ORGANOPHOSPHORUS ACID ANHYDRIDE HYDROLASE ACTIVITY IN HUMAN BUTYRYLCHOLINESTERASE - SYNERGY RESULTS IN A SOMANASE

Organophosphorus acid anhydride (OP) ''nerve agents'' are rapid, stoic hiometric, and essentially irreversible inhibitors of serine hydrolase s. By placing a His near the oxyanion hole of human butyrylcholinester ase (BChE), we made an esterase (G117H) that catalyzed the hydrolysis of several OF, including sarin and VX [Millard et al. (1995) Bibchemis try 34, 15925-15930], G117H was limited, however, because it was irrev ersibly inhibited by pinacolyl methylphosphonofluoridate (soman); soma n is among the most toxic synthetic poisons known. This limitation of G117H has been overcome by a new BChE (G117H/E197Q) that combines two engineered features: spontaneous dephosphonylation and slow aging (dea lkylation). G117H/E197Q was compared with the single mutants BChE G117 H and E197Q. Each retained cholinesterase activity with butyrylthiocho line as substrate, although k(cat)/K-m decreased 11-, 11- or 110-fold for purified G117H, E197Q, or G117H/E197Q, respectively, as compared w ith wild-type BChE. Only G117H/E197Q catalyzed soman hydrolysis; all f our soman stereoisomers as well as sarin and VX were substrates. Phosp honylation and dephosphonylation reactions were stereospecific. Double mutant thermodynamic cycles suggested that the effects of the His and Gin substitutions on phosphonylation were additive for PSCR or PRCR s oman, but were cooperative for the PSCS stereoisomer. Dephosphonylatio n limited overall OP hydrolysis with apparent rate constants of 0.006, 0.077, and 0.128 min(-1) for the PR/SCR, PSCS, and PRCS soman stereoi somers, respectively, at pH 7.5, 25 degrees C. We conclude that synerg istic protein design converted an archetypal ''irreversible inhibitor' ' into a slow substrate for the target enzyme.