Conformations and dynamics of the essential cysteinyl-cysteine ring derived from the acetylcholine receptor

The nicotinic acetylcholine receptor exists in different conformational states which are modulated by the binding of acetylcholine and are responsible for the regulation of ion transport across the cell membrane. How structural transitions are induced from the acetylcholine binding site to the ion channel is still a mystery. One hypothesis suggests that an invariant cysteinyl-cysteine ring in the N-terminal region of the alpha-subunit of all acetylcholine receptors may act as a molecular switch contributing to allosteric rearrangements. Since the cysteinyl-cysteine ring is a highly constrained structural element, we synthesized a peptide containing this ring, corresponding to amino acids 191 to 195 of the alpha-subunit of the receptor. This peptide (TCCPD) has four conformations, as characterized by both natural abundance C-13 and proton NMR spectroscopy in aqueous solution. Both cia and trans conformations for the amide bond between the two cysteines were observed. Exchange rates between the two cis conformations and between cis and trans conformations were measured by indirectly detected C-13 NMR. The Delta G double dagger at 20 degrees C was 15 and 21 kcal/mol, respectively. These barriers were within 1 kcal/mol of energy barriers calculated by molecular mechanics. Our results show the cysteine-cysteinyl ring can adopt multiple conformations and exchange between them. The eight-membered cysteinyl-cysteine may play in important role in allosteric rearrangements of the acetylcholine receptor and other proteins where this structural element is found.