Structure of a bacteriochlorophyll a-protein from the green photosynthetic bacterium Prosthecochloris aestuarii.

The three-dimensional structure of a water-soluble bacteriochlorophyll a-containing protein from the green photosynthetic bacterium Prosthecochloris aestuarii has been determined by X-ray crystallography from a 2.8 Å resolution electron density map based on four isomorphous derivatives. Details of the crystallographic procedures used to obtain the map are presented.The bacteriochlorophyll a-protein is shown to consist of three identical subunits, tightly packed around a 3-fold symmetry axis. Each subunit consists of a core of seven bacteriochlorophyll a molecules enclosed within a “bag” of protein. The polypeptide chain forms an extensive 15-strand β-sheet, which is almost planar in its central region, and twisted at its extremities, and wraps around the chlorophyll core to form an efficient amphipathic layer between the chlorophylls and the aqueous environment. There are extensive contacts between the phytyl chains of the seven bacteriochlorophylls within each subunit. These hydrocarbon chains constitute an inner hydrophobic core of the molecule which may be important in forming the complex. There are also extensive contacts between the protein and both the bacteriochlorophyll head groups and tails, but relatively few contacts between the respective head groups. The seven magnesiums all appear to be five co-ordinated. In five cases the presumed ligand is a histidine side-chain, in one case a polypeptide carbonyl oxygen, and in the other case a water molecule.At low temperature, both the absorption and circular dichroism spectra of the bacteriochlorophyll a-protein show splitting which can be interpreted in general terms as due to exciton interactions between the seven chromophores, but calculations of the expected splitting based on the bacteriochlorophyll co-ordinates determined crystallographically are in poor agreement with the observed spectra. Furthermore, the observed red shift of the Q_y absorption band of bacteriochlorophyll a, from about 770 nm in organic solvents to 809 nm in the bacteriochlorophyll a-protein, is not explained by the exciton calculations. It seems likely that the red shift is due to perturbations of the spectra of the individual bacteriochlorophylls by the protein environment, but, pending the determination of the amino acid sequence, it is not possible at this time to define in detail all the proteinchlorophyll interactions. It is suggested that the bacteriochlorophyll a-protein serves as a good model for the organization of chlorophyll in vivo, and that the types of interaction seen here between chlorophyll and protein are likely to be found in other chlorophyll proteins.     

Labeling of chromatophore membranes and reaction centers from the photosynthetic bacterium Rhodospirillum rubrum with the hydrophobic marker 5-[125I]iodonaphthyl-1-azide

Chromatophores of the photosynthetic bacterium Rhodospirillum rubrum and isolated reaction centers were labeled with the lipophilic membrane marker 5-[¹²⁵I]iodonaphthyl-1-azide. The two smaller reaction center proteins L and M bind more label than the larger subunit H, a fact supporting the proposed localisation of the 3 subunits obtained with hydrophilic labels. Besides these integral proteins the lipids, among them mainly the pigments and the quinones, are highly labeled suggesting a hydrophobic environment around these molecules and a preferred reactivity to iodonaphthylazide. Such a hydrophobic environment may be of great importance for the function of the photosynthetic reaction centers especially for the charge separation and the primary reactions in electron transport.     

The influence of hydrogen bonds on electron transfer rate in photosynthetic RCs

Hydrogen bonds formed between photosynthetic reaction centers (RCs) and their cofactors were shown to affect the efficacy of electron transfer. The mechanism of such influence is determined by sensitivity of hydrogen bonds to electron density rearrangements, which alter hydrogen bonds potential energy surface. Quantum chemistry calculations were carried out on a system consisting of a primary quinone Q_A, non-heme Fe²⁺ ion and neighboring residues_. The primary quinone forms two hydrogen bonds with its environment, one of which was shown to be highly sensitive to the Q_A state. In the case of the reduced primary quinone two stable hydrogen bond proton positions were shown to exist on [Q_A-His^M219] hydrogen bond line, while there is only one stable proton position in the case of the oxidized primary quinone. Taking into account this fact and also the ability of proton to transfer between potential energy wells along a hydrogen bond, theoretical study of temperature dependence of hydrogen bond polarization was carried out. Current theory was successfully applied to interpret dark P⁺/Q_A⁻ recombination rate temperature dependence.     

