(529b) The Role of Geobacterial Cytochromes In Uranium Reduction

Anthropogenic use of uranium has led to environmental contamination. Leaching of soluble uranium into the groundwater can lead to widespread human exposure and serious health hazards. One strategy to arrest groundwater contamination is to augment uranium precipitation via subsurface bacteria. Geobacter species have been shown to electrochemically reduce uranium from a soluble form U(VI) to an insoluble form U(IV). Geobacter cells use electron-carrying proteins to transfer electrons generated during respiration from the cytoplasm, through the cell membrane, to electron acceptors (mostly metal oxides) located outside the cell. Although Geobacter’s‘role in uranium reduction is widely recognized, the electron-transfer mechanism(s) used by these bacteria is not well understood. One distinctive feature of Geobacter species is the presence of more than 100 c-type cytochromes. These cytochromes are postulated to function as a heme network that can temporarily store electron produced during respiration and then shuttle the electrons to external electron acceptors, possibly with the involvement of conductive pili. Elucidation of Geobacter’s electron transfer mechanisms is particularly challenging due to the inherent complexity of Geobacter’s heme network and the possibility of pleiotropic effects following gene knockouts in whole cells.  In this study, we describe a biomimetic strategy, in which molecular architectures that mimic the structure and composition of Geobacter’s heme network are assembled on an electrode, and the resulting biomimetic interface is electrochemically characterized. This “bottom-up” approach enables hypotheses about structure-function relationships among key components of the electron transfer system to be tested. To illustrate the concept, a biomimetic interface was assembled that contained the periplasmic cytochrome PpcA, which is the only cytochrome that is conserved in all the members of the family Geobacteraceae. Recombinant PpcA from Geobacter sulfurreducens was overexpressed in Escherichia coli, purified, and immobilized on an alkanethiol self assembled monolayer bound to a gold electrode.  Redox activity of the PpcA-containing interface was characterized using cyclic voltammetry, amperometry and chronocoulometry. The relative rates of PpcA-mediated reduction of soluble iron and uranium species were determined. The results are discussed in terms of the PpcA’s possible role in groundwater uranium fixation.