Electrochemical Characterization of the Periplasmic PpcA c-Cytochrome of Geobacter sulfurreducens Reveals Its Affinity for Uranium

Abstract

Geobacter bacteria produce conductive pili appendages and an outer membrane-anchored lipopolysaccharide to immobilize uranium extracellularly. Yet, some radionuclide enters the cell envelope, where is rapidly mineralized by periplasmic c-cytochromes. To investigate this reaction, we constructed cell envelope biomimetic interfaces containing PpcA, the most conserved and abundant periplasmic c-cytochrome in Geobacter species. Cyclic voltammograms revealed much higher rate constants for uranium than for iron, the natural electron acceptor of Geobacter, independently of the metal-ligand complexation. These results are consistent with a much higher affinity of PpcA for uranium than for iron, providing a plausible explanation for the rapid mineralization of the radionuclide inside the cell envelope. The studies highlight the value of electrochemical biomimetic interfaces to dissect the contribution of cell envelope cytochromes to metal reduction and identify redox-active proteins with the affinity needed for sensory applications.