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Possible Pathway for Ubiquinone Shuttling in Rhodospirillum rubrum Revealed by Molecular Dynamics Simulation

A. Aird ^*, J. Wrachtrup ^*, K. Schulten  and C. Tietz ^*

^* Institute of Physics, University of Stuttgart, Stuttgart, Germany; and Beckman Institute and Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois

Correspondence: Address reprint requests to Carsten Tietz, Institute of Physics, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany. Tel.: 49-711-685-65231; Fax: 49-711-685-65281; E-mail: c.tietz{at}physik.uni-stuttgart.de.

In the last decade, the structures of many components of the photosynthetic apparatus of purple bacteria, as well as the mutual organization of these components within the purple membrane, were resolved. One key question that emerged concerned the assembly of the core complex consisting of the reaction center (RC) and the light-harvesting 1 (LH1) complex. In some species, like Rhodobacter sphaeroides, the ring-shaped LH1 complex was found to be open, whereas other species, like Rhodospirillum rubrum, have a closed ring surrounding the reaction center. This poses the question of how the ubiquinone molecule that transports electrons and protons from the RC to the cytochrome bc₁ complex overcomes the apparent barrier of the LH1 ring. In this study, we investigated how, in the case of a closed LH1 ring, the ubiquinone molecule diffuses through the LH1 ring. For this purpose, the LH1 structure of R. rubrum was modeled and the potential of mean force along the diffusion pathway through the LH1 was determined by steered molecular-dynamics simulations. The potential was reconstructed using the fluctuation theorem in combination with the stiff spring approximation. An upper limit for the mean first-passage time for diffusion of ubiquinone through the LH1 ring, based on a worst-case scenario potential, was calculated as 8 x 10^–3 s, which is still in agreement with known turnover rates of RC and RC-LH1 complexes in the range of 1000 Hz.

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