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

A conformational equilibrium controls the Q_A to Q_B electron transfer in bacterial RCs at low temperature. Nicolas Ginet^*,1, Jérôme Lavergne¹, ¹ LBC - UMR 6191, Saint-Paul-lez-Durance, France

ABSTRACT- We investigated the charge recombination processes following a single turnover flash in the 0 to −65 °C temperature range, using chromatophores of R. capsulatus devoid of a secondary electron donor. Two phases were observed: a fast phase in the tens of ms is indicative of the P⁺Q_A^- recombination, while a slow phase in the seconds range is due to the P⁺Q_B^- recombination. When lowering the temperature, the relative extent of the fast phase increases (from 100 % at 0 °C to 62 % at −65 °C). This behavior is generally interpreted in terms of a kinetic competition between the electron transfer from Q_A^- to P⁺ (with rate constant k_AP, little dependent on T) and the transfer from Q_A^- to Q_B (with a temperature dependent rate constant k_AB). The extent of the fast phase would then be k_AP /(k_AP + k_AB). We show that this interpretation, based on a homogeneous treatment of the reaction centers, is not consistent with the effect of multiple flashing. Such experiments indicate a marked heterogeneity of the RCs with respect to the Q_A^- to Q_B electron transfer. When lowering the temperature, the fraction of centers where this reaction is blocked increases. The effect of prolonged illumination, converting most of the RCs to the P⁺Q_B^- state, shows that the two conformations are slowly interconvertible. We thus interpret our results in terms of a temperature dependent equilibrium (log K_eq = H/RT - S/R) between the "passing" and "blocked" conformations.

KEY WORDS: temperature dependencie, bacterial reaction center, conformational equilibrium, acceptor side