Modelling of conformational change within photosynthetic reaction center of Rb. sphaeroides bacteria

A possible conformational change, which accompanies electron tranport in Rb. sphaeroides photosynthetic reaction center (RC), was studied using quantum-chemical approach. A kinetic model which takes into account two conformational states of RC is proposed. The model quantitatively describes experimental temperature dependencies of recombination reaction rate $\mathrm{P}^+\mathrm{Q}_A^- \to \mathrm{PQ}_A$. Quantum-chemical modeling of primary quinone ($\mathrm{Q}_A$) binding site permits one to propose a minor shift of $\mathrm{Q}_A$ as a conformational change of interest. The shift is accompanied by break of a hydrogen bond between 4-C=O group of $\mathrm{Q}_A$ and histidine $\mathrm{M}^{219}$, and formation of a new hydrogen bond between $\mathrm{Q}_A$ and hydroxyl group of threonine $\mathrm{M}^{222}$. Characteristics of this conformational change were obtained from quantum-chemical calculations and match parameters of kinetic model in qualitative fashion.