Natural engineering principles of electron tunnelling in biological oxidation-reduction

We have surveyed proteins with known atomic structure whose function involv es electron transfer; in these, electrons can travel up to 14 Angstrom betw een redox centres through the protein medium. Transfer over longer distance s always involves a chain of cofactors. This redox centre proximity alone i s sufficient to allow tunnelling of electrons at rates far faster than the substrate redox reactions it supports. Consequently, there has been no nece ssity for proteins to evolve optimized routes between redox centres. Instea d, simple geometry enables rapid tunnelling to high-energy intermediate sta tes. This greatly simplifies any analysis of redox protein mechanisms and c hallenges the need to postulate mechanisms of superexchange through redox c entres or the maintenance of,charge neutrality when investigating electron- transfer reactions. Such tunnelling also allows sequential electron transfe r in catalytic sites to surmount radical transition states without involvin g the movement of hydride ions, as is generally assumed. The 14 Angstrom or less spacing of redox centres provides highly robust engineering for elect ron transfer, and may reflect selection against designs that have proved mo re vulnerable to mutations during the course of evolution.