JBC, Vol. 250, Issue 18, 7280-7287, Sep, 1975
Some properties of mammalian cardiac cytochrome c1
L. S. Kaminsky, Y. L. Chiang and T. E. King
Investigations into the nature of the axial heme ligands, the strength of
the heme crevice, the reactivity with cyanide, and the ascorbate
reducibility of cytochrome c1 were performed to explore structure-function
relationships of cytochrome c1. The existence of an absorbance band at 690
nm, which was quenched by raising the pH with a pK of 9.2 corresponding to
a low spin-low transition, suggested that a methionine residue probably
functioned as one of the axial heme iron ligands in this cytochrome.
Spectral titrations of cytochrome c1 in the low pH range showed a markedly
elevated pK for the low spin-high spin transition relative to cytochrome c.
Denaturation studies with urea, the absence of any reaction with cyanide,
and the evidence from other lines would appear to indicate that the heme
group of cytochrome c1 was reduced by ascorbate at approximately 5% of the
rate of reduction of cytochrome c but this rate dramatically increased with
increasing pH concomitant with the disappearance of the 690 nm absorbance
band. Circular dichroic spectra substantiated that elevated pH produced
conformational changes localized to the heme crevice and probably also the
regions containing aromatic residues. The enhanced rate of ascorbate
reduction was perhaps a consequence of the increased accessibility of the
heme iron to ascorbate. Major unfolding of the protein in 8 M urea,
however, completely abolished the ascorbate reducibility of cytochrome c1.
The buried nature of the heme group of cytochrome c1 would probably
preclude transfer of an electron from cytochrome c1 to cytochrome c through
a direct Fe-Fe or a heme-heme interaction. This poses an important question
concerning the mechanism of this electron transfer between these two
cytochromes not only in mitochondria but also in solution.
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