Prostaglandin E synthase (PGES) catalyzes the isomerization of PGH(2) to PGE(2). We previously reported the identification and structural characterization of Bombyx mod PGES (bmPGES), which belongs to Sigma-class glutathione transferase. Here, we extend these studies by determining the structure of bmPGES in complex with glutathione sulfonic acid (GTS) at a resolution of 1.37 angstrom using X-ray crystallography. GTS localized to the glutathione-binding site. We found that electron-sharing network of bmPGES includes Asn95, Asp96, and Arg98. Site-directed mutagenesis of these residues to create mutant forms of bmPGES mutants indicate that they contribute to catalytic activity. These results are, to our knowledge, the first to reveal the presence of an electron-sharing network in bmPGES. (C) 2013 Elsevier Inc. All rights reserved.