Metabolite levels in the stroma of spinach chloroplasts exposed to osmotic stress: Effects of the pH of the medium and exogenous dihydroxyacetone phosphate

The levels of stromal photosynthetic intermediates were measured in isolated intact spinach (Spinacia oleracea L.) chloroplasts exposed to reduced osmotic potentials. Stressed chloroplasts showed slower rates of metabolite accumulation upon illumination than controls. Relative to other metabolites sedoheptulose-1,7-bisphosphate (SBP) and fructose-1,6-bisphosphate (FBP) accumulated in the stroma in the stressed treatments. Under these conditions 3-phosphoglycerate (3-PGA) efflux to the medium was restricted. Chloroplasts previously incubated with [³²P]KH₂PO₄ and [³²P]dihydroxyacetone phosphate ([³²P]DAP) in the dark were characterized by very high FBP and SBP levels prior to illumination. Metabolism of these pools upon illumination increased with increasing pH of the medium but was consistently inhibited in osmotically stressed chloroplasts. The responses of stromal FBP and SBP pools under hypertonic conditions are discussed in terms of both inhibited light activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) and sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37), and likely increases in stromal ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) active-site concentrations.Abbreviations and symbols DAP dihydroxyacetone phosphate - FBP fructose-1,6-bisphosphate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate - SBP sedoheptulose-1,7-bisphosphate - _s osmotic potential     

Fructose-1,6-bisphosphate as a metabolic substrate in hog ileum smooth muscle during hypoxial

Exogenously applied fructose-1,6-bisphosphate has been reported to be effective in preventing some damage to the small intestine during ischemia. To determine whether exogenously applied fructose-1,6-bisphosphate protects ileum smooth muscle from damage from hypoxia and from reoxygenation, we examined the effect of fructose-1,6-bisphosphate on the ability of hog ileum smooth muscle to maintain isometric force during hypoxia and to generate isometric force after reoxygenation in the presence of 5 mM glucose. After 180 min of hypoxia, tissues incubated with 20 mM fructose-1,6-bisphosphate maintained significantly greater levels of isometric force than tissues incubated in the absence of exogenous substrate (23% of pre-hypoxia force compared to 16%). During the first contraction following reoxygenation there was a significantly greater force generation in tissues incubated with 20 mM fructose-1,6-bisphosphate during the hypoxia period compared to tissues with no exogenous substrate included during the hypoxia period (29% of pre-hypoxia force compared to 19%). However, glucose always was a better metabolic substrate compared to fructose-1,6-bisphosphate under all experimental conditions. The presence of fructose-1,6-bisphosphate during hypoxia likely improved tissue function by fructose-1,6-bisphosphate entering the cells and acting as a glycolytic intermediate, since during a 120 min period of hypoxia, unmounted ileum smooth muscle metabolized 1,6-¹³C-fructose-1,6-bisphosphate to 3-¹³C-lactate. This conversion of 1,6-¹³C-fructose-1,6-bisphosphate to 3-¹³C-lactate was inhibited by the addition of 1 mM iodoacetic acid, a glycolytic inhibitor. We conclude that exogenously provided fructose-1,6-bisphosphate does provide modest protection of ileum smooth muscle from hypoxic damage by functioning as a glycolytic intermediate and improving the cellular energy state.This work was supported in part by NIH (HL48783 to CDH), NSF (Instrumentation Grant 8908304), and the Department of Physiology of the University of Missouri. T. Juergens was supported by the School of Medicine and the Department of Physiology of the University of Missouri.     

A radioassay for phosphofructokinase-1 activity in cell extracts and purified enzyme

Phosphofructokinase-1 plays a key role in the regulation of carbohydrate metabolism. Its activity can be used as an indicator of the glycolytic flux in a tissue sample. The method most commonly employed to determine phosphofructokinase-1 activity is based on oxidation of NADH by the use of aldolase, triosephosphate isomerase, and alpha-glycerophosphate dehydrogenase. This method suffers from several disadvantages, including interactions of the auxiliary enzymes with phosphofructokinase-1. Other methods that have been used also require auxiliary enzymes or are less sensitive than a coupled assay. Here, we propose a direct method to determine phosphofructokinase-1 activity, without the use of auxiliary enzymes. This method employs fructose-6-phosphate and ATP labeled with 32P in the gamma position ([gamma-32P]ATP), and leads to the formation of ADP and fructose-1,6-bisphosphate labeled with 32P ([1-32P]fructose-1,6-bisphosphate). Activated charcoal is used to adsorb unreacted [gamma-32P]ATP, and the radioactive product in the supernatant, [1-32P]fructose-1,6-bisphosphate, is analyzed on a liquid scintillation counter. The proposed method is precise and relatively inexpensive, and can be applied to determine phosphofructokinase-1 activity in cellular extracts as well as in the purified enzyme.     

