Hydrolysis of oxo- and thio-esters by human butyrylcholinesterase

Catalytic parameters of human butyrylcholinesterase (BuChE) for hydrolysis of homologous pairs of oxo-esters and thio-esters were compared. Substrates were positively charged (benzoylcholine versus benzoylthiocholine) and neutral (phenylacetate versus phenylthioacetate). In addition to wild-type BuChE, enzymes containing mutations were used. Single mutants at positions: G117, a key residue in the oxyanion hole, and D70, the main component of the peripheral anionic site were tested. Double mutants containing G117H and mutations on residues of the oxyanion hole (G115, A199), or the π-cation binding site (W82), or residue E197 that is involved in stabilization of tetrahedral intermediates were also studied. A mathematical analysis was used to compare data for BuChE-catalyzed hydrolysis of various pairs of oxo-esters and thio-esters and to determine the rate-limiting step of catalysis for each substrate. The interest and limitation of this method is discussed. Molecular docking was used to analyze how the mutations could have altered the binding of the oxo-ester or the thio-ester. Results indicate that substitution of the ethereal oxygen for sulfur in substrates may alter the adjustment of substrate in the active site and stabilization of the transition-state for acylation. This affects the k₂/k₃ ratio and, in turn, controls the rate-limiting step of the hydrolytic reaction. Stabilization of the transition state is modulated both by the alcohol and acyl moieties of substrate. Interaction of these groups with the ethereal hetero-atom can have a neutral, an additive or an antagonistic effect on transition state stabilization, depending on their molecular structure, size and enantiomeric configuration.