Hydrolysis of organophosphate compounds by mutant butyrylcholinesterase: A story of two histidines

This study is aimed at understanding the hydrolysis mechanism of organophosphate (OP) compounds by G117H-BChE. It is a theoretical study that focuses on the role of the G117H mutation in the dephosphorylation step. Various proposed mechanisms are examined. We show that His117 acts as a general base by activating a water molecule, and thus assisting its nucleophilic attack on the phosphate. The calculated reaction energy profile agrees well with the experimental data. Moreover, analysis of the reaction via its two hypothetical elementary steps, proton transfer and hydroxide attack, supports the role of His117 as a general base. Further support to the proposed mechanism is gained by structural comparison of the active site to RNAse A, which has similar composition of substrate and functional groups. The similarity between these enzymes extends beyond the structure and also becomes evident when comparing functionality of various active sites residues as well as rate-pH dependence obtained in the two cases. Moreover, it is demonstrated that an extended form of Bevilacqua's model (Biochemistry 2003;42:2259-2265) may resolve the apparent contradictions between the proposed mechanism and various experimental observations regarding rate-pH dependence. Finally, that same model is shown to rationalize the hydrolase activity of G117D BChE, an observation which is considered puzzling. It is concluded that G117H-BChE hydrolyzes echothiophate and possibly other OP compounds via a general acid-base mechanism. On the basis of this mechanism, one can now proceed with rational design aimed at improving the enzyme by exploiting both the structural and mechanistic knowledge.