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

1982; Elsevier BV; Volume: 257; Issue: 18 Linguagem: Inglês

10.1016/s0021-9258(18)33907-3

1083-351X

Stanford Moore, William P. Jencks,

Enzyme Structure and Function

Rateand equilibrium constants were determined for anhydride formation from the thiol esters p-nitrophenyl thiosuccinate (PTS) and N-acetyl-S-succinyl-fimercaptoethylamine, and from p-nitrophenyl thioacetate in the presence of substituted acetates.These thiol esters undergo thiol transfer reactions through ratedetermining anhydride formation that are models for the enzymic reaction catalyzed by CoA transferase.The same value of ,f,,c _ 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 1013 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 = 1011 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 °M, of which 6 X 105 M is caused by intramolecularity and 7 x 104 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 105 M from intramolecularity was observed for the reaction of N-acetyl-Ssuccinyl-/i-mercaptoethylamine compared with the corresponding acetate.The rate increase in the intramolecular reaction of PTS reflects more favorable values of TASt = 5.0 ± 1.4 kcal mol -and -AH = 2.9 + 1.3 kcal mol-' compared with the bimolecular reaction.We describe here a study of chemical thiol transfer reactions that are model systems for CoA transferase.This study is directed toward a better understanding of the mechanism and modes of rate acceleration which can be utilized by the enzyme.CoA transferase from pig heart catalyzes the transfer of coenzyme A between two acyl compounds as shown in Equation 1 with RICOO-= succinate and R 2 COO-= acetoacetate (1).