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PubMed2005

Structural and biochemical characterization of Gun4 suggests a mechanism for its role in chlorophyll biosynthesis.

Gun4 has been implicated in a developmental signaling pathway between the chloroplast and the nucleus involving magnesium protoporphyrin IX (MgP(IX)), the first dedicated intermediate in the chlorophyll biosynthetic pathway. Here we present the crystal structure of Thermosynechococcus elongatus Gun4 at 1.5 A, describe the binding affinities of Gun4 for substrate and product porphyrin molecules, and identify a likely (Mg)P(IX) binding site on the protein. Kinetic analyses show that Gun4 dramatically increases the efficiency of transformation of porphyrin substrate to metalloporphyrin product and that it also reduces the threshold Mg2+ concentration required for activity at low porphyrin concentration. Gun4 therefore controls magnesium chelatase at physiologically significant Mg2+ concentrations and likely acts as a molecular switch in vivo so that in its absence magnesium chelatase is inactive. This mechanism could allow Gun4 to mediate magnesium protoporphyrin levels both for chlorophyll biosynthesis and for signaling to the nucleus.

Amino Acid Sequence↗

PubMed1996

Assembly of light-harvesting chlorophyll a/b complex in vitro. Time-resolved fluorescence measurements.

The assembly kinetics of a pigment-binding membrane protein, the light- harvesting chlorophyll a/b complex of green plants, have been determined in vitro. Time-resolved fluorescence spectroscopy, with millisecond time resolution, has been used to monitor changes in both protein and chlorophyll (Chl) fluorescence, as well as in energy transfer from Chl b to Chl a, during complex assembly. Three reaction steps could be resolved after rapid, stopped-flow, mixing of the apoprotein (light-harvesting Chl a/b protein, LHCP) and pigments, solubilized in sodium dodecyl sulfate (SDS) and octyl glucoside (OG), respectively. A fast step in the range of 10 ms was detected regardless of whether the reaction mixture contained pigments or proteins, or both, and is interpreted as being connected with the formation of mixed SDS and OG detergent micelles. Two further steps were resolved: one with a time constant of about 1 min and another, slow step with a time constant of several minutes. Both of these were dependent on the presence of protein, Chls, and xanthophylls. Most, if not all, of the energy transfer from Chl b to Chl a was established during the slow step, indicating the the juxtaposition of these pigments, either by a structural rearrangement of the complex of by additional pigment binding, is the final stage in LHCII assembly in vitro.

Apoproteins↗

PubMed1996

UV-B-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence. Impairment of donor and acceptor side components.

Inhibition of photosystem II electron transport by UV-B radiation has been studied in isolated spinach photosystem II membrane particles using low-temperature EPR spectroscopy and chlorophyll fluorescence measurements. UV-B irradiation results in the rapid inhibition of oxygen evolution and the decline of variable chlorophyll fluorescence. These effects are accompanied by the loss of the multiline EPR signal arising from the S2 state of the water-oxidizing complex and the induction of Signal IIfast originating from stabilized Try-Z+. The EPR signals from the QA-Fe2+ acceptor complex, Tyr-D+, and the oxidized non-heme iron (Fe3+) are also decreased during the course of UV-B irradiation, but at a significantly slower rate than oxygen evolution and the multiline signal. The decrease of the Fe3+ signal at high g values (g = 8.06, g = 5.6) is accompanied by the induction of another EPR signal at g = 4.26 that arises most likely from the same Fe3+ ion in a modified ligand environment. UV-B irradiation also affects cytochrome b-559. The g = 2.94 EPR signal that arises from the dark- oxidized form is enhanced, whereas the light inducible g = 3.04 signal that arises from the photo-oxidizable population of cytochrome b-559 is diminished. UV-B irradiation also induces the degradation of the D1 reaction center protein. The rate of the D1 protein loss is slower than the inhibition of oxygen evolution and of the multiline signal but follows closely the loss of Signal IIslow, the QA-Fe2+ and the Fe3+ EPR signals, as well as the release of protein-bound manganese. It is concluded from the results that UV-B radiation affects photosystem II redox components at both the donor and acceptor side. The primary damage occurs at the water-oxidizing complex. Modification and/or inactivation of tyrosine-D, cytochrome b-559, and the QAFe2+ acceptor complex are subsequent events that coincide more closely with the UV-B-induced damage to the protein structure of the photosystem II reaction center.

