Induced conformational changes upon Cd ²⁺ binding at photosynthetic reaction centers

Abstract

Cd 2؉ 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 Cd 2؉ -bound bRC. Upon Cd 2؉ binding, we observed the release of two protons from His-H126͞128 at the Cd 2؉ 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 pK a upon Cd 2؉ binding. The direct electrostatic influence of the Cd 2؉ -positive charge on these pK a shifts was small. Instead, conformational changes of amino acid side chains induced electrostatically by Cd 2؉ binding were the main mechanism for these pKa shifts. The longrange electrostatic influence over Ϸ12 Å between Cd 2؉ and Asp-L213 is likely to originate from a set of Cd 2؉ -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.Cd͞Zn binding effect ͉ photosystem ͉ proton uptake ͉ revertant mutants ͉ secondary quinone

How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870

“…As described in previous applications (2,28,29), hydrogen atom positions were energetically optimized with CHARMM (30), keeping the positions of all nonhydrogen atoms fixed at crystallographically determined coordinates while all titratable groups were in their standard protonation states [i.e., acidic groups ionized and basic groups (including titratable histidines) protonated]. His residues that are ligands of Chla were treated as nontitratable with neutral charge.…”

Selenium-induced protection of photosynthesis activity in rape (Brassica napus) seedlings subjected to cadmium stress. Fluorescence and EPR measurements

“…The aim of the present experiments was to test the effect of selenium on the activity of the photosynthesis under stress conditions generated by Cd. Numerous data point to the interference of cadmium with the functioning of chloroplasts, especially its interaction with redox-active components of the PS2 system, as, e.g., with the quinone-iron complex (Q A -Fe-Q B ) on the PS2 side (Ishikita and Knapp 2005). It was shown that Cd inhibited the electron transfer between Q A and Q B probably via changing the protonation states of the histidine residues in the vicinity of Q B state (Paddock et al 2003).…”

Influence of Cd2+on the spin state of non-heme iron and on protein local motions in reactions centers from purple photosynthetic bacteriumRhodospirilium rubrum

“…These intriguing observations prompted us to investigate in detail NHFe in Rs. rubrum, in particular, how its low spin state is affected by fluctuations of protein matrix and by Cd 2+ ions, which are known to bind in the vicinity of the iron-quinone complex [22,23]. To this end, Mössbauer spectroscopy was applied to compare the valence and spin states of Fe atoms in native RC from Rs.…”