Possible Dynamically Gated Conductance along Heme Wires in Bacterial Multiheme Cytochromes

The staggered cross decaheme configuration of electron transfer cofactors in the outer-membrane cytochrome MtrF serves as a prototype for conformationally gated multiheme electron transport. Derived from the bacterium Shewanella oneidensis, the staggered cross configuration reveals intersecting c-type octaheme and tetraheme "wires" containing thermodynamic "hills" and "valleys" (Proc. Natl. Acad. Sci. U. S. A. 2014, 11, 611-616), suggesting that the protein structure may include a dynamical mechanism for conductance and pathway switching depending on enzymatic functional need. Here, we applied classical molecular and statistical mechanics calculations of large-amplitude protein dynamics in MtrF, to address its potential to modulate pathway conductance, including assessment of the effect of the total charge state. Explicit solvent molecular dynamics simulations of fully oxidized and fully reduced MtrF showed that the slowest mode of collective decaheme motion is 90% similar between the oxidized and reduced states and consists primarily of interheme separation with minor rotational contributions. The frequency of this motion is 1.7 × 10(7) s(-1), both for fully oxidized and fully reduced MtrF, slower than the downhill electron transfer rates between stacked heme pairs at the octaheme termini and faster than the electron transfer rates between parallel hemes in the tetraheme chain. This implies that MtrF uses slow conformational fluctuations to modulate electron flow along the octaheme pathway, apparently for the purpose of increasing the residence time of electrons on lowest potential hemes 4 and 9. This apparent gating mechanism should increase the success rate of electron transfer from MtrF to low potential environmental acceptors via these two solvent-exposed hemes.