Thermodynamic characterization of the redox centres in a representative domain of a novel c-type multihaem cytochrome

Multihaem cytochromes that could form protein nanowires were identified in the Geobacter sulfurreducens genome, and represent a new type of mulithaem cytochrome. The sequences of these proteins, two with 12 haems (GSU 1996, GSU0592) and one with 27 haems (GSU2210), suggest that they are formed with domains homologous to the trihaem cytochrome C7. Although all three haems have bis-His co-ordination in cytochromes C7, in each domain of the above polymers, the haem equivalent to haem IV has His-Met co-ordination. We previously determined the structure and measured the macroscopic redox potential of one representative domain (domain C) of a dodecahaem cytochrome (GSU1996). In the present study, the microscopic redox properties of the individual haem groups of domain C were determined using NMR and UV-visible spectroscopies. The reduction potentials of file haems for the fully reduced and protonated protein are different from each other (haem 1-106mV:haem III. - 136 mV and haem IV, - 125 mV) and are strongly modulated by redox interactions. This result is rather surprising since the His-Met co-ordinated haem TV does not have the highest potential as was expected. The polypeptide environment of each haem group and the strong haem pairwise redox interactions must play a dominant role in controlling, the individual haem potentials. The strong redox interactions between the haems extend the range of their operating potentials at physiological pH (haem 1, - 7 1 mV haem III, - 146 mV and haem IV, - 110 mV). Such a modulation in haem potentials is likely to have a functional significance in the metabolism of G. sulfurreducens.