Drugs-biomolecule interactions: binding study of substrate and inhibitors to acetylcholinesterase using NMR.

NMR was used to study the binding of acetylcholine, atropine, and physostigmine to acetylcholinesterase. Changes in the linewidth of the N-methyl resonance of acetylcholine, resulting from association with the enzyme during hydrolysis, were utilized to study the enzyme-substrate interaction. Physostigmine inhibited the binding of the substrate while atropine accelerated substrate hydrolysis without interfering with its binding. The dissociation constant, KD and the linewidth of the acetylcholinesterase-inhibitor complex, increment v bound, for atropine and physostigmine can be estimated from the linewidth changes of the N-methyl and phenyl group resonances of atropine and from the N-methyl and C-methyl group resonances of physostigmine resulting from association with the enzyme. The results indicate that there is at least one binding site on the enzyme surface for atropine and one for physostigmine. Further evidence that the two sites are distinct is indicated by the fact that gallamine displaces atropine from its site without competing with physostigmine.