Optimization of light harvesting and photoprotection: molecular mechanisms and physiological consequences

The distinctive lateral organization of the protein complexes in the thylakoid membrane investigated by Jan Anderson and co-workers is dependent on the balance of various attractive and repulsive forces. Modulation of these forces allows critical physiological regulation of photosynthesis that provides efficient light-harvesting in limiting light but dissipation of excess potentially damaging radiation in saturating light. The light-harvesting complexes (LHCII) are central to this regulation, which is achieved by phosphorylation of stromal residues, protonation on the lumen surface and de-epoxidation of bound violaxanthin. The functional flexibility of LHCII derives from a remarkable pigment composition and configuration that not only allow efficient absorption of light and efficient energy transfer either to photosystem II or photosystem I core complexes, but through subtle configurational changes can also exhibit highly efficient dissipative reactions involving chlorophyll–xanthophyll and/or chlorophyll–chlorophyll interactions. These changes in function are determined at a macroscopic level by alterations in protein–protein interactions in the thylakoid membrane. The capacity and dynamics of this regulation are tuned to different physiological scenarios by the exact protein and pigment content of the light-harvesting system. Here, the molecular mechanisms involved will be reviewed, and the optimization of the light-harvesting system in different environmental conditions described.     

Natural agents: cellular and molecular mechanisms of photoprotection

The skin is the largest organ of the body that produces a flexible and self-repairing barrier and protects the body from most common potentially harmful physical, environmental, and biological insults. Solar ultraviolet (UV) radiation is one of the major environmental insults to the skin and causes multi-tiered cellular and molecular events eventually leading to skin cancer. The past decade has seen a surge in the incidence of skin cancer due to changes in life style patterns that have led to a significant increase in the amount of UV radiation that people receive. Reducing excessive exposure to UV radiation is desirable; nevertheless this approach is not easy to implement. Therefore, there is an urgent need to develop novel strategies to reduce the adverse biological effects of UV radiation on the skin. A wide variety of natural agents have been reported to possess substantial skin photoprotective effects. Numerous preclinical and clinical studies have elucidated that natural agents act by several cellular and molecular mechanisms to delay or prevent skin cancer. In this review article, we have summarized and discussed some of the selected natural agents for skin photoprotection.     

Conservation and dissipation of light energy as complementary processes: homoiohydric and poikilohydric autotrophs

The relationship between photosynthetic energy conservation and thermal dissipation of light energy is considered, with emphasis on organisms which tolerate full desiccation without suffering photo-oxidative damage in strong light. As soon as water becomes available to dry poikilohydric organisms, they resume photosynthetic water oxidation. Only excess light is then thermally dissipated in mosses and chlorolichens by a mechanism depending on the protonation of a thylakoid protein and availability of zeaxanthin. Upon desiccation, another mechanism is activated which requires neither protonation nor zeaxanthin although the zeaxanthin-dependent mechanism of energy dissipation remains active, provided desiccation occurs in the light. Increased thermal energy dissipation under desiccation finds expression in the loss of variable, and in the quenching of, basal chlorophyll fluorescence. Spectroscopical analysis revealed the activity of photosystem II reaction centres in the absence of water. Oxidized beta-carotene (Car+) and reduced chlorophyll (Chl-), perhaps ChlD1 next to P680 within the D1 subunit, accumulates reversibly under very strong illumination. Although recombination between Car+ and Chl- is too slow to contribute significantly to thermal energy dissipation, a much faster reaction such as the recombination between P680+ and the neighbouring Chl- is suggested to form the molecular basis of desiccation-induced energy dissipation in photosystem II reaction centres. Thermal dissipation of absorbed light energy within a picosecond time domain deactivates excited singlet chlorophyll, thereby preventing triplet accumulation and the consequent photo-oxidative damage by singlet oxygen.     

