Inorganic pyrophosphatase and photosynthesis by isolated chloroplasts. II. The controlling influence of orthophosphate

Transport of inorganic pyrophosphate across the spinach chloroplast envelope

Spinach-leaf chloroplasts take up PPi at a rate of 1.9 mumol/h per mg of chlorophyll (Chl) in the dark and 1.6 mumol/h per mg of Chl in the light. The Km for PPi transport is 32 microM in the dark and 6 microM in the light. Uptake is inhibited by pyridoxal phosphate, 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid and imidodiphosphate, but not by NaF or EDTA. PPi does not appear to cross the chloroplast envelope in exchange for Pi, suggesting that it is not transported by the phosphate translocator. Exchange of PPi and adenine nucleotides across the chloroplast envelope is very slow and PPi does not competitively inhibit ATP uptake, suggesting that little, if any, PPi is transported by the adenine-nucleotide translocator. These results are consistent with the presence of a specific, high-affinity PPi translocator in the spinach chloroplast envelope. It is proposed that in vivo PPi is taken up into the chloroplast from the cytosol to replenish the Pi pool in the stroma.

Soaking in Cs2SO4 reveals a caesium-aromatic interaction in bovine-liver rhodanese

Soaking crystals of rhodanese (thiosulphate:cyanide sulphurtransferase) in 2 M caesium sulphate reveals three caesium binding sites of this enzyme. One of these had been described before as a binding site for sodium ions and is located in a cleft close to the active site. In this site the monovalent cation is coordinated by five oxygen atoms. The first additional binding site seems to be quite special. The caesium ion is bound to the phenyl ring of a tryptophan residue. It is further liganded by two oxygen atoms. The third binding site is a result of crystal packing effects: caesium is liganded by four oxygen atoms, provided by two rhodanese molecules and one sulphate ion. It is likely that the binding of caesium affects the fluorescence of the tryptophan residue with which it interacts. Such possible effects should also be kept in mind when caesium ions are used as a quencher in fluorescence studies of proteins in general.

A mathematical model of the Calvin photosynthesis cycle

1. A mathematical model is presented for photosynthetic carbohydrate formation in C3 plants under conditions of light and carbon dioxide saturation. The model considers reactions of the Calvin cycle with triose phosphate export and starch production as main output processes, and treats concentrations of NADPH, NAD+, CO2, and H+ as fixed parameters of the system. Using equilibrium approximations for all reaction steps close to equilibrium steady-state and transient-state relationships are derived which may be used for calculation of reaction fluxes and concentrations of the 13 carbohydrate cycle intermediates, glucose 6-phosphate, glucose 1-phosphate, ATP, ADP, and inorganic (ortho)phosphate. 2. Predictions of the model were examined with the assumption that photosynthate export from the chloroplast occurs to a medium containing orthophosphate as the only exchangeable metabolite. The results indicate that the Calvin cycle may operate in a single dynamically stable steady state when the external concentration of orthophosphate does not exceed 1.9 mM. At higher concentrations of the external metabolite, the reaction system exhibits overload breakdown; the excessive rate of photosynthate export deprives the system of cycle intermediates such that the cycle activity progressively approaches zero. 3. Reactant concentrations calculated for the stable steady state that may obtain are in satisfactory agreement with those observed experimentally, and the model accounts with surprising accuracy for experimentally observed effects of external orthophosphate on the steady-state cycle activity and rate of starch production. 4. Control analyses are reported which show that most of the non-equilibrium enzymes in the system have a strong regulatory influence on the steady-state level of all of the cycle intermediates. Substrate concentration control coefficients for cycle enzymes may be positive, such that an increase in activity of an enzyme may raise the steady-state concentration of the substrate is consumes. 5. Under optimal external conditions (0.15-0.5 mM orthophosphate), reaction flux in the Calvin cycle is controlled mainly by ATP synthetase and sedoheptulose bisphosphatase; the cycle activity approaches the maximum velocity that can be supported by the latter enzyme. At lower concentrations of external orthophosphate the cycle activity is controlled almost exclusively by the phosphate translocator.(ABSTRACT TRUNCATED AT 400 WORDS)

A rapid-equilibrium model for the control of the Calvin photosynthesis cycle by cytosolic orthophosphate

