Relative importance of driving force and electrostatic interactions in the reduction of multihaem cytochromes by small molecules

► The reduction of cytochrome c″ by dithionite was studied at various ionic strengths. ► The electron transfer involves a positively charged region close to the haem edge. ► Surface charge was changed by mutation and pH but had little effect on the rates. ► Using Marcus theory shows that the driving force is the main factor determining rates. ► This validates kinetic and thermodynamic characterizations of multihaem cytochromes.