Molecular Mechanism of a Hotdog‐Fold Acyl‐CoA Thioesterase

Thioesterases are enzymes that hydrolyze thioester bonds between a carbonyl group and a sulfur atom. They catalyze key steps in fatty acid biosynthesis and metabolism, as well as polyketide biosynthesis. The reaction molecular mechanism of most hotdog‐fold acyl‐CoA thioesterases remains unknown, but several hypotheses have been put forward in structural and biochemical investigations. The reaction of a human thioesterase (hTHEM2), representing a thioesterase family with a hotdog fold where a coenzyme A moiety is cleaved, was simulated by quantum mechanics/molecular mechanics metadynamics techniques to elucidate atomic and electronic details of its mechanism, its transition‐state conformation, and the free energy landscape of the process. A single‐displacement acid‐base‐like mechanism, in which a nucleophilic water molecule is activated by an aspartate residue acting as a base, was found, confirming previous experimental proposals. The results provide unambiguous evidence of the formation of a tetrahedral‐like transition state. They also explain the roles of other conserved active‐site residues during the reaction, especially that of a nearby histidine/serine pair that protonates the thioester sulfur atom, the participation of which could not be elucidated from mutation analyses alone.