NMR Analysis Reveals a Positively Charged Hydrophobic Domain as a Common Motif to Bound Acetylcholine and d-Tubocurarine

A complete ¹H assignment of d-tubocurarine was carried out using lD and 2D NMR techniques. Geometries of free acetylcholine (ACh) and d-tubocurarine were compared with those of the ligands bound to a recombinant cholinergic binding site (Tαl84–200 expressed as a fusion protein in Escherichia coli). The conformations of the free ligands were determined by NOESY experiments while those of the bound molecules were obtained by transferred NOESY. The complete relaxation matrix was solved yielding distance constraints which were further refined by a σ back-calculation. ACh bound to recombinant Tα184–200 closely resembled the conformation previously reported for ACh bound to the intact receptor. d-Tubocurarine in the bound state undergoes extensive induced conformational rearrangements generating a “cup”-shaped structure. A unique positively charged hydrophobic domain is identified as characteristic of both bound cholinergic ligands.