Chlorophyll↗

PubMed1997

Fluorescence quenching by chlorophyll cations in photosystem II.

Although fluorescence is widely used to study photosynthetic systems, the mechanisms that affect the fluorescence in photosystem II (PSII) are not completely understood. The aim of this study is to define the low-temperature steady-state fluorescence quenching of redox-active centers that function on the electron donor side of PSII. The redox states of the electron donors and acceptors were systematically varied by using a combination of pretreatments and illumination to produce and trap, at low temperature, a specific charge-separated state. Electron paramagnetic resonance spectroscopy and fluorescence intensity measurements were carried out on the same samples to obtain a correlation between the redox state and the fluorescence. It was found that illumination of PSII at temperatures between 85 and 260 K induced a fluorescence quenching state in two phases. At 85 K, where the fast phase was most prominent, only one electron-transfer pathway is active on the donor side of PSII. This pathway involves electron donation to the primary electron donor in PSII, P680, from cytochrome b559 and a redox-active chlorophyll molecule, ChlZ. Oxidized ChlZ was found to be a potent quencher of chlorophyll fluorescence with 15% of oxidized ChlZ sufficient to quench 70% of the fluorescence intensity. This implies that neighboring PSII reaction centers are energetically connected, allowing oxidized ChlZ in a few centers to quench most of the fluorescence. The presence of a well-defined quencher in PSII may make it possible to study the connectivity between antenna systems in different sample preparations. The other redox-active components on the donor side of PSII studied were the O2-evolving complex, the redox-active tyrosines (YZ and YD), and cytochrome b559. No significant changes in fluorescence intensity could be attributed to changes in the redox state of these components. The fast phase of fluorescence quenching is attributed to the rapid photooxidation of ChlZ, and the slow phase is attributed to multiple turnovers providing for further oxidation of ChlZ and irreversible photoinhibition. Significant photoinhibition only occurred at Chl concentrations below 0.7 mg/mL and above 150 K. The reversible oxidation of ChlZ in intact systems may function as a photoprotection mechanism under high-light conditions and account for a portion of the nonphotochemical fluorescence quenching.

Cations↗

PubMed1998

Identification of histidine 118 in the D1 polypeptide of photosystem II as the axial ligand to chlorophyll Z.

Chlorophyll Z (ChlZ) is a redox-active chlorophyll (Chl) which is photooxidized by low-temperature (<100 K) illumination of photosystem II (PSII) to form a cation radical, ChlZ+. This cofactor has been proposed to be an "accessory" Chl in the PSII reaction center and is expected to be buried in the transmembrane region of the PSII complex, but the location of ChlZ is unknown. A series of single-replacement site-directed mutants of PSII were made in which each of two potentially Chl-ligating histidines, D1-H118 or D2-H117, was substituted with amino acids which varied in their ability to coordinate Chl. Assays of the wild-type and mutant strains showed parallel phenotypes for the D1-118 and D2-117 mutants: noncoordinating or poorly coordinating residues at either position decreased photosynthetic competence and impaired assembly of PSII complexes. Only the mutants substituted with glutamine (D1-H118Q and D2-H117Q) had phenotypes comparable to the wild-type strain. The ChlZ+ cation was characterized by low-temperature electron paramagnetic resonance (EPR), near-infrared (IR) absorbance, and resonance Raman (RR) spectroscopies in wild-type, H118Q, and H117Q PSII core complexes. The quantum yield of ChlZ+ formation is the same (approximately 2.5% per saturating flash at 77 K) for wild-type, H118Q, and H117Q, indicating that its efficiency of photooxidation is unchanged by the mutations. Similarly, the EPR and near-IR absorbance spectra of ChlZ+ are insensitive to the mutations made at D1-118 and D2-117. In contrast, the RR signature of ChlZ+ in H118Q PSII, obtained by selective near-IR excitation into the ChlZ+ cation absorbance band, is significantly altered relative to wild-type PSII while the RR spectrum of ChlZ+ in the H117Q mutant remains identical to wild-type. Shifts in the RR spectrum of ChlZ+ in H118Q reflect a change in the structure of the Chl ring, most likely due to a perturbation of the core size and/or extent of doming caused by a change in the axial ligand to Mg(II). Thus, we conclude that the axial ligand to ChlZ is H118 of the D1 polypeptide. Furthermore, we propose that H117 of the D2 polypeptide is the ligand to a homologous redox-inactive accessory Chl which we term ChlD. The Chl Z and D terminology reflects the 2-fold structural symmetry of PSII which is apparent in the redox-active tyrosines, YZ and YD, and the active/inactive branch homology of the D1/D2 polypeptides with the L/M polypeptides of the bacterial reaction center.

