Midpoint Potential of the Interpolypeptide [4Fe-4S] Cluster FX in Reaction Centers from Heliobacterium Modesticaldum

Abstract

Heliobacteria are members of a class of poorly characterized phototrophic bacteria that utilize a photosynthetic reaction center (RC) containing Fe/S clusters to convert light into chemical energy. The homodimeric RC of heliobacteria contains ∼22 light harvesting bacteriochlorophyll g (BChl g) pigments, the primary donor P800 (a special pair of BChl g') and the electron transfer cofactors A0 (81-OH ChlaF) and FX (an interpolypeptide [4Fe-4S] cluster). This terminal Fe/S cluster has been characterized by electron paramagnetic resonance (EPR) and mossbauer spectroscopies as being in the [4Fe-4S]1+ oxidation state with S = 3/2 in isolated RCs. To further characterize this cluster isolated RCs were titrated with sodium dithionite and analyzed by low temperature, transient, and field-modulated time-resolved EPR. Titration of the S = 3/2 signal with low temperature EPR revealed a midpoint potential of −504 mV (v. SHE). These results were confirmed by analysis of the one-and-the same samples by both transient and field-modulated time-resolved EPR.Altogether these three techniques reveal a midpoint potential of −500 mV ± 5 mV (v. SHE) for the FX cluster at pH 11. In agreement with previously published work, the lifetime of the flash-induced field-modulated EPR signal of P800+ was ∼ 2 ms at 90 K. These experiments suggest that the midpoint potential of FX in heliobacteria is ∼ 200 mV more positive than the counterpart FX cluster in Photosystem I, and show recombination kinetics with the primary donor that are faster at 90 K (∼ 2 ms) than at room temperature (∼ 15 ms). Funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, under Grant DE-FG02-08ER15989.