Induced conformational changes upon Cd2+ binding at photosynthetic reaction centers

doi:10.1073/pnas.0503826102

Induced conformational changes upon Cd²⁺ binding at photosynthetic reaction centers

Institute of Chemistry and Biochemistry, Free University of Berlin, Takustrasse 6, D-14195 Berlin, Germany

Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved September 22, 2005 (received for review May 9, 2005)

Abstract

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 pK_a 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 pK_a upon Cd²⁺ binding. The direct electrostatic influence of the Cd²⁺-positive charge on these pK_a shifts was small. Instead, conformational changes of amino acid side chains induced electrostatically by Cd²⁺ binding were the main mechanism for these pK_a 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.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: knapp{at}chemie.fu-berlin.de.
Author contributions: E.-W.K. designed research; H.I. performed research; H.I. and E.-W.K. analyzed data; and H.I. and E.-W.K. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: bRC, bacterial photosynthetic reaction center; Cd-bRC, Cd²⁺-bound bRC; ET, electron transfer; PDB, Protein Data Bank; PSII, photosystem II; PT, proton transfer.
↵ † Paddock, M. L., Flores, M., Isaacson, R., Chang, C., Selvaduray, P., Feher, G. & Okamura, M. Y. (2005) Biophys. J. 88, 204 A (abstr.).