Activation of dihaloalkanes by thiol-dependent mechanisms

Dihaloalkanes constitute an important group of chemicals because of their widespread use in industry and agriculture and their potential for causing toxicity and cancer. Chronic toxic effects are considered to depend upon bioactivation, either by oxidation or thiol conjugation. Considerable evidence links genotoxicity and cancer with glutathione conjugations reactions, and some aspects of the mechanisms have been clarified with 1,2-dihaloalkanes and dihalomethanes. Recently the DNA repair protein O6-alkylguanine transferase has been shown to produce cytotoxicity and genotoxicity by means of a thiol-dependent process with similarities to the glutathione reactions.     

Activation of latent TGF-beta by thrombospondin-1: mechanisms and physiology

Regulation of the activation of latent TGF-beta is essential for health as too much or too little TGF-beta activity can have serious, deleterious consequences. The processes that control conversion of the precursor to the biologically active form of TGF-beta in vivo are not well characterized. We have identified a mechanism for the activation of latent TGF-beta that involves binding of the secreted and extracellular matrix protein, thrombospondin-1 (TSP-1), to the latent precursor. Specific sequences in TSP-1 and in the precursor portion (the latency associate peptide-LAP) have been determined to be essential for activation of latent TGF-beta by TSP-1. It is thought that binding of TSP-1 to the latent complex induces a conformational rearrangement of the LAP in such a manner as to prevent the LAP from conferring latency on the mature domain of TGF-beta. A TSP-dependent mechanism of activation may be locally important during wound healing and in post-natal development of epithelial structures. The possible involvement of TSP-1 in TGF-beta activation during several disease processes is also discussed.     

Plasticity in light reactions of photosynthesis for energy production and photoprotection

Plant photosynthesis channels some of the most highly reactive intermediates in biology, in a way that captures a large fraction of their energy to power the plant. A viable photosynthetic apparatus must not only be efficient and robust machinery, but also well integrated into the plant's biochemical and physiological networks. This requires flexibility in its responses to the dramatically changing environmental conditions and biochemical demands. First, the output of the energy-storing light reactions must match the demands of plant metabolism. Second, regulation of the antenna must be flexible to allow responses to diverse challenges that could result in excess light capture and subsequent photoinhibition. Evidence is presented for the interplay of two types of mechanistic flexibility, one that modulates the relative sensitivity of antenna down-regulation to electron flow, and the other, which primarily modulates the output ratio of ATP/NADPH, but also contributes to down-regulation.     

Conservation and dissipation of light energy in desiccation-tolerant photoautotrophs, two sides of the same coin

Conservation of light energy in photosynthesis is possible only in hydrated photoautotrophs. It requires complex biochemistry and is limited in capacity. Charge separation in reaction centres of photosystem II initiates energy conservation but opens also the path to photooxidative damage. A main mechanism of photoprotection active in hydrated photoautotrophs is controlled by light. This is achieved by coupling light flux to the protonation of a special thylakoid protein which activates thermal energy dissipation. This mechanism facilitates the simultaneous occurrence of energy conservation and energy dissipation but cannot completely prevent damage by light. Continuous metabolic repair is required to compensate damage. More efficient photoprotection is needed by desiccation-tolerant photoautotrophs. Loss of water during desiccation activates ultra-fast energy dissipation in mosses and lichens. Desiccation-induced energy dissipation neither requires a protonation reaction nor light but photoprotection often increases when light is present during desiccation. Two different mechanisms contribute to photoprotection of desiccated photoautotrophs. One facilitates energy dissipation in the antenna of photosystem II which is faster than energy capture by functional reaction centres. When this is insufficient for full photoprotection, the other one permits energy dissipation in the reaction centres themselves.     

Reaction centre quenching of excess light energy and photoprotection of photosystem II