Relationship between the oxidation potential of the bacteriochlorophyll dimer and electron transfer in photosynthetic reaction centers

The primary electron donor in the photosynthetic reaction center from purple bacteria is a bacteriochlorophyll dimer containing four conjugated carbonyl groups that may form hydrogen bonds with amino acid residues. Spectroscopic analyses of a set of mutant reaction centers confirm that hydrogen bonds can be formed between each of these carbonyl groups and histidine residues in the reaction center subunits. The addition of each hydrogen bond is correlated with an increase in the oxidation potential of the dimer, resulting in a 355-mV range in the midpoint potential. The resulting changes in the free-energy differences for several reactions involving the dimer are related to the electron transfer rates using the Marcus theory. These reactions include electron transfer from cytochrome c₂ to the oxidized dimer, charge recombination from the primary electron acceptor quinone, and the initial forward electron transfer.     

In vivo assembly of a truncated H subunit mutant of the <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis> photosynthetic reaction centre and direct electron transfer from the Q<Subscript>A</Subscript> quinone to an electrode

We address a challenge in the engineering of proteins to redirect electron transfer pathways, using the bacterial photosynthetic reaction centre (RC) pigment–protein complex. Direct electron transfer is shown to occur from the Q_A quinone of the Rhodobacter sphaeroides RC containing a truncated H protein and bound on the quinone side to a gold electrode. In previous reports of binding to the quinone side of the RC, electron transfer has relied on the use of a soluble mediator between the RC and an electrode, in part because the probability of Q_B quinone reduction is much greater than that of direct electron transfer through the large cytoplasmic domain of the H subunit, presenting a?~?25 Å barrier. A series of C-terminal truncations of the H subunit were created to expose the quinone region of the RC L and M proteins, and all truncated RC H mutants assembled in vivo. The 45M mutant was designed to contain only the N-terminal 45 amino acid residues of the H subunit including the membrane-spanning α-helix; the mutant RC was stable when purified using the detergent N-dodecyl-β-d-maltoside, contained a near-native ratio of bacteriochlorophylls to bacteriopheophytins, and showed a charge-separated state of \({{\text{P}}^{\text{+}}}{{\text{Q}}_{\text{A}}^-}\). The 45M-M229 mutant RC had a Cys residue introduced in the vicinity of the Q_A quinone on the newly exposed protein surface for electrode attachment, decreasing the distance between the quinone and electrode to ~?12 Å. Steady-state photocurrents of up to around 200 nA/cm² were generated in the presence of 20 mM hydroquinone as the electron donor to the RC. This novel configuration yielded photocurrents orders of magnitude greater than previous reports of electron transfer from the quinone region of RCs bound in this orientation to an electrode.     

Unfolding pathway and intermolecular interactions of the cytochrome subunit in the bacterial photosynthetic reaction center

Precise folding of photosynthetic proteins and organization of multicomponent assemblies to form functional entities are fundamental to efficient photosynthetic electron transfer. The bacteriochlorophyll b-producing purple bacterium Blastochloris viridis possesses a simplified photosynthetic apparatus. The light-harvesting (LH) antenna complex surrounds the photosynthetic reaction center (RC) to form the RC-LH1 complex. A non-membranous tetraheme cytochrome (4Hcyt) subunit is anchored at the periplasmic surface of the RC, functioning as the electron donor to transfer electrons from mobile electron carriers to the RC. Here, we use atomic force microscopy (AFM) and single-molecule force spectroscopy (SMFS) to probe the long-range organization of the photosynthetic apparatus from Blc. viridis and the unfolding pathway of the 4Hcyt subunit in its native supramolecular assembly with its functional partners. AFM images reveal that the RC-LH1 complexes are densely organized in the photosynthetic membranes, with restricted lateral protein diffusion. Unfolding of the 4Hcyt subunit represents a multi-step process and the unfolding forces of the 4Hcyt α-helices are approximately 121 picoNewtons. Pulling of 4Hcyt could also result in the unfolding of the RC L subunit that binds with the N-terminus of 4Hcyt, suggesting strong interactions between RC subunits. This study provides new insights into the protein folding and interactions of photosynthetic multicomponent complexes, which are essential for their structural and functional integrity to conduct photosynthetic electron flow.     

