Structural basis of heroin and cocaine metabolism by a promiscuous human drug-processing enzyme

Abstract

We present the first crystal structures of a human protein bound to analogs of cocaine and heroin. Human carboxylesterase 1 (hCE1) is a broad-spectrum bioscavenger that catalyzes the hydrolysis of heroin and cocaine, and the detoxification of organophosphate chemical weapons, such as sarin, soman and tabun. Crystal structures of the hCE1 glycoprotein in complex with the cocaine analog homatropine and the heroin analog naloxone provide explicit details about narcotic metabolism in humans. The hCE1 active site contains both specific and promiscuous compartments, which enable the enzyme to act on structurally distinct chemicals. A selective surface ligand-binding site regulates the trimer-hexamer equilibrium of hCE1 and allows each hCE1 monomer to bind two narcotic molecules simultaneously. The bioscavenger properties of hCE1 can likely be used to treat both narcotic overdose and chemical weapon exposure.

Crystal Structures of Human Carboxylesterase 1 in Covalent Complexes with the Chemical Warfare Agents Soman and Tabun,

“…These space groups have been observed in previous hCE1 structures (25)(26)(27)(28), and correspond either to two (P2 1 , hCE1-soman) or one (P2 1 2 1 2 1 , hCE1-tabun) trimer per asymmetric unit. While the cell constants of the hCE1-soman structure appear to be capable of adapting P2 1 2 1 2 1 space group symmetry, R sym values establish P2 1 as correct for this complex.…”

“…The active site of the enzyme is located at the interface of the three domains and is composed of the catalytic triad Ser221, His468, and Glu354. The hCE1 trimer exhibits C3 symmetry and is formed largely by contacts between the αβ domains of each monomer (not shown) (25,27,28). A hexamer has also been observed for hCE1 in which two trimers are stacked with their active sites facing in and with the Z-site loops pinterdigitating (26,27); neither of the structures described here form a hexamer, however.…”

“…It has been shown that AcChE, BuChE and mammalian CE exhibit a similar range of inhibition rate constants (log 10 k i ) of between 6.7 and 7.9 M −1 min −1 for soman (21). Several crystal structures of hCE1 have been determined, including complexes with heroin and cocaine analogues, the clinical therapeutics tamoxifen, tacrine, and mevastatin, and the endobiotics taurocholate, cholate, and coenzyme A (25)(26)(27)(28). These structures highlight the promiscuous ability of the enzyme to bind a broad spectrum of structurally diverse compounds, a characteristic that would be important for an OP hydrolase with specificity for a variety of OP compounds.…”

Liver prenylated methylated protein methyl esterase is the same enzyme as sus scrofa carboxylesterase

“…This may thus explain the low yields of the purified enzyme activity since separate purifications of the enzyme with and without Triton X-100 resulted in similar losses in activity following the use of high-salt concentrations. The active site characteristics are believed to be adaptations for binding the structurally diverse set of substrates such as cocaine and heroin [24,26]. The enzyme has been described as being promiscuous due to its large and flexible active site that renders it the ability to hydrolyze a wide variety of substrates [24,27].…”

Characterization of pyrethroid hydrolysis by the human liver carboxylesterases hCE-1 and hCE-2

“…From these results, the fluorescent pyrethroid-like substrates are very useful for the estimation of hydrolytic activity and stereoselectivity of esterases. A crystal structure of hCE-1 has recently been determined in the laboratory of Redinbo [31,32]. Our laboratory in collaboration with the Redinbo laboratory has examined the stereoselectivity of hCE-1 for the optical isomers of the pyrethroid-like substrates A3 and A4 by computer modeling [33].…”