The Ferrous Verdoheme−Heme Oxygenase Complex is Six-Coordinate and Low-Spin

A biosynthetic and enzymatic method was developed for the preparation of 13C-labeled verdoheme, which permits the 13C NMR spectroscopic characterization of this elusive intermediate in the heme oxidation path catalyzed by the enzyme heme oxygenase. The 13C NMR data indicate that the ferrous verdoheme complex of Neisseria meningitides heme oxygenase is hexacoordinate and diamagnetic, with a proximal histidine and likely a distal hydroxide as axial ligands. The coordination number and spin state of the ferrous verdoheme−heme oxygenase complex is in stark contrast to the pentacoordinate and paramagnetic nature of the heme−heme oxygenase complex and heme centers in general.