I just saw this review article by Brutinel and Gralnick on electron shuttling and the role of flavins. For the hardcore Shewanella folks, this will be a must-read paper.
The genus Shewanella contains Gram negative γ-proteobacteria capable of reducing
a wide range of substrates, including insoluble metals and carbon electrodes. The
utilization of insoluble respiratory substrates by bacteria requires a strategy
that is quite different from a traditional respiratory strategy because the cell
cannot take up the substrate. Electrons generated by cellular metabolism instead
must be transported outside the cell, and perhaps beyond, in order to reduce an
insoluble substrate. The primary focus of research in model organisms such as
Shewanella has been the mechanisms underlying respiration of insoluble
substrates. Electrons travel from the menaquinone pool in the cytoplasmic
membrane to the surface of the bacterial cell through a series of proteins
collectively described as the Mtr pathway. This review will focus on respiratory
electron transfer from the surface of the bacterial cell to extracellular
substrates. Shewanella sp. secrete redox-active flavin compounds able to transfer
electrons between the cell surface and substrate in a cyclic fashion-a process
termed electron shuttling. The production and secretion of flavins as well as the
mechanisms of cell-mediated reduction will be discussed with emphasis on the
experimental evidence for a shuttle-based mechanism. The ability to reduce
extracellular substrates has sparked interest in using Shewanella sp. for
applications in bioremediation, bioenergy, and synthetic biology.
