Homotropic and heterotropic interactions in cytochromes c3 from sulphate reducing bacteria

► Cytochromes c3 from sulphate reducing bacteria typically comprise 4 haems and about 120 aminoacids. ► Positive thermodynamic cooperativity is found between some haems as well as between haems and protonatable groups. ► Interactions of opposite sign to electrostatic effects arise from localised changes in conformation. ► Hence the cytochromes can accept pairs of electrons and protons from [NiFe]-hydrogenase and release them at the membrane. ► The interactions also allow electron to proton energy transduction and may be relevant to mechanisms of proton pumping.