High level expression in Escherichia coli,purification and properties of chloroplast fructose-1,6-bisphosphatase from rapeseed (Brassica napus) leaves

In chloroplasts, the light-modulated fructose-1,6-bisphosphatase catalyzes the formation of fructose 6-bisphosphate for the photosynthetic assimilation of CO₂ and the biosynthesis of starch. We report here the construction of a plasmid for the production of chloroplast fructose-1,6-bisphosphatase in a bacterial system and the subsequent purification to homogeneity of the genetically engineered enzyme. To this end, a DNA sequence that coded for chloroplast fructose-1,6-bisphosphatase of rapeseed (Brassica napus) leaves was successively amplified by PCR, ligated into the Ndel/EcoRI restriction site of the expression vector pET22b, and introduced into Escherichia coli cells. When gene expression was induced by isopropyl--d-thiogalactopyranoside, supernatants of cell lysates were extremely active in the hydrolysis of fructose 1,6-bisphosphate. Partitioning bacterial soluble proteins by ammonium sulfate followed by anion exchange chromatography yielded 10 mg of homogeneous enzyme per 1 of culture. Congruent with a preparation devoid of contaminating proteins, the Edman degradation evinced an unique N-terminal amino acid sequence [A-V-A-A-D-A-T-A-E-T-K-P-]. Gel filtration experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the (recombinant) rapeseed chloroplast fructose-1,6-bisphosphatases was a tetramer [160 kDa] comprised of four identical subunits. Like other chloroplast fructose-1,6-bisphosphatases, the recombinant enzyme was inactive at 1 mM fructose 1,6-bisphosphate and 1 mM Mg²⁺ but became fully active after an incubation in the presence of either 10 mM dithiothreitol or 1 mM dithiothreitol and chloroplast thioredoxin. However, at variance with counterparts isolated from higher plant leaves, the low activity observed in absence of reductants was not greatly enhanced by high concentrations of fructose 1,6-bisphosphate (3 mM) and Mg²⁺ (10 mM). In the catalytic process, all chloroplast fructose-1,6-bisphosphatases had identical features; viz., the requirement of Mg²⁺ as cofactor and the inhibition by Ca²⁺. Thus, the procedure described here should prove useful for the structural and kinetic analysis of rapeseed chloroplast fructose-1,6-bisphosphatase in view that this enzyme was not isolated from leaves.Abbreviation DTT dithiothreitol - PCR polymerase chain reaction - EDTA (ethylenedinitrilo)tetraacetic     

Enzymatic preparation of high-specific-activity β-d-[6,6′-H]fructose-2,6-bisphosphate: Application to a sensitive assay for fructose-2,6-bisphosphatase

β-d-Fructose-2,6-bisphosphate (Fru-2,6-P₂) is an important regulator of eukaryotic glucose homeostasis, functioning as a potent activator of 6-phosphofructo-1-kinase and inhibitor of fructose-1,6-bisphosphatase. Pharmaceutical manipulation of intracellular Fru-2,6-P₂ levels, therefore, is of interest for the treatment of certain diseases, including diabetes and cancer. [2-³²P]Fru-2,6-P₂ has been the reagent of choice for studying the metabolism of this effector molecule; however, its short half-life necessitates frequent preparation. Here we describe a convenient, economical, one-pot enzymatic preparation of high-specific-activity tritium-labeled Fru-2,6-P₂. The preparation involves conversion of readily available, carrier-free d-[6,6′-³H]glucose to [6,6′-³H]Fru-2,6-P₂ using hexokinase, glucose-6-phosphate isomerase, and 6-phosphofructo-2-kinase. The key reagent in this preparation, bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from human liver, was produced recombinantly in Escherichia coli and purified in a single step using an appendant C-terminal hexa-His affinity tag. Following purification by anion exchange chromatography using triethylammonium bicarbonate as eluant, radiochemically pure [6,6′-³H]Fru-2,6-P₂ having a specific activity of 50 Ci/mmol was obtained in yields averaging 35%. [6,6′-³H]Fru-2,6-P₂ serves as a stable, high-specific-activity substrate in a facile assay capable of detecting fructose-2,6-bisphosphatase in the range of 10⁻¹⁴ to 10⁻¹⁵ mol, and it should prove to be useful in many studies of the metabolism of this important biofactor.     

