Why is Aged Acetylcholinesterase So Difficult to Reactivate?

Area of Science:

• Biochemistry
• Toxicology
• Enzyme kinetics

Background:

• Organophosphorus agents are potent acetylcholinesterase inhibitors.
• Inhibition involves phosphylation of the active site serine, forming a neutral adduct.
• This adduct can undergo aging, forming a refractory anionic adduct.

Purpose of the Study:

• To review the reasons for the unreactivity of the aged acetylcholinesterase adduct.
• To discuss the challenges in reactivating aged acetylcholinesterase.
• To explore evolutionary mechanisms of transition state stabilization in acetylcholinesterase.

Main Methods:

• Review of existing literature on organophosphorus inhibition and acetylcholinesterase aging.
• Analysis of the chemical structures of inhibited and aged adducts.
• Discussion of enzyme catalysis and transition state theory.

Main Results:

• The aged adduct is an anionic phosphyl adduct, analogous to the deacylation transition state.
• This structural similarity makes the aged adduct refractory to nucleophilic reactivation.
• Evolutionary stabilization of the transition state contributes to the difficulty in reactivation.

Conclusions:

• The aged acetylcholinesterase adduct's resistance to reactivation is due to its transition-state analog structure.
• Reactivation strategies face significant hurdles due to enzyme evolution for efficient catalysis.
• Understanding these mechanisms is crucial for developing effective antidotes for organophosphate poisoning.