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PubMed1983

Reductive activation of fructose-1,6-bisphosphatase and the peroxide effect on chloroplast photosynthesis.

Studies of isolated intact spinach chloroplasts reveal most of the available sulfhydryl groups are associated with stromal protein as opposed to a thylakoid membrane fraction under nondenaturing conditions. Increases in sulfhydryl content of about 45% occurred with illumination and could be correlated with reductive activation of fructose-1,6-bisphosphatase during CO2 assimilation. The omission of catalase inhibited net photosynthesis and caused a lower sulfhydryl number along with diminished fructose-1,6-bisphosphatase activity. Subsequent additions of 0.3 mM NH4Cl accelerated oxygen evolution while both the sulfhydryls and fructose-1,6-bisphosphatase rate were restored to their control values. Peroxide additions extended the initial lag period before the onset of photosynthetic oxygen production and also inhibited the rate if added during continuous illumination. In both cases, fructose-1,6-bisphosphatase activity was similar to that observed in the absence of catalase. The relative sulfhydryl content also remained unchanged. Preincubation of extracts from illuminated chloroplasts in 0.66 mM peroxide did not significantly alter fructose-1,6-bisphosphatase rates whereas CuSO4 drastically inhibited activity and diminished the sulfhydryl number. These results suggest it is unlikely that peroxide alone induces re-formation of disulfide bridges in fructose-1,6-bisphosphatase and causes photosynthetic inhibition. However, the light-activated form of the enzyme is quite sensitive to a mild copper-induced oxidation in vitro and appears to be similarly affected within the chloroplast when both peroxide and the metal catalyst are present.

Catalase↗

PubMed1988

Photosynthesis in phosphoenolpyruvate carboxykinase-type C4 plants: activity and role of mitochondria in bundle sheath cells.

Mitochondria from bundle sheath cells of the phosphoenolpyruvate carboxykinase-type C4 species Urochloa panicoides were shown to have metabolic properties consistent with a role in C4 photosynthesis predicted from earlier studies. The rate of O2 uptake in response to added malate plus ADP was at least five times the activity observed with NADH, glycine, or succinate. With malate plus ADP the O2 uptake rate averaged about 150 nmol O2 min-1 mg-1 protein, equivalent to about 0.6 mumol min-1 mg-1 of extracted chlorophyll. About half of this activity was apparently phosphorylation-linked with ADP/O2 ratios of about 4. Studies with electron transport inhibitors suggested that about 65% of this malate oxidation is cytochrome oxidase-terminated with a minor component mediated via the alternative oxidase. These mitochondria supported rapid rates of pyruvate production from malate and this activity was also stimulated by ADP but blocked by inhibitors of electron transport. Adding oxaloacetate increased pyruvate production but inhibited O2 uptake. The results were consistent with the notion that in this subgroup of C4 species mitochondrial-located NAD malic enzyme contributes substantially to total C4 acid decarboxylation. This enzyme is apparently also the primary source of NADH necessary to generate the ATP required for phosphoenolpyruvate carboxykinase-mediated oxaloacetate decarboxylation.

Adenosine Diphosphate↗

PubMed1991

Regulation of C4 photosynthesis: modulation of mitochondrial NAD-malic enzyme by adenylates.

Effects of adenylates on the activity of mitochondrial NAD-malic enzyme from NAD-malic-enzyme (NAD-ME)-type and phosphoenolpyruvate-carboxykinase-(PKC)-type C4 plants are examined. At physiological concentrations, ATP, ADP, and AMP all inhibit the enzyme from Atriplex spongiosa and Panicum miliaceum (NAD-ME-type plants), with ATP the most inhibitory species. The degree of inhibition is greater with subsaturating levels of activator, malate, and Mn2+. NAD-malic enzyme from Urochloa panicoides (PCK-type) is activated by ATP (up to 10-fold) and inhibited by ADP and AMP. These effects are discussed in relation to regulation of C4 photosynthesis.

Adenine Nucleotides↗

PubMed1976

In vitro preparation and characterization of a 700 nm absorbing chlorophyll-water adduct according to the proposed primary molecular unit in photosynthesis.