In addition to the energy dissipation of excess light occurring in PSII antenna via the xanthophyll cycle, there is mounting evidence of a zeaxanthin-independent pathway for non-photochemical quenching based within the PSII reaction centre (reaction centre quenching) that may also play a significant role in photoprotection. It has been demonstrated that acclimation of higher plants, green algae and cyanobacteria to low temperature or high light conditions which potentially induce an imbalance between energy supply and energy utilization is accompanied by the development of higher reduction state of Q_A and higher resistance to photoinhibition (Huner et al., 1998). Although this is a fundamental feature of all photoautotrophs, and the acquisition of increased tolerance to photoinhibition has been ascribed to growth and development under high PSII excitation pressure, the precise mechanism controlling the redox state of Q_A and its physiological significance in developing higher resistance to photoinhibition has not been fully elucidated. In this review we summarize recent data indicating that the increased resistance to high light in a broad spectrum of photosynthetic organisms acclimated to high excitation pressure conditions is associated with an increase probability for alternative non-radiative P680⁺Q_A ^- radical pair recombination pathway for energy dissipation within the reaction centre of PSII. The various molecular mechanisms that could account for non-photochemical quenching through PSII reaction centre are also discussed.     

Activation of insect anti-oxidative mechanisms by mammalian glucagon

Resembling the main function of insect adipokinetic hormones (AKHs), the vertebrate hormone glucagon mobilizes energy reserves and participates in the control of glucose level in the blood. Considering the similarities, the effect of porcine glucagon was evaluated in an insect model species, the firebug Pyrrhocoris apterus. Using the mouse anti-glucagon antibody, presence of immunoreactive material was demonstrated for the first time in the firebug CNS and gut by ELISA. Mammalian (porcine) glucagon injected into the adult bugs showed no effect on hemolymph lipid level or on the level of AKH in CNS and hemolymph, however, it activated an antioxidant response when oxidative stress was elicited by paraquat, a diquaternary derivative of 4, 4′-bipyridyl. Glucagon elicited the antioxidant response by increasing glutathione and decreasing protein carbonyl levels in hemolymph, decreasing both protein carbonyl and protein nitrotyrosine levels in CNS. Additionally, when co-injected with paraquat, glucagon partially eliminated oxidative stress markers elicited by this redox cycling agent and oxidative stressor. This indicates that glucagon might induce an antioxidant defense in insects, as recently described for AKH. Failure of glucagon to alter AKH level in the bug's body indicates employment of an independent pathway without involving the native AKH.     

Photodegradation of DNA with fluorescent light in the presence of riboflavin, and photoprotection by flavin triplet-state quenchers

Superoxide anion was photogenerated upon illumination of nucleic acids with fluorescent light in a solution containing phosphate buffer, pH 7.8 and riboflavin. DNA was a better reducing substrate for this reaction than was RNA. A similar riboflavin-sensitized photoreaction caused single- and double-strand scissions of supercoiled PM2 DNA as detected by electrophoresis in agarose gels. None of specific scavengers or quenchers for superoxide anion and other active oxygen species prevented the DNA strand breaks. However, among the flavin triplet-state quenchers, potassium iodide, butylated hydroxyanisole, and ferricytochrome c protected the supercoiled DNA from photodegradation; butylated hydroxytoluene, alpha-tocopherol, tyrosine and hemoglobin did not have any protective effect. These results indicate that triplet-state riboflavin or a derivative formed from it participate directly in the observed riboflavin-sensitized DNA photodegradation and that active oxygen species are not directly involved.     

Activation of tat-defective human immunodeficiency virus by ultraviolet light

Ultraviolet light (UV) is known to cause activation of gene expression from the human immunodeficiency virus type 1 (HIV-1) promoter. To address the question of whether tat-defective HIV-1 provirus could be rescued by UV irradiation we examined its effect on HeLa cells containing integrated proviruses with tat mutations. Exposure of these cells to an optimal dose of UV resulted in the production of infectious viruses. The degree of UV activation and reversion to infectious virus appeared to depend on the nature of the original tat mutation. Two of the mutants required cocultivation with tat-expressing cells to fully generate replication competent viruses, while a third mutant required only cocultivation with H9 cells. Sequencing of cDNA from cells infected with this last mutant demonstrated that the parental mutant sequence was retained and that genotypic revertants to the wild-type as well as new mutant sequences were generated. These results suggest that tat-defective HIV-1 provirus can be activated by UV and can subsequently revert to wild-type virus. This study raises the possibility that UV exposure of immune cells in the skin plays a role in the activation of defective HIV-1 in vivo.     

