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A cytochrome c derivative from which iron is removed has been prepared and characterized. Several lines of evidente indicate that native and porphyrin cytochrome c have similar conformations: they have similar elution characteristics an Sephadex gel chromatography; in both proteins the tryptophan fluorescence is quenched and the pK values of protonation of the porphyrin are identical. Porphyrin cytochrome c does not substitute for native cytochrome c in Bither the oxidase reaction or in restoring electron transport in cytochrome‐c‐depleted mitochondria. It does however competitively inhibit native cytochrome c in these reactions, the Ki for inhibition being larger than the Km for reaction. The absorption and emission spectra, and the polarized excitation spectrum of the porphyrin cytochrome c are characteristic of free base porphyrin. The absente of fluorescence quenching of porphyrin cytochrome c when the protein is bound to cytochrome oxidase suggests that heure to heure distance between these proteins is larger than 0.5 to 0.9 nm depending upon orientation. Binding of the porphyrin cytochrome c to phospholipids or to mitochondria increases the fluorescence polarization of a positively polarized absorption band, which indicates that the bound form of the protein does not rotate freely within the time scale of relaxation from the excited state
Vanderkooi et al. (Mon,) studied this question.