Electrontunneling throughproteins

Gray, Harry B. ; R. Winkler, Jay R.;

Electron transfer processes are vital elements of energy transduction pathways
in living cells. More than a half century of research has produced a remarkably detailed
understanding of the factors that regulate these ‘ currents of life ’. We review investigations of
Ru-modified proteins that have delineated the distance- and driving-force dependences of
intra-protein electron-transfer rates. We also discuss electron transfer across protein–protein
interfaces that has been probed both in solution and in structurally characterized crystals. It
is now clear that electrons tunnel between sites in biological redox chains, and that protein
structures tune thermodynamic properties and electronic coupling interactions to facilitate these reactions. Our work has produced an experimentally validated timetable for electron tunneling across specified distances in proteins. Many electron tunneling rates in cytochrome c oxidase and photosynthetic reaction centers agree well with timetable predictions, indicating that the natural reactions are highly optimized, both in terms of thermodynamics and electronic coupling. The rates of some reactions, however, significantly exceed timetable predictions ; it is likely that multistep tunneling is responsible for these anomalously rapid charge transfer events.