Correlation between the stability and redox potential of three homologous cytochromes c from two thermophiles and one mesophile

The stability of the oxidized and reduced forms of three homologous cytochromes c from two thermophiles and one mesophile was systematically monitored by means of Soret absorption measurements in the presence of various concentrations of a denaturant, guanidine thiocyanate, at pH 7.0 at 25 °C. Thermophilic Hydrogenobacter thermophilus cytochrome c 552 was the most stable in both redox states, followed by moderately thermophilic Hydrogenophilus thermoluteolus cytochrome c 552 , and then mesophilic Pseudomonas aeruginosa cytochrome c 551 . Further stability and electrochemical analysis of the three proteins and the reciprocal variants, which exhibited a different hydrophobic interaction with the heme, showed that the one with the higher stability in both redox states had the lower redox potential. Consequently, these cytochromes c probably adapted to the cellular environments of the original bacteria with correlated stability and redox potential constraints, which are in part regulated by the hydrophobicity around the heme.