RESONANCE RAMAN-SPECTROSCOPIC CHARACTERIZATION OF ALPHA-HYDROXYHEME AND VERDOHEME COMPLEXES OF HEME OXYGENASE

Heme oxygenase (HO) is the microsomal enzyme that catalyzes the oxidat ive degradation of protoheme (iron protoporphyrin IX) and the generati on of carbon monoxide. The enzyme converts protoheme into biliverdin t hrough two known heme derivatives, alpha-hydroxyheme and verdoheme. To : gain insight into the degradation mechanisms of the two intermediate s, the resonance Raman spectra were observed for alpha-hydroxyheme and verdoheme complexes of HO and compared with those of apomyoglobin (ap e-Mb) complexes. The ferrous alpha-hydroxyheme complexed with both HO and apo-Mb shows a resonance Raman spectral pattern similar to that of the protoheme complexes. On the contrary, the ferric alpha-hydroxyhem e and ferrous verdoheme complexes of HO and ape-Mb show atypical Raman patterns, which are interpreted as the result of the symmetry lowerin g of the porphyrin-conJugated pi-electron system. The comparison of th e resonance Raman spectra of the verdoheme, complexed with HO and ape- Mb with those of the five- and six-coordinate model complexes of verdo heme shows that the ferrous forms of the verdoheme-protein complexes a re six-coordinate. The Fe-CO and Fe-CN stretching frequencies of ferro us verdoheme compounds are distinct from those of ferrous heme compoun ds. It is inferred that the positive charge of the verdoheme ring poss esses some of the charge density on the iron atom, causing unique char acteristics of the iron ligand stretching vibrations and altered ligan d binding properties.