  Shuttle-assisted charge transfer is pivotal for the efficient energy
transduction from the food-stuff electrons to protons in the respiratory chain
of animal cells and bacteria. The respiratory chain consists of four
metalloprotein Complexes (I-IV) embedded in the inner membrane of a
mitochondrion. Three of these complexes pump protons across the membrane,
fuelled by the energy of food-stuff electrons. Despite extensive biochemical
and biophysical studies, the physical mechanism of this proton pumping is still
not well understood. Here we present a nanoelectromechanical model of the
electron-driven proton pump related to the second loop of the respiratory
chain, where a lipid-soluble ubiquinone molecule shuttles between the Complex I
and Complex III, carrying two electrons and two protons. We show that the
energy of electrons can be converted to the transmembrane proton potential
gradient via the electrostatic interaction between electrons and protons on the
shuttle. We find that the system can operate either as a proton pump, or, in
the reverse regime, as an electron pump. For membranes with various
viscosities, we demonstrate that the uphill proton current peaks near the body
temperature $T \approx 37 ^{\circ}$C.
