PNNL

Abstract

The identification of electron transfer proteins that couple soluble redox carriers to electrode surfaces has great potential in permitting the development of scalable bioreactors. In this respect, the 85 kDa outer membrane decaheme cytochrome OmcA (SO1779) from Shewanella oneidensis MR-1 can reduce soluble Fe(III) chelates, and has previously been suggested to function in concert with other membrane proteins as one of the terminal electron donors in the metal reductase protein complex of Shewanella oneidensis MR-1.1 Shewanella is a facultative anaerobe that can reduce a range of different metal oxides, including iron [Fe(III)], manganese [Mn(III/IV)], chromium [Cr(IV)] and uranium [U(VI)]2, and whose metabolic diversity has considerable promise for both the bioremediation of organic and metal contaminants as well as in the design of microbial fuel cells3. Biofuel cells offer a potential means to couple the breakdown of bio-wastes to generate power4. Miniaturization of these fuel cells is dependent on the elimination of the currently necessary membrane between cathode and anode compartments.5 It has been demonstrated that the immobilization of redox active proteins, such as glucose oxidase, on electrodes with redox active polymer coatings renders the membrane unnecessary.6 The identification of a purified metal-reducing enzyme able to densely bind and directly donate electrons to iron-oxide coated electrodes (commonly used to increase electron transfer efficiencies7) has great potential to contribute to fuel cell design. To identify the terminal electron donors in the S. oneidensis metal reductase system, and to explore whether isolated proteins can directly bind and mediate electron transfer reactions to reduce solid metals, we have purified OmcA and measured its ability to bind and transfer electrons to solid Fe2O3 in the mineral hematite.