PARENT SESSION

Symposium S5A Type II reaction centres: Structure
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 210A
Chair: Dave Britt
Co-Chair: Rick Debus

Absence of large-scale displacement of functional Q_B in bacterial reaction centers. Jacques Breton^*,1, Eliane Nabedryk¹, ¹ Service de Bioénergétique; CEA-Saclay;, France

ABSTRACT- In photosynthetic bacterial RCs, electron transfer from the primary quinone Q_A to Q_B is gated by a protein conformational change. Based on a dramatic difference of location of Q_B in structures derived from crystals cooled to 90 K either under illumination or in the dark, a functional model for the gating mechanism was proposed whereby neutral Q_B moves 4.5 before receiving the electron from Q_A^-. Whilst neutral Q_B would be located in a distal site with only the C₄=O carbonyl hydrogen-bonded to the protein, Q_B^- occupies a proximal site with both the C₁=O and the C₄=O carbonyls interacting with the protein¹. In contrast, isotope-edited Fourier transform IR difference spectroscopy of Q_B photoreduction in native RCs has led to the conclusion that both carbonyls of neutral Q_B are hydrogen-bonded to the protein². Furthermore, the same isotope-edited IR fingerprints for the carbonyls of neutral Q_B have also been observed^3-5 for Rb. sphaeroides RC mutants at the Pro(L209), Ala(M260), or Glu(L212)/Asp(L213) sites for which X-ray crystallography has found neutral Q_B in the proximal position. Recently, we showed that the specific interactions of the carbonyls of Q_B and Q_B^- with the protein at a single binding site remain identical at both 290 and 85 K, thus demonstrating that Q_B does not move upon reduction⁶. Therefore, the different locations of Q_B reported in many X-ray crystal structures probably are unrelated to functional electron transfer from Q_A^- to Q_B. FTIR spectra of Q_B photoreduction in RC crystals and mother liquor solutions will be presented. 1. Stowell & al. 1997 Science 276 812-816. 2. Breton & al. 1995 Biochemistry 34 11606-11616. 3. Breton & al. 2002 ibid. 41 12921-12927. 4. Nabedryk & al. 2003 ibid. 42 5819-5827. 5. Breton & al. 2004 Biochim. Biophys. Acta, in press. 6. Breton 2004 Biochemistry 43 3318-3326.

KEY WORDS: quinone, FTIR difference spectroscopy, bacterial reaction center, electron transfer