Oxygen and temperature-dependent structural and redox changes in a novel cytochrome c4 from the purple sulfur photosynthetic bacterium Thiocapsa roseopersicina

A novel cytochrome c4, the first of this type in purple phototrophic bacteria has been discovered in Thiocapsa roseopersicina. The fact that cytochrome c4 has been found in an anaerobic organism puts in question the up hereto suggested role of cytochromes c4 in the aerobic respiratory metabolism. The structure of cytochrome c4 was studied under both aerobic and anaerobic conditions, using differential scanning calorimetry and a combination of redox potentiostatic measurements with CD and UV–Vis absorption techniques. Cytochrome c4 maintained its functional capability at high temperature (60 °C) if it was kept under anaerobic conditions. With increasing temperature under aerobic conditions, however, there are dramatic conformational changes in the protein and coordination changes on the iron side. Presumably oxygen binds to the iron at the position left vacant by the methionine and facilitates conformational changes with low reversibility.