Interaction between cytochrome c and the photosynthetic reaction center of purple bacteria: behaviour at low temperature.

In purple photosynthetic bacteria the electron donor to the special pair, after its oxidation by a light-induced reaction, is a c-type cytochrome: either a soluble monoheme cytochrome which forms a transitory complex with the reaction center, or a tetraheme cytochrome which remains permanently bound to the reaction center. The effects of low temperatures on electron transfer in the complex are presented and discussed. They provide estimates for the reorganization energy. The most prominent effect of low temperature is that a dominant fast phase of electron transfer becomes impossible at a temperature of around 250 K (monoheme cytochrome) or located between 250 K and 80 K according to the redox state (tetraheme cytochrome). This inhibition is attributed to a freezing-like transition of pools of water molecules which blocks structural changes of the protein which are normally associated with the cytochrome oxidation.