Prostaglandin endoperoxide H synthases 1 and 2, also known as cyclooxygenases (COXs) 1 and 2, convert arachidonic acid (AA) to prostaglandin endoperoxide H 2 . Prostaglandin endoperoxide H synthases are targets of nonspecific nonsteroidal anti-inflammatory drugs and COX-2-specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer has a peroxidase and a COX active site. We find that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (E cat ) and an allosteric monomer (E allo ). Heme binds tightly only to the peroxidase site of E cat , whereas substrates, as well as certain inhibitors (e.g. celecoxib), bind the COX site of E cat . E cat is regulated by E allo in a manner dependent on what ligand is bound to E allo . Substrate and nonsubstrate fatty acids (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the COX site of E allo . AA can bind to E cat and E allo , but the affinity of AA for E allo is 25 times that for E cat . Palmitic acid, an efficacious stimulator of human PGHS-2, binds only E allo in palmitic acid/murine PGHS-2 co-crystals. Nonsubstrate FAs can potentiate or attenuate actions of COX inhibitors depending on the FA and whether the inhibitor binds E cat or E allo . Our studies suggest that the concentration and composition of the free FA pool in the environment in which PGHS-2 functions in cells, the FA tone, is a key factor regulating PGHS-2 activity and its responses to COX inhibitors. We suggest that differences in FA tone occurring with different diets will likely affect both baseline prostanoid synthesis and responses to COX inhibitors.Prostaglandin endoperoxide H synthases (PGHSs), 2 also known generically as cyclooxygenases (COXs), convert arachidonic acid (AA) to prostaglandin H 2 (PGH 2 ) in the committed step of prostanoid biosynthesis (1-6). PGHS-1 and PGHS-2 are products of different genes. In general, PGHS-1 is constitutively expressed, whereas PGHS-2 expression is inducible (4, 5, 7). The enzymes are embedded in the luminal monolayer of the endoplasmic reticulum and inner membrane of the nuclear envelope (8 -11).PGHSs catalyze two different reactions, a COX reaction and a peroxidase (POX) reaction. COX catalysis begins with abstraction of the 13-pro-S-hydrogen from AA by the enzyme in the rate-determining step to generate an arachidonyl radical (3,6,12,13). Two molecules of O 2 are then sequentially added to the arachidonyl chain with concomitant rearrangements to form PGG 2 . PGG 2 can be reduced to PGH 2 by the POX activity. The purified isoforms are about equally efficient in catalyzing the conversion of AA to PGH 2 (1-6).PGHSs are homodimers composed of tightly associated monomers with identical sequences (14). Each monomer comprising a PGHS homodimer has a physically distinct COX and POX active site. Dissociation of the dimers into monomers only occurs upon denaturation (14). Kulmacz and Lands (15,16) provided the first evidence that the monomers of PGHS homodimers differ. They found that maximal COX activity of...

Background: Cyclooxygenase-2 (COX-2), a target of coxibs, aspirin, and related drugs, is a sequence homodimer that functions as a conformational heterodimer. Results: Kinetic and aspirin labeling studies indicate that COX-2 is composed of two equivalent, stable populations of conformational heterodimers. Conclusion: COX-2 is processed and folds into a pre-existent conformational heterodimer. Significance: COX-2 half-site functionality results from COX-2 folding into a stable conformational heterodimer.