Electron tunneling through proteins

Abstract

1. History 3422. Activation barriers 3432.1 Redox potentials 3442.2 Reorganization energy 3443. Electronic coupling 3454. Ru-modified proteins 3484.1 Reorganization energy 3494.1.1 Cyt c 3494.1.2 Azurin 3504.2 Tunneling timetables 3525. Multistep tunneling 3576. Protein–protein reactions 3596.1 Hemoglobin (Hb) hybrids 3596.2 Cyt c/cyt b5 complexes 3606.3 Cyt c/cyt c peroxidase complexes 3606.4 Zn–cyt c/Fe–cyt c crystals 3617. Photosynthesis and respiration 3627.1 Photosynthetic reaction centers (PRCs) 3627.2 Cyt c oxidase (CcO) 3648. Concluding remarks 3659. Acknowledgments 36610. References 366Electron 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.