Spectroscopic characterization of quinone-site mutants of the bacterial photosynthetic reaction center

Site-specific mutations in the quinone binding sites of the photosynthetic reaction center (RC) protein complexes of Rhodobacter (R.) capsulatus caused pronounced effects on sequential electron transfer. Conserved residues that break the twofold symmetry in this region of the RC - M246Ala and M247Ala in the Q(A) binding pocket, and L212Glu and L213Asp in the Q(B) binding pocket - were targeted. We constructed a Q(B)-site mutant, L212Glu-L213Asp → Ala-Ala, and a Q(A)-site mutant, M246Ala-M247Ala → Glu-Asp, to partially balance the differences in charge distribution normally found between the two quinone binding sites. In addition, two photocompetent revertants were isolated from the photosynthetically-incompetent M246Glu-M247Asp mutant: M246Ala-M247Asp and M246Gly-M247Asp. Sequential electron transfer was investigated by continuous light excitation and time-resolved electron paramagnetic resonance (EPR), and time-resolved optical techniques. Several lines of EPR evidence suggested that the forward electron transfer rate to Q(A), k(Q), was slowed in those strains containing altered Q(A) sites. The slower rates of secondary electron transfer were confirmed by time-resolved optical results with the M246Glu-M247Asp mutations in the Q(A) site resulting in a dramatically lowered secondary electron transfer efficiency [k(Q) < (2 ns)^-1] in comparison with either the native R. capsulatus RC or the Q(B) site mutant [k(Q) ≃ (200 ps)^-1]. Secondary electron transfer in the two revertants was intermediate between that of the native RC and the Q(A) mutant. The P⁺Q(A)^- → PQ(A) charge recombination rates were also changed in the strains that carried altered Q(A) sites. We show that local mutations in the Q(A) site, presumably through local electrostatic changes, significantly alter binding and electron transfer properties of Q(A).