title: Mechanism of Orlistat Hydrolysis by the Thioesterase of Human Fatty Acid Synthase.
abstract: Fatty acid synthase (FASN), the sole protein capable of de novo synthesis of free fatty acids, is overexpressed in a wide variety of human cancers and is associated with poor prognosis and aggressiveness of these cancers. Orlistat, an FDA-approved drug for obesity treatment that inhibits pancreatic lipases in the GI tract, also inhibits the thioesterase (TE) of human FASN. The cocrystal structure of TE with orlistat shows a pseudo TE dimer containing two different forms of orlistat in the active site, an intermediate that is covalently bound to a serine residue (Ser2308) and a hydrolyzed and inactivated product. In this study, we attempted to understand the mechanism of TE-catalyzed orlistat hydrolysis by examining the role of the hexyl tail of the covalently bound orlistat in water activation for hydrolysis using molecular dynamics simulations. We found that the hexyl tail of the covalently bound orlistat undergoes a conformational transition, which is accompanied by destabilization of a hydrogen bond between a hydroxyl moiety of orlistat and the catalytic His2481of TE that in turn leads to an increased hydrogen bonding between water molecules and His2481and increased chance for water activation to hydrolyze the covalent bond between orlistat and Ser2308. Thus, the conformation of the hexyl tail of orlistat plays an important role in orlistat hydrolysis. Strategies that stabilize the hexyl tail may lead to the design of more potent irreversible inhibitors that target FASN and block TE activity with greater endurance.
authors.affiliation: Department of Pharmacology and Toxicology and IU Simon Cancer Center, Indiana University School of Medicine , 980 West Walnut Street, Indianapolis, Indiana 46202, United States.
authors.forename: Valerie E
authors.initials: VE
authors.lastname: Fako
authors.affiliation: Department of Pharmacology and Toxicology and IU Simon Cancer Center, Indiana University School of Medicine , 980 West Walnut Street, Indianapolis, Indiana 46202, United States ; Department of Pharmacology and Toxicology and IU Simon Cancer Center, Indiana University School of Medicine , 980 West Walnut Street, Indianapolis, Indiana 46202, United States.
authors.forename: Jian-Ting
authors.initials: JT
authors.lastname: Zhang
authors.affiliation: Department of Pharmacology and Toxicology and IU Simon Cancer Center, Indiana University School of Medicine , 980 West Walnut Street, Indianapolis, Indiana 46202, United States ; Department of Computer and Information Science, Indiana University-Purdue University , 635 Barnhill Drive, Indianapolis, Indiana 46202, United States.
authors.forename: Jing-Yuan
authors.initials: JY
authors.lastname: Liu
keywords: fatty acid synthase; orlistat; thioesterase