Exploring the molecular mechanisms of electron shuttling across the microbe/metal space

Abstract

Dissimilatory metal reducing organisms play key roles in the biogeochemical cycle of metals as well as in the durability of submerged and buried metallic structures. The molecular mechanisms that support electron transfer across the microbe-metal interface in these organisms remain poorly explored. It is known that outer membrane proteins, in particular multiheme cytochromes, are essential for this type of metabolism, being responsible for direct and indirect, via electron shuttles, interaction with the insoluble electron acceptors. Soluble electron shuttles such as flavins, phenazines, and humic acids are known to enhance extracellular electron transfer. In this work, this phenomenon was explored. All known outer membrane decaheme cytochromes from Shewanella oneidensis MR-1 with known metal terminal reductase activity and a undecaheme cytochrome from Shewanella sp. HRCR-6 were expressed and purified. Their interactions with soluble electron shuttles were studied using stopped-flow kinetics, NMR spectroscopy, and molecular simulations. The results show that despite the structural similarities, expected from the available structural data and sequence homology, the detailed characteristics of their interactions with soluble electron shuttles are different. MtrC and OmcA appear to interact with a variety of different electron shuttles in the close vicinity of some of their hemes, and with affinities that are biologically relevant for the concentrations typical found in the medium for this type of compounds. All data support a view of a distant interaction between the hemes of MtrF and the electron shuttles. For UndA a clear structural characterization was achieved for the interaction with AQDS a humic acid analog. These results provide guidance for future work of the manipulation of these proteins toward modulation of their role in metal attachment and reduction.

How thermophilic Gram-positive organisms perform extracellular electron transfer: characterization of the cell surface terminal reductase OcwA

“…These spectral changes arise from binding of FMN to OcwA in slow exchange regime on the NMR timescale, which suggests a strong binding. This behavior is also different from what was previously reported for the outer-membrane MHCs from S. oneidensis MR-1, where the binding was transient and occurred in the fast regime on the NMR timescale (28).…”

“…Oxidation by electron shuttles was monitored by measuring the absorption changes at 552 nm upon mixing reduced OcwA with each of them. The temperature of the kinetic experiments was maintained at 25ºC using an external circulating bath (28).…”

“…Shewanella where the binding of soluble redox shuttles to MtrC, OmcA and UndA caused a disturbance of the heme methyl signals (28).…”

Molecular interaction studies revealed the bifunctional behavior of triheme cytochrome PpcA from Geobacter sulfurreducens toward the redox active analog of humic substances

“…This sets the heme substituents as excellent targets for monitoring relevant interactions for electron transfer [18,30,31]. Therefore, in the present work the interactions between AH 2 QDS and PpcA were further evaluated by monitoring the chemical shift perturbation on the heme substituents by 2D-1 H NOESY experiments.…”

Comparative structure-potentio-spectroscopy of the Shewanella outer membrane multiheme cytochromes

“…For OmcA mutation of the distal axial ligand of heme 7 in OmcA has been shown to lower the affinity of OmcA for FMN, indicating a similar interaction [46]. Different groups have utilised molecular dynamic simulations to identify riboflavin and FMN binding sites on OMMC [43,47,48].…”