title: Specific bonds between an iron oxide surface and outer membrane cytochromes MtrC and OmcA from Shewanella oneidensis MR-1.
abstract: Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration. A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface. Atomic force microscopy (AFM) was used to detect whether a specific bond forms between a hematite (Fe(2)O(3)) thin film, created with oxygen plasma-assisted molecular beam epitaxy, and recombinant MtrC or OmcA molecules coupled to gold substrates. Force spectra displayed a unique force signature indicative of a specific bond between each cytochrome and the hematite surface. The strength of the OmcA-hematite bond was approximately twice that of the MtrC-hematite bond, but direct binding to hematite was twice as favorable for MtrC. Reversible folding/unfolding reactions were observed for mechanically denatured MtrC molecules bound to hematite. The force measurements for the hematite-cytochrome pairs were compared to spectra collected for an iron oxide and S. oneidensis under anaerobic conditions. There is a strong correlation between the whole-cell and pure-protein force spectra, suggesting that the unique binding attributes of each cytochrome complement one another and allow both MtrC and OmcA to play a prominent role in the transfer of electrons to Fe(III) in minerals. Finally, by comparing the magnitudes of binding force for the whole-cell versus pure-protein data, we were able to estimate that a single bacterium of S. oneidensis (2 by 0.5 microm) expresses approximately 10(4) cytochromes on its outer surface.
authors.affiliation: Environmental Dynamics and Simulation Group, Pacific Northwest National Laboratory, Richland, WA 99352, USA. brian.lower@pnl.gov
authors.forename: Brian H
authors.initials: BH
authors.lastname: Lower
authors.forename: Liang
authors.initials: L
authors.lastname: Shi
authors.forename: Ruchirej
authors.initials: R
authors.lastname: Yongsunthon
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Droubay
authors.forename: David E
authors.initials: DE
authors.lastname: McCready
authors.forename: Steven K
authors.initials: SK
authors.lastname: Lower
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D003574:Cytochrome c Group / Q000737:chemistry / Q000378:metabolism*; D005290:Ferric Compounds / Q000737:chemistry / Q000378:metabolism*; D018625:Microscopy, Atomic Force; D008954:Models, Biological; D011485:Protein Binding; D017510:Protein Folding; D020592:Shewanella / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D003574:Cytochrome c Group; D005290:Ferric Compounds; C000499:ferric oxide; C433527:MtrC protein, Shewanella