Role of the polarity of the heme environment for the CO stretch modes in cytochrome P-450cam-CO.

The CO stretch mode of various substrate complexes of cytochrome P-450cam-CO was measured using FT infrared spectroscopy. At room temperature most of the complexes show a single, but often asymmetric infrared band. The representative wavenumber of this band for the various complexes increases when the high-spin content, induced by the substrates in the oxidized protein, decreases. Additionally, the increase of the CO stretch wavenumber (1939 to 1956 cm-1) correlates with the decrease of the Soret band wavenumber (22440 to 22373 cm-1). It is suggested that the polarity of the heme pocket is modulated by the substrates due to changed accessibility of the heme environment for water molecules. The increased water content compensates positive electrostatic potentials near the CO ligand, which results in loosening the contact of CO to the I helix.