Protein dynamics enhance electronic coupling in electron transfer complexes

Electron-transferring flavoproteins (ETFs) from human and Paracoccus denitr ificans have been analyzed by small angle x-ray scattering, showing that ne ither molecule exists in a rigid conformation in solution. Both ETFs sample a range of conformations corresponding to a large rotation of domain II wi th respect to domains I and III. A model of the human ETF-medium chain acyl -CoA dehydrogenase complex, consistent with x-ray scattering data, indicate s that optimal electron transfer requires domain Il of ETF to rotate by sim ilar to 30 to 50 degrees toward domain I relative to its position in the x- ray structure. Domain motion establishes a new "robust engineering principl e" for electron transfer complexes, tolerating multiple configurations of t he complex while retaining efficient electron transfer.