A dynamic ASP-Arg interaction is essential for catalysis in microsomal prostaglandin E₂ synthase

Microsomal prostaglandin E₂ synthase type 1 (mPGES-1) is responsible for the formation of the potent lipid mediator prostaglandin E₂ under proinflammatory conditions, and this enzyme has received considerable attention as a drug target. Recently, a high-resolution crystal structure of human mPGES-1 was presented, with Ser-127 being proposed as the hydrogen-bond donor stabilizing thiolate anion formation within the cofactor, glutathione (GSH).We have combined site-directed mutagenesis and activity assays with a structural dynamics analysis to probe the functional roles of such putative catalytic residues.We found that Ser-127 is not required for activity, whereas an interaction between Arg-126 and Asp-49 is essential for catalysis. We postulate that both residues, in addition to a crystallographicwater, serve critical roles within the enzymatic mechanism. After characterizing the size or charge conservative mutations Arg-126-Gln, Asp-49-Asn, and Arg- 126-Lys, we inferred that a crystallographic water acts as a general base during GSH thiolate formation, stabilized by interaction with Arg-126, which is itself modulated by its respective interaction with Asp-49. We subsequently found hidden conformational ensembles within the crystal structure that correlate well with our biochemical data. The resulting contact signaling network connects Asp-49 to distal residues involved in GSH binding and is ligand dependent. Our work has broad implications for development of efficientmPGES-1 inhibitors, potential anti-inflammatory and anticancer agents.