Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: I. A review

The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at approximately 730 nm (Frese et al., Biochemistry 42:9205-9213, 2003). It was found (Heber and Shuvalov, Photosynth Res 84:84-91, 2005) that two fluorescent bands at 685 and 720 nm are observed in different organisms. These two forms are registered in the action spectrum of Q(A) photoreduction. Similar results were obtained in core complexes of PSII at low temperature (Hughes et al., Biochim Biophys Acta 1757: 841-851, 2006). In all cases the far-red absorption and emission can be interpreted as indication of the state with charge transfer character in which the chlorophyll monomer plays a role of an electron donor. The role of bacteriochlorophyll monomers (B(A) and B(B)) in BRCs can be revealed by different mutations of axial ligand for Mg central atoms. RCs with substitution of histidine L153 by tyrosine or leucine and of histidine M182 by leucine (double mutant) are not stable in isolated state. They were studied in antennaless membrane by different kinds of spectroscopy including one with femtosecond time resolution. It was found that the single mutation (L153HY) was accompanied by disappearance of B(A) molecule absorption near 802 nm and by 14-fold decrease of photochemical activity measured with ms time resolution. The lifetime of P(870)* increased up to approximately 200 ps in agreement with very low rate of the electron transfer to A-branch. In the double mutant L153HY + M182HL, the B(A) appears to be lost and B(B) is replaced by bacteriopheophytin Phi(B) with the absence of any absorption near 800 nm. Femtosecond measurements have revealed the electron transfer to B-branch with a time constant of approximately 2 ps. These results are discussed in terms of obligatory role of B(A) and Phi(B) molecules located near P for efficient electron transfer from P*.     

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.     

The role of plastoquinone and β-carotene in the primary reaction of plant Photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll : plastoquinone ratio, 900 : 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of β-carotene. The addition of β-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that β-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.     

Role of β-carotene in the reaction centres of Photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl₄, n-heptane) and the β-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b₆ in the ratio 1:2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP⁺ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn²⁺ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of β-carotene in photochemistry separate from its roles in energy transfer and photoprotection.Removal of the fraction of β-carotene closely associated with the Photo-system I reaction centre caused the rate of NADP⁺ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of β-carotene, photochemistry can still be observed, however the specific association of β-carotene with the reaction centre is required for maximal rates. We propose that β-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state β-carotene and chlorophyll in the first excited state.     

A routine method to determine the chlorophyll a,pheophytin a and β-carotene contents of isolated Photosystem II reaction center complexes

The most simple way in which the stoichiometry of chlorophyll a, pheophytin a and -carotene in isolated Photosystem II reaction center complexes can be determined is by analysis of the spectrum of the extracted pigments in 80% acetone. We present two different calculation methods using the extinction coefficients of the purified pigments in 80% acetone at different wavelengths. One of these methods also accounts for the possible presence of chlorophyll b. The results are compared with results obtained with HPLC pigment analysis, and indicate that these methods are suitable for routine determination of the pigment stoichiometry of isolated Photosystem II reaction center complexes.     

Involvement of sulfoquinovosyl diacylglycerol in the structural integrity and heat-tolerance of photosystem II

To examine the role of sulfoquinovosyl diacylglycerol (SQDG) in thylakoid membranes, we compared the structural and functional properties of photosystem II (PSII) between a mutant of Chlamydomonas reinhardtii defective in SQDG ( hf-2) and the wild type. The PSII core complex of hf-2, as compared with that of the wild type, showed structural fragility when solubilized with a detergent, dodecyl beta- d-maltoside, suggesting that the physical properties of the PSII complex were altered by the loss of SQDG. On the other hand, exposure of the cells to 41 degrees C for 120 min in the dark decreased the PSII activity to 70% and 50% of the initial levels in the wild type and hf-2, respectively, which implies that the PSII activity, in the absence of SQDG, becomes less stable under heat-stress conditions. PSII inactivated to 60% of the initial level by dark incubation at 41 degrees C was reactivated by following illumination even at 41 degrees C to more than 90% in the wild type, but only to 70% in hf-2. These results suggest that PSII inactivated by heat recovers through some mechanism dependent on light, and that SQDG participates in functioning of the mechanism. The conformational disorder of PSII caused by the defect in SQDG might be correlated with the increased susceptibility of its activity to heat-stress.     

