title: Thermodynamics of the Electron Acceptors in Heliobacterium modesticaldum: An Exemplar of an Early Homodimeric Type I Photosynthetic Reaction Center.
abstract: The homodimeric type I reaction center in heliobacteria is arguably the simplest known pigment-protein complex capable of conducting (bacterio)chlorophyll-based conversion of light into chemical energy. Despite its structural simplicity, the thermodynamics of the electron transfer cofactors on the acceptor side have not been fully investigated. In this work, we measured the midpoint potential of the terminal [4Fe-4S](2+/1+) cluster (FX) in reaction centers from Heliobacterium modesticaldum. The FX cluster was titrated chemically and monitored by (i) the decrease in the level of stable P800 photobleaching by optical spectroscopy, (ii) the loss of the light-induced g ≈ 2 radical from P800(+•) following a single-turnover flash, (iii) the increase in the low-field resonance at 140 mT attributed to the S = (3)/2 ground spin state of FX(-), and (iv) the loss of the spin-correlated P800(+) FX(-) radical pair following a single-turnover flash. These four techniques led to similar estimations of the midpoint potential for FX of -502 ± 3 mV (n = 0.99), -496 ± 2 mV (n = 0.99), -517 ± 10 mV (n = 0.65), and -501 ± 4 mV (n = 0.84), respectively, with a consensus value of -504 ± 10 mV (converging to n = 1). Under conditions in which FX is reduced, the long-lived (∼15 ms) P800(+) FX(-) state is replaced by a rapidly recombining (∼15 ns) P800(+)A0(-) state, as shown by ultrafast optical experiments. There was no evidence of the presence of a P800(+) A1(-) spin-correlated radical pair by electron paramagnetic resonance (EPR) under these conditions. The midpoint potentials of the two [4Fe-4S](2+/1+) clusters in the low-molecular mass ferredoxins were found to be -480 ± 11 mV/-524 ± 13 mV for PshBI, -453 ± 6 mV/-527 ± 6 mV for PshBII, and -452 ± 5 mV/-533 ± 8 mV for HM1_2505 as determined by EPR spectroscopy. FX is therefore suitably poised to reduce one [4Fe-4S](2+/1+) cluster in these mobile electron carriers. Using the measured midpoint potential of FX and a quasi-equilibrium model of charge recombination, the midpoint potential of A0 was estimated to be -854 mV at room temperature. The midpoint potentials of A0 and FX are therefore 150-200 mV less reducing than their respective counterparts in Photosystem I of cyanobacteria and plants. This places the redox potential of the FX cluster in heliobacteria approximately equipotential to the highest-potential iron-sulfur cluster (FA) in Photosystem I, consistent with its assignment as the terminal electron acceptor.
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Bryan
authors.initials: B
authors.lastname: Ferlez
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: John
authors.initials: J
authors.lastname: Cowgill
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Weibing
authors.initials: W
authors.lastname: Dong
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Christopher
authors.initials: C
authors.lastname: Gisriel
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Su
authors.initials: S
authors.lastname: Lin
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Marco
authors.initials: M
authors.lastname: Flores
authors.affiliation: Department of Chemistry, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Karim
authors.initials: K
authors.lastname: Walters
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Daniel
authors.initials: D
authors.lastname: Cetnar
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Kevin E
authors.initials: KE
authors.lastname: Redding
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.|Department of Chemistry, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: John H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D000068016:Eubacteriales / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004583:Electrons; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000378:metabolism*; D055503:Protein Multimerization; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D045331:Photosystem I Protein Complex