title: Conformational analysis of the eight-membered ring of the oxidized cysteinyl-cysteine unit implicated in nicotinic acetylcholine receptor ligand recognition.
abstract: Nicotinic acetylcholine receptors (nAChRs) are membrane-bound, pentameric ligand-gated ion channels associated with a variety of human disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia, and pain. Most known nAChRs contain an unusual eight-membered disulfide-containing cysteinyl-cysteine ring, ox-[Cys-Cys], as does the soluble acetylcholine binding protein (AChBP) found in the snail Lymnaea stagnalis. The cysteinyl-cysteine ring is located in a region implicated in ligand binding, and conformational changes involving this ring may be important for modulation of nAChR function. We have studied the preferred conformations of Ac-ox-[Cys-Cys]-NH2 by NMR in water and computationally by Monte Carlo simulations using the OPLS-AA force field and GB/SA water model. ox-[Cys-Cys] adopts four distinct low-energy conformers at slightly above 0 degrees C in water. Two populations are dependent on the peptide omega2 dihedral angle, with the trans amide favored over the cis amide by a ratio of ca. 60:40. Two ox-[Cys-Cys] conformers with a cis amide bond (C+ and C-) differ from each other primarily by variation of the chi3 dihedral angle, which defines the orientation of the helicity about the S-S bond (+/- 90 degrees ). Two trans amide conformers have the same S-S helicity (chi3 approximately -90 degrees ), but are distinguished by a backbone rotation about phi2 and psi1 (T- and T'-). The ratio of T-/T'-/C+/C- is 47:15:29:9. The orientation of the pendant moieties from the eight-membered ring is more compact for the major trans conformer (T-) than for the extended conformations adopted by T'-, C+, and C-. These conformational preferences are also observed in tetrapeptide and undecapeptide fragments of the human alpha7 subtype of the nAChR that contains the ox-[Cys-Cys] unit. Conformer T- is nearly identical to the conformation seen in the X-ray structure of ox-[Cys(187)-Cys(188)] found in the unliganded AChBP, and is a Type VIII beta-turn.
authors.affiliation: Drug Discovery Division, R. W. Johnson Pharmaceutical Research Institute, Welsh and McKean Roads, P.O. Box 776, Spring House, Pennsylvania 19477-0776, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Creighton
authors.forename: C H
authors.initials: CH
authors.lastname: Reynolds
authors.forename: D H
authors.initials: DH
authors.lastname: Lee
authors.forename: G C
authors.initials: GC
authors.lastname: Leo
authors.forename: A B
authors.initials: AB
authors.lastname: Reitz
mesh_terms: D003198:Computer Simulation; D003545:Cysteine / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000737:chemistry*; D011487:Protein Conformation; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D008024:Ligands; D010446:Peptide Fragments; D010456:Peptides, Cyclic; D011978:Receptors, Nicotinic; D003545:Cysteine