Role of glutamine 148 of human 15-hydroxyprostaglandin dehydrogenase in catalytic oxidation of prostaglandin E₂

NAD⁺-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH), a member of the short-chain dehydrogenase/reductase (SDR) family, catalyzes the first step in the catabolic pathways of prostaglandins and lipoxins. This enzyme oxidizes the C-15 hydroxyl group of prostaglandins and lipoxins to produce 15-keto metabolites which exhibit greatly reduced biological activities. A three-dimensional (3D) structure of 15-PGDH based on the crystal structures of the levodione reductase and tropinone reductase-II was generated and used for docking study with NAD⁺ coenzyme and PGE₂ substrate. Three well-conserved residues among SDR family which correspond to Ser-138, Tyr-151, and Lys-155 of 15-PGDH have been shown to participate in the catalytic reaction. Based on the molecular interactions observed from 3D structure of 15-PGDH, we further propose that Gln-148 in 15-PGDH is important in properly positioning the 15-hydroxyl group of PGE₂ by hydrogen bonding with the side-chain oxygen atom of Gln-148. This residue is found to be less conserved and replaceable by glutamyl, histidinyl, and asparaginyl residues in SDR family. Accordingly, site-directed mutagenesis of Gln-148 of 15-PGDH to alanine, glutamic acid, histidine, and asparagine (Q148A, Q148E, Q148H, and Q148N) was carried out. The activity of mutant Q148A was not detectable, whereas those of mutants Q148E, Q148H, and Q148N were comparable to or higher than the wild type. This indicates that the side-chain oxygen or nitrogen atom at position 148 of 15-PGDH plays an important role in anchoring C-15 hydroxyl group of PGE₂ through hydrogen bonding for catalytic reaction.