Interaction of nitric oxide with prostaglandin endoperoxide H synthase-1: Implications for Fe-His bond cleavage in heme proteins

We have investigated the cleavage of the bond between the heme iron and the proximal heme ligand, His388, in ferrous ovine prostaglandin endoperoxide H synthase-1 (oPGHS-1) on nitric oxide (NO) binding by using resonance Raman spectroscopy. The Fe-NO and N~O vibrations are observed at 526 and 1667 cm^-1, respectively, which is indicative of a five-coordinated, heme-NO complex. We also observed NO photolysis under low-power, continuous wave conditions, which reflects nongeminate rebinding of NO to the heme cofactor on the millisecond time scale. Furthermore, we attempted to make a six-coordinated heme-NO complex by adding imidazole (Im) to ferrous oPGHS-1-NO. However, ferrous oPGHS-1-NO remained fivecoordinated even in the presence of 500 mM Im. The heme-NO complex remains bound in the heme pocket after Fe-His bond cleavage, and subsequent protein backbone movement is relatively small. Finally, we compared the structures of several heme proteins that form either five- or six-coordinated complexes with NO. Inspection of the heme-proximal His geometry did not reveal any common structural parameter that may be critical for Fe-His bond cleavage. However, the Fe-His stretching frequency [v(Fe-His)] shows a good correlation with Fe-His bond cleavage and is modulated by hydrogen bonding to the N_δ proton of the His. We propose that the strength of this hydrogen bond is the primary factor determining Fe-His bond cleavage: a strong hydrogen bond gives the His imidazolate character, which allows it to compensate for the NO trans ligand effect and to avoid Fe-His bond cleavage.