Redox Conduction Through Cytochrome 'Nanowires' Can Sustain Cellular Respiration

Micron-scale electron transfer through polymeric cytochrome nanowires powers prokaryotic life from hydrothermal vents to terrestrial soils in ways not fully understood. Herein, six reduction potentials from recently reported spectroelectrochemistry are each assigned with <0.04 eV to the cryogenic electron microscopy structure of the hexa-heme homopolymeric outer-membrane cytochrome type S (OmcS) from Geobacter sulfurreducens using hybrid quantum/classical computations. The unambiguous assignments define a reversible free energy roller-coaster that is dynamically modulated by <0.1 V under the flow of electrons due to redox cooperativities between adjacent hemes. A physiologically relevant tens to hundreds of filaments are predicted to suffice for cellular respiration by pairing, in the context of non-adiabatic Marcus theory, the free energy landscape with reorganization energies that account for active site or protein-water electronic polarizability, and electronic couplings characteristic of the highly conserved heme packing motifs. General considerations on protein electron transfer and comparison to all known cytochrome nanowires suggest the mechanistic insights are broadly applicable to multi-heme cytochromes in all kingdoms of life.