1. A simple model based on rapid-equilibrium assumptions is derived which relates the steady-state activity of the Calvin cycle for photosynthetic carbohydrate formation in C3 plants to the kinetic properties of a single cycle enzyme (fructose bisphosphatase) and of the phosphate translocator which accounts for the export of photosynthate from the chloroplast. Depending on the kinetic interplay of these two catalysts, the model system may exhibit a single or two distinct modes of steady-state operation, or may be unable to reach a steady state. 2. The predictions of the model are analysed with regard to the effect of external orthophosphate on the steady-state rate of photosynthesis in isolated chloroplasts under conditions of saturating light and CO2. Due to the possible existence of two distinct steady states, the model may account for the stimulatory as well as the inhibitory effects of external phosphate observed in experiments with intact chloroplasts. Stability arguments indicate, however, that only the steady-state case corresponding to phosphate inhibition of the rate of photosynthesis could be of physiological interest. 3. It is concluded that chloroplasts under physiological conditions most likely operate in a high-velocity steady state characterized by a negative Calvin cycle flux control coefficient for the phosphate translocator. This means that any factor enhancing the export capacity of the phosphate translocator can be anticipated to decrease the actual steady-state rate of photosynthate export due to a decreased steady-state rate of cyclic photosynthate production.

The influence of inorganic phosphate on photosynthesis in intact chloroplasts from Mesembryanthemum crystallinum L. plants exhibiting C3 photosynthesis or crassulacean acid metabolism

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C3 or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O2 evolution and CO2 fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75-85% of the (14)CO2 fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of (14)CO2 fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.

Isolation of intact chloroplasts with high CO2 fixation capacity from sugarbeet leaves containing calcium oxalate

Intact chloroplasts were isolated from sugarbeet leaves by the mechanical disruption technique normally used for spinach. The chloroplast pellet contained a ring of white irregularly shaped crystals which were identified as calcium oxalate. The chloroplasts were greater than 90% intact yet good rates of CO2 fixation were only obtained when inorganic pyrophosphate or 3-phosphoglycerate were added to the assay medium. Chloroplasts free of calcium oxalate were prepared by purification on a three step Percoll gradient. These purified chloroplasts were highly intact and showed high rates of CO2 fixation without adding inorganic pyrophosphate or 3-phosphoglycerate. With optimal assay conditions (0.2 mM orthophosphate and pH 8.0) rates of 110-130 μ mole per milligram chlorophyll per hour were routinely obtained. It is concluded that intact chloroplasts capable of high rates of CO2 fixation can be prepared from sugarbeet leaves using a simple three step Percoll gradient.

Isolation of intact chloroplasts with high CO2 fixation capacity from sugarbeet leaves containing calcium oxalate

Intact chloroplasts were isolated from sugarbeet leaves by the mechanical disruption technique normally used for spinach. The chloroplast pellet contained a ring of white irregularly shaped crystals which were identified as calcium oxalate. The chloroplasts were greater than 90% intact yet good rates of CO2 fixation were only obtained when inorganic pyrophosphate or 3-phosphoglycerate were added to the assay medium. Chloroplasts free of calcium oxalate were prepared by purification on a three step Percoll gradient. These purified chloroplasts were highly intact and showed high rates of CO2 fixation without adding inorganic pyrophosphate or 3-phosphoglycerate. With optimal assay conditions (0.2 mM orthophosphate and pH 8.0) rates of 110-130 μ mole per milligram chlorophyll per hour were routinely obtained. It is concluded that intact chloroplasts capable of high rates of CO2 fixation can be prepared from sugarbeet leaves using a simple three step Percoll gradient.

Effects of inorganic phosphate on the photosynthetic carbon reduction cycle in extracts from the stroma of pea chloroplasts

1. Turnover of the photosynthetic carbon reduction cycle has been demonstrated in chlorophyll-free reaction mixtures containing chloroplast stromal extract, as evidenced by the fixation of CO2 following addition of small amounts of 3-phosphoglycerate. 2. The activity of the photosynthetic carbon reduction cycle in this system is inhibited by inorganic phosphate (Pi), with activity reduced to 50% by about 6.5 mM Pi. Pi also increased the lag period which elapsed before a steady rate of CO2 fixation was obtained. 3. The effect of Pi on the rate of 3-phosphoglycerate reduction following the addition of substrate amounts of some cycle intermediates was investigated. Substantial inhibition was observed with fructose 1,6-bisphosphate, sedoheptulose 1,7-bisphosphate and erythrose 4-phosphate as substrates. Pi also affected the activity of ribulose-bisphosphate carboxylase, with stimulation at Pi concentrations below 2.5 mM and inhibition at higher concentrations. 4. The results showed that the sedoheptulose bisphosphatase reaction is inhibited more strongly by Pi than the fructose bisphosphatase reaction. 5. It is concluded that the previously established inhibitory effects of Pi on photosynthesis by intact isolated chloroplasts may be partly due to these inhibitory effects of Pi on the reactions of the photosynthetic carbon reduction cycle.