Inhibition of fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate

Rat liver fructose-1,6-bisphosphatase, which was assayed by measuring the release of 32P from fructose 1,6-[1-32P]bisphosphate at pH 7.5, exhibited hyperbolic kinetics with regard to its substrate. beta-D-Fructose 2,6-bisphosphate, an activator of hepatic phosphofructokinase, was found to be a potent inhibitor of the enzyme. The inhibition was competitive in nature and the Ki was estimated to be 0.5 microM. The Hill coefficient for the reaction was 1.0 in the presence and absence of fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate also enhanced inhibition of the enzyme by the allosteric inhibitor AMP. The possible role of fructose 2,6-bisphosphate in the regulation of substrate cycling at the fructose-1,6-bisphosphatase step is discussed.     

Carbon-13 as a tool for the study of carbohydrate structures,conformations and interactions

The application of ¹³C-NMR spectroscopy to problems involving the structures and interactions of carbohydrates is described. Both ¹³C-enriched and natural abundance compounds were used and some advantages of the use of the stable isotope are described. Carbon-carbon and carbon-proton coupling constants obtained from 1-¹³ C enriched carbohydrates were employed in the assignment of their chemical shifts and to establish solution conformation. In all cases studied thus far, C-3 couples to C-1 only in the β-anomers while C-5 couples to C-1 only in the α-anomers. C-6 and C-2 always couple to C-1 in both anomeric species. The alkaline degradation of glucose [1-¹³ C] to saccharinic acids was followed by ¹³C-NMR. The conversion of glucose [1-¹³ C] to fructose-1,6-bisphosphate [1,6-¹³ C] by enzymes of the glycolytic pathway was shown as an example of the use of ¹³C-enriched carbohydrates to elucidate biochemical pathways. In a large number of glycosyl phosphates the ³¹P to H-1 and ³¹P to C-2 coupling constants demonstrate that in the preferred conformation the phosphate group lies between the O-5 and the H-1 of the pyranose ring. The influence of paramagnetic Mn²⁺ ions on the proton decoupled ¹³C-NMR spectra of uridine diphosphate N-acetylglucosamine indicates that the Mn²⁺ interacts strongly with the pyrophosphate moiety and with the carbonyl groups of the uracil and N-acetyl groups.     

Control of CO2 fixation regulation of stromal fructose-1,6-bisphosphatase in spinach by pH and Mg2+ concentration

The effect of pH and of Mg²⁺ concentration on the light activated form of stromal fructose-1,6-bisphosphatase (FBPase) was studied using the enzyme rapidly extracted from illuminated spinach chloroplasts. The (fructose-1,6-bisphosphate^4-)(Mg²⁺) complex has been identified as the substrate of the enzyme. Therefore, changes of pH and Mg²⁺ concentrations have an immediate effect on the activity of FBPase by shifting the pH and Mg²⁺ dependent equilibrium concentration of the substrate. In addition, changes of pH and Mg²⁺ concentration in the assay medium have a delayed effect on FBPase activity. A correlation of the activities observed using different pH and Mg²⁺ concentrations indicates, that the effect is not a consequence of the pH and Mg²⁺ concentration as such, but is caused by a shift in the equilibrium concentration of a hypothetical inhibitor fructose-1,6-bisphosphate^3- (uncomplexed), resulting in a change of the activation state of the enzyme. The interplay between a rapid effect on the concentration of the substrate and a delayed effect on the activation state enables a rigid control of stromal FBPase by stromal Mg²⁺ concentrations and pH. Fructose-1,6-bisphosphatase is allosterically inhibited by fructose-6-phosphate in a sigmoidal fashion, allowing a fine control of the enzyme by its product.Abbreviations Fru1,6 bis P fructose-1,6-bisphosphate - Fru6P fructose-6-phosphate - FBPase fructose-1,6-bisphosphatase Some of these results have been included in a preliminary report (Heldt et al. 1984)     

Identification of a phosphoenolpyruvate:fructose phosphotransferase system (fructose-1-phosphate forming) in Listeria monocytogenes.

