Characterization of mercury(II)-induced inhibition of photochemistry in the reaction center of photosynthetic bacteria

Mercuric contamination of aqueous cultures results in impairment of viability of photosynthetic bacteria primarily by inhibition of the photochemistry of the reaction center (RC) protein. Isolated reaction centers (RCs) from Rhodobacter sphaeroides were exposed to Hg 2+ ions up to saturation concentration (~ 10 3 [Hg 2+ ]/[RC]) and the gradual time- and concentration-dependent loss of the photochemical activity was monitored. The vast majority of Hg 2+ ions (about 500 [Hg 2+ ]/[RC]) had low affinity for the RC [binding constant K b ~ 5 mM −1 ] and only a few (~ 1 [Hg 2+ ]/[RC]) exhibited strong binding ( K b ~ 50 μM −1 ). Neither type of binding site had specific and harmful effects on the photochemistry of the RC. The primary charge separation was preserved even at saturation mercury(II) concentration, but essential further steps of stabilization and utilization were blocked already in the 5 < [Hg 2+ ]/[RC]