Conference paper

Non-Competitive Inhibition of Flyhead Acetylcholinesterase by Oxime Carbamates. Kinetic Evidence for Non-productive Binding to the Catalytic Site

P. J. Jewess ; Shell Research Limited, Woodstock Laboratory, Sittingbourne, Kent, Great Britain
N. R. McFarlane ; Shell Research Limited, Woodstock Laboratory, Sittingbourne, Kent, Great Britain

Abstract

Reversible inhibition of flyhead acetylcholinesterase (E. C.
3.1.1.7) by two oxime carbamates possessing large N-substituents
(isopropyl and allyl) was found to follow competitive kinetics of a
biphasic nature with acetylcholine as the substrate. The derived
values for the substrate dissociation constants of high and low
affinity were in approximate agreement with the Michaelis and the
non-competitive substrate inhibition constant for acetylcholine respectively.
Data for the dependence of carbamoylation rates of the
enzyme upon substrate concentration did not agree with a model
derived from reversible inhibition kinetics. Reversible inhibition
studies indicated low active site competitive inhibition constants,
showing good binding to the active site. Studies upon the carbamoylation
rates indicated (i) a non competitive interaction, (ii) very
low concentrations of a reversibly-formed enzyme/carbamate complex
prior to carbamoylation of the active site. A possible explanation for the discrepancy is discussed whereby a reversible Ki determined from inhibition rate saturation by a »Main plot« measures the concentration of carbamate aligned in the active site following an induced shift of enzyme conformation, whereas competitive inhibition constants (Ki) determined from reversible inhibition experiments determine all binding modes at the active site which interfere with substrate attachment. Carbamates with large N-substituents show this effect more because
overlap of the carbamoyl moiety with the catalytic site is less
likely due to steric hinderance.

Keywords

Hrčak ID:

196606

URI

https://hrcak.srce.hr/196606

Publication date:

3.12.1975.

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