Bound Flavin Model Suggests Similar Electron‐Transfer Mechanisms in Shewanella and Geobacter

Certain microbes can produce energy for sustaining life by transporting electrons from cell interior-respiratory electron chain to insoluble electron acceptors located outside of the cell, a process referred to as extracellular electron transport (EET). Bacteria capable of EET are currently utilized as “living anode catalysts” in microbial fuel cells (MFCs). Differing from anode catalysts in chemical fuel cells, these bacteria attach to the anode and operate as catalysts for the oxidation of diverse types of biomass. Thus, MFC technology has received much attention as a potential tool for wastewater treatment and/or other bioremediation uses. Given that EET is the rate-determining process at the MFC anode, a number of organic redox molecules, such as quinones and metal-centered porphyrin-ring derivatives, have been used to enhance the rate of EET. These redox mediators are capable of extracting electrons from outer-membrane (OM) enzymes at the edge of bacterial respiratory electron chain and diffusing to the anode surface to terminate the EET. For example, 100 mm anthraquinone-2,6-disulfonate (AQDS) can increase the current production (and thus the rate of EET) through an electron-shuttling mechanism. Unlike these redox species, riboflavin (RF) and flavin mononucleotide (FMN) enhance EET more than five times, with much lower concentrations than those needed for AQDS shuttling. Initially, flavin was proposed to be a redox-shuttling molecule, as in the high concentration case. However, it has recently been shown that flavins also work as cofactors bound to OM enzymes, leading to more rapid kinetics than can be obtained through electron shuttling. Owing to the involvement of self-secreted flavins being a relatively recent finding, and because there has been some disagreement in the literature, we review our recent work, presenting a “bound-flavin cofactor” model, which is a testable model that, we believe, provides a suitable explanation for all of the published data to date.

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