Thermodynamics of the Electron Acceptors in Heliobacterium modesticaldum: An Exemplar of an Early Homodimeric Type i Photosynthetic Reaction Center

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 (F_X) in reaction centers from Heliobacterium modesticaldum. The F_X 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 F_X^-, and (iv) the loss of the spin-correlated P₈₀₀⁺ F_X^- radical pair following a single-turnover flash. These four techniques led to similar estimations of the midpoint potential for F_X 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 F_X is reduced, the long-lived (∼15 ms) P₈₀₀⁺ F_X^- 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. F_X is therefore suitably poised to reduce one [4Fe-4S]^2+/1+ cluster in these mobile electron carriers. Using the measured midpoint potential of F_X 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 F_X 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 F_X cluster in heliobacteria approximately equipotential to the highest-potential iron-sulfur cluster (F_A) in Photosystem I, consistent with its assignment as the terminal electron acceptor.