Identification of charge‐transfer transitions in the optical spectrum of low‐spin ferric cytochrome P‐450 Bacillus megaterium

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Abstract
The optical, low temperature magnetic circular dichroism (MCD) and EPR spectra of low‐spin Fe(III) cytochrome P‐450 BM‐3 from Bacillus megaterium, and its imidazole adduct have been measured. The MCD spectra locate new transitions over 600–700 am and 800–1300 nm. The latter are assigned to porphyrin (a1u)–Fe(III) (dyz) charge‐transfer (CT) transitions. In the case of the imidazole adduct the energy of this transition 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‐ordination by a strongly H‐bonded water ligand and the axial thiolate form of cysteine. Two transitions between 600–700 nm are detected in both derivatives. A theoretical analysis and fit of the MCD magnetisation properties shows that these transitions are polarised XY and XZ, respectively. They are assigned as thiolate sulphur py–d‐shell and pz–d‐shell CT transitions, analogous to the well known 695 nm band of methioninehistidine co‐ordinated haem as in cytochrome c. They should prove usefully diagnostic of cysteine/Fe(III) conformational changes or protonation.

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