Figure 4.

Part of the RC structure with a bound Cd²⁺ showing the region between the bound metal ion and Q_B⨪ [modified from Axelrod et al. (25)]. The metal (large sphere) is coordinated (solid lines) to Asp-H124, His-H126, His-H128, and three water molecules. Located in this region are Asp-L210, Asp-M17, Asp-L213, Ser-L223 (as indicated), and several water molecules (small spheres). The kinetic results presented in this work show that Asp-L210 and Asp-M17 are involved in proton transfer to Q_B⨪ in the presence of a bound Cd²⁺. The most likely proton transfer pathways connecting the surface near Cd²⁺ to Q_B⨪ are indicated by the dashed lines. In the absence of a bound Cd²⁺, the same pathways are expected to predominate with an even greater effectiveness.