Electron tunneling in protein crystals

The current understanding of electron tunneling through proteins has come f rom work on systems where donors and accepters are held at fixed distances and orientations. The factors that control electron flow between proteins a re less well understood, owing to uncertainties in the relative orientation s and structures of the reactants during the very short time that tunneling occurs. As we report here, the way around such structural ambiguity is to examine oxidation-reduction reactions in protein crystals. Accordingly, we have measured and analyzed the kinetics of electron transfer between native and Zn-substituted tuna cytochrome c (cyt c) molecules in crystals of know n structure. Electron transfer rates [(320 s(-1) for *Zn-cyt c --> Fe(III)- cyt c; 2000 s(-1) for Fe(II)-cyt c --> Zn-cyt c(+))] over a Zn-Fe distance of 24.1 Angstrom closely match those for intraprotein electron tunneling ov er similar donor-acceptor separations. Our results indicate that van der Wa als interactions and water-mediated hydrogen bonds are effective coupling e lements for tunneling across a protein-protein interface.