Listeria monocytogenes is a gram-positive bacterium whose carbohydrate metabolic pathways are poorly understood. We provide evidence for an inducible phosphoenolpyruvate (PEP):fructose phosphotransferase system (PTS) in this pathogen. The system consists of enzyme I, HPr, and a fructose-specific enzyme II complex which generates fructose-1-phosphate as the cytoplasmic product of the PTS-catalyzed vectorial phosphorylation reaction. Fructose-1-phosphate kinase then converts the product of the PTS reaction to fructose-1,6-bisphosphate. HPr was shown to be phosphorylated by [32P]PEP and enzyme I as well as by [32P]ATP and a fructose-1,6-bisphosphate-activated HPr kinase like those found in other gram-positive bacteria. Enzyme I, HPr, and the enzyme II complex of the Listeria PTS exhibit enzymatic cross-reactivity with PTS enzyme constituents from Bacillus subtilis and Staphylococcus aureus.     

In vitro phosphorylation of fructose-1,6-bisphosphatase from rabbit and pig liver with cyclic AMP-dependent protein kinase

Homogeneous preparations of fructose-1,6-bisphosphatase from mouse, man, rabbit, pig, and rat were tested as substrates for cyclic AMP-dependent protein kinase. Up to 1 mol of [32P]phosphate per mole enzyme subunit was incorporated into fructose-1,6-bisphosphatase from pig and rabbit liver, which should be compared with 2.6 mol of phosphate per mole enzyme subunit in the case of the rat liver enzyme. The phosphorylation of fructose-1,6-bisphosphatase from the livers of man and mouse was negligible. Phosphorylation of pig and rabbit fructose-1,6-bisphosphatase decreased the apparent Km for fructose-1,6-bisphosphate, but in contrast to the case of the rat liver enzyme it did not change the inhibition constants for AMP and fructose-2,6-bisphosphate. The phosphorylation sites in rabbit and pig liver fructose-1,6-bisphosphatase were located close to the carboxyterminal of the polypeptide chains, since trypsin treatment of the phosphorylated enzyme quantitatively removed all of the protein-bound radioactivity without significantly altering the subunit molecular weight and with a maintained neutral pH optimum.     

Ion-Exchange Chromatography Separates Activities Synthesizing and Degrading Fructose 2,6-Bisphosphate from C3 and C4 Leaves but Not from Rat Liver

Fructose-6-phosphate,2-kinase and fructose-2,6-bisphosphatase were separated on the basis of charge from leaves of C₃ (spinach, lettuce, and pea) and C₄ (sorghum and amaranthus) plants but not from rat liver—a tissue known to contain a bifunctional enzyme with both activities. [2-³²P]Fructose 2,6-bisphosphate binding experiments also suggest that the major forms of these activities reside on different proteins in leaves.     

Carbohydrate oxidation by leucoplasts from suspension cultures of soybean (Glycine max L.)

The aim of this work was to discover how leucoplasts from suspension cultures of soybean (Glycine max L.) oxidize hexose monophosphates. Leucoplasts were isolated from protoplast lysates on a continuous gradient of Nycodenz with a yield of 28% and an intactness of 80%. Incubation of the leucoplasts with ¹⁴C-labelled substrates led to ¹⁴CO₂ production, that was dependent upon leucoplast intactness, from [U-¹⁴C]glucose 6-phosphate, [U-¹⁴C]glucose 1-phosphate, [U-¹⁴C] fructose 6-phosphate and [U-¹⁴C]glucose+ATP, but not from [U-¹⁴C]fructose-1,6-bisphosphate or [U-¹⁴C]triose phosphate. The yield from [U-¹⁴C]glucose 6-phosphate was at least four times greater than that from any of the other substrates. When [1-¹⁴C]-, [2-¹⁴C]-, [3,4-¹⁴C]-, and [6-¹⁴C]glucose 6-phosphate were supplied to leucoplasts significant ¹⁴CO₂ production that was dependent upon leucoplast intactness was found only for [1-¹⁴C]glucose 6-phosphate. It is argued that soybean cell leucoplasts oxidize glucose 6-phosphate via the oxidative pentose phosphate pathway with very little recycling, and that in these plastids glycolysis to acetyl CoA is negligible.S.A.C. thanks the Science and Engineering Research Council for a research studentship.     