Cell Division↗

PubMed1999

Characterization of the fast and slow reversible components of non-photochemical quenching in isolated pea thylakoids by picosecond time-resolved chlorophyll fluorescence analysis.

The fast and slow reversible components of non-photochemical chlorophyll fluorescence quenching commonly assigned to the qE and the qI mechanism have been studied in isolated pea thylakoids which were prepared from leaves after a moderate photoinhibitory treatment. Chlorophyll fluorescence decays were measured at picosecond resolution and analyzed on the basis of the heterogeneous exciton/radical pair equilibrium model. Our results show that the fast reversible non-photochemical quenching is completely assigned to the PS II antenna and is related to zeaxanthin. The slow reversible qI type quenching is located at the PS II reaction center and involves enhanced nonradiative decay of the primary charge separated state to its ground state and/or triplet excited state. Apart from its independence from the proton gradient, the qI quenching shows striking similarities to a particular form of qE quenching which is also located at the PS II reaction center and has resently been resolved in isolated thylakoids from dark-adapted leaves [Wagner, B., et al. (1996) J. Photochem. Photobiol., B 36, 339-350]. Our data suggest that during exposure to the supersaturating light the reaction center qE component was replaced by qI quenching. This qE to qI transition is supposed to be part of the mechanism of the long-term downregulation of PS II during photoinhibition. It is also evident that under the conditions used in our study zeaxanthin-dependent antenna quenching is not involved in the slow reversible downregulation of PS II but that it retains its dependence on the proton gradient during exposure to strong light.

Chlorophyll↗

PubMed2003

Pulsed high-frequency EPR study on the location of carotenoid and chlorophyll cation radicals in photosystem II.

When the primary electron-donation pathway from the water-oxidation complex in photosystem II (PS II) is inhibited, chlorophyll (Chl(Z) and Chl(D)), beta-carotene (Car) and cytochrome b(559) are alternate electron donors that are believed to function in a photoprotection mechanism. Previous studies have demonstrated that high-frequency EPR spectroscopy (at 130 GHz), together with deuteration of PS II, yields resolved Car(+) and Chl(+) EPR signals (Lakshmi et al. J. Phys. Chem. B 2000, 104, 10 445-10 448). The present study describes the use of pulsed high-frequency EPR spectroscopy to measure the location of the carotenoid and chlorophyll radicals relative to other paramagnetic cofactors in Synechococcus lividus PS II. The spin-lattice relaxation rates of the Car(+) and Chl(+) radicals are measured in manganese-depleted and manganese-depleted, cyanide-treated PS II; in these samples, the non-heme Fe(II) is high-spin (S = 2) and low-spin (S = 0), respectively. The Car(+) and Chl(+) radicals exhibit dipolar-enhanced relaxation rates in the presence of high-spin (S = 2) Fe(II) that are eliminated when the Fe(II) is low-spin (S = 0). The relaxation enhancements of the Car(+) and Chl(+) by the non-heme Fe(II) are smaller than the relaxation enhancement of Tyr(D)(*) and P(865)(+) by the non-heme Fe(II) in PS II and in the reaction center from Rhodobactersphaeroides, respectively, indicating that the Car(+)-Fe(II) and Chl(+)-Fe(II) distances are greater than the known Tyr(D)(*)-Fe(II) and P(865)(+)-Fe(II) distances. The Car(+) radical exhibits a greater relaxation enhancement by Fe(II) than the Chl(+) radical, consistent with Car being an earlier electron donor to P(680)(+) than Chl. On the basis of the distance estimates obtained in the present study and by analogy to carotenoid-binding sites in other pigment-protein complexes, possible binding sites are discussed for the Car cofactors in PS II. The relative location of Car(+) and Chl(+) radicals determined in this study provides valuable insight into the sequence of electron transfers in the alternate electron-donation pathways of PS II.

