Mechanism of Proton Transfer Inhibition by Cd²⁺ Binding to Bacterial Reaction Centers: Determination of the pK_A of Functionally Important Histidine Residues

The bacterial photosynthetic reaction center (RC) uses light energy to catalyze the reduction of a bound quinone molecule QB to quinol QBH2. In RCs from Rhodobacter sphaeroides the protons involved in this process come from the cytoplasm and travel through pathways that involve His-H126 and His-H128 located near the proton entry point. In this study, we measured the pH dependence from 4.5 to 8.5 of the binding of the proton transfer inhibitor...