Accommodation of physostigmine and its analogs by acetylcholinesterase is dominated by hydrophobic interactions

摘要

The role of the functional architecture of the human acetylcholinesterase (HuAChE) in reactivity toward the carbamates pyridostigmine, rivastigmine and several analogs of physostigmine, that are currently used or considered for use as drugs for Alzheimer’s disease, was analysed using over 20 mutants of residues that constitute the interaction subsites in the active center. Both steps of the HuAChE carbamylation reaction, formation of the Michaelis complex as well as the nucleophilic process, are sensitive to accommodation of the ligand by the enzyme. For certain carbamate - HuAChE combinations, the mode of inhibition shifted from a covalent to a noncovalent type, according to the balance between dissociation and covalent reaction rates. Whereas the charged moieties of pyridostigmine and rivastigmine contribute significantly to the stability of the corresponding HuAChE complexes, no such effect was observed for physostigmine and its analogs, phenserine and cymserine. Moreover, physostigmine-like ligands carrying oxygen instead of nitrogen at position – 1 of the tricyclic moiety (physovenine and tetrahydrofurobenzofuran analogs) displayed comparable structure – function characteristics toward the various HuAChE enzymes. The essential role of the HuAChE hydrophobic pocket, comprising mostly of residues Trp86 and Tyr337, in accommodating (−)-physostigmine and in conferring ~300-fold stereoselectivity toward physostigmines, was elucidated through examination of the reactivity of selected HuAChE mutations toward enantiomeric pairs of different physostigmine analogs. The present study demonstrates that certain charged and uncharged ligands, like analogs of physostigmine and physovenine, seem to be accommodated by the enzyme mostly through hydrophobic interactions.