Rate‐determining step of butyrylcholinesterase‐catalyzed hydrolysis of benzoylcholine and benzoylthiocholine

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Abstract
The rate‐limiting step for hydrolysis of the positively charged oxoester benzoylcholine (BzCh) by human butyrylcholinesterase (BuChE) is deacylation (k3), whereas it is acylation (k2) for hydrolysis of the homologous thioester benzoylthiocholine (BzSCh). Steady‐state hydrolysis of BzCh and BzSCh by wild‐type BuChE and its peripheral anionic site mutant D70G was investigated at different hydrostatic pressures, which allowed determination of volume changes associated with substrate binding, and the activation volumes for the chemical steps. A differential nonlinear pressure‐dependence of the catalytic parameters for hydrolysis of both substrates by both enzymes was shown. Nonlinearity of the plots may be explained in terms of compressibility changes or rate‐limiting changes. To distinguish between these two possibilities, enzyme phosphorylation by diisopropylfluorophosphate (DFP) in the presence of substrate (BzSCh) under pressure was studied. There was no pressure dependence of volume changes for DFP binding or for phosphorylation of either wild‐type or D70G. Analysis of the pressure dependence for steady‐state hydrolysis of substrates, and for phosphorylation by DFP provided evidence that no enzyme compressibility changes occurred during the catalyzed reactions. Thus, the nonlinear pressure dependence of substrate hydrolysis reflects changes in the rate‐limiting step with pressure. Change in rate‐determining step occurred at a pressure of 100 MPa for hydrolysis of BzCh by wild‐type and at 75 MPa for D70G. For hydrolysis of BzSCh the change occurred at higher pressures because k2 << k3 at atmospheric pressure for this substrate. Elementary volume change contributions upon initial binding, productive binding, acylation and deacylation were calculated from the pressure differentiation of kinetic constants. This analysis shed light on the molecular events taking place along the hydrolysis pathways of BzCh and BzSCh by wild‐type BuChE and the D70G mutant. In addition, volume change differences between wild‐type and D70G provided new evidence that residue D70 in the peripheral site controls hydration of the active site gorge and the dynamics of the water molecule network during catalysis. Finally, a steady‐state kinetic study of the oxyanion hole mutant (G117H) showed that substitution of the ethereal sulfur for oxygen in the substrate alters the final adjustment of substrate in the active site and stabilization of the acylation transition state.

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