Energetic and Molecular Constraints on the Mechanism of Environmental Fe(III) Reduction by Geobacter

This review aims to discuss how Geobacter and its relatives are shaped by the nature of their electron donor and acceptor, where electrons liberated during complete cytoplasmic oxidation of organics must travel far beyond the cell to reduce extracellular metals without the aid of soluble shuttles. This sequence of reactions must often occur in permanently anoxic habitats where reactant concentrations lower the ∆G to only tens of kJ/mol, severely limiting the energy available for protein synthesis. Extracellular Fe(III) reduction is additionally challenging, from a bioenergetic perspective, as oxidation of organic matter (releasing protons and electrons) occurs in the cell interior, but only the negatively charged electrons are transferred outside the cell. Finally, the low amount of energy available from metals in direct contact with a cell predicts that Geobacter must organize electron transfer proteins to extend outward, to take advantage of the Fe(III) in the volume available a few microns beyond its outer membrane. This review will discuss these thermodynamic constraints on environmental metal reduction, and briefly mention recently described aspects of the molecular mechanism of electron transfer by Geobacter spp. when viewed through this lens.