The respiration of dissimilatory iron-reducing bacteria, such as Shewanella oneidensis MR-1, on insoluble Fe(III) oxides represents one of the most important geochemical mineral cycling processes.  In MR-1, electron transfer from the inner-membrane through the periplasmic space to the outer-membrane is accomplished by a series of multi-heme proteins, CymA, MtrA, MtrB, MtrC and OmcA. CymA is an inner-membrane quinol dehydrogenase that oxidizes quinol in the inner-membrane and transfers the released electrons to MtrA either directly or indirectly via other periplamsic proteins. MtrA is a c-cytochrome that is thought to be inserted into MtrB. Together, MtrAB mediate electron transfer across the bacterial outer-membrane to MtrC and OmcA.  Evidences have suggested that in addition to direct electron transfer with the outer-membrane proteins, MtrC and OmcA, indirect electron transfer via flavins also occurs, enhancing the reduction of insoluble Fe(III) minerals. However, there has been no systematic investigation on the effect of flavins on Fe(III)-oxide mineral phases and the role of flavins in such process is still up to debate.  In this work, we applied a methyl viologen-encapsulated MtrABC proteoliposome system for investigating interfacial electron transfer reactions from cytochromes to the minerals we developed recently to examine the effect of flavin mononucleotide (FMN) on the reduction kinetics of nano-sized goethite, hematite and lepidocrocite at different FMN concentrations and under two FMN-Fe(III) oxide reduction-mixing scenarios. Our results showed that the reduction rate of Fe(III) oxide by reduced methyl viologen largely depends on the reduction potentials of the Fe(III) oxide, emphasizing the role of the thermodynamic driving force on the reduction kinetics.  The Fe(III) reduction occurred in two stages in the presence of FMN; FMN accelerates the first stage of the reaction but inhibits the second stage, implying a complex role of FMN in the electron transfer processes.

Kinetic study of the effect of FMN on Fe(III) oxide reduction by MtrABC proteoliposome system

247th National Meeting (2014)

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