Location of inhibitor binding sites in the human inducible prostaglandin E synthase, MPGES1.

The inducible microsomal prostaglandin E(2) synthase 1 (MPGES1) is an integral membrane protein coexpressed with and functionally coupled to cyclooxygenase 2 (COX-2) generating the pro-inflammatory molecule PGE(2). The development of effective inhibitors of MPGES1 holds promise as a highly selective route for controlling inflammation. In this paper, we describe the use of backbone amide H/D exchange mass spectrometry to map the binding sites of different types of inhibitors of MPGES1. The results reveal the locations of specific inhibitor binding sites that include the GSH binding site and a hydrophobic cleft in the protein thought to accommodate the prostaglandin H(2) substrate. In the absence of three-dimensional crystal structures of the enzyme-bound inhibitors, the results provide clear physical evidence that three pharmacologically active inhibitors bind in a hydrophobic cleft composed of sections of transmembrane helices Ia, IIb, IIIb, and IVb at the interface of subunits in the trimer. In principle, the H/D exchange behavior of the protein can be used as a preliminary guide for optimization of inhibitor efficacy. Finally, a comparison of the structures and H/D exchange behavior of MPGES1 and the related enzyme MGST1 in the presence of glutathione and the inhibitor glutathione sulfonate confirms the unusual observation that two proteins from the same superfamily harbor GSH binding sites in different locations.