Oxidation and formation of oxidation products of β-carotene at boiling temperature

β-Carotene is one of the most important lipid component extensively used in food industries as source of pro-vitamin A and colorant. During processing and storage β-carotene is oxidized and degraded to various oxidation compounds. Some of these compounds are also the key aroma compounds in certain flowers, vegetables and fruits. The methods for analysis and determination of these oxidized products formed during food boiling or preparation are key to the understanding the chemistry of these compounds. This paper presents a novel analytical method incorporating high performance liquid chromatography with diode array and mass spectrometric detection for the characterization of oxidation, isomerization and oxidation products of β-carotene in toluene at boiling temperature. HPLC and APCI-MS was optimized using oxidized sample and flow injection analysis of the standard β-carotene respectively. β-Carotene was oxidized in the Rancimat at 110°C for 30, 60 and 90 min. The oxidized samples were than analyzed by HPLC system at 450 nm and 350 nm as well as scanning and single ion monitoring mass spectrometry. A total of ten oxidation products and three Z-isomers were reported. Extensive isomerization was observed during treatment at the control accelerated conditions. The oxidation products include five apo-carotenals, three diepoxides, one mono-epoxide and one short chain species. Results show that the method was reproducible, accurate and reliable for the separation and identification of oxidation products of β-carotene.     

Biosynthesis of plastoquinone and β-carotene in isolated chloroplasts

Intact, isolated spinach chloroplasts incorporated ¹⁴C from ¹⁴CO₂ into plastoquinone and β-carotene under photosynthetic conditions. Addition of unlabelled l-tyrosine, p-hydroxyphenylpyruvate, or homogentisate increased the incorporation of ¹⁴C into plastoquinone, but decreased that into β-carotene.     

Photochemical reactions of chlorophyll in dehydrated Photosystem II: two chlorophyll forms (680 and 700 nm)

Lichens and phototolerant poikilohydric mosses differ from spinach leaves, fern fronds or photosensitive mosses in that they show strongly decreased F_o chlorophyll fluorescence after drying. This desiccation-induced fluorescence loss is rapidly reversible under rehydration. Fluorescence emission from Photosystem II at 685 nm was decreased more strongly by dehydration than 720 nm emission. Reaction centers of Photosystem II lose activity on dehydration and regain it on hydration. Heating of desiccated lichens increased F_o chlorophyll fluorescence. The activation energy for the reversible part of the temperature-dependent fluorescence increase was 0.045 eV, which corresponds to the energy difference between the 680 and 697 nm absorption bands. In desiccated chlorolichens such as Parmelia sulcata, heating induces the appearance of positive variable fluorescence related to the reversible reduction of Q_A due to overcoming the energy barrier. This is interpreted to provide information on the mechanism of photoprotection: energy is dissipated by changing Chl680 or P680 into a chlorophyll form, which absorbs at 700 nm and emits light at 720 nm (Chl-720 or P680(700)) with a low quantum yield. Dissipation of light energy in this trap is activated by desiccation.     

Mechanisms of chlorophyll fluorescence revisited: Prompt or delayed emission from photosystem II with closed reaction centers?

This paper proposes a model which correlates the exciton decay kinetics observed in picosecond fluorescence studies with the primary processes of charge separation in the reaction center of photosystem II. We conclude that the experimental results from green algae and chloroplasts from higher plants are inconsistent with the concept that delayed luminescence after charge recombination should account for the long-lived (approx. 2 ns) fluorescence decay component of closed photosystem II centers. Instead, we show that the experimental data are in agreement with a model in which the long-lived fluorescence is also prompt fluorescence. The model suggests furthermore that the rate constant of primary charge separation is regulated by the oxidation state of the quinone acceptor Q_A.     

