Thermodynamic characterization of the redox centers in a representative domain of a novel c-type multiheme cytochrome
Résumé
Multiheme cytochromes that could form protein “nanowires” were identified in Geobacter sulfurreducens genome and represent a new type of multiheme cytochromes. The sequences of these proteins, two with 12-hemes (GSU1996, GSU0592) and one with 27-hemes (GSU2210) suggest that they are formed by domains homologous to triheme cytochromes c7. While all three hemes have bis-His coordination in cytochromes c7, in each domain of the above polymers the heme equivalent to heme IV has His-Met coordination. We previously determined the structure and measured the macroscopic redox potential of one representative domain (domain C) of a dodecaheme cytochrome (GSU1996). In the present study, the microscopic redox properties of the individual heme groups of domain C were determined using NMR and UV-visible spectroscopies. The reduction potentials of the hemes for the fully reduced and protonated protein are different from each other (heme I, -106 mV; heme III, -136 mV; heme IV, -125 mV) and are strongly modulated by redox-interactions. This result is rather surprising since the His-Met coordinated heme IV does not have the highest potential as was expected. The polypeptide environment of each heme group and the strong heme pairwise redox interactions must play a dominant role in controlling the individual heme potentials. The strong redox interactions between the hemes extend the range of their operating potentials at physiological pH (heme I, -71 mV, heme III, -146 mV and heme IV -110 mV). Such a modulation in heme potentials is likely to have a functional significance in the metabolism of Geobacter sulfurreducens.
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