Metabolic Fluxes in Corynebacterium glutamicum during Lysine Production with Sucrose as Carbon Source

Metabolic fluxes in the central metabolism were determined for lysine-producing Corynebacterium glutamicum ATCC 21526 with sucrose as a carbon source, providing an insight into molasses-based industrial production processes with this organism. For this purpose, ¹³C metabolic flux analysis with parallel studies on [1-¹³C^Fru]sucrose, [1-¹³C^Glc]sucrose, and [¹³C₆^Fru]sucrose was carried out. C. glutamicum directed 27.4% of sucrose toward extracellular lysine. The strain exhibited a relatively high flux of 55.7% (normalized to an uptake flux of hexose units of 100%) through the pentose phosphate pathway (PPP). The glucose monomer of sucrose was completely channeled into the PPP. After transient efflux, the fructose residue was mainly taken up by the fructose-specific phosphotransferase system (PTS) and entered glycolysis at the level of fructose-1,6-bisphosphate. Glucose-6-phosphate isomerase operated in the gluconeogenetic direction from fructose-6-phosphate to glucose-6-phosphate and supplied additional carbon (7.2%) from the fructose part of the substrate toward the PPP. This involved supply of fructose-6-phosphate from the fructose part of sucrose either by PTS^Man or by fructose-1,6-bisphosphatase. C. glutamicum further exhibited a high tricarboxylic acid (TCA) cycle flux of 78.2%. Isocitrate dehydrogenase therefore significantly contributed to the total NADPH supply of 190%. The demands for lysine (110%) and anabolism (32%) were lower than the supply, resulting in an apparent NADPH excess. The high TCA cycle flux and the significant secretion of dihydroxyacetone and glycerol display interesting targets to be approached by genetic engineers for optimization of the strain investigated.     

Degradation of 2-carboxyarabinitol 1-phosphate by a specific chloroplast phosphatase

The catalytic degradation of 2-carboxyarabinitol 1-phosphate (CA 1-P), a naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), was investigated by chromatographic and spectroscopic analyses of the reaction products. Carboxy-labeled [¹⁴C]CA 1-P was incubated with a partially purified tobacco (Nicotiana rustica) chloroplast protein that has been shown previously to catalyze metabolism of CA 1-P to a form incapable of inhibiting Rubisco (ME Salvucci, GP Holbrook, JC Anderson, and G Bowes [1988] FEBS Lett 231: 197-201). In the presence and absence of NADPH, ion-exchange chromatography showed a progressive conversion of [2′-¹⁴C]CA 1-P to a labeled compound which coeluted with authentic carboxyarabinitol. Parallel assays with unlabeled CA 1-P showed a concomitant decrease in the ability of reaction samples to inhibit Rubisco activity. In separate experiments, a 1:1 stoichiometry was found between the release of inorganic phosphate from [2′-¹⁴C]CA 1-P and accumulation of the ¹⁴C-labeled product. Liberation of inorganic phosphate was not observed when the tobacco enzyme was incubated with ribulose-1,5-bisphosphate, fructose-1,6-bisphosphate, glucose-1-phosphate, glucose-6-phosphate, or 6-phosphogluconate. Proton nuclear magnetic resonance spectroscopy of the labeled CA 1-P reaction product established its identity as carboxyarabinitol. We therefore propose that light-stimulated degradation of CA 1-P is catalyzed in vivo by a specific phosphatase, 2-carboxyarabinitol 1-phosphatase. Carboxyarabinitol 1-phosphatase activity was detected in the absence of NADPH, but increased threefold when 2 millimolar NADPH was present. Thus, while not required for the reaction, NADPH may play an important role in the regulation of CA 1-P degradation.     

