5.B.14.  The Flavin-based Extracellular Electron Transfer (F-EET) Family  

Extracellular electron transfer (EET) describes microbial bioelectrochemical processes in which electrons are transferred from the cytosol to the exterior of the cell or vice versa. Mineral-respiring Gram-negaive bacteria use elaborate haem-based electron transfer mechanisms (see TC#s 5.B.8 and 5.B.9). Light et al. 2018 showed that the food-borne pathogen, Listeria monocytogenes, uses a flavin-based EET mechanism to deliver electrons to extracellular iron or an electrode. By performing a forward genetic screen to identify L. monocytogenes mutants with diminished extracellular ferric iron reductase activity, an eight-gene locus was identified that is responsible for EET. This locus encodes a specialized cytoplasmic NADH dehydrogenase that segregates EET from aerobic respiration by channelling electrons to a discrete membrane-localized quinone pool. Other proteins facilitate the assembly of an extracellular flavoprotein that, in conjunction with free-molecule flavin shuttles, mediates electron transfer to extracellular acceptors such as Fe³⁺. This system thus establishes a simple electron conduit that is compatible with the single-membrane structure of the Gram-positive cell. Activation of EET supports growth on non-fermentable carbon sources, and an EET mutant exhibited a competitive defect within the mouse gastrointestinal tract. Orthologues of the genes responsible for EET are present in hundreds of species across the Firmicutes phylum, including multiple pathogens and commensal members of the intestinal microbiota, and correlate with EET activity in assayed strains (Light et al. 2018). In fact, EET is probably associated with half of all bacteria, and the process requires extracytoplasmic flavinylation by AbpE, a  mediator of extracytosolic electron transfer (Méheust et al. 2021).

Type II NADH dehydrogenase—or Ndh1 in L. monocytogenes—catalyses electron exchange from cytosolic NADH to a lipid-soluble quinone derivative, which is the first step in the respiratory electron transport chain. Ndh2, which is encoded by one of the genes in the EET locus, is a protein with an N-terminal type II NADH dehydrogenase domain and a unique transmembrane C-terminal domain that is absent from functionally characterized enzymes. Consistent with Ndh2 being a novel NADH dehydrogenase, Light et al. 2018 observed that EET activation correlated with cellular NAD⁺ levels. Furthermore, the proteins DmkA and DmkB—which are encoded by two other genes in the EET locus—are paralogues of the highly conserved microbial enzymes MenA and HepT, which catalyse terminal steps in the production of the quinone demethylmenaquinone. In Escherichia coli, three different quinones—demethylmenaquinone, menaquinone and ubiquinone—are used to selectively channel electrons to different electron acceptors. Thus, a distinct quinone derivative and NADH dehydrogenase functionally segregate electron fluxes for EET and aerobic respiration (Light et al. 2018).