Effect of pH on chloroplast photosynthesis. Inhibition of O2 evolution by inorganic phosphate and magnesium

1. The pH optimum of CO2-dependent O2 evolution by barley (Hordeum vulgare L.) chloroplasts was found to be between 7.8 and 8.2. The addition of 1 mM MgCl2 in the dark inhibited O2 evolution over the entire pH range tested and resulted in a much sharper pH profile centered around pH 8.2. 2. The pH optimum for O2 evolution, in the presence and absence of 1 mM MgCl2, was acid-shifted 0.3--0.4 pH units by 2 mM NH4Cl. The pH optimum of O2 evolution, with and without 1 mM MgCl2, was base-shifted by 2 mM sodium acetate, approx. 0.5 pH units relative to the controls. 3. O2 evolution in the presence of bicarbonate plus 3-phosphoglycerate or ribose-5-phosphate was considerably less sensitive to pH than CO2-dependent O2 evolution in the absence of substrate. With these substrates, both in the presence and absence of 1 mM MgCl2, the pH optimum was broad and was centered around pH 7.8. 4. Inhibition of CO2-dependent O2 evolution by inorganic phosphate and magnesium increased as the pH of the reaction mixture was decreased below the optimum. Decreasing the pH from 8.2 to 7.6, reduced over 3-fold the concentration of inorganic phosphate required to inhibit O2 evolution completely. For magnesium, a similar change in pH reduced the concentration required to inhibit O2 evolution 50% approx. 5-fold. At pH 8.2, magnesium inhibition required inorganic phosphate. Magnesium was not required for inhibition of O2 evolution by inorganic phosphate, but incresaed the relative inhibition observed. 5. Illumination of intact barley chloroplasts increased the activity of NADP-glyceraldehyde-3-P dehydrogenase, phosphoribulokinase and fructose-1,6-diphosphatase. MgCl2 and inorganic phosphate prevented this increase in enzyme activity at concentrations that completely inhibited CO2-dependent O2 evolution. 6. The results obtained suggest that magnesium inhibition of O2 evolution may be caused by enhanced phosphate exchange across the chloroplast envelope.

Light-dependent changes of the Mg2+ concentration in the stroma in relation to the Mg2+ dependency of CO2 fixation in intact chloroplasts

(1) Light-dependent changes of the Mg2+ content of thylakoid membranes were measured at pH 8.0 and compared with earlier measurements at pH 6.6. In a NaCl and KCl medium, the light-dependent decrease in the Mg2+ content of the thylakoid membranes at pH 8.0 is found to be 23 nmol Mg2+ per mg chlorophyll, whereas in a sorbitol medium it is 83 nmol Mg2+ per mg chlorophyll. (2) A light dependent increase in the Mg2+ content of the stroma was detected wjem chloroplasts were subjected to osmotic shock, amounting to 26 nmol/mg chlorophyll. Furthermore, a rapid and reversible light-dependent efflux of Mg2+ has been observed in intact chloroplasts when the divalent cation ionophore A 23 187 was added, indicating a light-dependent transfer of about 60 nmol of Mg2+ per mg chlorophyll from the thylakoid membranes to the stroma. (3) CO2 fixation, but not phosphoglycerate reduction, could be completely inhibited when A 23 187 was added to intact chloroplasts in the absence of external Mg2+. If Mg2+ was then added to the medium, CO2 fixation was restored. Half of the maximal restoration was achieved with about 0.2 mM Mg2+, which is calculated to reflect a Mg2+ concentration in the stroma of 1.2 mM. The further addition of Ca2+ strongly inhibits CO2 fixation. (4) The results suggest that illumination of intact chloroplasts causes an increase in the Mg2+ concentration of 1-3 mM in the stroma. Compared to the total Mg2+ content of chloroplasts, this increase is very low, but it appears to be high enough to have a possible function in the light regulation of CO2 fixation.

Availability of monovalent and divalent cations within intact chloroplasts for the action of ionophores nigericin and A23187

1. A23187 will uncouple electron transport by broken chloroplasts in a divalent cation dependent manner provided that they have been treated with a low concentration of EDTA. 2. A23187 stimulates oxaloacetate-dependent oxygen evolution and inhibits phosphoglycerate reduction by intact chloroplasts isolated in a cation-free medium whereas the full effect of nigericin was dependent on the presence of external K+. 3. Uncoupling of oxaloacetate reduction by A23187 in intact chloroplasts is inhibited by EDTA and this effect is overcome by excess Mg2+. 4. The results suggest that divalent and not monovalent cations are available for collapsing the light-induced H+ gradient within the intact organelle.