The G117H mutant of human butyrylcholinesterase (EC 3.1.1.8) was expressed in Chinese hamster ovary cells. Substitution of Gly 117 with His to make the G117H mutant endowed butyrylcholinesterase with the ability to catalyze the hydrolysis of organophosphate esters. G117H was still able to hydrolyze butyrylthiocholine, benzoylcholine, and o-nitrophenyl butyrate, but in addition it had acquired the ability to hydrolyze the antiglaucoma drug echothiophate and the pesticide paraoxon. Wild-type butyrylcholinesterase was irreversibly inhibited by echothiophate and paraoxon, but G117H regained 100% activity within 2-3 min following reaction with these compounds. On a polyacrylamide gel, the same bands that stained for activity with butyrylthiocholine also stained for activity with echothiophate. G117H is the only enzyme known that hydrolyzes echothiophate. Diethoxyphosphorylated G117H aged with a half-time of 5.5 h, a rate 600 times slower than the rate of hydrolysis. Echothiophate and paraoxon were hydrolyzed with the same kcat of 0.75 min-1. This calculates to a rate acceleration of 100,000-fold for hydrolysis of echothiophate and paraoxon by the G117H mutant compared to the nonenzymatic rate.
        
Title: Design and expression of organophosphorus acid anhydride hydrolase activity in human butyrylcholinesterase Millard CB, Lockridge O, Broomfield CA Ref: Biochemistry, 34:15925, 1995 : PubMed
Serine esterases and proteases are rapidly and irreversibly inhibited by organophosphorus (OP) nerve agents. To overcome this limitation, we selected several residues that were predicted to be within 3-10 A of both the active site Ser O gamma and the oxyanion hole of human butyrylcholinesterase for mutation to His (G115H, G117H, Q119H, and G121H). In remarkable contrast with wild-type (WT) and all other His mutants tested, G117H underwent spontaneous reactivation following OP inhibition to regain 100% of original esterase activity with maximum k3 values of approximately 6.8 x 10(-5) and 16 x 10(-5) s-1 for GB (sarin) and VX, respectively, in 0.1 M Bis-Tris, 25 degrees C. The free energy of activation for k3 was 19 kcal mol-1, and measurement of pH dependence suggested that reactivation resulted from an acidic group with pKa 6.2. To evaluate further the importance of His in achieving this result, we changed the same Gly to Lys (G117K) and compared its substrate and inhibitor kinetics with those of G117H. Both mutants retained esterase activity with Km values similar to those of WT for neutral ester hydrolysis, but G117K did not reactivate. Complete reactivation proves that G117H is not irreversibly inhibited but instead functions as a catalyst for OP hydrolysis. Dephosphonylation is the rate-limiting step, and G117H effects overall rate constant enhancements of approximately 100- and 2000-fold above the uncatalyzed hydrolysis of GB and VX, respectively, at pH 6.0, 25.0 degrees C. We conclude that an appropriately positioned imidazolium ion in the oxyanion hole catalyzes dephosphonylation and, thereby, confers a novel organophosphorus acid anhydride hydrolase activity upon butyrylcholinesterase.