Electron Paramagnetic Resonance Investigation of Nitrite Binding in Myoglobin

Abstract

It has been proposed that myoglobin (Mb) may act as a nitrite reductase under hypoxic conditions. Any mechanism describing such activity should take into account the binding geometry of the ligand to the heme. Crystal structures of horse-heart Mb and human hemoglobin-nitrite complexes suggest that the anion adopts an uncommon O-nitrito binding mode. Electron Paramagnetic Resonance (EPR) spectroscopy was employed to investigate the nature of nitrite binding to Mb at pH values ranging from 6.5 to 10.8. Results suggest that for ferric Mb at low pH, nitrite binds in the O-bound nitrito mode resulting in a low-spin (LS) iron center. Further a high-spin (HS) iron center is observed at high pH in Mb-Nitrite with spectral values different to that of purely HS-Mb that is proposed to be due to an N-bound nitrite. The yields of these two species were found to be influenced by pH.\n\nBackgroundMyoglobin has been theorized to have a role as a nitrite reductase.\n\nResultsO-bound nitrite produces a low-spin ferric heme complex, whilst at high pH a high-spin species is found proposed to be the N-bound form.\n\nConclusionNitrite may bind to heme in myoglobin via N-nitro or O-nitrito mode.\n\nSignificanceThe mechanism of any nitrite reduction will depend on its binding to the heme cofactor.