Structural and functional studies on the tetraheme cytochrome subunit and its electron donor proteins: the possible docking mechanisms during the electron transfer reaction

The photosynthetic reaction centers (RCs) classified as the group II possess a peripheral cytochrome (Cyt) subunit, which serves as the electron mediator to the special-pair. In the cycle of the photosynthetic electron transfer reactions, the Cyt subunit accepts electrons from soluble electron carrier proteins, and re-reduces the photo-oxidized special-pair of the bacteriochlorophyll. Physiologically, high-potential cytochromes such as the cytochrome c₂ and the high-potential iron–sulfur protein (HiPIP) function as the electron donors to the Cyt subunit. Most of the Cyt subunits possess four heme c groups, and it was unclear which heme group first accepts the electron from the electron donor. The most distal heme to the special-pair, the heme-1, has a lower redox potential than the electron donors, which makes it difficult to understand the electron transfer mechanism mediated by the Cyt subunit. Extensive mutagenesis combined with kinetic studies has made a great contribution to our understanding of the molecular interaction mechanisms, and has demonstrated the importance of the region close to the heme-1 in the electron transfer. Moreover, crystallographic studies have elucidated two high-resolution three-dimensional structures for the RCs containing the Cyt subunit, the Blastochloris viridis and Thermochromatium tepidum RCs, as well as the structures of their electron donors. An examination of the structural data also suggested that the binding sites for both the cytochrome c₂ and the HiPIP are located adjacent to the solvent-accessible edge of the heme-1. In addition, it is also indicated by the structural and biochemical data that the cytochrome c₂ and the HiPIP dock with the Cyt subunit by different mechanisms although the two electron donors utilize the same region for the interactions; cytochrome c₂ is recognized through electrostatic interactions while hydrophobic interactions are important in the HiPIP docking.     

The role of a conserved tyrosine in the 49-kDa subunit of complex I for ubiquinone binding and reduction

Iron–sulfur cluster N2 of complex I (proton pumping NADH:quinone oxidoreductase) is the immediate electron donor to ubiquinone. At a distance of only ～ 7 Å in the 49-kDa subunit, a highly conserved tyrosine is found at the bottom of the previously characterized quinone binding pocket. To get insight into the function of this residue, we have exchanged it for six different amino acids in complex I from Yarrowia lipolytica. Mitochondrial membranes from all six mutants contained fully assembled complex I that exhibited very low dNADH:ubiquinone oxidoreductase activities with n-decylubiquinone. With the most conservative exchange Y144F, no alteration in the electron paramagnetic resonance spectra of complex I was detectable. Remarkably, high dNADH:ubiquinone oxidoreductase activities were observed with ubiquinones Q₁ and Q₂ that were coupled to proton pumping. Apparent K_m values for Q₁ and Q₂ were markedly increased and we found pronounced resistance to the complex I inhibitors decyl-quinazoline-amine (DQA) and rotenone. We conclude that Y144 directly binds the head group of ubiquinone, most likely via a hydrogen bond between the aromatic hydroxyl and the ubiquinone carbonyl. This places the substrate in an ideal distance to its electron donor iron–sulfur cluster N2 for efficient electron transfer during the catalytic cycle of complex I.     