Binding Sites↗

PubMed2005

Effects of chlorophyll-related compounds on hydrogen peroxide induced DNA damage within human lymphocytes.

Chlorophylls (Chl's) are the most abundant natural plant pigments. Four chlorophyll-related compounds (CRCs), including chlorophyllide a and b (Chlide a and b) and pheophorbide a and b (Pho a and b), were investigated for their antioxidative capacities to protect human lymphocyte DNA from hydrogen peroxide (H(2)O(2)) induced strand breaks and oxidative damage ex vivo. Lymphocytes exposed to H(2)O(2) at concentrations of 10 and 50 microM revealed an increased frequency of DNA single-strand breaks (ssb's; as measured by the comet assay) and also an increased level of oxidized nucleoside (as measured by 8-hydroxydeoxyguanosine, 8-OHdG). All Chl's reduced the level of DNA ssb's and 8-OHdG within human lymphocytes following exposure to 10 microM H(2)O(2). Only Pho a and b were able to decrease DNA ssb's and 8-OHdG following treatment of lymphocytes with 50 microM H(2)O(2), in a concentration-dependent fashion. It was demonstrated herein that Pho a and b were more antioxidative than others. We applied DPPH free-radical scavenge assays in vitro, and got similar results. Pho a and b had higher ability in scavenging capacities than others. We conclude that water-extract Chl's are able to enhance the ability of human lymphocytes to resist H(2)O(2)-induced oxidative damage, especially for Pho a and b.

Antioxidants↗

PubMed1992

Characterization of silkworm chlorophyll metabolites as an active photosensitizer for photodynamic therapy.

Silkworm excreta containing chlorophyll metabolites have long been used for their medicinal activities in the Far East. The major chlorophyll derivative (CpD) fraction in the Me2CO extract from the silkworm excreta exhibits strong photodynamic action on mice and human tumor cells in vitro. The main CpD component was isolated, purified, and subjected to extensive structural and characterization studies to assess its photodynamic therapeutic activity. According to the spectral analyses, the structure of the major unknown CpD was determined to be 10-hydroxypheophytin a [1]. The CpD component possessed a high quantum efficiency (50%) for the photosensitized production of singlet molecular oxygen, thus providing a molecular basis for its photodynamic therapeutic efficiency. A further advantage is that the "photooxidation/degradation" product(s) from this CpD retains all the characteristics of a photodynamic sensitizer.

Animals↗

PubMed2001

Biosynthesis of chlorophyll of photosystem II reaction centers in plant leaves in early stages of etiolation.

Spectral methods were used to study the sequences of chlorophyll biosynthesis reactions in etiolated pea, bean, and maize plants in early stages (3-4 days) of growth. For these juvenile plants, along with the reaction chain known for mature (7-9 day-old) plants, a new reaction chain was found which started with phototransformation of the long-wavelength form PChld 686/676 into PChld 653/648. (PChld 653/648 differs from the main known precursor form PChld 655/650). The subsequent photoreduction of PChld 653/648 leads to the formation of Chld 684/676, which is transformed into Chl 688/680 in the course of a dark reaction. After completion of this reaction, fast (20-30 sec) quenching of the fluorescence of the reaction product is observed with the formation of non-fluorescent Chl 680. The reaction accompanied by pigment fluorescence quenching is absent in pea mutants with depressed function of Photosystem II reaction centers. This suggests that the newly found reaction chain leads to the formation of chlorophyll of the Photosystem II reaction center.

