Source of endoproteolytic activity associated with purified ribulose bisphosphate carboxylase

The association of endoproteolytic activity with purified ribulose bisphosphate carboxylase (RuBPCase) from barley (Hordeum vulgare var Numar) leaves was investigated. RuBPCase purified by chromatography on agarose gel and diethylaminoethyl-cellulose was free of associated endoproteolytic activity. The addition of leupeptin and casein to the extraction buffer completely eliminated proteolysis during initial extraction of RuBPCase. Endoproteolytic activity previously found associated with RuBPCase was identified as being due to contamination of endoproteinases from broken vacuoles.     

Storage and maintaining activity of ribulose bisphosphate carboxylase/oxygenase

Purified ribulose-1,5-bisphosphate carboxylase/oxygenase in 50% saturated (NH₄)₂SO₄ was stable when frozen as small beads in liquid nitrogen and stored at −80 C. When stored as a slurry at 4 C most of the activity was lost within four weeks. This loss was due not only to enzyme polymerization. Activity in old preparations purified from spinach leaves, but not tobacco or tomato leaves, can be restored to the level of newly purified enzyme after storage at 4 C by treatment with 50 to 100 millimolar dithiothreitol for several hours followed by dialysis against buffer and 1 millimolar dithiothreitol before CO₂ and Mg²⁺ activation and assay. Some enzyme oligomers that had been formed were not converted back to native enzyme by treatment with 100 millimolar dithiothreitol.     

Light-dependent changes in ribulose bisphosphate carboxylase activase activity in leaves

An assay for the activity of ribulose bisphosphate carboxylase (Rubisco) activase in crude leaf extracts was developed. The assay is based on a spectrophotometric assay of Rubisco, and activase activity (in nanomoles activated Rubisco per minute per milligram chlorophyll) was calculated from the rate of increase in Rubisco activity over time. Activase activity measurements were made using samples from spinach (Spinacia oleracea) leaves undergoing (a) steady-state photosynthesis at various photon flux density (PFD) values and (b) nonsteady-state photosynthesis following an increase from darkness to a high PFD. Analysis of these samples showed that steady-state Rubisco activase activity was relatively low in darkness, increased with PFD, and saturated below 300 micromoles per square meter per second. Rubisco activity (measured spectrophotometrically) was also found to be low in darkness and to increase with PFD, but it saturated at much higher PFD values (approximately 1000 micromoles per square meter per second) along with the rate of photosynthesis. Following an increase in PFD from darkness to 650 micromoles per square meter per second, activase activity increased more or less linearly over a period of 5 to 6 minutes, after which it was constant. Rubisco activity, however, increased more slowly. The light-dependence of Rubisco activase is consistent with previous gas-exchange data showing two interdependent processes in the activation of Rubisco following an increase in PFD.     

Photorespiration-induced reduction of ribulose bisphosphate carboxylase activation level

Leaf photosynthesis and ribulose bisphosphate carboxylase activation level were inhibited in several mutants of the C₃ crucifer Arabidopsis thaliana which possess lesions in the photorespiratory pathway. This inhibition occurred when leaves were illuminated under a photorespiratory atmosphere (50% O₂, 350 microliters per liter CO₂, balance N₂), but not in nonphotorespiratory conditions (2% O₂, 350 microliters per liter CO₂, balance N₂). Inhibition of carboxylase activation level was observed in strains with deficient glycine decarboxylase, serine transhydroxymethylase, serine-glyoxylate aminotransferase, glutamate synthase, and chloroplast dicarboxylate transport activities, but inhibition did not occur in a glycolate-P phosphatase-deficient strain. Also, the photorespiration inhibitor aminoacetonitrile produced a decline in leaf and protoplast ribulose bisphosphate carboxylase activation level, but was without effect on intact chloroplasts. Fructose bisphosphatase, a light-activated enzyme which is strongly dependent on stromal pH and Mg²⁺ for regulation, was unaffected by conditions which caused inhibition of ribulose bisphosphate carboxylase. Thus, the mechanism of inhibition does not appear to involve changes in stromal Mg²⁺ and pH but rather is associated with metabolite flux through the photorespiratory pathway.     

