PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Electron transfer in bacterial reaction centers in the 0 °C to − 60 °C temperature range. Nicolas Ginet^*,1, Jérôme Lavergne¹, ¹ LBC - UMR 6191, Saint-Paul-lez-Durance, France

ABSTRACT- We investigated flash-induced absorption and fluorescence yield changes in the 0 to −60 °C range with chromatophores of R. capsulatus devoid of the physiological cytochrome donors to the RC. Reduced TMPD (10 mM) was added as an exogenous electron donor. Unexpectedly, rapid reduction of P⁺ by TMPD was observed well below the freezing temperature of the medium (no glycerol or antifreeze was present), suggesting that the internal space of the chromatophores (or a layer at the membrane interface) remained in the liquid state down to −60 °C. Under such conditions, a pronounced binary oscillation with flash number was observed for the rate of Q_A^- reduction. At −25 °C the half time for this reaction was about 150 ms following odd-numbered flashes and < 5 ms following even-numbered flashes. This shows that the whole turnover of the two-electron gate Q_B (two electron and two proton reduction followed by release of the quinol and binding of a fresh quinone) is unimpaired in this temperature region. The "first" electron transfer (Q_A^- to Q_B) displays a marked slowing down at low temperature (large activation energy), in contrast with the "second" electron transfer (Q_A^- to Q_B^-). The most spectacular finding is the marked difference for the Q_A^- to Q_B electron transfer depending on the presence of TMPD. In its absence, the reaction occurs with t ≤ 4 ms (see our accompanying poster) and it becomes slowed down more than 40-fold in the presence of this donor. This effect is indicative of an interaction between the donor and acceptor sides of the RC. In the presence of P⁺ (no TMPD present) the Q_A^- to Q_B transfer is fast; when P⁺ is rapidly reduced by TMPD the RC adopts a different conformation where the acceptor side reaction becomes much slower. This effect is reminiscent of the effect of P/P⁺ on the G between Q_A and Q_B that we reported previously [Ginet, N. and Lavergne, J. (2000) Biochemistry 39, 16252-16262].

KEY WORDS: low temperature turn over, bacterial reaction center, donor / acceptor side interaction, low temperature