1. This study characterized chlorophyll a-H2O adducts in vitro in order to establish their generic relationship to the recently proposed [15, 18-20, 31] primary molecular adduct in photosynthesis. The effects of water titration and temperature on the absorption, fluorescence, excitation, and redox properties of the various in vitro chlorophyll a aggregate species are investigated. 2. From fluorescence measurements, we conclude that the driest chlorophyll a sample contains an equimolar amount of water. This conclusion is consistent with earlier experimental work [2, 3, 14, 17, 31], and clarifies the origin of the controversial [15] Katz model [14] of chlorophyll a-H2O interactions. 3. With increasing water concentration or as the temperature is lowered below room temperature, the A-663 monohydrate chlorophyll a-H2O (species absorbing at 663 nm) is favored at the expense of the A-678 anhydrous aggregate according to the equilibrium 2H2O+chlorophyll a2in equilibrium2 chlorophyll a-H2O. Under excess water conditions, A-663 is converted to A-743 (chlorophyll a-2H2O)n. 4. On slow sample cooling to T less than or approximately 200 degrees K, we observe the growth of A-700 at the expense of A-663. There is a direct correspondence between the increasing (decreasing) absorption by A-700 (A-633) and increasing (decreasing) fluorescence at 720 nm (664 nm). 5. It is concluded that A-700 is most probably the dimer participating in the equilibrium 2 chlorophyll a-H2O in equilibrium (chlorophyll a-H2O)2. The A-700 band consists of two exciton components (separated by approximately 280 cm1) that are interpretable in terms of the dimeric origin of A-700. 6. The deconvoluted A-700 absorption spectrum and the excitation spectrum of the 720 nm fluorescence are compared with the light-minus-dark spectra of P-700. 7. It is found that A-700 is reversibly bleached by I2 (E0 equals 0.54 V). The significance of this observation is discussed in terms of the redox properties of monomeric chlorophyll a and P-700.

Binding Sites↗

PubMed1976

Energy transfer in photosynthesis.

A theoretical model is presented to account for the physical mechanism of energy transfer from antenna molecules to the reaction centers in photosynthesis. The energy transfer is described by a generalized transport equation or "master equation". The solution of this equation for the proposed model gives a relationship between the antennae interaction energy and the transfer rate. The results are shown to be in agreement with inter-antenna transfer rates calculated from experimental fluorescence lifetimes. Previous theories were based either on the Förster mechanism, which is valid for very small interaction energies, or an exciton model valid for very large interactions, but experimental results seemed to indicate that the actual situation was intermediate between these two. The Förster theory and the exciton model are limiting cases of the master equation.

Electron Transport↗

PubMed1978

Electron transfer and spin exchange contributing to the magnetic field dependence of the primary photochemical reaction of bacterial photosynthesis.

The yield phiT of triplet products "PR" generated in reaction centers of Rhodopseudomonas sphaeroides in which the "primary" acceptor is reduced had been found to depend on external magnetic fields. The magnetic field dependence varies, however, between different reaction center preparations. By means of a theoretical description of the primary electron transfer processes and hyperfine coupling-induced electron spin motion the factors influencing the magnetic field behaviour of the triplet products are studied. The following quantities characteristic of the primary electron transfer in photosynthesis have a strong effect on phiT: (1) the rate constants of reversible electron transfer between the initially excited singlet state of the reaction center and an intermediate radical ion pair state; (2) the rate constants of irreversible electron transfer of the radical pair to the ground and excited triplet state of the reaction center; (3) the electron exchange interactions between the radical pair and the "primary" acceptor. From the observed magnetic field dependence of phiT estimates for these quantities are obtained. A temperature dependence of the magnetic field behaviour of phiT and a magnetic field effect on the fluorescence quantum yield of the reaction center are predicted.

Bacteriochlorophylls↗

PubMed1981

Correlation between photosynthesis and the transthylakoid proton gradient.

In isolated intact chloroplasts, maximal rates of photosynthetic O2 evolution (in saturating HCO3) are associated with a critical transthylakoid proton gradient as a result of the stoichiometric consumption of 2 mol NADPH and 3 mol ATP/mol CO2 fixed. Studies with the fluorescent probe 9-aminoacridine reveal that in the illuminated steady state the critical delta pH is 3.9. CO2-dependent O2 evolution is inhibited by increases of 0.1-0.2 in delta pH that occur when catalase is omitted from the medium, NO2- is included as an electron acceptor, or when chloroplasts are illuminated under low partial pressures of O2. Low concentrations of antimycin (0.33 microM) or NH4Cl (0.33 mM)decrease delta pH and relieve this inhibition of electron flow. The energy transfer inhibitor quercetin lowers the high ATP/ADP ratio associated with these conditions, but does not lower delta pH or relieve the inhibition. A decrease of delta pH below 3.9 by weaker illumination, millimolar levels of NH4Cl or micromolar levels of antimycin, results in lower rates of photosynthesis owing to limitation by the phosphorylation rate. These findings show that in absence of rate limitation by the carbon cycle, the extent of thylakoid energization is related to the ratio of ATP to NADPH production and in turn, the rate of CO2 assimilation.