Modification of ribulose bisphosphate carboxylase by 2,3-butadione

D-ribulose-1,5-bisphosphate carboxylases purified from barley or formate-grown $\text{Pseudomonas oxalaticus}$ were inactivated by 2,3-butadione. Pseudo first-order inactivation depended on the presence of borate and was reduced by product 3-phosphoglycerate. The half-times at 30°C and pH 8.3 in the presence of 2 mM 2,3-butadione are 10 and 60 minutes for the enzymes from $\text{P. oxalaticus}$ and barley, respectively. Saturation kinetics and arginine modification were demonstrated for the enzyme from $\text{P. oxalaticus}$.     

Interactions of hydrogen peroxide with ribulose bisphosphate carboxylase oxygenase

Hydrogen peroxide inhibited both carboxylase and oxygenase activities of purified, and fully activated, spinach ribulose-1,5-bisphosphate (RuP2) carboxylase-oxygenase. Inhibition of the carboxylase reaction was mixed competitive with respect to CO2 (Ki = 1.2 mM) and uncompetitive with respect to RuP2. For the oxygenase reaction, H2O2 was a competitive inhibitor with respect to O2 (Ki = 2.1 mM) and an uncompetitive inhibitor with respect to RuP2. H2O2 did not alter the stoichiometry between CO2 and RuP2 in the carboxylase reaction, indicating that H2O2 was not itself a substrate for the enzyme. RuP2 decreased the rate of deactivation of the enzyme which occurred at limiting CO2 concentrations. H2O2 greatly enhanced this stabilizing effect of RuP2 but had no effect on the rate of deactivation in the absence of RuP2. The inhibitory and stabilizing effects of H2O2 varied similarly with H2O2 concentration. These instantaneous, reversible effects of H2O2 were readily distinguishable from an irreversible inhibitory effect which occurred quite slowly, and in the absence of RuP2. These observations are discussed in relation to the enzyme's catalytic mechanism and its activation-deactivation transformations.     

Activation state of ribulose bisphosphate carboxylase in soybean leaves

Conditions for extraction and assay of ribulose-1,5-bisphophate carboxylase present in an in vivo active form (initial activity) and an inactive form able to be activated by Mg²⁺ and CO₂ (total activity) were examined in leaves of soybean, Glycine max (L.) Merr. cv Will. Total activity was highest after extracts had preincubated in NaHCO₃ (5 millimolar saturating) and Mg²⁺ (5 millimolar optimal) for 5 minutes at 25°C or 30 minutes at 0°C before assay. Initial activity was about 70% of total activity. K_act (Mg²⁺) and K_act (CO₂) were approximately 0.3 millimolar and 36 micromolar, respectively. The carry-over of endogenous Mg²⁺ in the leaf extract was sufficient to support considerable catalytic activity. While Mg²⁺ was essential for both activation and catalysis, Mg²⁺ levels greater than 5 millimolar were increasingly inhibitory of catalysis. Similar inhibition by high Mg²⁺ was also observed in filtered, centrifuged, or desalted extracts and partially purified enzyme. Activities did not change upon storage of leaves for up to 4 hours in ice water or liquid nitrogen before homogenization, but were about 20% higher in the latter. Activities were also stable for up to 2 hours in leaf extracts stored at 0°C. Initial activity quickly deactivated at 25°C in the absence of high CO₂. Total activity slowly declined irreversibly upon storage of leaf homogenate at 25°C.     

