Heme coordination of prostaglandin H synthase.

The heme coordination of ovine prostaglandin H synthase (PGHS) has been characterized by EPR, magnetic circular dichroism, resonance Raman, and optical spectroscopies. The EPR spectrum of ferric PGHS is consistent with an equilibrium mixture of high-spin and low-spin heme species. Both species disappear on reaction of the synthase with hydroperoxides. The high-spin to low-spin interconversion is temperature- and concentration-dependent. Correlation between the axial and rhombic ligand fields of the low-spin heme species suggests that it has bishistidine axial ligation. Magnetic circular dichroism spectra of PGHS also show a temperature-dependent spin transition. Resonance Raman spectra indicate that the enzyme exists as a mixture of six-coordinate low-spin and six-coordinate high-spin ferric heme species. No Raman bands attributable to five-coordinate high-spin heme species are detectable. The magnetic circular dichroism spectra of the fluoride, azide, cyanide, and imidazole derivatives of PGHS resemble those of the corresponding metmyoglobin derivatives and are very different from those of the catalase derivatives. EPR spectra of the imidazole derivative of these three proteins provide additional evidence that the heme coordination structure of PGHS is similar to that of metmyoglobin rather than that of catalase. The midpoint potential of the PGHS Fe(III)/Fe(II) pair is in the range observed for hemeproteins with mono- or bishistidine coordination. These data provide a convincing case that the axial heme ligands of PGHS-1 are a pair of histidine residues, with the distal histidine weakly associated and possibly exchangeable with a weak-field ligand.