Methyl jasmonate vapor promotes β-carotene synthesis and chlorophyll degradation in Golden Delicious apple peel

Methyl jasmonate (JAMe) vapors (8 ppm) for 4 h at 25°C dramatically increased Golden Delicious apple peel -carotene synthesis by nearly threefold to 35 ng/mm², while control fruits remained nearly constant at 11 ng/mm² during the 10 day measurement period. Chlorophyll a and to a lesser extent chlorophyll b and lutein degradation were accelerated by JAMe treatment, but all showed some recovery after 6 days. Peel chlorophyll ab ratio held almost constant at 4.2–4.5 in control fruits during 10 days, while JAMe-treated apple chl ab ratio decreased linearly to 2.9 during 10 days.     

Energy dissipation in photosynthesis: Does the quenching of chlorophyll fluorescence originate from antenna complexes of photosystem II or from the reaction center?

Dissipation of light energy was studied in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst., and in leaves of Spinacia oleracea L. and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reaction. Maximum chlorophyll fluorescence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated spinach leaves, as produced by saturating light and studied between +5 and −20 °C, revealed an activation energy ΔE of 0.11 eV. As this suggested recombination fluorescence produced by charge recombination between the oxidized primary donor of photosystem II and reduced pheophytin, a mathematical model explaining fluorescence, and based in part on known characteristics of primary electron-transport reactions, was developed. The model permitted analysis of different modes of fluorescence quenching, two localized in the reaction center of photosystem II and one in the light-harvesting system of the antenna complexes. It predicted differences in the relationship between quenching of variable fluorescence F _v and quenching of basal, so-called F ₀ fluorescence depending on whether quenching originated from antenna complexes or from reaction centers. Such differences were found experimentally, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the moss Rhytidiadelphus, whereas reaction-center quenching appeared to be important in spinach and Arabidopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended on or was strongly enhanced by zeaxanthin. De-protonation permitted relaxation of this quenching with half-times below 1 min. More slowly reversible quenching, tentatively identified as so-called q _I or photoinhibitory quenching, required protonation but persisted for prolonged times after de-protonation. It appeared to originate in reaction centers. Received: 8 April 2000 / Accepted: 31 August 2000     

Transcriptome profiling of β-carotene biosynthesis genes and β-carotene accumulation in leaf blades and petioles of celery cv. Jinnanshiqin

β-carotene is a kind of carotenoids and has many biological functions.Proper amount of β-carotene is beneficial to promote the synthesis of vitamin A [1].The unsaturated double bonds in β-carotene structure make it have strong antioxidant capacity.     

Interaction of chlorophyll a′ with the 65 kDa subunit protein of photosystem I reaction center

The 65 kDa polypeptide subunit depleted of P700 was prepared from a photosystem I reaction center preparation and mixed with chlorophyll a′ (C-10 epimer of chlorophyll a) to yield a complex exhibiting a tripleheaded spectrum with absorbance maxima at 673, 692 and 707 nm. The difference spectra (oxidized-minus-untreated and light-minus-dark) had a major trough at 707 nm and minor ones at 690 and 430 nm. The overall shape of the spectra resembled well that of P700 with a small red shift. A rapidly decaying flash-induced absorbance change was observed at 430 nm with a half decay time of less than 500 μs in a preparation supplemented with an electron donor system.     

