High yield of secondary B-side electron transfer in mutant Rhodobacter capsulatus reaction centers |
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Authors: | Lucas Kressel Kaitlyn M. Faries Marc J. Wander Charles E. Zogzas Rachel J. Mejdrich Deborah K. Hanson Dewey Holten Philip D. Laible Christine Kirmaier |
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Affiliation: | 1. Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA;2. Department of Chemistry, Washington University, St. Louis, MO 63130, USA |
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Abstract: | From the crystal structures of reaction centers (RCs) from purple photosynthetic bacteria, two pathways for electron transfer (ET) are apparent but only one pathway (the A side) operates in the native protein-cofactor complex. Partial activation of the B-side pathway has unveiled the true inefficiencies of ET processes on that side in comparison to analogous reactions on the A side. Of significance are the relative rate constants for forward ET and the competing charge recombination reactions. On the B side, these rate constants are nearly equal for the secondary charge-separation step (ET from bacteriopheophytin to quinone), relegating the yield of this process to < 50%. Herein we report efforts to optimize this step. In surveying all possible residues at position 131 in the M subunit, we discovered that when glutamic acid replaces the native valine the efficiency of the secondary ET is nearly two-fold higher than in the wild-type RC. The positive effect of M131 Glu is likely due to formation of a hydrogen bond with the ring V keto group of the B-side bacteriopheophytin leading to stabilization of the charge-separated state involving this cofactor. This change slows charge recombination by roughly a factor of two and affords the improved yield of the desired forward ET to the B-side quinone terminal acceptor. |
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Keywords: | Photosynthetic reaction center Charge recombination High-throughput screening Ultrafast spectroscopy Directed evolution Transmembrane electron transfer |
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