Formation of a carboxyarabinitol bisphosphate complex with ribulose bisphosphate carboxylase/oxygenase and theoretical specific activity of the enzyme

¹⁴C-Labeled 2-carboxyarabinitol-1,5-bisphosphate was bound to both nonactivated and CO₂and Mg²⁺ activated forms of ribulose bisphosphate carboxylase/oxygenase. The complex could be precipitated with 20% polyethylene glycol and 20 mm MgCl₂ for quantitation of the moles of the affinity label bound per mole of enzyme. The [¹⁴C]carboxyarabinitol-P₂ bound to the nonactivated enzyme could be exchanged with a 100-fold excess of the unlabeled compound. With the activated enzyme the binding of [¹⁴C]carboxyarabinitol-P₂ was so tight that it did not exchange with the unlabeled compound and a binding stoichiometry of one molecule per active site was assumed. This tight binding was dependent upon pretreatment of the enzyme with both CO₂ and MgCl₂ in the same manner that enzyme activation depended on CO₂ and Mg²⁺ concentrations. Various enzyme preparations from spinach leaves tightly bound [¹⁴C]carboxyarabinitol-P₂ in proportion to their specific activities. By extrapolating to a maximum binding of 8 mol of [¹⁴C]carboxyarabinitol-P₂ per mole of this A₈B₈ enzyme a theoretical specific activity of 2.8 μmol · min^?1 · mg protein^?1 was indicated. Enzyme preparations purified from spinach leaves generally have a specific activity in the range of 1.0 to 2.3.     

Is ribulose bisphosphate carboxylase present in guard cell chloroplasts

Evidence for and against the presence of ribulose bisphosphate carboxylase/oxygenase (EC 4.1.1.39) in guard cell chloroplasts is presented. Methods to investigate this question, including immunocytochemistry, are compared.     

Induction and regulation of ribulose bisphosphate carboxylase activity in Rhizobium japonicum during formate-dependent growth

Rhizobium japonicum CJ1 was capable of growing using formate as the sole source of carbon and energy. During aerobic growth on formate a cytoplasmic NAD⁺-dependent formate dehydrogenase and ribulose bisphosphate carboxylase activity was demonstrated in cell-free extracts, but hydrogenase enzyme activity could not be detected. Under microaerobic growth conditions either formate or hydrogen metabolism could separately or together support ribulose bisphosphate carboxylase-dependent CO₂ fixation. A number of R. japonicum strains defective in hydrogen uptake activity were shown to metabolise formate and induce ribulose bisphosphate carboxylase activity. The induction and regulation of ribulose bisphosphate carboxylase is discussed.Abbreviations hup hydrogen uptake - MOPS 3-(N-morpholino)-propanesulphonate - TSA tryptone soya agar - RuBP ribulose 1,5-bisphosphate - FDH formate dehydrogenase     

Purification and degradation of ribulose bisphosphate carboxylase from soybean leaves

A rapid method is described for the preparation of up to 500 milligrams of pure ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase) from 250 grams of field-grown soybean leaves. Leaves were extracted in 20 millimolar phosphate (pH 6.9) at 4°C, containing 4% (w/v) polyvinylpolypyrrolidone, 10 micromolar leupeptin, 1 millimolar phenylmethyl sulfonylfluoride, 1 millimolar diethyldithiocarbamate, 5 millimolar MgCl₂, 1 millimolar dithiothreitol, 0.2 millimolar ethylene-diaminetetraacetic acid, 50 millimolar 2-mercaptoethanol. The extract was incubated in the presence of 5 millimolar ATP at 58°C for 9 minutes, then centrifuged and concentrated. Sucrose gradient centrifugation into 8 to 28% (w/v) sucrose on a vertical rotor for 2.5 hours yielded pure enzyme with a specific activity of 1.1 to 1.3 micromoles per minute per milligram protein at pH 8.0, 25°C. Soybean plants of the same line grown (at 400 microeinsteins per square meter per second) in growth chambers yielded enzyme with a specific activity of 0.6 to 0.7 micromoles per minute per milligram protein. During prolonged purification procedures a proteolytic degradation of RuBP carboxylase caused complete loss of catalytic activity. Without destroying the quaternary structure of the enzyme, a 3 kilodalton peptide was removed from all large subunits before further breakdown (removal of a 5 kilodalton peptide) occurred. Catalytic competence of the enzyme was abolished with the loss of the first (3 kilodalton) peptide.     

