Absence of a link between stabilized charge-separated state and structural changes proposed from crystal structures of a photosynthetic reaction center

Abstract

Structural differences between illuminated and unilluminated crystal structures led to the proposal that the charge-separated state was stabilized by structural changes in its membrane extrinsic protein subunit H in a bacterial photosynthetic reaction center [Katona, G. et al. Nat. Struct. Mol. Biol. 2005, 12, 630–631]. Here, we explored the proposal by titrating all titratable sites and calculating the redox potential (Em) values in these crystal structures. Contrary to the expected charge-separated states, Em for quinone, Em(QA/QA•–), is even lower in the proposed charge-separated structure than in the ground-state structure. The subunit-H residues, which were proposed to exhibit electron-density changes in the two crystal structures, contribute to an Em(QA/QA•–) difference of only <0.5 mV. Furthermore, the protonation states of the titratable residues in the entire reaction center are practically identical in the two structures. These findings indicate that the proposed structural differences are irrelevant to explaining the significant prolongation of the charge-separated-state lifetime. In bacterial photosynthetic reactions, charge-separated states are thought to be stabilized by structural changes in the membrane extrinsic protein subunit H. Here, the authors probe all titratable sites and calculate values of cofactors, and find that the redox potential for quinone is lower in the proposed charge-separated than in the ground-state structure and that the subunit-H residues contribute to a redox potential difference below 0.5 mV.