CAREER: Manipulating Directionality of Electron Transfer Within Type 1 Photosynthetic Reaction Centers

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Photosynthetic reaction centers (RCs) are one of life's most ancient and useful devices, allowing the biosphere to exploit the abundant solar energy continuously striking our planet and to diversify into a huge number of species with distinct bioenergetic strategies. All known RCs have symmetric structures, using two similar or identical integral membrane subunits to form a dimeric core, which binds the cofactors through which electrons are transferred across the membrane. This symmetric arrangement gives rise to two similar branches of cofactors down which light-driven electron transfer could proceed. The first two members of each branch are chlorins, while the third is a quinone. It is known that the initial electron transfer occurs almost exclusively along one of the two branches in the well-characterized type 2 RCs, although the origins of this strong asymmetry are still debated. Photosystem I (PS1) is still the best-characterized representative of the type 1 RCs, but many aspects of the direction of electron transfer in PS1 remain unknown. Recent optical work clearly suggests that electron transfer can make use of both cofactor branches of PS1 at ambient temperature, while electron paramagnetic resonance (EPR) data indicate that only one branch is active at low temperature. The purpose of this project is to explore the nature, origins, and degree of bi-directionality of electron transfer in PS1. This will be accomplished mainly by genetic manipulation of the PS1 core polypeptides to bias the partitioning of electrons between the two pathways. The effect of the manipulations will be assessed by time-resolved optical and EPR spectroscopy, which should be capable of distinguishing between the phylloquinone molecules in each branch of PS1, thus providing an assessment of the relative use of each pathway. Mutations near the primary electron donor and acceptors may influence the degree to which ET occurs down the two pathways in predictable ways. The role of external conditions and temperature in determining the utilization of the two branches will also be explored. Ultimately, the long-term goal is to deduce the general rules used by nature to direct light-driven electron transfer.Broader impacts of this work include the training of graduate students and undergraduate students in interdisciplinary science at the interface of chemistry, biology, and physics. It will strengthen ties between several research groups, especially the Redding group and that of Fabrice Rappaport and Pierre Joliot at the Institut de Biologie Physico-Chimique (IBPC, Paris). Moreover, this particular international collaboration will be enlarged by the engagement of a postdoctoral fellow who will spend a considerable amount of time at the IBPC, using their instruments. This project will help to build up the EPR facilities at the University of Alabama, which has already attracted collaborative efforts from several nearby institutes. Finally, the knowledge and expertise gained may have practical aspects, such as the ability to re-engineer RCs, so that partitioning between the two branches becomes responsive to external stimuli (e.g. small ions, binding of hydrophobic ligands, electric field, etc.) and may be directed in a controlled way.