Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

Photosynthetic bacterial reaction centers convert light excitation into chemical free energy. The initial electron transfer leads to the consecutive semireductions of the primary (Q(A)) and secondary (Q(B)) quinone acceptors. The Q [Formula: see text] and Q [Formula: see text] formations induce proton uptake from the bulk. Their magnitudes (H(+)/Q [Formula: see text] and H(+)/Q [Formula: see text] , respectively) probe the electrostatic interactions within the complex. The pH dependence of H(+)/Q [Formula: see text] and H(+)/Q [Formula: see text] were studied in five single mutants modified at the L209 site (L209P→F,Y,W,E,T). This residue is situated at the border of a continuous chain of water molecules connecting Q(B) to the bulk. In the wild type (WT), a proton uptake band is present at high pH in the H(+)/Q [Formula: see text] and H(+)/Q [Formula: see text] curves and is commonly attributed to a cluster of acidic groups situated nearby Q(B). In the H(+)/Q [Formula: see text] curves of the L209 variants, this band is systematically absent but remains in the H(+)/Q [Formula: see text] curves. Moreover, notable increase of H(+)/Q [Formula: see text] is observed in the L209 mutants at neutral pH as compared with the WT. The large effects observed in all L209 mutants are not associated with significant structural changes (Kuglstatter, A., Ermler, U., Michel, H., Baciou, L. & Fritzsch, G. Biochemistry (2001) 40, 4253–4260). Our data suggest that, in the L209 mutants, the Q(B) cluster does not respond to the Q [Formula: see text] formation as observed in the WT. We propose that, in the mutants, removal of the rigid proline L209 breaks a necessary hydrogen bonding connection between the quinone sites. These findings suggest an important role for structural rigidity in ensuring a functional interaction between quinone binding sites.