IDENTIFICATION OF CHARGE-TRANSFER TRANSITIONS IN THE OPTICAL-SPECTRUMOF LOW-SPIN FERRIC CYTOCHROME-P-450 BACILLUS-MEGATERIUM

The optical, low temperature magnetic circular dichroism (MCD) and EPR spectra of low-spin Fe(III) cytochrome P-450 BM-3 from Bacillus megat erium, and its imidazole adduct have been measured. The MCD spectra lo cate new transitions over 600-700 nm and 800-1300 nm. The latter are a ssigned to porphyrin (a1u)-Fe(III) (d(yz)) charge-transfer (CT) transi tions. In the case of the imidazole adduct the energy of this transiti on fits well to the theoretical model of Gadsby and Thomson [Gadsby, P . M. A. & Thomson, A. J. (1990) J. Amer. Chem. Soc 112, 5003 - 5011 ]. For the native enzyme, the energy of the CT band suggests co-ordinati on by a strongly H-bonded water ligand and the axial thiolate form of cysteine. Two transitions between 600-700 nm are detected in both deri vatives. A theoretical analysis and fit of the MCD magnetisation prope rties shows that these transitions are polarised XY and XZ, respective ly. They are assigned as thiolate sulphur p(y)-d-shell and p(z) d-shel l CT transitions, analogous to the well known 695 nm band of methionin e-histidine co-ordinated haem as in cytochrome c. They should prove us efully diagnostic of cysteine/Fe(III) conformational changes or proton ation.