Widespread extracellular electron transfer pathways for charging microbial cytochrome OmcS nanowires via periplasmic cytochromes PpcABCDE

Abstract

Extracellular electron transfer (EET) via microbial nanowires drives globally-important environmental processes and biotechnological applications for bioenergy, bioremediation, and bioelectronics. Due to highly-redundant and complex EET pathways, it is unclear how microbes wire electrons rapidly (>106 s−1) from the inner-membrane through outer-surface nanowires directly to an external environment despite a crowded periplasm and slow (<105 s−1) electron diffusion among periplasmic cytochromes. Here, we show that Geobacter sulfurreducens periplasmic cytochromes PpcABCDE inject electrons directly into OmcS nanowires by binding transiently with differing efficiencies, with the least-abundant cytochrome (PpcC) showing the highest efficiency. Remarkably, this defined nanowire-charging pathway is evolutionarily conserved in phylogenetically-diverse bacteria capable of EET. OmcS heme reduction potentials are within 200 mV of each other, with a midpoint 82 mV-higher than reported previously. This could explain efficient EET over micrometres at ultrafast (<200 fs) rates with negligible energy loss. Engineering this minimal nanowire-charging pathway may yield microbial chassis with improved performance.

Redox Conduction Through Cytochrome ‘Nanowires’ Can Sustain Cellular Respiration

“…Indeed, the unphysically negative potentials computed at 310 K created a large (∼|0.44| eV) free energy difference that was larger than any free energy difference computed at 270 K. The result was a severe underestimation of redox conductivity at 310 K, and the apparent agreement with experiment that conductivity is higher at the lower temperature (anti-Arrhenius kinetics); the right result for an experimentally disproven reason. Note that the discrepancy of the Dahl et al work relative to spectroelectrochemistry performed in the same laboratory was known to the authors up to two years before publication, 44,45 and some of the same authors on the Portela et al 24 study do not comment on how the discrepancy invalidates their prior work.…”

“…The experimental titration curve is reproduced from Portela et al 24 The computed titration curves are simulated from redox potentials reported by Guberman-Pfeffer 33 and Dahl et al 30 The figure shows the fraction of reduced hemes in the OmcS filament as a function of the solution potential. The blue, green and black solid-line titration curves are respectively and chronologically the redox profiles computationally predicted by Dahl et al, 30 Guberman-Pfeffer, 33 and measured using spectroelectrochemistry by Portela et al 24 The computed solid-line titration curves represent an approximation to a cathodic electrochemical sweep in which each heme was separately reduced while all other hemes were held in the oxidized state. The blue dashed-line titration curve represents an anodic electrochemical sweep simulated by Dahl et al 30 in which each heme was separately oxidized while all other hemes were fixed in the reduced state.…”

“…50 In contrast to an evolutionary design principle proposed from the analysis of MtrF, however, the electronic coupling ሺ‫ۦ‬Hۧ, blue open diamonds in Figure 6ሻ is greatest for the exergonic, instead of the endergonic steps. The open and closed green circles represent the reaction free energy based on the computed redox potentials of Guberman-Pfeffer, 33 or the experimental spectroelectrochemical redox potentials from Portela et al, 24 after assigning them to the CryoEM structure with the aid of the computations. .…”

“…6 This 'rock breathing' strategy for anaerobic life, known as extracellular electron transfer, is ancient, 7 ubiquitous, 8,9 environmentally significant, [10][11][12][13] and holds promise for sustainable technologies, [14][15][16][17][18][19][20][21] but only if its mechanistic underpinnings are elucidated. A fundamental question beyond the limits of current experimental techniques 4,[22][23][24] that is addressed herein by hybrid quantum/classical computations is: What is the electron transfer mechanism that connects the atomic structures of filamentous cytochromes to their physiological role as mesoscopic electrical conductors? All known cytochrome filaments 4,[25][26][27][28] have the basic anatomy summarized in Figure 1.…”

“…For the comparison to be non-arbitrary, the computed and measured redox potentials were both sorted in the same way before making a 1:1 mapping. The computed and experimental redox potentials are reproduced, respectively, from Portela et al24 and Guberman-Pfeffer 33. The heme sequence comes from PDB 6EF8 26.…”