Structural insights into xenobiotic and organophosphate binding by human carboxylesterase 1 and efforts made towards the characterization of the androgen receptor modulator MAGE-11

The processing and elimination of harmful exogenous compounds is important for the successful survival of an organism in its environment. Several proteins classified as drug metabolism enzymes have evolved to provide this protection by catalyzing reactions that increase the polarity of lipophilic molecules, facilitating excretion. The drug metabolism enzyme human carboxylesterase hCE1 works to cleave ester, thioester, and amide linkages in many structurally distinct compounds. The crystal structures of hCE1 bound to tamoxifen, mevastatin, ethyl acetate, and benzil are presented here. These complexes show that hCE1 binds and metabolizes these ligands differently, highlighting its substrate promiscuity. We have additionally sought out to utilize this promiscuity in developing hCE1 as a protein based therapeutic for exposure to chemical warfare agents. Organophosphate nerve agents work by permanently inhibiting human acetylcholinesterase, an enzyme responsible for processing the neurotransmitter acetylcholine and thus terminating cholinergic nerve impulses. Current treatments for nerve agent exposure are limited, and must be administered quickly to be effective. Therefore, developing an enzyme towards the prophylactic treatment of nerve agent exposure is essential. Crystal structures of hCE1 covalently bound to the chemical weapons soman and tabun are presented here. These structures show that hCE1 is stereoselective towards these nerve agents, and its active site architecture may provide it resistance to permanent inhibition by these compounds. The Androgen Receptor (AR) is an intracellular transcription factor responsible for the regulation of androgen-responsive genes. AR activity is modulated by co-activators that bind to the activation function (AF-2) region of its ligand binding domain through an LxxLL motif. AR, however, exhibits a novel N/C-terminal self-association between the AF-2 and an FxxLF N-terminal motif that precludes the recruitment of these co-activators. The melanoma antigen protein MAGE-11 disrupts this interaction, facilitating AR mediated transactivation. The efforts made towards the purification and biophysical characterization of MAGE-11 are presented here. MAGE-11 was found to bind the FxxLF motif of AR an order of magnitude stronger than the AR LBD, providing an explanation for its role in AR mediated gene expression