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

Abstract

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.

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“…The carbamate position is essential for the activity of physostigmine, because when the ester bond of physostigmine is hydrolysed, forming eseroline, the inhibitory activity is not observed. Furthermore, it is known that the carbonyl group of the carbamate portion interacts with a hydroxyl group of a serine residue in AChE, forming an ester, and is then slowly hydrolysed, regenerating the active parent form of the enzyme . Several changes were made to the physostigmine skeleton in an attempt to potentiate the activity or to change the polarity of the compound.…”

Cholinesterases, a Target of Pharmacology and Toxicology

“…AChE becomes active again. Carbamates are probably able to bind through non-covalent interactions 81 . The mechanism of AChE inhibition by an organophosphonate is depicted below (Fig.…”

Interaction of metal chelators with different molecular forms of acetylcholinesterase and its significance in Alzheimer's disease treatment

“…Several other inhibitors were shown to inhibit the enzyme activity by accommodating through hydrophobic interactions. For example, the charged physostigmine and its structural analogue physovenine which is uncharged, both show similar affinity towards AChE in bringing about activity inhibition . Yet another example where the uncharged aromatic chelating agent, e.g.…”