Photosynthesis Research

, Volume 136, Issue 3, pp 379–392 | Cite as

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

  • Gábor Sipka
  • Mariann Kis
  • Péter MarótiEmail author
Original Article


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 Hg2+ ions up to saturation concentration (~ 103 [Hg2+]/[RC]) and the gradual time- and concentration-dependent loss of the photochemical activity was monitored. The vast majority of Hg2+ ions (about 500 [Hg2+]/[RC]) had low affinity for the RC [binding constant Kb ~ 5 mM−1] and only a few (~ 1 [Hg2+]/[RC]) exhibited strong binding (Kb ~ 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 < [Hg2+]/[RC] < 50 range whose locations were revealed. (1) The proton gate at the cytoplasmic site had the highest affinity for Hg2+ binding (Kb ~ 0.2 μM−1) and blocked the proton uptake. (2) Reduced affinity (Kb ~ 0.05 μM−1) was measured for the mercury(II)-binding site close to the secondary quinone that resulted in inhibition of the interquinone electron transfer. (3) A similar affinity was observed close to the bacteriochlorophyll dimer causing slight energetic changes as evidenced by a ~ 30 nm blue shift of the red absorption band, a 47 meV increase in the redox midpoint potential, and a ~ 20 meV drop in free energy gap of the primary charge pair. The primary quinone was not perturbed upon mercury(II) treatment. Although the Hg2+ ions attack the RC in large number, the exertion of the harmful effect on photochemistry is not through mass action but rather a couple of well-defined targets. Bound to these sites, the Hg2+ ions can destroy H-bond structures, inhibit protein dynamics, block conformational gating mechanisms, and modify electrostatic profiles essential for electron and proton transfer.


Bacterial photosynthesis Bacteriochlorophyll absorption spectroscopy Bacteriochlorophyll fluorescence spectroscopy Bioenergetics Mercury(II) contamination Quinones 






cyt c2+

Reduced cytochrome c


Delayed fluorescence


Lauryl dimethylamine N-oxide




Bacteriochlorophyll dimer




Prompt fluorescence


Primary acceptor (ubiquinone)


Secondary acceptor




Reaction center



We are grateful to Prof. James Smart (Department of Biological Sciences, University of Tennessee at Martin, USA) for valuable discussions and gratefully acknowledge financial support from GINOP-2.3.2-15-2016-00001, OTKA-K 112688, Photosynthesis—Life from Light—Foundation (Hungary) (GS), COST (CM1306), and EFOP-3.6.2-16-2017-0005 (MK and PM).


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Authors and Affiliations

  1. 1.Department of Medical PhysicsUniversity of SzegedSzegedHungary
  2. 2.Department of Plant BiologyHungarian Academy of Science, Biological Research CentreSzegedHungary

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