Chloroplasts↗

PubMed1987

Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate.

In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP+ by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, tau L = 0.4 s, which is nearly two orders of magnitude shorter than the corresponding value, tau D = 11 +/- 3 s, for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible light-dark cycles of the NADP+/NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitol-1-phosphate binding to the enzyme active site.

Carbon↗

PubMed1988

Non-cyclic photoreductive carbon fixation in photosynthesis. Light and dark transients of the glycerate-3-P special pair.

It is demonstrated that carbon fixation in photosynthesis is regulated in two kinetically coupled pathways involving the specialized pair of non-equivalent, enzyme-bound glycerate-3-P (3-PGA) molecules obtained from ribulose 1,5-bisphosphate (RuBP) carboxylation in the light. A non-cyclic pathway is suggested (reaction 2) for the direct biosynthesis of sucrose from the 3-PGA obtained from C-3, C-4 and C-5 of the six-carbon carboxylation adduct. Concomitant to the appearance of sucrose as the principal product, the Mg2+-bound 3-PGA molecule formed from C-1, C-2 and C-2' of the C6 intermediate is released and subsequently reduced in regenerating the RuBP. It is proposed that the nocturnal inhibitor, 2-carboxyarabinitol-1-phosphate (1-PCA) is obtained from a condensation of 3-PGA and glyceraldehyde.

Carbon↗

PubMed1991

Chloramphenicol is an inhibitor of photosynthesis.

Chloramphenicol inhibited significantly but incompletely photosynthesis in leaf segments of rice. Fluorescence and polarographic experiments indicated that chloramphenicol competes with the CO2 reducing cycle for electrons from photosystem I because it serves as an electron acceptor of photosystem I and its reduction intermediate transfers its electron to molecular oxygen.

Chloramphenicol↗

PubMed1996

Modelling canopy photosynthesis in response to environmental conditions.

Physiological models in the plant and crop sciences provide a means of integrating different aspects of the system, in particular the interaction between plant processes and environmental factors. This paper focuses on the response of canopy photosynthesis, including adaptation, to environmental conditions. Adaptation is likely to be important when considering controlled ecological life support systems since physiological characteristics are affected by past as well as current environmental conditions. In particular, the level of photosynthetic enzymes in a plant is generally greater for plants grown in high irradiance levels than for similar plants grown in low irradiance. The models have been developed to apply to 'normal' growing conditions, although the principles will apply to closed bioregenerative systems.

Adaptation, Physiological↗

PubMed1992

How does protein phosphorylation regulate photosynthesis?

Phosphorylation of light-harvesting antenna proteins redirects absorbed light energy between reaction centres of photosynthetic membranes. A generally accepted explanation for this is that electrostatic forces drive the more negatively charged, phosphorylated antenna proteins between membrane domains that differ in surface charge. However, structural studies on soluble phosphoproteins indicate that phosphorylated amino acid side chains have specific effects on molecular recognition, by ligand blocking or by intramolecular interactions which alter protein structure. These studies suggest alternative mechanisms for phosphorylation in control of pairwise protein-protein interactions in biological membranes. Thus, in photosynthesis, the surface charge model is only one possible interpretation.

Amino Acid Sequence↗

PubMed1979

Effects of simulated hypogravity on respiration and photosynthesis of higher plants.

Clinostat rotation about a horizontal axis mechanically cancels the directional component of the gravity force vector which is biologically sensed by plants. However, efficiency of clinostats as simulators of weightlessness for prolonged periods has not been demonstrated conclusively. Morphological appearance of plants in orbital flight may resemble their counterparts on earth-based clinostats, but physiological responses may differ. Photosynthesis experiments with algae have been performed in satellites orbiting the earth, but similar experiments with higher plants have not been conducted. This paper describes an experimental apparatus for measurement of photosynthetic and respiratory rates of whole plants on rotating clinostats. Initial experiments show an enhancement of gas exchange during rotation about a horizontal axis.

Asteraceae↗

PubMed2006

Commentary on: "Photosynthesis and negative entropy production" by Jennings and coworkers.

This commentary argues against the view that photochemical energy conversion violates the second law of thermodynamics, as expressed in a recent paper [R.C. Jennings, E. Engelmann, F. Garlaschi, A.P. Casazza, G. Zucchelli. Photosynthesis and negative entropy production. Biochim. Biophys. Acta 1709 (2005) 251-255]. The basic principles of free energy conversion by a photo-electrochemical cell are outlined, emphasizing the fact that the potential depends on the relative population of the excited state and thus on the illumination intensity.