Cyclization of lycopene in the biosynthesis of β-carotene

Mycobacterium marinum produces carotenoids when exposed to light or when antimycin A is added. Although the major pigment synthesized is β-carotene, lycopene is accumulated when the induced bacteria are incubated in the presence of nicotine (5 mM) or 2-(4-chlorophenylthio)-triethylamine hydrochloride (CPTA) (50 μM). Both of these compounds inhibit β-carotene synthesis by blocking the cyclization of lycopene. When nicotine is removed by washing the cells, the accumulated lycopene is cyclized to form β-carotene. The cyclization of lycopene is not an energy-requiring reaction and, furthermore, does not require oxygen or any other electron acceptor. Chloramphenicol addition also does not inhibit the conversion of lycopene to β-carotene indicating that no de novo protein synthesis is involved. Nicotine appears to act by inhibiting the activity of the enzyme required for the cyclization of lycopene.Although the mode of action of CPTA is similar to nicotine, it cannot be removed by washing once the cells have been incubated in its presence, suggesting that the molecule is tightly bound to the enzyme. The possible active molecular sites of nicotine and CPTA are discussed.     

Kinetics of photoaccumulation of β-carotene in Phycomyces blakesleeanus

Upon carbon starvation the -carotene content of Phycomyces mycelium grown on minimal agar medium disappears with a time lag of about 90 min and a T_1/2 of 68–75 min. If continuous light is given 2 h after starvation, there is an increase in -carotene content with respect to the dark control. This increase has a time lag of 20–25 min. The fluence rate-response curve of wt is biphasic and two mutants in the gene madA (madA7, madA35) and in the gene madB (madB101, madB104) have higher thresholds than wt; madB mutants are blinder than madA mutants. Only blue light is effective and we suggest that it has an effect solely on the catabolism of -carotene.Abbreviations D dark - L light - wt wild type     

Enzymatic production and in?situ separation of natural β-ionone from β-carotene

A biotechnological process concept for generation and in?situ separation of natural β-ionone from β-carotene is presented. The process employs carotenoid cleavage dioxygenases (CCDs), a plant-derived iron-containing nonheme enzyme family requiring only dissolved oxygen as cosubstrate and no additional cofactors. Organophilic pervaporation was found to be very well suited for continuous in?situ separation of β-ionone. Its application led to a highly pure product despite the complexity of the reaction solution containing cell homogenates. Among three different pervaporation membrane types tested, a polyoctylmethylsiloxane active layer on a porous polyetherimide support led to the best results. A laboratory-scale demonstration plant was set up, and a highly pure aqueous–ethanolic solution of β-ionone was produced from β-carotene. The described process permits generation of high-value flavor and fragrance compounds bearing the desired label “natural” according to US and European food and safety regulations and demonstrates the potential of CCD enzymes for selective oxidative cleavage of carotenoids.     

Measurements of peripheral and deep body temperature in cattle – A review

Automation of the measurement of the physiological and behavioural parameters of livestock has become an important goal for both scientists and farmers. Accurate data and knowledge about farmed animals, especially in cattle breeding, are needed. Proper early diagnosis of a cow's health status in real time allows for preventing the development of infection, oestrus detection and leads to reduced environmental stress. Thus, it contributes to more effective herd management. Among the physiological parameters, body temperature and its fluctuations are key indicators of health and well-being in animals. Currently, along with the development of technical solutions and their implementation, increasingly more attention is being paid to the continuously measurement of body core and peripheral temperature in animals. Recently there has been an increased number of publications devoted to this subject. However, there is a need to systematise this knowledge as these studies have had different purposes, have been performed in various environmental conditions, and the measurements were taken using different methods and equipment. As such, the results obtained by the different authors often may not be comparable. For this reason, this paper has two main purposes: to present the most widely used continuous methods of peripheral and body core temperature measurement, and to show its references values which characterise the individual locations of the cattle body in thermoneutral ambient. An analysis of the professional publications regarding measurements of peripheral and deep body temperature led to the conclusion that these methods have high research and diagnostic potential. However, it is necessary to standardised research to enable better and more comparable results, including among others; different cattle groups, animal age, health and environmental conditions.