Revealing the Involvement of Extended Hydrogen Bond Networks in the Cooperative Function between Distant Sites in Bacterial Reaction Centers

In reaction center proteins of photosynthetic bacteria, the amplitude of proton uptake induced by the one-electron reduction of either of the two quinone electron acceptors (QA and QB) is an intrinsic observable of the electrostatic interactions associated with the redox function of the complex. We report here that, in Rhodobacter capsulatus, complete restoration of proton uptake (upon formation of QA− and QB−) to the level found in the wild type is observed in a mutant reaction center in which a tyrosine substitution in the QA environment (AlaM247 → Tyr) is coupled with mutations of acidic residues near QB (GluL212 → Ala/AspL213 → Ala) that initially cancel the proton uptake above pH 8. This result demonstrates that proton uptake occurs by strong cooperation between structural motifs, such as hydrogen-bonded networks, that span the 18 Å distance between the two quinone acceptors.