Molecular mechanisms of heme based sensors from sediment organisms capable of extracellular electron transfer

Bioelectrochemical systems (BES) rely on the metabolism of sediment bacteria capable of forming electrogenic biofilms to generate electrical work. The environment across the thickness of the biofilm is variable and in order for the cells to maintain their viability they require molecular sensors that allow them to adapt their metabolism to their respective environment. The DcrA sensor from Desulfovibrio vulgaris and the GSU 0582 and 0935 sensor domains from Geobacter sulfurreducens appear to function as redox sensors. The SO2144 sensor domain from Shewanella oneidensis MR-1 and the cytochrome c″ from Methylophilus methylotrophus appear to function as NO sensors. Although M. methylotrophus is not known to colonize electrodes on BES, the characterization of cytochrome c″ serves to illustrate the general mechanism of NO sensing similar to that of the heme based sensors of sediment bacteria used in BES. In all cases, conformational changes initiated by the signal trigger the response. What appears to set these two groups of proteins apart is the poise of the heme in the sensors. In the case of redox sensors the hemes appear to be low spin iron II axially coordinated by two residues of the protein. In the case of NO sensors the heme appears to be high spin iron II with the distal coordination site vacant. Understanding of the molecular bases for signal and ligand discrimination may enable a fine control of biofilm formation in bioelectrochemical systems or the development of novel biosensors.