Proton Transport in the Outer‐Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

Abstract

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D2O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D2O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET.

Microbial interactions in the anaerobic oxidation of methane: model simulations constrained by process rates and activity patterns

“…Our additional simulations with artificially accelerated proton diffusion allowed for higher rates, indicating the potential role of pH variations to limit AOM at high rates and large aggregate size. We note that in addition to the modelled thermodynamic impact here, a decrease in pH at higher AOM rates may also impede proton transport across outer cell membranes, impacting extracellular electron transport as documented by Okamoto et al (2017) for Shewanella oneidensis MR-1.…”

“…We note that in addition to the modelled thermodynamic impact here, a decrease in pH at higher AOM rates may also impede proton transport across outer cell membranes, impacting extracellular electron transport as documented by Okamoto et al . () for Shewanella oneidensis MR‐1.…”

Basicity of N5 in semiquinone enhances the rate of respiratory electron outflow inShewanella oneidensisMR-1

“…1C). The diffusion-limited kinetics of electron transfer from α-AQS to the electrode was previously confirmed by voltammetric analysis at scan rates of 1– 100 mVs −1 in the presence of the S. oneidensis MR-1 biofilm (47). With an N5-containing molecule (N5 molecule), MB, I c enhancement did not follow Fick’s law and occurred at low concentrations (Fig.…”

“…To analyze the rate-limiting step in EET, we evaluated the kinetic isotope effect (KIE) using a highly reproducible S. oneidensis MR-1 monolayer biofilm (47, 60). We added deuterated water (≤ 4%; subtoxic concentration) to the bulk solution during current production of the MR-1 monolayer biofilm in the presence of 2.0 μM of each cofactor molecule.…”

“…We added deuterated water (≤ 4%; subtoxic concentration) to the bulk solution during current production of the MR-1 monolayer biofilm in the presence of 2.0 μM of each cofactor molecule. A previously reported experimental setup was used (47, 60). As the i c was limited by the EET process, the KIE could characterize the rate-limiting proton transfer process through cofactor-bound Cyts (47).…”