The electrocatalytic iron release from P. furiosus ferritin upon reduction with a series of electron mediators was studied. The observed iron release rate as a function of mediator midpoint potentials is described by a two-step model, in which electron transfer from the mediator to ferritin is rate limiting at low driving force, and the protein's overall catalytic rate of k(cat)= 701 electrons per s is limiting at high driving force (low mediator potentials). The upper limit of the mediator potential at which the reductive iron release activity of P. furiosus ferritin has been observed in the electrochemical cell is -47 mV vs. SHE.