Intramolecular and intracomplex electron transfer in redox proteins.

The properties of intramolecular and intracomplex electron transfer are discussed for four different redox protein systems: cytochrome c-flavodoxin, ferredoxin NADP+: reductase-ferredoxin, cytochrome c-cytochrome c oxidase and cytochrome c-cytochrome c peroxidase. It is shown that in these rather diverse systems several factors in addition to distance and thermodynamic driving force play an important role in the kinetics of electron transfer between prosthetic groups. These include dynamic motions of the reactants within the electron transfer complex, orientation of the prosthetic groups relative to each other, and the nature of the intervening media, that is, the amino acid side chains and solvent between the redox centers. It became apparent such factors were involved when distance and driving force, which play significant roles in the kinetics of electron transfer in diffusion-controlled systems, showed no obvious correlations to the kinetics of intracomplex electron transfer. Although these parameters are probably also important, they are apparently masked by the dominant contribution of the other factors (dynamics, intervening media and orientation). It can also be concluded that hypothetical computer generated protein-protein complexes are not entirely consistent with experimental data.