Effect of tetramethylammonium, choline and edrophonium on insect acetylcholinesterase: test of a kinetic model.
Cholinesterases display a non-Michaelian behaviour with respect to substrate concentration. With the insect enzyme, there is an activation at low substrate concentrations and an inhibition at high concentrations. Previous studies allow us to propose a kinetic model involving a secondary non-productive binding site for the substrate. Unexpectedly, this secondary site has a very high affinity for the substrate when the enzyme is free. On the contrary, when the catalytic site of the enzyme is occupied a strong decrease of this affinity was observed. Moreover, a substrate molecule bound to the peripheral site results in a global decrease of the acylation and/or the deacylation step. Kinetic studies with three reversible inhibitors, tetramethylammonium, edrophonium and choline supported the kinetic model and enable its further refinement.
Institute of Biochemistry, Medical Faculty, Ljubljana, Slovenia.
J
J
Stojan
V
V
Marcel
D
D
Fournier
D000110:Acetylcholinesterase / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D016367:Baculoviridae / Q000201:enzymology / Q000235:genetics; D001665:Binding Sites; D002384:Catalysis; D002794:Choline / Q000494:pharmacology*; D002800:Cholinesterase Inhibitors / Q000494:pharmacology*; D004330:Drosophila / Q000201:enzymology; D004491:Edrophonium / Q000494:pharmacology*; D004789:Enzyme Activation; D005731:Ganglionic Stimulants / Q000378:metabolism / Q000494:pharmacology*; D019476:Insect Proteins / Q000378:metabolism*; D007700:Kinetics; D008954:Models, Biological; D018697:Nootropic Agents / Q000494:pharmacology*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism / Q000494:pharmacology*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism
D002800:Cholinesterase Inhibitors; D005731:Ganglionic Stimulants; D019476:Insect Proteins; D018697:Nootropic Agents; D000644:Quaternary Ammonium Compounds; D011994:Recombinant Proteins; D004491:Edrophonium; D000110:Acetylcholinesterase; C027917:tetramethylammonium; D002794:Choline