Human Cyclooxygenase-2 Is a Sequence Homodimer That Functions as a Conformational Heterodimer

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.