Structural basis of cyanobacterial photosystem II Inhibition by the herbicide terbutryn

Herbicides that target photosystem II (PSII) compete with the native electron acceptor plastoquinone for binding at the Q_B site in the D1 subunit and thus block the electron transfer from Q_A to Q_B. Here, we present the first crystal structure of PSII with a bound herbicide at a resolution of 3.2 Å. The crystallized PSII core complexes were isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The used herbicide terbutryn is found to bind via at least two hydrogen bonds to the Q_B site similar to photosynthetic reaction centers in anoxygenic purple bacteria. Herbicide binding to PSII is also discussed regarding the influence on the redox potential of Q_A, which is known to affect photoinhibition. We further identified a second and novel chloride position close to the water-oxidizing complex and in the vicinity of the chloride ion reported earlier (Guskov, A., Kern, J., Gabdulkhakov, A., Broser, M., Zouni, A., and Saenger, W. (2009) Nat. Struct. Mol. Biol. 16, 334–342). This discovery is discussed in the context of proton transfer to the lumen.     

Reaction Mechanism of Superoxide Generation during Ubiquinol Oxidation by the Cytochrome bc1 Complex

In addition to its main functions of electron transfer and proton translocation, the cytochrome bc₁ complex (bc₁) also catalyzes superoxide anion (O₂^˙̄) generation upon oxidation of ubiquinol in the presence of molecular oxygen. The reaction mechanism of superoxide generation by bc₁ remains elusive. The maximum O₂^˙̄ generation activity is observed when the complex is inhibited by antimycin A or inactivated by heat treatment or proteinase K digestion. The fact that the cytochrome bc₁ complex with less structural integrity has higher O₂^˙̄-generating activity encouraged us to speculate that O₂^˙̄ is generated inside the complex, perhaps in the hydrophobic environment of the Q_P pocket through bifurcated oxidation of ubiquinol by transferring its two electrons to a high potential electron acceptor, iron-sulfur cluster, and a low potential heme b_L or molecular oxygen. If this speculation is correct, then one should see more O₂^˙̄ generation upon oxidation of ubiquinol by a high potential oxidant, such as cytochrome c or ferricyanide, in the presence of phospholipid vesicles or detergent micelles than in the hydrophilic conditions, and this is indeed the case. The protein subunits, at least those surrounding the Q_P pocket, may play a role either in preventing the release of O₂^˙̄ from its production site to aqueous environments or in preventing O₂ from getting access to the hydrophobic Q_P pocket and might not directly participate in superoxide production.     

Orientation of the chromophores in the reaction center of Rhodopseudomonas viridis. Comparison of low-temperature linear dichroism spectra with a model derived from X-ray crystallography

