Electrical circuitry in biology: emerging principles from protein structure

The rate of electron transfer by tunnelling decreases exponentially with distance, but is generally not rate limiting for distances up to 14A, enabling the robust design of redox systems. Fast transfer over distances greater than 14A is accomplished using diffusible electron carriers, an array of closely spaced redox centres or large-scale motion of a redox-active domain. Recent advances indicate that all three mechanisms are used in interprotein electron transfer. The classic stratagem, diffusible electron carriers, may be extended with either of the other designs. The use of an array of solvent-exposed, closely spaced redox centres can maximise productive collisions. Also, the use of conformational sampling via domain motion within the electron transfer complex optimises tunnelling probability.