Figure 4.

Part of the RC structure near the secondary quinone, Q_B binding site, as determined for the Q_B⁻ state by Stowell et al. (18). Possible proton transfer pathways (P1–P3) proposed by Abresch et al. (19) are shown by dashed lines. One carbonyl oxygen of Q_B is located near Ser-L223 and the backbone NH of Ile-L224 (not shown); the other carbonyl oxygen of Q_B is located near His-L190. Nearby are two carboxylic acid groups Asp-L213 and Glu-L212 that have been implicated in proton transfer to reduced Q_B (reactions 2a and 2b, respectively) (8–13) and to which the proton transfer pathways lead. Also shown are a His cluster (consisting of H68, H126, and H128) and a carboxylic acid cluster (consisting of Asp-L213, Asp-L210, Asp-M17, Glu-H173, Asp-H170, and Asp-M124).