Photosynthesis of terrestrial cyanobacteria under light and desiccation stress as expressed by chlorophyll fluorescence and gas exchange

Terrestrial mats of cyanobacteria with other associated microscopicalcryptogams were obtained from various sites in the tropics,i.e. rocks of mountains and rock-outcrops and bare soil surfaces,a valley in the Austrian Alps and a glasshouse. Species diversityof each sample was analysed qualitatively. The samples camefrom very different light climates. Responses to light and desiccationstress were studied using the saturation pulse method for recordingchlorophyll fluorescence variables as well as by measuring netCO² and O² exchange in order to confirm results by independentmethods. Under light stress, shade and high-light, samples showeda reduction of gas exchange and of the fluorescence variablesphotochemical fluorescence quenching coefficient (qp), potentialquantum yield of photosystem II (F_vIF^m) and effective quantumyield (     

Activation of c-Raf kinase by ultraviolet light. Regulation by retinoids

The present study highlights retinoids as modulators of c-Raf kinase activation by UV light. Whereas a number of retinoids, including retinol, 14-hydroxyretroretinol, anhydroretinol (AR), and retinoic acid bound the c-Raf cysteine-rich domain (CRD) with equal affinity in vitro as well as in vivo, they displayed different, even opposing, effects on UV-mediated kinase activation; retinol and 14-hydroxyretroretinol augmented responses, whereas retinoic acid and AR were inhibitory. Oxidation of thiol groups of cysteines by reactive oxygen, generated during UV irradiation, was the primary event in c-Raf activation, causing the release of zinc ions and, by inference, a change in CRD structure. Retinoids modulated these oxidation events directly: retinol enhanced, whereas AR suppressed, zinc release, precisely mirroring the retinoid effects on c-Raf kinase activation. Oxidation of c-Raf was not sufficient for kinase activation, productive interaction with Ras being mandatory. Further, canonical tyrosine phosphorylation and the action of phosphatase were essential for optimal c-Raf kinase competence. Thus, retinoids bound c-Raf with high affinity, priming the molecule for UV/reactive oxygen species-mediated changes of the CRD that set off GTP-Ras interaction and, in context with an appropriate phosphorylation pattern, lead to full phosphotransferase capacity.     

Activation of epithelial Na channels by hormonal and autoregulatory mechanisms of action

Methods of blocker-induced noise analysis were used to investigate the way in which forskolin and vasopressin stimulate Na transport at apical membranes of short-circuited frog skin transporting Na at spontaneous rates of transport. Experiments were done under conditions where the apical Ringer solution contained either 100 mM Na or a reduced Na concentration of 5 or 10 mM Na and buffered with either HCO3 or HEPES. Reduction of apical solution Na concentration caused a large autoregulatory increase of Na channel density (NT) similar in magnitude to that observed previously in response to blocker (amiloride) inhibition of apical membrane Na entry. Forskolin at 2.5 microM caused maximal and reversible large increases of NT, which were larger than could be elicited by 30 mU/ml vasopressin. In both the absence and presence of the autoregulatory increase of NT (caused by reduction of apical Na concentration), forskolin caused large increases of NT. Although the fractional increases of NT in response to forskolin were roughly similar, the absolute increases of NT were considerably larger in those tissues studied at reduced Na concentration and where baseline values of NT were markedly elevated by reduction of apical Na concentration. Because the effects on NT were additive, it is likely that the cAMP-dependent and autoregulatory mechanism that lead to changes of NT are distinct. We speculate that autoregulation of NT may involve change of the size of a cytosolic pool of Na-containing vesicles that are in dynamic balance with the apical membranes. cAMP-dependent regulation of NT may involve change of the dynamic balance between vesicles and the apical membranes of these epithelial cells. Alternative hypotheses cannot at present be ruled out, but will require incorporation of the idea that regulation of NT can occur both by hormonal and nonhormonal (autoregulatory) mechanisms of action.     

Anesthetic mechanisms: worms light the way

In the nematode C. elegans, immobility induced by the anesthetic halothane is coupled to its ability to modulate neuronal resting membrane potential, perhaps through effects on leak channels; a similar anesthetic, isoflurane, appears to work a different way.