Magnetic‐Circular‐Dichroism Studies of Cytochrome c and Cytochrome b2

In spite of the very different conformation of the polypeptide chain around the heme‐ligand chelate, the magnetic circular dichroism (MCD) spectra of native cytochrome c and of its heme undecapeptide dissolved in 3.3 MN‐acetyl‐dl‐methionine are identical in the Soret region. This indicates that the MCD spectrum of the heme group (in the Soret band) is not sensitive to changes in the protein conformation as long as the proper ligands are bound to the iron atom. On the contrary, the MCD spectrum of holocytochrome b2 differs from that of its derivatives lacking the FMN group; in the Soret region a MCD band at 365 nm, observed only with the holo‐enzyme, is assigned to the δ absorption band of the heme group rather than to a flavin transition. If, as shown with cytochrome c, the MCD spectra are independent of the protein conformation, the appearance of this band upon binding of FMN can be tentatively explained by a perturbation exerted on the heme group by the flavin itself. This would imply a close proximity of the two prosthetic groups.