Bioelectric motors: bridging the gap

A series of membrane-bound proteins called the cellular respiratory chain (or "electron transport chain") serves as the power system sustaining all known oxygen-breathing life on Earth. This system is essentially a chain of electric motors, powered by the flow of tunneling electrons. Thanks to the advent of sophisticated experimental and numerical techniques, biological molecules have become a particularly attractive medium in which to study quantum transport. The bacterium Paracoccus denitrificans is believed to share a common ancestor with the modern mitochondrion (the "powerhouse of the cell"). I present numerical characterizations of molecular motion at an inter-protein electron transfer interface in the respiratory chain of Paracoccus denitrificans. I am motivated by a recent experiment in which anomalous electron transfer rates were measured for this tunneling step [Ma et al., Biochemistry 46, 11137 (2007)]. I argue that specific amino acids are likely to harness nearby water molecules to bridge the inter-protein gap, enhancing the tunneling rate.