Dark/Light modulation of ribulose bisphosphate carboxylase activity in plants from different photosynthetic categories

Ribulose bisphosphate carboxylase/oxygenase (RuBPCase) from several plants had substantially greater activity in extracts from lightexposed leaves than dark leaves, even when the extracts were incubated in vitro with saturating HCO₃⁻ and Mg²⁺ concentrations. This occurred in Glycine max, Lycopersicon esculentum, Nicotiana tabacum, Panicum bisulcatum, and P. hylaeicum (C₃); P. maximum (C₄ phosphoenolpyruvate carboxykinase); P. milioides (C₃/C₄); and Bromelia pinguin and Ananas comosus (Crassulacean acid metabolism). Little or no difference between light and dark leaf extracts of RuBPCase was observed in Triticum aestivum (C₃); P. miliaceum (C₄ NAD malic enzyme); Zea mays and Sorghum bicolor (C₄ NADP malic enzyme); Moricandia arvensis (C₃/C₄); and Hydrilla verticillata (submersed aquatic macrophyte). It is concluded that, in many plants, especially Crassulacean acid metabolism and C₃ species, a large fraction of ribulose-1,5-bisphosphate carboxylase/oxygenase in the dark is in an inactivatable state that cannot respond to CO₂ and Mg²⁺ activation, but which can be converted to an activatable state upon exposure of the leaf to light.     

Prospects for manipulating the substrate specificity of ribulose bisphosphate carboxylase/oxygenase

Pierce, J. 1988. Prospects for manipulating the substrate specificity of ribulose bisphosphate carboxylase/oxygenase. - Physiol. Plant. 72: 690–698.
The idea of enhancing plant productivity by minimizing the apparently wasteful process of photorespiration has been an enduring one. Since the relative fluxes of carbon through the competing pathways of photosynthesis and photorespiration are determined by the kinetic properties of a single enzyme, ribulose bisphosphate carboxylase/oxygenase, it has been conjectured that genetic modification of this protein could provide more productive plants. Recent advances in techniques for studying ribulose bisphosphate carboxylase/oxygenase hold promise for determining whether such modifications will prove possible.     

In vivo photoinactivation of ribulose bisphosphate carboxylase in the gas-vacuolate cyanobacteriumMicrocystis aeruginosa

Irradiation of buoyant, gas-vacuolate cells of the cyanobacteriumMicrocystis aeruginosa by 5·10⁴ Wm^–2 of blue light for 1 h caused a 5% loss of extractable ribulose bisphosphate carboxylase activity compared to dark and red-light controls. Ribulose bisphosphate carboxylase activity was unaffected by blue light in similar experiments conducted with cells containing collapsed gas vacuoles.Abbreviations RuBP Ribulose 1,5-bis-phosphate carboxylase     

Manipulating ribulose bisphosphate carboxylase/oxygenase in the chloroplasts of higher plants

Transgenic manipulation of the photosynthetic CO2-fixing enzyme, ribulose bisphosphate carboxylase/oxygenase (Rubisco) in higher plants provides a very specific means of testing theories about photosynthesis and its regulation. It also encourages prospects for radically improving the efficiencies with which photosynthesis and plants use the basic resources of light, water, and nutrients. Manipulation was once limited to variation of the leaf's total content of Rubisco by transforming the nucleus with antisense genes directed at the small subunit. More recently, technology for transforming the small genome of the plastid of tobacco has enabled much more precise manipulation and replacement of the plastome-encoded large subunit. Engineered changes in Rubisco's properties in vivo are reflected as profound changes in the photosynthetic gas-exchange properties of the leaves and the growth requirements of the plants. Unpredictable expression of plastid transgenes and assembly requirements of some foreign Rubiscos that are not satisfied in higher-plant plastids provide challenges for future research.