Prostaglandin F_2α Formation from Prostaglandin H₂ by Prostaglandin F Synthase (PGFS):  Crystal Structure of PGFS Containing Bimatoprost^†,‡

Prostaglandin H₂ (PGH₂) formed from arachidonic acid is an unstable intermediate and is efficiently converted into more stable arachidonate metabolites by the action of enzymes. Prostaglandin F synthase (PGFS) has dual catalytic activities:  formation of PGF_2α from PGH₂ by the PGH₂ 9,11-endoperoxide reductase activity and 9α,11β-PGF₂ (PGF_2αβ) from PGD₂ by the PGD₂ 11-ketoreductase activity in the presence of NADPH. Bimatoprost (BMP), which is a highly effective ocular hypotensive agent, is a PGF_2α analogue that inhibits both the PGD₂ 11-ketoreductase and PGH₂ 9,11-endoperoxide reductase activities of PGFS. To examine the catalytic mechanism of PGH₂ 9,11-endoperoxide reductase, a crystal structure of PGFS[NADPH + BMP] has been determined at 2.0 Å resolution. BMP binds near the PGD₂ binding site, but the α- and ω-chains of BMP are locate on the ω- and α-chains of PGD₂, respectively. Consequently, the bound BMP and PGD₂ direct their opposite faces of the cyclopentane moieties toward the nicotinamide ring of the bound NADP. The α- and ω-chains of BMP are involved in H-bonding with protein residues, while the cyclopentane moiety is surrounded by water molecules and is not directly attached to either the protein or the bound NADPH, indicating that the cyclopentane moiety is movable in the active site. From the complex structure, two model structures of PGFS containing PGF_2α and PGH₂ were built. On the basis of the model structures and inhibition data, a putative catalytic mechanism of PGH₂ 9,11-endoperoxide reductase of PGFS is proposed. Formation of PGF_2α from PGH₂ most likely involves a direct hydride transfer from the bound NADPH to the endoperoxide of PGH₂ without the participation of specific amino acid residues.