Chlorophyll↗

PubMed1999

Light-regulated expression of the gsa gene encoding the chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransferase in carotenoid-deficient Chlamydomonas reinhardtii cells.

Expression of the Chlamydomonas reinhardtii gsa gene encoding the chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransferase was previously shown to be induced by blue light. Possible blue light photoreceptors include flavins and carotenoids. Light induction of gsa was investigated in carotenoid-deficient mutant C. reinhardtii cells. Strain CC-2682 cells are sensitive to light, produce only small amounts of chlorophyll, and do not exhibit phototaxis. Solvent extracts show the absence of carotenoids and carotenoid precursors beyond phytoene in dark-grown mutant cells. Although apparently devoid of carotenoids, the cells did show light induction of gsa. The gsa transcript level was very low in dark-grown cells but increased significantly after 2 h of exposure to dim (1.5 x 10(-5) mol m(-2) s(-1)) green (480-585 nm) light. This light regime was previously determined not to injure these photosensitive cells and to fully induce gsa in wild-type cells. Exposure to this light did not cause the mutant cells to produce measurable carotenoids or to become phototactic. Growth of the mutant cells in the presence of exogenous beta-carotene or all-trans retinol restored phototaxis but did not affect the degree of gsa induction by light. The induction of gsa by light in the absence of carotenoids, and the fact that incorporation of physiologically usable carotenoids (as indicated by the restoration of phototaxis) did not affect the degree of light induction, indicate that the photoreceptor for light induction of gsa in C. reinhardtii is not a carotenoid. The flavin antagonist diphenyleneiodonium blocked light induction of gsa in both wild-type and mutant cells under conditions where respiration was not inhibited. These results suggest that the photoreceptor or a signal transduction effector for light induction of the C. reinhardtii gsa gene is a flavoprotein.

Animals↗

PubMed2002

Biosynthesis of chlorophyll from protochlorophyll(ide) in green plant leaves.

Using spectral methods, the biosynthesis of protochlorophyll(ide) and chlorophyll(ide) in green plant leaves was studied. The main chlorophyll precursors in the green leaves (as in etiolated leaves) were photoactive photocholorophyll(ide) forms Pchl(ide)655/650(448) and Pchl(ide)653/648(440). The contributions into Chl biosynthesis of the shorter-wavelength precursor forms, which were accumulated in darkened green leaves as well, were completely absent (of Pchl(ide) 633/628(440)) or insignificant (of Pchl(ide) 642/635(444)).

Chlorophyll↗

PubMed2004

Chlorophyllide a Oxygenase mRNA and Protein Levels Correlate with the Chlorophyll a/b Ratio in Arabidopsis thaliana.

Plants can change the size of their light harvesting complexes in response to growth at different light intensities. Although these changes are small compared to those observed in algae, their conservation in many plant species suggest they play an important role in photoacclimation. A polyclonal antibody to the C-terminus of the Arabidopsis thaliana chlorophyllide a oxygenase (CAO) protein was used to determine if CAO protein levels change under three conditions which perturb chlorophyll levels. These conditions were: (1) transfer to shaded light intensity; (2) limited chlorophyll synthesis, and (3) during photoinhibition. Transfer of wild-type plants from moderate to shaded light intensity resulted in a slight reduction in the Chl a/b ratio, and increases in both CAO and Lhcb1 mRNA levels as well as CAO protein levels. CAO protein levels were also measured in the cch1 mutant, a P642L missense mutation in the H subunit of Mg-chelatase. This mutant has reduced total Chl levels and an increased Chl a/b ratio when transferred to moderate light intensity. After transfer to moderate light intensity, CAO mRNA levels decreased in the cch1 mutant, and a concomitant decrease in CAO protein levels was also observed. Measurements of tetrapyrrole intermediates suggested that decreased Chl synthesis in the cch1 mutant was not a result of increased feedback inhibition at higher light intensity. When wild-type plants were exposed to photoinhibitory light intensity for 3 h, total Chl levels decreased and both CAO mRNA and CAO protein levels were also reduced. These results indicate that CAO protein levels correlate with CAO mRNA levels, and suggest that changes in Chl b levels in vascular plants, are regulated, in part, at the CAO mRNA level.

