Mutants of human butyrylcholinesterase with organophosphate hydrolase activity; evidence that His117 is a general base catalyst for hydrolysis of echothiophate

Abstract

Human butyrylcholinesterase (BChE, EC 3.1.1.8) is an efficient scavenger of nerve agents and organophosphorus (OP) pesticides; one molecule of BChE inactivates one molecule of OP in a suicide reaction that irreversibly inhibits BChE. By contrast the BChE mutant, G117H, inactivates many molecules of OP. The OP makes a covalent bond with the active site serine and then the serine is dephosphorylated by the action of His117. In an effort to understand the mechanism by which His 117 achieves dephosphorylation, 62 new mutants of human BChE were tested for OP hydrolase activity, using a new screening assay. It was found that not only G117H, but also G117D, G117E, and L286H mutants were OP hydrolases. These results support the hypothesis that a hydrogen-bond acceptor acts as a general base to activate a water molecule which in turn dephosphorylates the active site serine. The screening assay provides a convenient means for identifying cholinesterase mutants with OP hydrolase activity.