to the difference in free energy of agonist binding to active versus resting receptors (?GB1). The two binding sites behave independently in both adult and fetal AChRs. For four different agonists, including ACh and choline, ?GB1 is ?-2 kcal/mol more favorable at ?? compared with at ?? and ??. Only three of the aromatics contribute significantly to ?GB1 at the adult sites (?Y190, ?Y198, and ?W149), but all five do so at ?? (as well as ?Y93 and ?W55). ?W55 makes a particularly large contribution only at ?? that is coupled energetically to those contributions of some of the ?-subunit aromatics. The hydroxyl and benzene groups of loop C residues ?Y190 and ?Y198 behave similarly with regard to ?GB1 at all three kinds of site. ACh binding energies estimated from molecular dynamics simulations are consistent with experimental values from electrophysiology and suggest that the ?? site is more compact, better organized, and less dynamic than ?? and ??. We speculate that the different sensitivities of the fetal ?? site versus the adult ?? and ?? sites to choline and ACh are important for the proper maturation and function of the neuromuscular synapse.

SUBMITTER: Nayak TK 

PROVIDER: S-EPMC4267328 | biostudies-literature | 2014 Dec

REPOSITORIES: biostudies-literature