Structural analysis of NSAID binding by prostaglandin H-2 synthase: Time-dependent and time-independent inhibitors elicit identical enzyme conformations

Nonsteroidal antiinflammatory drugs (NSAIDs) block prostanoid biosynthesis by inhibiting prostaglandin H-2 synthase (EC 1.14.99.1). NSAIDs are either rapidly reversible competitive inhibitors or slow tight-binding inhibitors of this enzyme. These different modes of inhibition correlate with clinical ly important differences in isoform selectivity. Hypotheses have been advan ced to explain the different inhibition kinetics. but no structural data ha ve been available to test them, We present here crystal structures of posta glandin H-2 synthase-1 in complex with the inhibitors ibuprofen? methyl flu rbiprofen, flurbiprofen, and alclofenac at resolutions ranging from 2.6 to 2.75 Angstrom. These structures allow direct comparison of enzyme complexes with reversible competitive inhibitors (ibuprofen anal methyl flurbiprofen ) and slow tight-binding inhibitors (alclofenac and flurbiprofen). The four inhibitors bind to the same site and adopt similar conformations. In all f our complexes. the enzyme structure is essentially unchanged, exibiting onl y minimal differences in the inhibitor binding site. These results argue st rongly against hypotheses that explain the difference between slow tight-bi nding and fast reversible competitive inhibition by invoking global conform ational differences or different inhibitor binding sites. Instead, they sug gest that the different apparent modes of NSAID binding may result from dif ferences ill the speed and efficiency with which inhibitors can perturb the hydrogen bonding network around Arg-120 and Tyr-355.