Kinetic evidence for thermally induced conformational change of butyrylcholinesterase.

The effect of temperature on the kinetics of human plasma butyrylcholinesterase-catalyzed reactions was studied. The Arrhenius plot of o-nitrophenylbutyrate hydrolysis presents a break at 21 degrees C. However, nucleophilic competition data indicate that there is no change in the rate-limiting step of the overall reaction. In addition, the temperature dependence of the bimolecular rate constant of enzyme carbamylation shows a break at 18 degrees C. These results argue for the existence of thermally induced conformational active states of the enzyme tetramer. It is suggested that the effects of this transition on kinetics arise at the acylation step.