title: Cyclic lactam analogues containing the main immunogenic region of Torpedo acetylcholine receptor.
abstract: The majority of autoantibodies against the nicotinic acetylcholine receptor (AChR) bind to an extracellular region of the AChR's alpha-subunit, named main immunogenic region (MIR), with the sequence W67-N-P-A-DY-G-G-I-K76 for the Torpedo californica electric organ. We report on the synthesis and the biological and 1H-NMR studies of two cyclic MIR compounds--namely, [D71,K76]-MIR-NH2 and Ac-[Orn68,D71,A76]-MIR-NH2. The relatively small chemical shift differences between [D71,K76]-MIR-NH2 and the biologically active [A76]-analogue suggest that both MIR derivatives possess similar conformations. Thus, the observed limited anti-MIR MAb binding capacity of [D71,K76]-MIR-NH2 is attributed to the D71,K76 side-chain blockage, through lactam. Formation of the Orn68,D71 cycle in the Ac-[Orn68,D71,A76]-MIR-NH2 preserves, unchanged, the low antigenicity of the linear Ac-[Orn68,A76]-MIR-NH2, thus confirming the key role of position 68. The low temperature coefficient value of A70-NH and the observed NOE effect between P69-C delta H2 and A70-NH in Ac-[Orn68,D71,A76]-MIR-NH2 argue in favor of a type I beta-turn in the Trp67-Orn-P-A70 sequence. However, the N-terminus beta-folding and the Orn68,D71 cycle appear ineffective for optimal antibody molecular recognition.
authors.affiliation: Department of Chemistry, University of Ioannina, Greece.
authors.forename: E
authors.initials: E
authors.lastname: Detsikas
authors.forename: V
authors.initials: V
authors.lastname: Tsikaris
authors.forename: M
authors.initials: M
authors.lastname: Sakarellos-Daitsiotis
authors.forename: C
authors.initials: C
authors.lastname: Sakarellos
authors.forename: M T
authors.initials: MT
authors.lastname: Cung
authors.forename: M
authors.initials: M
authors.lastname: Marraud
authors.forename: E
authors.initials: E
authors.lastname: Vatzaki
authors.forename: S J
authors.initials: SJ
authors.lastname: Tzartos
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D000939:Epitopes / Q000737:chemistry; D007769:Lactams / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011950:Receptors, Cholinergic; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000276:immunology; D014101:Torpedo
chemical_list: D000911:Antibodies, Monoclonal; D000939:Epitopes; D007769:Lactams; D011950:Receptors, Cholinergic; D011978:Receptors, Nicotinic