MAGNETIC-CIRCULAR-DICHROISM STUDIES OF ESCHERICHIA-COLI CYTOCHROME BO- IDENTIFICATION OF HIGH-SPIN FERRIC, LOW-SPIN FERRIC AND FERRYL [FE(IV)] FORMS OF HEME-O

Room-temperature (295 K) magnetic-circular-dichroism spectra at 280-25 00 nm have been recorded for Escherichia coli cytochrome bo in its fas t form (which has a g = 3.7 EPR signal and reacts rapidly with cyanide ) and for its formate, fluoride, cyanide and hydrogen-peroxide derivat ives. The spectra of all forms are dominated by signals from low-spin ferric heme b. These include a porphyrin-to-ferric ion charge-transfer transition in the near-infrared region (the near-infrared charge-tran sfer band) at 1610 nm. High-spin ferric heme o gives rise to a negativ e magnetic-circular-dichroism feature at 635, 642 and 625 nm (correspo nding to a shoulder observed in the electronic absorption spectral and a derivative charge-transfer feature at 1100, 1180 and 940 nm for the fast, formate and fluoride forms, respectively. The energies of these bands confirm that fluoride and formate are ligands to heme o. The en ergies of the analogous bands in the spectrum of fast cytochrome bo ar e typical for high-spin ferric hemes with histidine and water axial li gands. Addition of cyanide ion to fast cytochrome bo causes a red shif t in the position of the Soret absorption peak, from 406.5 nm to 413 n m, and results in the loss of the 635-nm feature from the magnetic-cir cular-dichroism spectrum and of the corresponding shoulder in the elec tronic absorption spectrum. In the magnetic-circular-dichroism spectru m, the intensities of the Soret and alpha,beta bands are significantly increased. New near-infrared charge-transfer intensity is observed at 1000-2300 nm with a peak near 2050 nm. These changes are interpreted as resulting from a high-spin to low-spin transition at ferric heme o brought about by the binding of cyanide ion. The energy of the near-in frared charge-transfer band suggests that the cyanide ion is bridged t o the Cu-B of the binuclear site. Treatment of fast cytochrome bo with hydrogen peroxide also causes a red shift in the position of the Sore t absorbance, to 412 nm, and a loss of the 625-nm absorption shoulder. Changes in the magnetic-circular-dichroism spectrum at 450-600 nm are observed, but there is no significant increase in the intensity of th e magnetic-circular-dichroism Soret band and no new near-infrared char ge-transfer bands are detected, ruling out a similar high-spin to low- spin transition at heme o. Subtraction of the contribution of low-spin ferric heme b from the magnetic-circular-dichroism spectrum reveals a spectrum characteristic of ferryl [Fe(IV)I heme o.