CAO↗

PubMed2005

Ecology: a niche for cyanobacteria containing chlorophyll d.

The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.

Animals↗

PubMed2006

Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum.

Deep chlorophyll maxima (DCMs) are widespread in large parts of the world's oceans. These deep layers of high chlorophyll concentration reflect a compromise of phytoplankton growth exposed to two opposing resource gradients: light supplied from above and nutrients supplied from below. It is often argued that DCMs are stable features. Here we show, however, that reduced vertical mixing can generate oscillations and chaos in phytoplankton biomass and species composition of DCMs. These fluctuations are caused by a difference in the timescales of two processes: (1) rapid export of sinking plankton, withdrawing nutrients from the euphotic zone and (2) a slow upward flux of nutrients fuelling new phytoplankton production. Climate models predict that global warming will reduce vertical mixing in the oceans. Our model indicates that reduced mixing will generate more variability in DCMs, thereby enhancing variability in oceanic primary production and in carbon export into the ocean interior.

Biomass↗

PubMed1967

Incorporation of DL-[2-14C]mevalonic acid lactone into beta-carotene and the phytol side chain of chlorophyll in cotyledons of four species of pine seedlings.

1. The incorporation of dl-[2-(14)C]mevalonic acid lactone into beta-carotene and the phytol side chain of chlorophyll has been investigated in cotyledons of four species of pine seedlings (Pinus silvestris, P. contorta, P. radiata and P. jeffrei) grown in darkness and in light. 2. The relative incorporation of label into beta-carotene and the phytol side chain of chlorophyll is similar to that observed in experiments on monocotyledons and dicotyledons. 3. The relative incorporation of (14)CO(2) into beta-carotene and phytol is much higher than the incorporation of [2-(14)C]mevalonic acid.

Carbon Dioxide↗

PubMed2005

Role of electron-transfer quenching of chlorophyll fluorescence by carotenoids in non-photochemical quenching of green plants.

NPQ (non-photochemical quenching) is a fundamental photosynthetic mechanism by which plants protect themselves against excess excitation energy and the resulting photodamage. A discussed molecular mechanism of the so-called feedback de-excitation component (qE) of NPQ involves the formation of a quenching complex. Recently, we have studied the influence of formation of a zeaxanthin-chlorophyll complex on the excited states of the pigments using high-level quantum chemical methodology. In the case of complex formation, electron-transfer quenching of chlorophyll-excited states by carotenoids is a relevant quenching mechanism. Furthermore, additionally occurring charge-transfer excited states can be exploited experimentally to prove the existence of the quenching complex during NPQ.

Carotenoids↗

PubMed2000

Cloning and functional expression of the gene encoding the key enzyme for chlorophyll b biosynthesis (CAO) from Arabidopsis thaliana.

Chlorophyll (Chl) biosynthesis and degradation are the only biochemical processes on Earth that can be directly observed from satellites or other planets. The bulk of the Chls is found in the light-harvesting antenna complexes of photosynthetic organisms. Surprisingly little is known about the biosynthesis of Chl b, which is the second most abundant Chl pigment after Chl a. We describe here the expression and properties of the chlorophyllide a oxygenase gene (CAO) from Arabidopsis thaliana, which is apparently the key enzyme in Chl b biosynthesis. The recombinant enzyme produced in Escherichia coli catalyses an unusual two-step oxygenase reaction that is the 'missing link' in the chlorophyll cycle of higher plants.

Arabidopsis↗