A putative kinetic model for substrate metabolisation by Drosophila acetylcholinesterase

Insect acetylcholinesterase, an enzyme whose catalytic site is located at t he bottom of a gorge, can metabolise its substrate in a wide range of conce ntrations (from 1 mu M to 200 mM) since it is activated at low substrate co ncentrations. It also presents inhibition at high substrate concentrations. Among the various rival kinetic models tested to analyse the kinetic behav iour of the enzyme, the simplest able to explain all the experimental data suggests that there are two sites for substrate molecules on the protein, B inding on the catalytic site located at the bottom of the gorge seems to be irreversible, suggesting that each molecule of substrate which enters the active site gorge is metabolised, Reversible binding at the peripheral site of the free enzyme has high affinity (2 mu M), suggesting that this bindin g increases the probability of the substrate entering the active site gorge . Peripheral site occupation decreases the entrance rate constant of the se cond substrate molecule to the catalytic site and strongly affects the cata lytic activity of the enzyme. On the other hand, catalytic site occupation lowers the affinity of the peripheral site for the substrate (34 mM), These effects between the two sites result both in apparent activation at low su bstrate concentration and in general inhibition at high substrate concentra tion. (C) 1998 Federation of European Biochemical Societies.