Structural basis for the variation of pH-dependent redox potentials of Pseudomonas cytochromes c-551

Abstract

The redox potentials of many c-type cytochromes vary with pH over the physiological pH range. The pH dependence of redox potential was investigated for the 4 homologous cytochromes c-551 from P. aeruginosa, P. stutzeri strain 221, P. stutzeri strain 224 and P. mendocina. The pH dependence is due to an ionizable group that ionizes with pKox in ferricytochrome c-551 but with a higher pK, pKred, in ferrocytochrome c-551. For P. aeruginosa cytochrome c-551, this ionizable group is one of the heme propionic acid substituents but the values of pKox and pKred are significantly lower in this protein than in the other 3 cytochromes. NMR and chemical modification studies show that for the 2 P. stutzeri cytochromes c-551 and P. mendocina cytochrome c-551, this propionic acid substituent is again important for the pH dependence of the redox potential. A histidine occurring at position 47 in their sequences H bonds to the propionic acid and raises its pK. In P. aeruginosa cytochrome c-551, His-47 is substituted by Arg-47. H-bonding schemes involving His-57 and the propionic acid are proposed.