Water Can Ease Electron Transfer between Proteins

Redox Chemistry

Water molecules ease electron transfer between proteins docked at moderate distances-neither in contact nor too far apart-to an unexpected degree, according to a new study. The findings could lead to a better fundamental understanding of biological oxidation-reduction (redox) processes, such as photosynthesis and respiration.

Up to now, theoretical models of electron transfer between proteins in aqueous solution assumed uniform exponential decreases in electron-transfer rates as the proteins get farther apart. However, experimental data obtained by several groups have deviated from this model in unexplained ways.

Chemistry professor David N. Beratan and coworkers at Duke University now find that those earlier models are correct only at short and long interprotein distances (Science 2005, 310, 1311). At intermediate interprotein distances (shown), the researchers find, water molecules between proteins adopt an organized cluster structure that facilitates electron transfer to an unexpected extent. Transfer rates change little, if at all, with distance within that range.

The Duke team made the discovery by carrying out molecular dynamics and quantum mechanics studies of electron transfer between two molecules of cytochrome b₅, a redox enzyme. Our analysis explains a range of otherwise puzzling biological electron-transfer kinetic data and provides a framework for including explicit water-mediated tunneling [quantum mechanical] effects on electron-transfer kinetics, the researchers note.