Resonance Raman and EPR spectroscopic studies on heme-heme oxygenase complexes.
The binding of ferrous and ferric hemes and manganese(II)- and manganese(III)-substituted hemes to heme oxygenase has been investigated by optical absorption, resonance Raman, and EPR spectroscopy. The results are consistent with the presence of a six-coordinate heme moiety ligated to an essential histidine ligand and a water molecule. The latter ionizes with a pKa approximately 8.0 to give a mixture of high-spin and low-spin six-coordinate hydroxo adducts. Addition of excess cyanide converts the heme to a hexacoordinate low-spin species. The resonance Raman spectrum of the ferrous heme-heme oxygenase complex and that of the Mn(II)protoporphyrin-heme oxygenase complex shows bands at 216 and 212 cm-1, respectively, that are assigned to the metal-histidine stretching mode. The EPR spectrum of the oxidized heme-heme oxygenase complex has a strongly axial signal with g parallel of approximately 6 and g perpendicular approximately 2. 14NO and 15NO adducts of ferrous heme-heme oxygenase exhibit EPR hyperfine splittings of approximately 20 and approximately 25 Gauss, respectively. In addition, both nitrosyl complexes show additional superhyperfine splittings of approximately 7 Gauss from spin-spin interaction with the proximal histidine nitrogen. The heme environment in the heme-heme oxygenase enzyme-substrate complex has spectroscopic properties similar to those of the heme in myoglobin. Hence, there is neither a strongly electron-donating fifth (proximal) ligand nor an electron-withdrawing network on the distal side of the heme moiety comparable to that for cytochromes P-450 and peroxidases. This observation has profound implications about the nature of the oxygen-activating process in the heme-->biliverdin reaction that are discussed in this paper.
Sun
J
J
Department of Chemistry, Biochemistry, and Molecular Biology, Oregon Graduate Institute of Science and Technology, Portland 97291-1000.
Wilks
A
A
Ortiz de Montellano
P R
PR
Loehr
T M
TM
D002413:Cations, Divalent; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds; D006418:Heme / Q000737:chemistry*; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry; D009569:Nitric Oxide / Q000737:chemistry; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman
D002413:Cations, Divalent; D005290:Ferric Compounds; D009569:Nitric Oxide; D006418:Heme; D008345:Manganese; D006419:Heme Oxygenase (Decyclizing)