title: 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 αδ and either αε (adult) or αγ (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 (ΔGB1). The two binding sites behave independently in both adult and fetal AChRs. For four different agonists, including ACh and choline, ΔGB1 is ∼-2 kcal/mol more favorable at αγ compared with at αε and αδ. Only three of the aromatics contribute significantly to ΔGB1 at the adult sites (αY190, αY198, and αW149), but all five do so at αγ (as well as αY93 and γW55). γW55 makes a particularly large contribution only at αγ that is coupled energetically to those contributions of some of the α-subunit aromatics. The hydroxyl and benzene groups of loop C residues αY190 and αY198 behave similarly with regard to ΔGB1 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 αγ site is more compact, better organized, and less dynamic than αε and αδ. We speculate that the different sensitivities of the fetal αγ site versus the adult αε and αδ sites to choline and ACh are important for the proper maturation and function of the neuromuscular synapse.
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214; and.
authors.forename: Tapan K
authors.initials: TK
authors.lastname: Nayak
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214; and.
authors.forename: Iva
authors.initials: I
authors.lastname: Bruhova
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214; and Department of Physics, State University of New York at Buffalo, Buffalo, NY 14260.
authors.forename: Srirupa
authors.initials: S
authors.lastname: Chakraborty
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214; and.
authors.forename: Shaweta
authors.initials: S
authors.lastname: Gupta
authors.affiliation: Department of Physics, State University of New York at Buffalo, Buffalo, NY 14260.
authors.forename: Wenjun
authors.initials: W
authors.lastname: Zheng
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214; and auerbach@buffalo.edu.
authors.forename: Anthony
authors.initials: A
authors.lastname: Auerbach
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002794:Choline / Q000737:chemistry; D003198:Computer Simulation; D004594:Electrophysiology; D018507:Gene Expression Regulation, Developmental; D057809:HEK293 Cells; D006801:Humans; D006868:Hydrolysis; D008024:Ligands; D008195:Lymnaea; D051379:Mice; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D009132:Muscles / Q000378:metabolism*; D009154:Mutation; D018377:Neurotransmitter Agents / Q000737:chemistry*; D011485:Protein Binding; D011950:Receptors, Cholinergic / Q000737:chemistry / Q000502:physiology*; D009435:Synaptic Transmission; D013816:Thermodynamics; D014101:Torpedo; D014364:Tryptophan / Q000737:chemistry
chemical_list: D008024:Ligands; D018377:Neurotransmitter Agents; D011950:Receptors, Cholinergic; D014364:Tryptophan; D002794:Choline; D000109:Acetylcholine
keywords: allosteric protein; ion channel; ligand binding sites; single-channel electrophysiology; synaptic maturation