Site-directed mutagenesis of a phenylalanine residue strictly conserved in cytochromes c3

Saraiva, LM, Salgueiro CA, Legall J, van Dongen WMAM, Xavier AV.  1996.  Site-directed mutagenesis of a phenylalanine residue strictly conserved in cytochromes c3. Journal of Biological Inorganic Chemistry. 1(6):542-550.


Reduction of the haems in tetrahaem cytochromes c3 is a cooperative process, i.e., reduction of each of the haems depends on the redox states of the other haems. Furthermore, electron transfer is coupled to proton transfer (redox-Bohr effect). Two of its haems and a strictly conserved nearby phenylalanine residue, F20, in Desulfovibrio vulgaris (Hildenborough) cytochrome c3 form a structural motif that is present in all cytochromes c3 and also in cytochrome c oxidase. A putative role for this phenylalanine residue in the cooperativity of haem reduction was investigated. Therefore, this phenylalanine was replaced, with genetic techniques, by isoleucine and tyrosine in D. vulgaris (Hildenborough) cytochrome c3. Cyclic voltammetry studies revealed a small increase (30 mV) in one of the macroscopic redox potentials in the mutated cytochromes. EPR showed that the main alterations occurred in the vicinity of haem I, the haem closest to residue 20 and one of the haems responsible for positive cooperativities in electron transfer of D. vulgaris cytochrome c3. NMR studies of F20I cytochrome c3 demonstrated that the haem core architecture is maintained and that the more affected haem proton groups are those near the mutation site. NMR redox titrations of this mutated protein gave evidence for only small changes in the relative redox potentials of the haems. However, electron/electron and proton/electron cooperativity are maintained, indicating that this aromatic residue has no essential role in these processes. Furthermore, chemical modification of the N-terminal amino group of cytochrome c3 backbone, which is also very close to haem I, had no effect on the network of cooperativities.

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