On the role of endogenous electron shuttles in extracellular electron transfer

As a result of cellular metabolism, microbes dramatically alter the chemistry of environments in which they live. Microbes directly influence cycling of metals in the environment via respiratory redox transformations, often influencing solubility and toxicity of these metals by altering their redox state. Metal oxides and a number of other potential electron acceptors are inaccessible to most organisms due to poor solubility at neutral pH. Insoluble substrates cannot diffuse into the cell and therefore require specific electron transfer strategies. The primary focus of research in model organisms has been the mechanisms underlying electron transfer to insoluble, extracellular substrates. Two distinct mechanisms, which are not mutually exclusive, have been championed to explain how organisms transfer electrons from the surface of the cell to an extracellular substrate. In the first mechanism, electrons are transferred during direct contact between the insoluble substrate and redox active proteins associated with the cell surface. The second mechanism involves small redox active molecules termed 'endogenous electron shuttles' secreted by the organism. These molecules are reduced at the cell surface and react abiotically with the insoluble substrate in a cyclic fashion. In this chapter the discovery, characterization, and implications of endogenous electron shuttles are discussed with emphasis on the experimental evidence for shuttle-based electron transfer mechanisms. Model systems of Shewanella oneidensis, Pseudomonas sp., and Geothrix fermentans are examined in detail to illustrate the current state of the field.