Organophosphorus (OP) nerve agents are potent toxins that inhibit cholinesterases and produce a rapid and lethal cholinergic crisis. Development of protein-based therapeutics is being pursued with the goal of preventing nerve agent toxicity and protecting against the long-term side effects of these agents. The drug-metabolizing enzyme human carboxylesterase 1 (hCE1) is a candidate protein-based therapeutic because of its similarity in structure and function to the cholinesterase targets of nerve agent poisoning. However, the ability of wild-type hCE1 to process the G-type nerve agents sarin and cyclosarin has not been determined. We report the crystal structure of hCE1 in complex with the nerve agent cyclosarin. We further use stereoselective nerve agent analogs to establish that hCE1 exhibits a 1700- and 2900-fold preference for the P(R) enantiomers of analogs of soman and cyclosarin, respectively, and a 5-fold preference for the P(S) isomer of a sarin analog. Finally, we show that for enzyme inhibited by racemic mixtures of bona fide nerve agents, hCE1 spontaneously reactivates in the presence of sarin but not soman or cyclosarin. The addition of the neutral oxime 2,3-butanedione monoxime increases the rate of reactivation of hCE1 from sarin inhibition by more than 60-fold but has no effect on reactivation with the other agents examined. Taken together, these data demonstrate that hCE1 is only reactivated after inhibition with the more toxic P(S) isomer of sarin. These results provide important insights toward the long-term goal of designing novel forms of hCE1 to act as protein-based therapeutics for nerve agent detoxification.
The organophosphorus nerve agents sarin, soman, tabun, and VX exert their toxic effects by inhibiting the action of human acetylcholinesterase, a member of the serine hydrolase superfamily of enzymes. The current treatments for nerve agent exposure must be administered quickly to be effective, and they often do not eliminate long-term toxic side effects associated with organophosphate poisoning. Thus, there is significant need for effective prophylactic methods to protect at-risk personnel from nerve agent exposure, and protein-based approaches have emerged as promising candidates. We present the 2.7 A resolution crystal structures of the serine hydrolase human carboxylesterase 1 (hCE1), a broad-spectrum drug metabolism enzyme, in covalent acyl-enzyme intermediate complexes with the chemical weapons soman and tabun. The structures reveal that hCE1 binds stereoselectively to these nerve agents; for example, hCE1 appears to react preferentially with the 10(4)-fold more lethal PS stereoisomer of soman relative to the PR form. In addition, structural features of the hCE1 active site indicate that the enzyme may be resistant to dead-end organophosphate aging reactions that permanently inactivate other serine hydrolases. Taken together, these data provide important structural details toward the goal of engineering hCE1 into an organophosphate hydrolase and protein-based therapeutic for nerve agent exposure.
Human carboxylesterase 1 (hCE1) exhibits broad substrate specificity and is involved in xenobiotic processing and endobiotic metabolism. We present and analyze crystal structures of hCE1 in complexes with the cholesterol-lowering drug mevastatin, the breast cancer drug tamoxifen, the fatty acyl ethyl ester (FAEE) analogue ethyl acetate, and the novel hCE1 inhibitor benzil. We find that mevastatin does not appear to be a substrate for hCE1, and instead acts as a partially non-competitive inhibitor of the enzyme. Similarly, we show that tamoxifen is a low micromolar, partially non-competitive inhibitor of hCE1. Further, we describe the structural basis for the inhibition of hCE1 by the nanomolar-affinity dione benzil, which acts by forming both covalent and non-covalent complexes with the enzyme. Our results provide detailed insights into the catalytic and non-catalytic processing of small molecules by hCE1, and suggest that the efficacy of clinical drugs may be modulated by targeted hCE1 inhibitors.
Title: Stereoselective hydrolysis of pyrethroid-like fluorescent substrates by human and other mammalian liver carboxylesterases Huang H, Fleming CD, Nishi K, Redinbo MR, Hammock BD Ref: Chemical Research in Toxicology, 18:1371, 2005 : PubMed
Mammalian hepatic carboxylesterases (CEs) play important roles in the detoxification of ester-containing pyrethroids, which are widely used for the control of agricultural pests and disease vectors such as mosquitoes. Pyrethroids and pyrethroid-like fluorescent substrates exhibit a consistent pattern of stereoselective hydrolysis by a recombinant murine hepatic CE. We sought to understand whether this pattern is maintained in other hepatic CEs and to unravel the origin of the stereoselectivity. We found that all hepatic CEs tested displayed a consistent pattern of stereoselective hydrolysis: the chiral center(s) in the acid moiety more strongly influenced stereoselective hydrolysis than the chiral center in the alcohol moiety. For cypermethrin analogues with a cyclopropane ring in the acid moiety, trans-isomers were generally hydrolyzed faster than the corresponding cis-isomers. For fenvalerate analogues without a cyclopropane ring in the acid moiety, 2R-isomers were better substrates than 2S-isomers. These general hydrolytic patterns were examined by modeling the pyrethroid-like analogues within the active site of the crystal structure of human carboxylesterase 1 (hCE1). Stereoselective steric clashes were found to occur between the acid moieties and either the catalytic Ser loop (residues 219-225) or the oxyanion hole (residues140-144). These clashes appeared to explain the stereopreference between trans- and cis-isomers of cypermethrin analogues, and the 2R- and 2S-isomers of fenvalerate analogues by hCE1. The implications these findings have on the design and use of effective pesticides are discussed.
