Induced conformational changes upon Cd2+ binding at photosynthetic reaction centers

Cd2+ binding at the bacterial photosynthetic reaction center (bRC) from Rhodobacter sphaeroides is known to inhibit proton transfer (PT) from bulk solvent to the secondary quinone Q(B). To elucidate this mechanism, we calculated the pK(a) for residues along the water channels connecting Q(B) with the stromal side based on the crystal structures of WT-bRC and Cd2+-bound bRC. Upon Cd2+ binding, we observed the release of two protons from His-H126/128 at the Cd2+ binding site and significant pKa shifts for residues along the PT pathways. Remarkably, Asp-L213 near Q(B), which is proposed to play a significant role in PT, resulted in a decrease in pKa upon Cd2+ binding. The direct electrostatic influence of the Cd2+-positive charge on these pKa shifts was small. instead, conformational changes of amino acid side chains induced electrostatically by Cd2+ binding were the main mechanism for these pKa shifts. The long-range electrostatic influence over approximate to 12 angstrom between Cd2+ and Asp-L213 is likely to originate from a set of Cd2+-induced successive reorientations of side chains (Asp-H124, His-H126, His-H128, Asp-H170, Glu-H173, Asp-M17, and Asp-L210), which propagate along the PT pathways as a "domino" effect.