7 Cytochromes c

Publisher Summary The term cytochrome c is both a spectral and a structural classification, related more to the heme and its attachment to the polypeptide chain than to the protein which surrounds it. The most familiar member of the class is cytochrome c from the mitochondria1 respiratory chain, which has a single heme group per chain of 103–113 amino acids, and a reduction potential of +260 mV. Cytochromes all have a characteristic three-banded absorption spectrum in the reduced state. One of the most striking features of the heme group is the delocalization of electrons among the π orbitals of the porphyrin ring. The Soret band in the absorption spectrum represents the excitation of delocalized π electrons to unoccupied levels of the porphyrin ring of similar angular momentum. The absorption spectrum of cytochrome c, which is the basis for classification, depends on the side groups around the porphyrin ring and the way that they are connected to the protein chain. All cytochromes c are oxidation-reduction proteins involved in either respiration or photosynthesis.