The role of cytochrome c diffusion in mitochondrial electron transport.

We have compared the modes and rates of cytochrome c diffusion to the rates of cytochrome c-mediated electron transport in isolated inner membranes and in whole intact mitochondria. For inner membranes, an increasing ionic strength results in an increasing rate of cytochrome c diffusion, a decreasing concentration (affinity) of cytochrome c near the membrane surface as well as near its redox partners, and an increasing rate of electron transport. For intact mitochondria, an increasing ionic strength results in a parallel, increasing rate of cytochrome c-mediated electron transport. In both inner membranes and intact mitochondria the rate of cytochrome c-mediated electron transport is highest at physiological ionic strength (100-150 mM), where the diffusion rate of cytochrome c is highest and its diffusion mode is three-dimensional. In intact mitochondria, succinate and duroquinol-driven reduction of endogenous cytochrome c is greater than 95% at all ionic strengths, indicating that cytochrome c functions as a common pool irrespective of its diffusion mode. Using a new treatment to obtain bimolecular diffusion-controlled collision frequencies in a heterogenous diffusion system, where cytochrome c diffuses laterally, pseudo-laterally, or three-dimensionally while its redox partners diffuse laterally, we determined a high degree of collision efficiency (turnover/collisions) for cytochrome c with its redox partners for all diffusion modes of cytochrome c. At physiological ionic strength, the rapid diffusion of cytochrome c in three dimensions and its low concentration (affinity) near the surface of the inner membrane mediate the highest rate of electron transport through maximum collision efficiencies. These data reveal that the diffusion rate and concentration of cytochrome c near the surface of the inner membrane are rate-limiting for maximal (uncoupled) electron transport activity, approaching diffusion control.