title: High yield of secondary B-side electron transfer in mutant Rhodobacter capsulatus reaction centers.
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.
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Lucas
authors.initials: L
authors.lastname: Kressel
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO 63130, USA.
authors.forename: Kaitlyn M
authors.initials: KM
authors.lastname: Faries
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Marc J
authors.initials: MJ
authors.lastname: Wander
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Zogzas
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Rachel J
authors.initials: RJ
authors.lastname: Mejdrich
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hanson
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO 63130, USA.
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
authors.affiliation: Biosciences Division, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Philip D
authors.initials: PD
authors.lastname: Laible
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO 63130, USA. Electronic address: kirmaier@wustl.edu.
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
keywords: Charge recombination; Directed evolution; High-throughput screening; Photosynthetic reaction center; Transmembrane electron transfer; Ultrafast spectroscopy