Open AccessInduced conformational changes upon Cd 2+ binding at photosynthetic reaction centers
Author(s) -
Hiroshi Ishikita,
ErnstWalter Knapp
Publication year - 2005
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.0503826102
Subject(s) - chemistry , rhodobacter sphaeroides , stereochemistry , crystallography , binding site , side chain , conformational change , binding energy , photosynthesis , biochemistry , organic chemistry , polymer , physics , nuclear physics
Cd2+ binding at the bacterial photosynthetic reaction center (bRC) fromRhodobacter sphaeroides is known to inhibit proton transfer (PT) from bulk solvent to the secondary quinone QB . To elucidate this mechanism, we calculated the pKa for residues along the water channels connecting QB 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 QB , 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 ≈12 Å 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.