Export or recombination of charges in reaction centers in intact cells of photosynthetic bacteria

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 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 K_b ~ 5 mM^-1] and only a few (~ 1 [Hg²⁺]/[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 < [Hg²⁺]/[RC] < 50 range whose locations were revealed. (1) The proton gate at the cytoplasmic site had the highest affinity for Hg²⁺ binding (K_b ~ 0.2 μM^-1) and blocked the proton uptake. (2) Reduced affinity (K_b ~ 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.

Fluorescence relaxation in intact cells of photosynthetic bacteria: donor and acceptor side limitations of reopening of the reaction center

The dark relaxation of the yield of variable BChl fluorescence in the 10(-5)-10 s time range is measured after laser diode (808 nm) excitation of variable duration in intact cells of photosynthetic bacteria Rba. sphaeroides, Rsp. rubrum, and Rvx. gelatinosus under various treatments of redox agents, inhibitors, and temperature. The kinetics of the relaxation is complex and much wider extended than a monoexponential function. The longer is the excitation, the slower is the relaxation which is determined by the redox states, sizes, and accessibility of the pools of cytochrome [Formula: see text] and quinone for donor and acceptor side-limited bacterial strains, respectively. The kinetics of fluorescence decay reflects the opening kinetics of the closed RC. The relaxation is controlled preferentially by the rate of re-reduction of the oxidized dimer by mobile cytochrome [Formula: see text] in Rba. sphaeroides and Rsp. rubrum and by the rate constant of the [Formula: see text] interquinone electron transfer, (350 μs)(-1) and/or the quinol/quinone exchange at the acceptor side in Rvx. gelatinosus. The commonly used acceptor side inhibitors (e.g., terbutryn) demonstrate kinetically limited block of re-oxidation of the primary quinone. The observations are interpreted in frame of a minimum kinetic and energetic model of electron transfer reactions in bacterial RC of intact cells.

Assembly of photosynthetic apparatus in Rhodobacter sphaeroides as revealed by functional assessments at different growth phases and in synchronized and greening cells

The development of photosynthetic membranes of intact cells of Rhodobacter sphaeroides was tracked by light-induced absorption spectroscopy and induction and relaxation of the bacteriochlorophyll fluorescence. Changes in membrane structure were induced by three methods: synchronization of cell growth, adjustment of different growth phases and transfer from aerobic to anaerobic conditions (greening) of the bacteria. While the production of the bacteriochlorophyll and carotenoid pigments and the activation of light harvesting and reaction center complexes showed cell-cycle independent and continuous increase with characteristic lag phases, the accumulation of phospholipids and membrane potential (electrochromism) exhibited stepwise increase controlled by cell division. Cells in the stationary phase of growth demonstrated closer packing and tighter energetic coupling of the photosynthetic units (PSU) than in their early logarithmic stage. The greening resulted in rapid (within 0-4 h) induction of BChl synthesis accompanied with a dominating role for the peripheral light harvesting system (up to LH2/LH1 ~2.5), significantly increased rate (~7·10(4) s(-1)) and yield (F v/F max ~0.7) of photochemistry and modest (~2.5-fold) decrease of the rate of electron transfer (~1.5·10(4) s(-1)). The results are discussed in frame of a model of sequential assembly of the PSU with emphasis on crowding the LH2 complexes resulting in an increase of the connectivity and yield of light capture on the one hand and increase of hindrance to diffusion of mobile redox agents on the other hand.

The reaction center is the sensitive target of the mercury(II) ion in intact cells of photosynthetic bacteria

The sensitivity of intact cells of purple photosynthetic bacterium Rhodobacter sphaeroides wild type to low level (<100 μM) of mercury (Hg²⁺) contamination was evaluated by absorption and fluorescence spectroscopies of the bacteriochlorophyll-protein complexes. All assays related to the function of the reaction center (RC) protein (induction of the bacteriochlorophyll fluorescence, delayed fluorescence and light-induced oxidation and reduction of the bacteriochlorophyll dimer and energization of the photosynthetic membrane) showed prompt and later effects of the mercury ions. The damage expressed by decrease of the magnitude and changes of rates of the electron transfer kinetics followed complex (spatial and temporal) pattern according to the different Hg²⁺ sensitivities of the electron transport (donor/acceptor) sites including the reduced bound and free cytochrome c₂ and the primary reduced quinone. In contrast to the RC, the light harvesting system and the bc₁ complex demonstrated much higher resistance against the mercury pollution. The 850 and 875 nm components of the peripheral and core complexes were particularly insensitive to the mercury(II) ions. The concentration of the photoactive RCs and the connectivity of the photosynthetic units decreased upon mercury treatment. The degree of inhibition of the photosynthetic apparatus was always higher when the cells were kept in the light than in the dark indicating the importance of metabolism in active transport of the mercury ions from outside to the intracytoplasmic membrane. Any of the tests applied in this study can be used for detection of changes in photosynthetic bacteria at the early stages of the action of toxicants.

Light-induced quinone reduction in photosystem II

The photosystem II core complex is the water:plastoquinone oxidoreductase of oxygenic photosynthesis situated in the thylakoid membrane of cyanobacteria, algae and plants. It catalyzes the light-induced transfer of electrons from water to plastoquinone accompanied by the net transport of protons from the cytoplasm (stroma) to the lumen, the production of molecular oxygen and the release of plastoquinol into the membrane phase. In this review, we outline our present knowledge about the "acceptor side" of the photosystem II core complex covering the reaction center with focus on the primary (Q(A)) and secondary (Q(B)) quinones situated around the non-heme iron with bound (bi)carbonate and a comparison with the reaction center of purple bacteria. Related topics addressed are quinone diffusion channels for plastoquinone/plastoquinol exchange, the newly discovered third quinone Q(C), the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of Q(A) in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.

Early detection of mercury contamination by fluorescence induction of photosynthetic bacteria

The induction (sudden dark-to-light transition) of fluorescence of photosynthetic bacteria has proved to be sensitive tool for early detection of mercury (Hg(2+)) contamination of the culture medium. The major characteristics of the induction (dark, variable and maximum fluorescence levels together with rise time) offer an easier, faster and more informative assay of indication of the contamination than the conventional techniques. The inhibition of Hg(2+) is stronger in the light than in the dark and follows complex kinetics. The fast component (in minutes) reflects the damage of the quinone acceptor pool of the RC and the slow component (in hours) is sensitive to the disintegration of the light harvesting system including the loss of the structural organization and of the pigments. By use of fluorescence induction, the dependence of the diverse pathways and kinetics of the mercury-induced effects on the age and the metabolic state of the bacteria were revealed.

Kinetic bacteriochlorophyll fluorometer

A pump and probe fluorometer with a laser diode as single light source has been constructed for measurement of fast induction and relaxation of the fluorescence yield in intact cells, chromatophores and isolated reaction centers of photosynthetic bacteria. The time resolution of the fluorometer is limited by the repetition time of the probing flashes to 20 micros. The apparatus offers high sensitivity, excellent performance and can become a versatile device for a range of demanding applications. Some of them are demonstrated here including fast and easy investigation of the (1) organization and redox state of the photosynthetic apparatus of the intact cells of different bacterial strains and mutants and (2) electron transfer reactions on donor and acceptor sides of isolated reaction centers. The compact design of the mechanics, optics, electronics, and data processing makes the device easy to use as outdoor instrument or to integrate into larger measuring systems.