The origin of 15R-prostaglandins in the Caribbean coral Plexaura homomalla: Molecular cloning and expression of a novel cyclooxygenase

The highest concentrations of prostaglandins in nature are found in the Car ibbean gorgonian Plexaura homomalla. Depending on its geographical location , this coral contains prostaglandins with typical mammalian stereochemistry (15S-hydroxy) or the unusual 15R-prostaglandins. Their metabolic origin ha s remained the subject of mechanistic speculations for three decades. Here, we report the structure of a type of cyclooxygenase (COX) that catalyzes t ransformation of arachidonic acid into 15R-prostaglandins. Using a homology -based reverse transcriptase-PCR strategy, we cloned a cDNA corresponding t o a COX protein from the R variety of P, homomalla. The deduced peptide seq uence shows 80% identity with the 15S-specific coral COX from the Arctic so ft coral Gersemia fruticosa and approximate to 50% identity to mammalian CO X-1 and COX-2. The predicted tertiary structure shows high homology with ma mmalian COX isozymes having all of the characteristic structural units and the amino acid residues important in catalysis. Some structural differences are apparent around the peroxidase active site, in the membrane-binding do main, and in the pattern of glycosylation. When expressed in Sf9 cells, the P, homomalla enzyme forms a 15R-prostaglandin endoperoxide together with 1 1R-hydroxyeicosatetraenoic acid and 15R-hydroxyeicosatetraenoic acid as byp roducts. The endoperoxide gives rise to 15R-prostaglandins and 12R-hydroxyh eptadecatrienoic acid, identified by comparison to authentic standards. Eva luation of the structural differences of this 15R-COX isozyme should provid e new insights into the substrate binding and stereospecificity of the diox ygenation reaction of arachidonic acid in the cyclooxygenase active site.