The axial ligands of heme in cytochromes: a near-infrared magnetic circular dichroism study of yeast cytochromes c, c1, and b and spinach cytochrome f

Abstract

Room temperature near-infrared magnetic circular dichroism and low-temperature electron paramagnetic resonance measurements have been used to characterize the ligands of the heme iron in mitochondrial cytochromes c, c1, and b and in cytochrome f of the photosynthetic electron transport chain. The MCD data show that methionine is the sixth ligand of the heme of oxidized yeast cytochrome c1; the identify of this residue is inferred to be the single conserved methionine identified from a partial alignment of the available cytochrome c1 amino acid sequences. A different residue, which is most likely lysine, is the sixth heme ligand in oxidized spinach cytochrome f. The data for oxidized yeast cytochrome b are consistent with bis-histidine coordination of both hemes although the possibility that one of the hemes is ligated by histidine and lysine cannot be rigorously excluded. The neutral and alkaline forms of oxidized yeast cytochrome c have spectroscopic properties very similar to those of the horse heart proteins, and thus, by analogy, the sixth ligands are methionine and lysine, respectively.

Spectroscopic identification of the axial ligands of cytochrome b560 in bovine heart succinate‐ubiquinone reductase

“…The combination of EPR and MCD spectroscopies provides 20 ~~ 1 a powerful and convenient method for assessing axial ligation 4 I ~ '\ of low spin Fe(III) hemes [19][20][21][22]. Low temperature greatly enhances the sensitivity of MCD spectroscopy for detecting low "7 0 -0 1> spin Fe(III) hemes and facilitates investigation under analo-~: o 3 gous conditions to those required for EPR measurements.…”

“…The MCD spectra were recorded at 4.2 K with a magnetic field of 5 T. nances are also apparent in the SQR samples. The relative intensities of the g = 3.46, 3.07, and 2.92 signals were not significantly altered in spectra recorded at 9 K with 1-mW microwave power indicating that none are saturated under the conditions visible region, see While the UV/visible MCD is relatively insensitive to the ~7 ~ nature of the heme axial ligands, the energy and intensity of the porphyrin 0r)-to-iron charge MCD bands in the near-IR region have been found to be extremely sensitive [19][20][21][22]. This is to be expected since the nature and relative orientation of the axial " I ,,~/~' rQPs ligands will have a direct affect on the energies of the Fe d-/ orbitals.…”

“…Hence a ramped line shape for the low-field EPR feature with g > 3.3, coupled with 2cx in the ligation [19][20][21][22] intensity and the HALS-type EPR signal [20]. are provided by alkaline horse heart cytochrome c (His/Lys)…”

Cloning and DNA sequencing of the fbc operon encoding the cytochrome bc1 complex from Rhodobacter sphaeroides

“…6). The conserved sequence C-X-X-C-H beginning at Cys57 in R. sphaeroides is a likely site for heme c attachment, and His61 and Met205 are the probable axial heme ligands (see [50]). …”

Spectroscopic identification of the heme axial ligation of cytochrome b558 in the NADPH oxidase of porcine neutrophils

“…Low spin ferric heroes have two porphyrin(lc)-to-Fe(III) charge transfer bands in the near-IR region. Both exhibit positively-signed, temperature-dependent MCD transitions and the lower energy component almost invariably has the larger intensity [18][19][20][21]. The energy of these transitions, as assessed by the wavelength of the MCD maxima (2CT), has been shown to be extremely sensitive to the nature of the axial ligands [18][19][20][21].…”