Electron tunneling in structurally engineered proteins

Abstract

Photosynthesis, respiration, nitrogen fixation, drug metabolism, DNA synthesis, and immune response are among the scores of biological processes that rely heavily on long-range (10 to 25 Å) protein electron-transfer (ET) reactions. Semiclassical theory predicts that the rates of these reactions depend on the reaction driving force −ΔG°, a nuclear reorganization parameter λ, and the electronic-coupling strength H_(AB) between reactants and products at the transition state: ET rates (k°_(ET)) reach their maximum values when the nuclear factor is optimized (−ΔG° = λ); these k_(ET)° values are limited only by the strength (H_(AB)^2) of the electronic interaction between the donor (D) and acceptor (A). Coupling-limited Cu^+ to Ru^(3+) and Fe^(2+) to Ru^(3+) ET rates have been extracted from kinetic studies on several Ru-modified proteins. In azurin, a blue copper protein, the distant D/A pairs are relatively well coupled (k°_(ET) decreases exponentially with R(CuRu); the decay constant is 1.1 Å^(−1)). In contrast to the extended peptides found in azurin and other β-sheet proteins, helical structures have tortuous covalent pathways owing to the curvature of the peptide backbone. The decay constants estimated from ET rates for D/A pairs separated by long sections of the α helix in myoglobin and the photosynthetic reaction center are between 1.25 and 1.6 Å^(−1).