Kinetic and Mechanism of Mineral Respiration: How Iron-Hemes Synchronize Electron Transfer Rates.

Anaerobic microorganisms of the Geobacter genera are effective electron sources for the synthesis of nanoparticles, for bioremidiation of polluted water, and for the production of electrical currents in fuel cells. In multistep reactions, electrons are transferred via iron/heme cofactors of c-type cytochromes from the inner cell membrane to extracellular metal ions, which are bound to outer membrane cytochromes. We measured electron production and electron flux rates to 5•105 e-/s per G. sulfurreducens. Remarkably, these rates are independent of the oxidants, and follow a zero order kinetic. It turned out that the microorganisms regulate electron flux rates by increasing their Fe2+:Fe3+ ratios in the multiheme cytochromes whenever the activity or concentration of the extracellular metal oxidants is diminished. By this mechanism the respiration remains constant even when oxidizing conditions are changing. Such a homeostasis is a vital condition for living systems, and makes G. sulfurreducens to a versatile electron source.