Linear dichroism (LD) and absorption (A) spectra of reaction centers from Rhodopseudomonas viridis included in the native chromatophores or reconstituted in planar aggregates have been recorded at 10 K. The samples were oriented in squeezed polyacrylamide gels and the primary donor P was in the reduced or (chemically) oxidized state. The LD spectra of reaction centers in these two states are in favor of a dimeric model of P in which excitonic coupling between the two non-parallel Q_Y transitions leads to a main transition at 990 nm (parallel to the membrane plane) and another one of smaller oscillator strength at 850 nm (tilted at approx. 60° out of the membrane plane). These assignments are in close agreement with the ones proposed in a previous LD study at room temperature (Paillotin, G., Verméglio, A. and Breton, J. (1979) Biochim. Biophys. Acta 545, 249–264). The main Q_X excitonic component of P has a broad absorption peaking at 620 nm and it corresponds to dipoles exhibiting the same orientation as those responsible for the 850 nm transition. On the basis of the present LD study and of CD data of chemically oxidized-minus-reduced reaction centers, we proposed that the minor Q_X excitonic component of P is oriented close to the membrane plane and absorbs around 660 nm. The two monomeric bacteriochlorophylls exhibit a positive LD for both their Q_Y transitions (unresolved at 834 nm) and their Q_X transitions (resolved at 600 and 607 nm), indicating that the planes of these molecules are only slightly tilted out of the membrane plane. The two bacteriopheophytins exhibit strong negative LD with identical LD/A values for their Q_Y transitions (resolved at 790 and 805 nm) and small positive LD for their Q_X transitions (resolved at 534 and 544 nm), demonstrating that these two molecules are strongly tilted out of the membrane plane with each of the Q_Y transitions tilted at approx. 50° out of that plane. A comparison of these LD data with the structural model derived from X-ray crystallography (Deisenhofer, J., Epp, O., Miki, K., Huber, R. and Michel, H. (1984) J. Mol. Biol. 180, 385–398) clearly suggests that a good agreement exists between the results of the two techniques under the following conditions: (i) the C-2 symmetry axis of the reaction center runs along the membrane normal; (ii) excitonic coupling is present only in the primary donor special pair; and (iii) the direction of the optical transitions of the monomeric bacteriochlorophylls and of the bacteriopheophytins is not significantly perturbed by the interactions among the pigments. In addition, a carotenoid is detected in the isolated reaction center with an orientation rather perpendicular to the C-2 symmetry axis. Finally, a comparison of these data with similar ones obtained on the bacteriochlorophyll a-containing reaction center of Rhodopseudomonas sphaeroides 241 points towards a geometrical arrangement of the chromophores which is indistinguishable from the one observed in the reaction center of Rps. viridis.     

Electric field effect on absorption spectra of reaction centers of Rb. sphaeroides and Rps. viridis

Comparison of the Stark effect on the Q_y transitions of the pigments in reaction centers of Rb. sphaeroides and Rps. viridis at 77 K shows great similarity in the long-wavelength absorption of the bacteriochlorophyll dimer but differs significantly in the absorption region of the accessory bacteriochlorophylls and bacteriopheophytins.     

Control of bacteriochlorophyll accumulation by light in Rhodobacter capsulatus.

Oxygen levels which control induction of the assembly of the pigment-protein photosynthetic polypeptides in dark-grown Chloroflexus aurantiacus were determined. The induction signal by low-oxygen tension is not directly related to the respiratory competence of these photosynthetic cells. Cytochrome c554, the primary electron donor to P865+ of the reaction center, is not present in dark-grown respiratory cells but is induced in parallel with bacteriochlorophylls a and c and at similar oxygen partial pressure. The development of these components of the photosynthetic apparatus and its electron transport chain is completely independent of the presence of any detectable light or bacteriochlorophyll c or a pigments in C. aurantiacus.     

The three-dimensional structure of the FMO protein from <Emphasis Type="Italic">Pelodictyon phaeum</Emphasis> and the implications for energy transfer

The Fenna–Matthews–Olson (FMO) antenna protein from the green bacterium Pelodictyon phaeum mediates the transfer of energy from the peripheral chlorosome antenna complex to the membrane-bound reaction center. The three-dimensional structure of this protein has been solved using protein crystallography to a resolution limit of 2.0 Å, with R _work and R _free values of 16.6 and 19.9%, respectively. The structure is a trimer of three identical subunits related by a threefold symmetry axis. Each subunit has two beta sheets that surround 8 bacteriochlorophylls. The bacteriochlorophylls are all five-coordinated, with the axial ligand being a histidine, serine, backbone carbonyl, or bound water molecule. The arrangement of the bacteriochlorophylls is generally well conserved in comparison to other FMO structures, but differences are apparent in the interactions with the surrounding protein. In this structure the position and orientation of the eighth bacteriochlorophyll is well defined and shows differences in its location and the coordination of the central Mg compared to previous models. The implications of this structure on the ability of the FMO protein to perform energy transfer are discussed in terms of the experimental optical measurements.     

