Effect of pH on the thermostability and redox properties of cytochrome c ₅₅₂ from Wolinella succinogenes

Cytochrome c ₅₅₂ from Wolinella succinogenes is one of the few examples of a low reduction potential class I c -type cytochrome with a mixture of high/low spin state populations observed in its visible spectrum. Analysis of its structural model suggests that the heme is Met/His coordinated and highly solvent-exposed. This supports the hypothesis that it is the solvent accessibility of the propionate groups that controls the reduction potential of small c -type cytochromes. The visible spectra obtained at different pH values reveal the presence of a protonable group with a p K _a of 7.3, which also influences the reduction potential of this small cytochrome c ₅₅₂ (E _m ^0’ of 97 ± 5 mV, pH 7.0) and can be either an H ₂ O/OH ^- group distantly coordinating the heme iron, or one of the propionate groups. The thermostability of cytochrome c ₅₅₂ has been studied by circular dichroism and differential scanning calorimetry, indicating a highly stable protein at pH 5-7 (90 °C to 77 °C).