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A Brownian Dynamics Study: The Effect of a Membrane Environment on an Electron Transfer System

Max Planck Institute of Biophysics, Frankfurt, Germany

Correspondence: Address reprint requests to Volkhard Helms, E-mail: Volkhard.Helms{at}bioinformatik.uni-saarland.de.

During the past few years, three-dimensional crystal structures of many of the important integral membrane proteins responsible for the bioenergetic processes of photosynthesis and respiration have been determined. Moreover, a few crystal structures of protein-protein complexes have become available that characterize the interaction between those membrane proteins and the electron carrier protein cytochrome c. Here, we address the association kinetics for binding of cytochrome c to cytochrome c oxidase (COX) from Paracoccus denitrificans by Brownian dynamics simulations. The effects of ionic strength and protein mutations were studied for two different cytochrome c species: the positively charged, dipolar horse heart cytochrome c and the negatively charged physiological electron transfer partner cytochrome c₅₅₂. We studied association toward "naked" COX and toward membrane-embedded COX where the membrane is represented as an uncharged DPPC bilayer modeled in atomistic detail. For the nonnatural association toward "naked" COX, the association rates are >100 times larger for horse heart cytochrome c than for cytochrome c₅₅₂. Interestingly, the presence of the lipid bilayer leads to a dramatic decrease of the association rate of horse heart cytochrome c, but slightly enhances association of cytochrome c₅₅₂, leading to very similar association rates of both proteins to membrane-embedded COX. This finding from computational modeling studies may reflect the optimization of surface patches and of the total net charge on electron transfer pairs in nature.

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