Crystal Structure of Human Prostaglandin F Synthase (AKR1C3)

Abstract

Prostaglandin H2 (PGH2) formed from arachidonic acid is an unstable intermediate and is efficiently converted into more stable arachidonate metabolites (PGD2, PGE2, and PGF 2) by the action of three groups of enzymes. Prostaglandin F synthase (PGFS) was first purified from bovine lung and catalyzes the formation of 9α,11β-PGF2 from PGD2 and PGF 2α from PGH2 in the presence of NADPH. Human PGFS is 3α-hydroxysteroid dehydrogenase (3α-HSD) type II and has PGFS activity and 3α-HSD activity. Human lung PGFS has been crystallized with the cofactor NADP+ and the substrate PGD2, and with the cofactor NADPH and the inhibitor rutin. These complex structures have been determined at 1.69 Å resolution. PGFS has an (α/β)8 barrel structure. The cofactor and substrate or inhibitor bind in a cavity at the C-terminal end of the barrel. The cofactor binds deeply in the cavity and has extensive interactions with PGFS through hydrogen bonds, whereas the substrate (PGD2) is located above the bound cofactor and has little interaction with PGFS. Despite being largely structurally different from PGD2, rutin is located at the same site of PGD2, and its catechol and rhamnose moieties are involved in hydrogen bonds with PGFS. The catalytic site of PGFS contains the conserved Y55 and H117 residues. The carbonyl O11 of PGD2 and the hydroxyl O13 of rutin are involved in hydrogen bonds with Y55 and H117. The cyclopentane ring of PGD2 and the phenyl ring of rutin face the re-side of the nicotinamide ring of the cofactor. On the basis of the catalytic geometry, a direct hydride transfer from NADPH to PGD2 would be a reasonable catalytic mechanism. The hydride transfer is facilitated by protonation of carbonyl O11 of PGD2 from either H117 (at low pH) or Y55 (at high pH). Since the substrate binding cavity of PGFS is relatively large in comparison with those of AKR1C1 and AKR1C2, PGFS (AKR1C3) could catalyze the reduction and/or oxidation reactions of various compounds over a relatively wide pH range.