Stigmatellin Probes the Electrostatic Potential in the QB Site of the Photosynthetic Reaction Center

The electrostatic potential in the secondary quinone (Q_B) binding site of the reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides determines the rate and free energy change (driving force) of electron transfer to Q_B. It is controlled by the ionization states of residues in a strongly interacting cluster around the Q_B site. Reduction of the Q_B induces change of the ionization states of residues and binding of protons from the bulk. Stigmatellin, an inhibitor of the mitochondrial and photosynthetic respiratory chain, has been proven to be a unique voltage probe of the Q_B binding pocket. It binds to the Q_B site with high affinity, and the pK value of its phenolic group monitors the local electrostatic potential with high sensitivity. Investigations with different types of detergent as a model system of isolated RC revealed that the pK of stigmatellin was controlled overwhelmingly by electrostatic and slightly by hydrophobic interactions. Measurements showed a high pK value (>11) of stigmatellin in the Q_B pocket of the dark-state wild-type RC, indicating substantial negative potential. When the local electrostatics of the Q_B site was modulated by a single mutation, L213Asp→Ala, or double mutations, L213Asp-L212Glu→Ala-Ala (AA), the pK of stigmatellin dropped to 7.5 and 7.4, respectively, which corresponds to a >210 mV increase in the electrostatic potential relative to the wild-type RC. This significant pK drop (ΔpK > 3.5) decreased dramatically to (ΔpK > 0.75) in the RC of the compensatory mutant (AA+M44Asn→AA+M44Asp). Our results indicate that the L213Asp is the most important actor in the control of the electrostatic potential in the Q_B site of the dark-state wild-type RC, in good accordance with conclusions of former studies using theoretical calculations or light-induced charge recombination assay.     

C-H...O hydrogen bonds in the nuclear receptor RARgamma--a potential tool for drug selectivity

Hydrogen bonds between polarized atoms play a crucial role in protein interactions and are often used in drug design, which usually neglects the potential of C-H...O hydrogen bonds. The 1.4 A resolution crystal structure of the ligand binding domain of the retinoic acid receptor RARgamma complexed with the retinoid SR11254 reveals several types of C-H...O hydrogen bonds. A striking example is the hydroxyl group of the ligand that acts as an H bond donor and acceptor, leading to a synergy between classical and C-H...O hydrogen bonds. This interaction introduces both specificity and affinity within the hydrophobic ligand pocket. The similarity of intraprotein and protein-ligand C-H...O interactions suggests that such bonds should be considered in rational drug design approaches.     

Mutant reaction centers of <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis> I(L177)H with strongly bound bacteriochlorophyll <Emphasis Type="Italic">a</Emphasis>: Structural properties and pigment-protein interactions

Methods of photoinduced Fourier transform infrared (FTIR) difference spectroscopy and circular dichroism were employed for studying features of pigment-protein interactions caused by replacement of isoleucine L177 by histidine in the reaction center (RC) of the site-directed mutant I(L177)H of Rhodobacter sphaeroides. A functional state of pigments in the photochemically active cofactor branch was evaluated with the method of photo-accumulation of reduced bacteriopheophytin H _A ^? . The results are compared with those obtained for wild-type RCs. It was shown that the dimeric nature of the radical cation of the primary electron donor P was preserved in the mutant RCs, with an asymmetric charge distribution between the bacteriochlorophylls P_A and P_B in the P⁺ state. However, the dimers P in the wild-type and mutant RCs are not structurally identical due probably to molecular rearrangements of the P_A and P_B macrocycles and/or alterations in their nearest amino acid environment induced by the mutation. Analysis of the electronic absorption and FTIR difference P⁺Q^?/PQ spectra suggests the 17³-ester group of the bacteriochlorophyll P_A to be involved in covalent interaction with the I(L177)H RC protein. Incorporation of histidine into the L177 position does not modify the interaction between the primary electron acceptor bacteriochlorophyll B_A and the bacteriopheophytin H_A. Structural changes are observed in the monomer bacteriochlorophyll B_B binding site in the inactive chromophore branch of the mutant RCs.     