A radiotracer enzyme assay for phosphofructokinase using ( , , -32P) adenosine triphosphate

A radiotracer enzyme assay for phosphofructokinase using adenosine 5′-triphosphate[α,β,γ-³²P] is described in this paper. Here the rates of appearance of both [1-³²P]d-fructose 1,6-diphosphate and [α,β-³²P] adenosine 5′-diphosphate were followed to establish enzyme activity. The unique advantages of multiple rate determinations in a single reaction sequence which accrue from the use of a readily available multiply labeled cosubstrate are discussed. By an extension of this approach other labeled(¹) nucleotides of the type, N(¹P)_n, and enzymes in the Enzyme Commision categories, EC 2.7(phosphotransferases) and EC 6.1–6.4(ligases) are equally amenable to radionuclide assay.     

Conditions Required for the Rapid Activation In Vitro of the Chloroplast Fructose-1,6-bisphosphatase

Conditions required for the reductive activation of purified, spinach chloroplast fructose-1,6-bisphosphatase (EC 3.1.3.11) have been determined in vitro. Full reductive activation was observed only when fructose-1,6-bisphosphate and Mg²⁺ were present at the same time as the reducing agent (dithiothreitol). Reduction in the absence either of fructose-1,6-bisphosphate or of Mg²⁺ slowly and irreversibly inactivated the enzyme. The concentration of fructose-1,6-bisphosphate that must be present during reduction for maximum activation depends upon the divalent cation present: it is highest with Mg²⁺, lower with Ca²⁺, and lowest when both Mg²⁺ and Ca²⁺ are present. A scheme for the reductive activation and inactivation of the enzyme is presented.     

Identification of the 64 kilodalton chloroplast stromal phosphoprotein as phosphoglucomutase

Phosphorylation of the 64 kilodalton stromal phosphoprotein by incubation of pea (Pisum sativum) chloroplast extracts with [γ-³²P]ATP decreased in the presence of Glc-6-P and Glc-1,6-P₂, but was stimulated by glucose. Two-dimensional gel electrophoresis following incubation of intact chloroplasts and stromal extracts with [γ-³²P]ATP, or incubation of stromal extracts and partially purified phosphoglucomutase (EC 2.7.5.1) with [³²P]Glc-1-P showed that the identical 64 kilodalton polypeptide was labeled. A 62 kilodalton polypeptide was phosphorylated by incubation of tobacco (Nicotiana sylvestris) stromal extracts with either [γ-³²P]ATP or [³²P]Glc-1-P. In contrast, an analogous polypeptide was not phosphorylated in extracts from a tobacco mutant deficient in plastid phosphoglucomutase activity. The results indicate that the 64 (or 62) kilodalton chloroplast stromal phosphoprotein is phosphoglucomutase.     

Inositol-containing lipids in suspension-cultured plant cells: an isotopic study

Polar lipids were extracted from suspension-cultured tomato (Lycopersicon esculentum Mill.) cells and analyzed by thin layer chromatography. Four major inositol-containing compounds were found, and incorporation of [³²P]orthosphosphate, [2-³H]glycerol, and myo-[2-³H]inositol was studied. Results showed that phosphatidylinositol-monophosphate is the phospholipid in these cells displaying the most rapid incorporation of [³²P]orthophosphate. We suggest that the tracer is incorporated primarily into the phosphomonoester group. Two inositol-containing lipids showed chromatographic behavior similar to phosphatidylinositol-4,5-bisphosphate when using standard thin layer chromatography techniques. The labeling pattern of these compounds, however, reveals that it is unlikely that either of these is identical to phosphatidylinositol-4,5-bisphosphate. Should phosphatidylinositol-bisphosphate be present in suspension cultured plant cells, our data indicate chemical abundancies substantially lower than previously reported.     

