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Eur. J. Biochem. 270, 3603–3609 (2003) Ó FEBS 2003 Does different orientation of the methoxy groups of ubiquinone-10in the reaction centre of Rhodobacter sphaeroides cause differentbinding at QA and QB? Andre´ Remy1, Rutger B. Boers2, Tatiana Egorova-Zachernyuk2, Peter Gast3, Johan Lugtenburg2and Klaus Gerwert1 1Lehrstuhl fu¨r Biophysik, Ruhr-Universita¨t Bochum, Germany; 2Department of Chemistry, Gorlaeus Laboratories, Leiden University,the Netherlands; 3Department of Biophysics, Huygens Laboratory, Leiden University, the Netherlands The different roles of ubiquinone-10 (UQ10) at the primary 1288 cm)1 and 1264 cm)1 were assigned to the methoxy and secondary quinone (QA and QB) binding sites of Rho- vibrations. They did not shift in frequency at either the QA or dobacter sphaeroides R26 reaction centres are governed by QB binding sites, as compared with unbound UQ10. As the the protein microenvironment. The 4C¼O carbonyl group frequencies of these vibrations and their coupling are sensi- of QA is unusually strongly hydrogen-bonded, in contrast to tive to the conformations of the methoxy groups, different QB. This asymmetric binding seems to determine their dif- conformations of the C(5) and C(6) methoxy groups at the ferent functions. The asymmetric hydrogen-bonding at QA QA and QB binding sites can now be excluded. Both methoxy can be caused intrinsically by distortion of the methoxy groups are oriented out of plane at QA and QB. Therefore, groups or extrinsically by binding to specific amino-acid side hydrogen-bonding to His M219 combined with electrostatic groups. Different X-ray-based structural models show con- interactions with the Fe2+ ion seems to determine the strong tradictory orientations of the methoxy groups and do not provide a clear picture. To elucidate if distortion of Keywords: electron transfer; Fourier-transform infrared the methoxy groups induces this hydrogen-bonding, their spectroscopy; isotopic labelling; photosynthetic reaction (ring-)C-O vibrations were assigned by use of site-specifically labelled [5-13C]UQ10 and [6-13C]UQ10 reconstituted at eitherthe QA or the QB binding site. Two infrared bands at The photosynthetic reaction centre (RC) of the purple are accepted here to form a hydroquinone (QBH2), which nonsulphur bacterium Rhodobacter sphaeroides is a trans- is finally released from the RC; for a recent review see [5].
membrane pigment–protein complex, the structure of To elucidate the protein–cofactor interactions that deter- which has been determined with up to 2.2 A˚ resolution mine the different functions of UQ10 at QA and QB, Fourier- [1–4]. Upon light excitation, an electron is transferred from transform infrared (FTIR) difference spectroscopy has been the primary donor P (bacteriochlorophyll a dimer) via a applied [6–9]. By the use of UQ10 specifically 13C-labelled at monomeric bacteriochlorophyll a and a bacteriopheo- the ring positions 1, 2, 3, and 4, the 1C¼O and 4C¼O and phytin a molecule to the primary quinone QA and finally 2/3C¼C stretching vibrations of UQ10 in the RC have been assigned in the QA ) QA and QB ) QB difference spectra (UQ10) is found at QA and QB, the two molecules differ in [10–13]. At the QA site, the mode dominated by the 4C¼O function: QA is tightly bound to the RC. By accepting one vibration is dramatically downshifted compared with electron, a semiquinone anion radical Q–• unbound UQ10, indicating unusually strong hydrogen- quickly transfers the electron to QB. QB is less tightly bonding to the protein environment [10,11]. In contrast, bound. After the formation of a nonprotonated semiqui- the 1C¼O group is only weakly bound to the protein. This asymmetric binding is conserved in the charge-separatedstate [10,11]. At the QB site, two fractions of UQ10 arefound. The minor fraction is loosely bound and almostunaffected by the protein. In the major fraction, both C¼Ovibrations show symmetric hydrogen-bonding, but weaker Correspondence to K. Gerwert, Lehrstuhl fu¨r Biophysik, Ruhr- Universita¨t Bochum, Postfach 102148, 44780 Bochum, Germany.
Fax: + 49 234 321 4626, Tel.: + 49 234 322 4461, These results for the charge-separated state are supported by EPR [14] and NMR spectroscopy [15].
Abbreviations: FTIR, Fourier-transform infrared; IR, infrared; It is proposed that this difference in binding governs the molecular origin of the strong binding of the 4C B, secondary acceptor quinone; Rb., Rhodobacter; RC, reaction centre; UQ10, ubiquinone-10.
not clear. The conformation of the C(5) and C(6) methoxy (Received 14 April 2003, revised 25 June 2003, accepted 8 July 2003) substituents of UQ10 may differ at both binding sites as

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