Modification of protein hydrogen bonds influences the efficiency of picosecond electron transfer in bacterial photosynthetic reaction centers

Picosecond absorption spectroscopy was used to monitor laser-induced oxidation-reductions of reaction center (RC) bacteriochlorophyll (P) and bacteriopheophytin (I) in Rhodopseudomonas sphaeroides RC preparations on exposure to different chemicals. The D₂O isotope substitution of H₂O or partial substitution of water by organic solvents (ethylene glycol, glycerol, propylene glycol, dimethyl sulfoxide) causes the appearance of a fast, nanosecond component of P⁺ reduction, the result of an increased probability of recombination of the primary ion-radical products P⁺I⁻ → PI. The effect is accompanied by a noticeable slowing down of electron transfer from photoreduced bacteriopheophytin to the primary quinone acceptor Q_A. The effect of the organic solvents, known as cryoprotectors, is correlated with their degree of hydrophobicity, i.e. the ability to penetrate the RC protein and interact with bound water and protein hydrogen bonds. The conclusion drawn from the data is that the dielectric relaxation processes through which the intermediate energy levels of the carriers in the PIQ_A system are lowered to levels necessary for the stabilization of the photochemically separated charges proceed with the involvement of protons of the nearest water-protein surrounding of the RC pigments and electron transport cofactors.     

Photosynthetic response of clusterbean chloroplasts to UV-B radiation: Energy imbalance and loss in redox homeostasis between QA and QB of photosystem II

The effects of ultraviolet-B (UV-B: 280-320 nm) radiation on the photosynthetic pigments, primary photochemical reactions of thylakoids and the rate of carbon assimilation (P_n) in the cotyledons of clusterbean (Cyamopsis tetragonoloba) seedlings have been examined. The radiation induces an imbalance between the energy absorbed through the photophysical process of photosystem (PS) II and the energy consumed for carbon assimilation. Decline in the primary photochemistry of PS II induced by UV-B in the background of relatively stable P_n, has been implicated in the creation of the energy imbalance_. The radiation induced damage of PS II hinders the flow of electron from Q_A to Q_B resulting in a loss in the redox homeostasis between the Q_A to Q_B leading to an accumulation of Q_A⁻. The accumulation of Q_A⁻ generates an excitation pressure that diminishes the PS II-mediated O₂ evolution, maximal photochemical potential (F_v/F_m) and PS II quantum yield (Φ_{PS II}). While UV-B radiation inactivates the carotenoid-mediated protective mechanisms, the accumulation of flavonoids seems to have a small role in protecting the photosynthetic apparatus from UV-B onslaught. The failure of protective mechanisms makes PS II further vulnerable to the radiation and facilitates the accumulation of malondialdehyde (MDA) indicating the involvement of reactive oxygen species (ROS) metabolism in UV-B-induced damage of photosynthetic apparatus of clusterbean cotyledons.     

Heliobacterial Rieske/cytb complex

Data on structure and function of the Rieske/cytb complex from Heliobacteria are scarce. They indicate that the complex is related to the b ₆ f complex in agreement with the phylogenetic position of the organism. It is composed of a diheme cytochrome c, and a Rieske iron–sulfur protein, together with transmembrane cytochrome b ₆ and subunit IV. Additional small subunits may be part of the complex. The cofactor content comprises heme c _i, first discovered in the Q_i binding pocket of b ₆ f complexes. The redox midpoint potentials are more negative than in b ₆ f complex in agreement with the lower redox midpoint potentials (by about 150 mV) of its reaction partners, menaquinone, and cytochrome c ₅₅₃. The enzyme is implicated in cyclic electron transfer around the RCI. Functional studies are favored by the absence of antennae and the simple photosynthetic reaction chain but are hampered by the high oxygen sensitivity of the organism, its chlorophyll, and lipids.