Electron Tunneling in Proteins: Coupling Through a β Strand

Electron coupling through a β strand has been investigated by measurement of the intramolecular electron-transfer (ET) rates in ruthenium-modified derivatives of the β barrel blue copper protein Pseudomonas aeruginosa azurin. Surface histidines, introduced on the methionine-121 β strand by mutagenesis, were modified with a Ru(2,2′-bipyridine)$_2$(imidazole)$^{2+}$ complex. The Cu$^+$ to Ru$^{3+}$ rate constants yielded a distance-decay constant of 1.1 per angstrom, a value close to the distance-decay constant of 1.0 per angstrom predicted for electron funneling through an idealized β strand. Activation-less ET rate constants in combination with a tunneling-pathway analysis of the structures of azurin and cytochrome c confirm that there is a generally efficient network for coupling the internal (native) redox center to the surface of both proteins.