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202371 https://hdl.handle.net/2286/R.2.N.202371 http://rightsstatements.org/vocab/InC/1.0/ All Rights Reserved 2025 183 pages Doctoral Dissertation Academic theses en Granstrom, Jesse Redding, Kevin Mazor, Yuval Jones, Anne Arizona State University Partial requirement for: Ph.D., Arizona State University, 2025 Field of study: Chemistry The photochemical reaction center (RC), i.e. the enzyme responsible for converting light into chemical energy, is the enzyme that drives photosynthesis. The RC found in heliobacteria, called the heliobacterial reaction center (HbRC) has significance as a system for studying biological electron transfer (ET) and evolution because of its unique structural/ functional characteristics relative to the RCs found in algae, plants, and cyanobacteria. Herein, several questions that remain about the HbRC are investigated, including the role of quinones as well as the ability of the HbRC to facilitate fast, long-distance ET between A0 and FX. The first part of this dissertation (Chapter 2) describes an experimental investigation into the secondary acceptor of the HbRC. The primary acceptor, an 81-OH Chl a called A0, is generally accepted to pass an electron to a [4Fe-4S] cluster called FX, the terminal electron acceptor in the HbRC. Recently, it was found using a modern theoretical/ computational approach that the ~18 Å center-to-center distance between A0 and FX is too long to be reconciled with the experimentally reported rate of electron transfer (ET, ~600-700 ps). To explain this discrepancy, I investigated the possibility of redox-active cofactors between A0 and FX in the form of (1) menaquinone and (2) Tyr550. Removal of these potential acceptors had no effect on ET. Long-distance ET from A0 to FX was then tested by making another point-mutation in PshA, converting Thr440 to a Val, a mutation that was intended to decrease the redox potential of FX by removing a stabilizing interaction between the hydroxyl group in the side chain and an S atom in FX. HbRC-T440V caused a nearly two-fold decrease in the rate of ET from A0 to FX, indicating that the ET from A0 to FX is direct and does not require additional intermediates. The properties of these mutant RCs are investigated in detail. The second major part of this dissertation (Chapter 3) covers research that was performed to investigate quinones as terminal electron acceptors in the HbRC. Biochemistry Microbiology Physical Chemistry Electron transfer Photosynthesis phototrophy Site-directed Mutagenesis Characterization of the Secondary Electron Acceptor in the Heliobacterial Reaction Center and Menaquinone as a Terminal Electron Acceptor