Tuna ferricytochrome

The last of the critical eight aromatic rings is phenylalanine-82, totally invariant over all 60 eukaryotic sequences and the two bacterial cytochromes whose three-dimensional structures are known. It is found nested against the heme, closing the upper left of the heme crevice as shown in Fig. 6. Although this residue was formerly believed from the two-derivative, 2.8 A resolution electron density map of horse oxidized cytochrome to be swung out and away from the heme in ferricytochrome (12-1J ), the four-derivative, 2.0 A map of tuna ferricytochrome has revealed this to be incorrect (15). In both oxidation states, phenylalanine-82 appears to lie next to the heme, and the crevice remains "closed in the sense observed in Fig. 7. 

Figure 10. Front view of the structure of tuna ferricytochrome c (17). The larger eircles are a carbon atoms the smaller eircles represent side-chain atoms.

Ferricytochrome c outer (tuna) Ferricytochrome c inner (tuna) Ferrocytochrome c (bonito)... 

The structures of ferrocytochrome c from tuna (17) and bonito (23) are in essential agreement, and Fig. 9 of ref. 23 shows a stereo comparison of the heme and left channel aromatic ring orientations in these two molecules and in horse ferricytochrome. Because of the poor quality of the horse map, these latter orientations must be considered provisional, and in a sense this chapter is one year premature. A detailed comparison of accurate ring orientations in oxidized and reduced cytochrome will soon be possible, but cannot yet be made. Although the crystal forms obtained... 

A series of exciting papers on cytochrome c by Dickerson et al. includes a report on the structures at 2.8 A resolution of horse and bonito ferricyto-chromes, a speculative discussion on chain flexibility in ferricytochrome, a report of tuna ferrocytochrome at 2.45 A resolution, and a discussion of sequence and structure homologies in bacterial and mammalian-type cytochromes. ... 

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