Cytochrome 'Nanowires' are Physically Limited to Sub-Picoamp Currents that Suffice for Cellular Respiration

Abstract

Mineral-respiring microorganisms from hydrothermal vents to terrestrial soils express filaments that electrically connect intracellular respiration to extracellular geochemistry. Filaments dubbed "cytochrome nanowires" (CNs) have been resolved by CryoEM, but whether they are the two-decades-long sought-after physiological nanowires remains unproven. To assess their functional competence, we analyzed biological redox conduction in all CNs by computing driving forces in the presence of redox anti-cooperativities, reorganization energies with electronic polarizability, and Marcus rates for diffusive and protein-limited flux models. The chain of heme cofactors in any CN must be densely packed to realize weak ([≤]0.01 eV) electronic coupling for electron transfer, as evidenced by a single Soret band produced from coincidental absorptions on multiple hemes. Dense packing, in turn, has three consequences: (1) limited driving forces ([≤]|0.3| eV) due to shared electrostatic microenvironments, (2) strong ([≤]0.12 eV) redox anti-cooperativities that would accentuate the free energy landscape if the linear heme arrangement did not dictate a contra-thermodynamic oxidation order, and (3) an entropic penalty that is offset by thioether tethers of the hemes to the protein backbone. These linkages physically necessitate the rate-throttling T-stacked motif (10-fold slower than the other highly conserved slip-stacked motif). If the sequence of slip- and T-stacked hemes in the CNs had the fastest known nanosecond rates at every step, a micron-long filament would carry a diffusive 0.02 pA current at a physiological 0.1 V, or a protein-limited current of 0.2 pA. Actual CNs have sub-optimal ([≤]102-fold lower), but sufficient conductivities for cellular respiration, with at most thousands of filaments needed for total cellular metabolic flux. Since cells likely discharge less than the 1.0 pA assumed here, and there are multiple pathways besides CNs for expelling electrons, the micro-to-milli-Siemens/cm conductivities are more than sufficient. Reported conductivities once used to argue for metallic-like pili against the cytochrome hypothesis and now illogically attributed to CNs remain inconsistent by 102-105-fold with the physical constraints imposed on biological redox conduction through multiheme architectures.