Energy Transfer↗

PubMed2006

Virtual imprinting as a tool to design efficient MIPs for photosynthesis-inhibiting herbicides.

Molecular modelling and computational screening were used to identify functional monomers capable of interacting with several different photosynthesis-inhibiting herbicides. The process involved the design of a virtual library of molecular models of functional monomers containing polymerizable residues and residues able to interact with the template through electrostatic, hydrophobic, Van der Waals forces and dipole-dipole interactions. Each of the entries in the virtual library was probed for its possible interactions with molecular models of the template molecules. It was anticipated that the monomers giving the highest binding score would represent good candidates for the preparation of affinity polymers. Strong interactions were computationally determined between acidic functional monomers like methacrylic acid (MAA) or itaconic acid (IA) with triazines, and between vinylimidazole with bentazone and bromoxynil. Nevertheless, weaker interactions were seen with phenylureas. The corresponding blank polymers were prepared using the selected monomers and tested in the solid phase extraction (SPE) of herbicides from chloroform solutions. A good correlation was found between the binding score of the monomers and the affinities of the corresponding polymers. The use of computationally designed blanks can potentially eliminate the need for molecular imprinting, (adding a template to the monomer mixture to create specific binding sites). Data also showed that some monomers have a natural selectivity for some herbicides, which can be further enhanced by imprinting. Thus, in regard to retention on the blank polymer, we can estimate if the resulting imprinted polymer will be effective or not.

Drug Design↗

PubMed2005

Response of growth, photosynthesis and photoinhibition of the edible cyanobacterium Nostoc sphaeroides colonies to thiobencarb herbicide.

Effects of thiobencarb on growth, photosynthesis and photoinhibition of Nostoc sphaeroides colonies were examined. Thiobencarb concentrations higher than 2 mg/l led to a significant decrease in phycoerythrin, phycocyanin and allophycocyanin content, but no significant effect on chlorophyll a (chl a) content. Biomass yield, protein content and photosynthetic rate were reduced only in the highest thiobencarb concentration (10 mg/l). Colonies exposed to high thiobencarb dose (10 mg/l) were more sensitive to high light than those exposed to the control and 4 mg/l thiobencarb. Dim light induced recovery from photoinhibition to more great extent in colonies grown in the control and 4 mg/l thiobencarb than 10 mg/l thiobencarb. Darkness resulted in a insignificant recovery regardless of thiobencarb treatments.

Bacterial Proteins↗

PubMed2004

Comparing aquatic risk assessment methods for the photosynthesis-inhibiting herbicides metribuzin and metamitron.

Three different risk assessment procedures are described that aim to protect freshwater habitats from risks of the photosynthesis-inhibiting herbicides metribuzin and metamitron. These procedures are (1) the first-tier approach, based on standard toxicity tests and the application of an assessment factor, (2) the Species Sensitivity Distribution (SSD) approach, based on laboratory tests with a wider array of species and the application of a statistical model to calculate the HCx (the Hazardous Concentration for x% of the species), and (3) the model ecosystem approach, based on the evaluation of treatment-related effects in field enclosures. A comparison of the risk assessment procedures reveals that the first-tier approach is the most conservative for metamitron and metribuzin, and that HC5 values (and even HC10 values) based on acute EC50 values of algae and aquatic vascular plants may be used to derive maximum permissible concentrations for single applications. For both compounds these HC5 values were very similar to the ecological threshold concentrations in the enclosure studies. In contrast to model ecosystem experiments, however, HCx values based on lab toxicity tests do not provide information on the recovery potential of sensitive endpoints and on indirect effects, which may be important for regulatory decision-making. In the enclosure study, indirect effects of metribuzin on invertebrate populations were observed at an exposure concentration that was approximately 20 times lower than the corresponding HC5 value based on lab toxicity data for aquatic invertebrates.

Animals↗

PubMed2005

The long-wavelength limit of plant photosynthesis.

It is a common knowledge that the photosynthesis efficiency drops rapidly under the long-wavelength light excitation above 680 nm. We discovered that in sunflower leaves attached to the plant the initial fall is replaced by an unexpected increase at much longer wavelengths, so that a detectable O(2) evolution is remained till 780 nm. The quantum yield of O(2) evolution at the local maximum at 745 nm reaches almost 20% of the yield at 650 nm. We conclude that extreme long-wavelength chlorophylls may be present in the intact photosystem II antenna system, similarly to photosystem I.

Biophysical Phenomena↗