Microcalorimetric assay of acetylcholinesterase

Enzyme activity determination on macromolecular substrates by isothermal titration calorimetry: application to mesophilic and psychrophilic chitinases

Isothermal titration calorimetry has been applied to the determination of the kinetic parameters of chitinases (EC 3.2.1.14) by monitoring the heat released during the hydrolysis of chitin glycosidic bonds. Experiments were carried out using two different macromolecular substrates: a soluble polymer of N-acetylglucosamine and the insoluble chitin from crab shells. Different experimental temperatures were used in order to compare the thermodependence of the activity of two chitinases from the psychrophile Arthrobacter sp. TAD20 and of chitinase A from the mesophile Serratia marcescens. The method allowed to determine unequivocally the catalytic rate constant k(cat), the activation energy (E(a)) and the thermodynamic activation parameters (DeltaG(#), DeltaH(#), DeltaS(#)) of the chitinolytic reaction on the soluble substrate. The catalytic activity has also been determined on insoluble chitin, which displays an effect of substrate saturation by chitinases. On both substrates, the thermodependence of the activity of the psychrophilic chitinases was lower than that observed with the mesophilic counterpart.

Microcalorimetric determination of binding sites of acetylcholinesterase

Acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7), phosphorylated with dichlorvos, showed relatively more reactivity toward the substrate indophenyl acetate than the enzyme that was carbamylated with carbaryl. When the anionic subsite of the phosphorylated or carbamylated enzyme was alkylated with an aziridinium ion, the reaction velocity toward indophenyl acetate increased in the phosphorylated enzyme but not in the carbamylated enzyme. The organophosphate binding site--which appears to be different from that of carbamate or indophenyl acetate, but probably the same as that of acetylcholine -- is apparently alkylated in such a way that the modified (phosphorylated) enzyme is better-fit for the binding (and hydrolysis) of indophenyl acetate. The modified conformation presumably results in the release of the phosphoryl group from the esteratic subsite.