Model reactions for CoA transferase involving thiol transfer. Anhydride formation from thiol esters and carboxylic acids.

Rate and equilibrium constants were determined for anhydride formation from the thiol esters p-nitrophenyl thiosuccinate (PTS) and N-acetyl-S-succinyl-beta-mercaptoethylamine, and from p-nitrophenyl thioacetate in the presence of substituted acetates. These thiol esters undergo thiol transfer reactions through rate-determining anhydride formation that are models for the enzymic reaction catalyzed by CoA transferase. The same value of beta nuc approximately equal to 1.0 for the nucleophilic reactions of substituted acetates with p-nitrophenyl thioacetate and with enzyme-CoA supports the anhydride mechanism for the enzymic reactions. The bimolecular reaction of succinyl-CoA with enzyme is 10(13) times faster than the nonenzymic reaction of acetyl-CoA with acetate; the intramolecular reaction of bound substrate is accelerated by a factor of kcat/kN - 10(11) M. The nonenzymic intramolecular reaction of PTS is faster than the nucleophilic reaction of acetate with acetyl-CoA by a similar factor of 4.5 X 10(10) M, of which 6 X 10(5) M is caused by intramolecularity and 7 X 10(4) by the increased chemical reactivity of PTS compared to acetyl-CoA. The enzyme may utilize binding energy to increase the reaction rate by making the reaction intramolecular (decreasing the requirement for loss of entropy) and by destabilizing the bound substrate relative to the transition state in an analogous manner. An identical rate increase of 6 X 10(5) M from intramolecularity was observed for the reaction of N-acetyl-S-succinyl-beta-mercaptoethylamine compared with the corresponding acetate. The rate increase in the intramolecular reaction of PTS reflects more favorable values of T delta S identical = 5.0 +/- 1.4 kcal mol-1 and -delta H identical to = 2.9 +/- 1.3 kcal mol-1 compared with the bimolecular reaction.