HETEROLOGOUS EXPRESSION OF Escherichia coli FRUCTOSE-1,6-BISPHOSPHATASE IN Corynebacterium glutamicum AND EVALUATING THE EFFECT ON CELL GROWTH AND L-LYSINE PRODUCTION

Fructose-1,6-bisphosphatase (FBPase), which is mainly used to supply NADPH, has an important role in increasing L-lysine production by Corynebacterium glutamicum. However, C. glutamicum FBPase is negatively regulated at the metabolic level. Strains that overexpressed Escherichia coli fructose-1,6-bisphosphatase in C. glutamicum were constructed, and the effects of heterologous FBPase on cell growth and L-lysine production during growth on glucose, fructose, and sucrose were evaluated. The heterologous fructose-1,6-bisphosphatase is insensitive to fructose 1-phosphate and fructose 2,6-bisphosphate, whereas the homologous fructose-1,6-bisphosphatase is inhibited by fructose 1-phosphate and fructose 2,6-bisphosphate. The relative enzyme activity of heterologous fructose-1,6-bisphosphatase is 90.8% and 89.1% during supplement with 3 mM fructose 1-phosphate and fructose 2,6-bisphosphate, respectively. Phosphoenolpyruvate is an activator of heterologous fructose-1,6-bisphosphatase, whereas the homologous fructose-1,6-bisphosphatase is very sensitive to phosphoenolpyruvate. Overexpression of the heterologous fbp in wild-type C. glutamicum has no effect on L-lysine production, but fructose-1,6-bisphosphatase activities are increased 9- to 13-fold. Overexpression of the heterologous fructose-1,6-bisphosphatase increases L-lysine production in C. glutamicum lysC ^T311I by 57.3% on fructose, 48.7% on sucrose, and 43% on glucose. The dry cell weight (DCW) and maximal specific growth rate (μ) are increased by overexpression of heterologous fbp. A “funnel-cask” diagram is first proposed to explain the synergy between precursors supply and NADPH supply. These results lay a definite theoretical foundation for breeding high L-lysine producers via molecular target.     

Short-term water stress leads to a stimulation of sucrose synthesis by activating sucrose-phosphate synthase

The aim of this work was to identify which aspects of photosynthetic metabolism respond most sensitively to leaf water deficit. Spinach (Spinacia oleracea L.) leaf discs were floated on sorbitol concentrations of increasing molarity and changes of the protoplast volume were estimated using [¹⁴C]sorbitol and ³H₂O penetration. Detached leaves were also wilted until 10% of their fresh weight was lost. Photosynthesis was studied at very high external CO₂ concentrations, to eliminate the effect of closing stomata. There was no large inhibition of CO₂ fixation after wilting leaves, or until the external water deficit was greater than-1.2 MPa. However, partitioning changed markedly at these moderate water deficits: more sucrose and less starch was made. When an inhibition of CO₂-saturated photosynthesis did appear at a water deficit of-2.0 MPa and above, measurements of chlorophyll-fluorescence quenching and metabolite levels showed the thylakoid reactions were not especially susceptible to short-term water stress. The inhibition was accompanied by a small increase of the triose phosphate: ribulose-1,5-bisphosphate ratio, showing regeneration of ribulose-1,5-bisphosphate was affected. However, there was also a general increase of the estimated concentrations of most metabolites, indicating that there is no specific site for the inhibition of photosynthesis. Increasing water deficit led to a large increase of fructose-2,6-bisphosphate. This is explained in terms of a simultaneous increase of fructose-6-phosphate and inorganic phosphate as the cell shrinks. The high fructose-2,6-bisphosphate led to the accumulation of triose phosphates, and the potential significance of this for protection against photoinhibition is discussed. There was an increase in the extractable activity of sucrose-phosphate synthase. This was only detected when the enzyme was assayed in conditions which distinguish between different kinetic forms which have previously been identified in spinach leaves. It is proposed that activation of sucrose-phosphate synthase is one of the first sites at which spinach leaves respond to a rising water deficit. This could be of importance for osmoregulation.Abbreviations Chl chlorophyll - Fru1,6bisP fructose-1,6-bisphosphate - Fru2,6bisP fructose-2,6-bisphosphate - Fru6P fructose-6-phosphate - Glc6P glucose-6-phosphate - PGA glycerate-3-phosphate - Pi inorgamic phosphate - Ru1,5bisP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - triose-P sum of glyceraldehyde-3-phosphate and dehydroxyacetone phosphate - UDPGlc uridine diphosphoglucose