Charge separation induces conformational changes in the photosynthetic reaction centre of purple bacteria

Abstract

X-ray structures of the wild-type reaction centre from Rhodobacter sphaeroides have been determined to a resolution of 1.87 Å in the neutral (dark) state and to 2.06 Å in the charge-separated (lightexcited) state. Whereas the overall protein structures of both states are rather similar, the domain around the secondary quinone shows significant shifts. The quinone molecule itself is observed at two different positions. In the neutral state, 55% of the quinone is located distally and 45% proximally to the cytoplasmic side. After excitation by light, however, at least 90% of the quinone is found at the proximal position. Results presented by Stowell et al. (1997) are confirmed, but the quality of crystallographic data has been improved. We compare the data with the structure of the mutant RC L209 PY that keeps the Q B molecule in the proximal position even in the charge-neutral state.

Time-resolved crystallographic studies of light-induced structural changes in the photosynthetic reaction center

“…At the resolution limit sufficient to at least 20% photoactivation within the crystal, our result does not support a large-scale motion of Q B between the PQ B and P ϩ Q B Ϫ states at pH 6.8 in B. viridis. This finding differs from observations made at pH 8.0-8.5 in the R. sphaeroides RC (16)(17)(18). However, it is in agreement with a previous attempt to measure light-induced structural changes in B. viridis using monochromatic x-ray diffraction (40) and with recent investigations of secondary electron transfer in R. sphaeroides at pH 7 by Fourier transform IR spectroscopy (41)(42)(43).…”

“…In B. viridis, spontaneous movement from the distal site to the proximal site depended on the initial orientation of the quinone in the distal site in both the PQ A Q B and P ϩ Q B Ϫ states (23). Neither simulation reproduced the 180°rotation of the aromatic ring between dark and light structures observed in the freeze-trapping experiments (16)(17)(18).…”

“…Recently, several crystallographic freeze-trapping experiments in reaction centers from Rhodobacter sphaeroides have suggested a significant rearrangement within the Q B binding site in response to electron transfer (16)(17)(18). The ubiquinone binds in a ''distal'' binding site in the dark, and moves approximately 4.5 Å to a ''proximal'' binding site upon illumination (Fig.…”

Trapped Conformational States of Semiquinone (D+•QB-•) Formed by B-Branch Electron Transfer at Low Temperature in Rhodobacter sphaeroides Reaction Centers

“…Since binding to these two sites is nearly isoenergetic, small changes in the binding free energy resulting from temperature or external solvent could lead to a change in the relatively occupancy. This may be one reason for the reported differences in the binding position of Q B (26)(27)(28)(29)(30)69) and the FTIR observation that Q B prefers to bind at the proximal position at room temperature (46). …”

“…Furthermore, the preferred binding position may be a function of the cryoprotectant (if the crystal is frozen) and temperature (30). In contrast, Q B −• , the reduced anionic semiquinone, has always been found to be bound in the proximal site (27,28) consistent with the stabilization of the negative charge by H-bonds. This binding position is supported by EPR and ENDOR (31,32) and FTIR (33,34) spectroscopic studies.…”