NEW CATALYTIC ROLES FOR SERINE ESTERASES - A F-19-NMR STUDY OF THE INTERACTION OF 3,3,3-TRIFLUORO-2,2-DIHYDROXY-1-PHENYL-1-PROPANONE WITH CHICKEN LIVER CARBOXYLESTERASE

The reactions of 3,3,3-trifluoro-2,2-dihydroxy-1-phenyl-1-propanone (T DPP) with chicken liver carboxylesterase have shown that this ketone h ydrate is not only a potent inhibitor of the enzyme, but also a substr ate for a number of enzyme-catalyzed reactions. The kinetics of inhibi tion are consistent with a mechanism in which the bound hydrate is ini tially dehydrated in a rate-limiting step catalyzed by the enzyme. Nuc leophilic attack by the active-site serine on the parent ketone then p roduces a hemiketal adduct. However, the slow reactivation (by dialysi s) of TDPP-inhibited enzyme indicates that the interaction with this i nhibitor is more complex. At equilibrium, a dissociation constant of 2 .4 pM was obtained for this interaction. F-19-NMR studies of the enzym e-TDPP complex show that after pre-equilibration, the major adduct is not the hemiketal adduct. It is proposed that this final adduct is a c ross-linked adduct formed between TDPP, the active-site serine and the active-site histidine. F-19-NMR studies reveal that chicken liver car boxylesterase catalyses the cleavage of TDPP to yield either fluoride ion or trifluoroacetate, and also the benzilic acid rearrangement of T DPP to alpha-trifluoromethylmandelate. These products have also been i dentified in model studies of the reaction between TDPP and imidazole.