Plant cytosolic pyruvate kinase: a kinetic study

FaGAPC2/FaPKc2.2 and FaPEPCK reveal differential citric acid metabolism regulation in late development of strawberry fruit

Citric acid is the primary organic acid that affects the taste of strawberry fruit. Glycolysis supplies key substrates for the tricarboxylic acid cycle (TCA cycle). However, little is known about the regulatory mechanisms of glycolytic genes on citric acid metabolism in strawberry fruits. In this study, the citric acid content of strawberry fruit displayed a trend of rising and decreasing from the initial red stage to the full red stage and then dark red stage. Thus, a difference in citric acid metabolic regulation was suspected during strawberry fruit development. In addition, overexpression of either cytoplasm glyceraldehyde-3-phosphate dehydrogenase (FxaC_14g13400, namely FaGAPC2) or pyruvate kinase (FxaC_15g00080, namely FaPKc2.2) inhibited strawberry fruit ripening and the accumulation of citric acid, leading to a range of maturity stages from partial red to full red stage. The combined transcriptome and metabolome analysis revealed that overexpression of FaGAPC2 and FaPKc2.2 significantly suppressed the expression of phosphoenolpyruvate carboxykinase (FxaC_1g21491, namely FaPEPCK) but enhanced the content of glutamine and aspartic acid. Meanwhile, the activities of PEPCK and glutamate decarboxylase (GAD) were inhibited, but the activities of glutamine synthase (GS) were increased in FaGAPC2/FaPKc2.2-overexpressed fruit. Further, functional verification demonstrated that overexpression of FaPEPCK can promote strawberry fruit ripening, resulting in a range of maturity stage from full red to dark red stage, while the citric acid synthase (CS) activities and citric acid content were significantly decreased. Overall, this study revealed that FaGAPC2/FaPKc2.2 and FaPEPCK perform an important role in reducing citric acid content in strawberry fruit, and FaGAPC2/FaPKc2.2 mainly by promoting the GS degradation pathway and FaPEPCK mainly by inhibiting the CS synthesis pathway.

Nitrate nutrition influences multiple factors in order to increase energy efficiency under hypoxia in Arabidopsis

BACKGROUND AND AIMS

Nitrogen (N) levels vary between ecosystems, while the form of available N has a substantial impact on growth, development and perception of stress. Plants have the capacity to assimilate N in the form of either nitrate (NO3-) or ammonium (NH4+). Recent studies revealed that NO3- nutrition increases nitric oxide (NO) levels under hypoxia. When oxygen availability changes, plants need to generate energy to protect themselves against hypoxia-induced damage. As the effects of NO3- or NH4+ nutrition on energy production remain unresolved, this study was conducted to investigate the role of N source on group VII transcription factors, fermentative genes, energy metabolism and respiration under normoxic and hypoxic conditions.

METHODS

We used Arabidopsis plants grown on Hoagland medium with either NO3- or NH4+ as a source of N and exposed to 0.8 % oxygen environment. In both roots and seedlings, we investigated the phytoglobin-nitric oxide cycle and the pathways of fermentation and respiration; furthermore, NO levels were tested using a combination of techniques including diaminofluorescein fluorescence, the gas phase Griess reagent assay, respiration by using an oxygen sensor and gene expression analysis by real-time quantitative reverse transcription-PCR methods.

KEY RESULTS

Under NO3- nutrition, hypoxic stress leads to increases in nitrate reductase activity, NO production, class 1 phytoglobin transcript abundance and metphytoglobin reductase activity. In contrast, none of these processes responded to hypoxia under NH4+ nutrition. Under NO3- nutrition, a decreased total respiratory rate and increased alternative oxidase capacity and expression were observed during hypoxia. Data correlated with decreased reactive oxygen species and lipid peroxidation levels. Moreover, increased fermentation and NAD+ recycling as well as increased ATP production concomitant with the increased expression of transcription factor genes HRE1, HRE2, RAP2.2 and RAP2.12 were observed during hypoxia under NO3- nutrition.

CONCLUSIONS

The results of this study collectively indicate that nitrate nutrition influences multiple factors in order to increase energy efficiency under hypoxia.

Barley grains, deficient in cytosolic small subunit of ADP-glucose pyrophosphorylase, reveal coordinate adjustment of C:N metabolism mediated by an overlapping metabolic-hormonal control

The barley Risø16 mutation leads to inactivation of cytosolic ADP-Glc pyrophosphorylase, and results in decreased ADP-Glc and endospermal starch levels. Here we show that this mutation is accompanied by a decrease in storage protein accumulation and seed size, which indicates that alteration of a single enzymatic step can change the network of storage metabolism as a whole. We used comprehensive transcript, metabolite and hormonal profiling to compare grain metabolism and development of Risø16 and wild-type endosperm. Despite increased sugar availability in mutant endosperm, glycolytic intermediates downstream of hexose phosphates remained unchanged or decreased, while several glycolytic enzymes were downregulated at the transcriptional level. Metabolite and transcript profiling also indicated an inhibition of the tricarboxylic acid cycle at the level of mitochondrial nicotinamide adenine dinucleotide (NAD)-isocitrate dehydrogenase and an attendant decrease in alpha-ketoglutarate and amino acids levels in Risø16, compared with wild type. Decreased levels of cytokinins in Risø16 endosperm suggested co-regulation between starch synthesis, abscisic acid (ABA) deficiency and cytokinin biosynthesis. Comparative cis-element analysis in promoters of jointly downregulated genes in Risø16 revealed an overlap between metabolic and hormonal regulation, which leds to a coordinated downregulation of endosperm-specific and ABA-inducible gene expression (storage proteins) together with repression by sugars (isocitrate dehydrogenase, amylases). Such co-regulation ensured that decreased carbon fluxes into starch lead to a coordinated inhibition of glycolysis, amino acid and storage proteins biosynthesis, which is useful in the prevention of osmotic imbalances and oxidative stress due to increased accumulation of sugars.

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.

Decreased expression of cytosolic pyruvate kinase in potato tubers leads to a decline in pyruvate resulting in an in vivo repression of the alternative oxidase

The aim of this work was to investigate the effect of decreased cytosolic pyruvate kinase (PKc) on potato (Solanum tuberosum) tuber metabolism. Transgenic potato plants with strongly reduced levels of PKc were generated by RNA interference gene silencing under the control of a tuber-specific promoter. Metabolite profiling showed that decreased PKc activity led to a decrease in the levels of pyruvate and some other organic acids involved in the tricarboxylic acid cycle. Flux analysis showed that this was accompanied by changes in carbon partitioning, with carbon flux being diverted from glycolysis toward starch synthesis. However, this metabolic shift was relatively small and hence did not result in enhanced starch levels in the tubers. Although total respiration rates and the ATP to ADP ratio were largely unchanged, transgenic tubers showed a strong decrease in the levels of alternative oxidase (AOX) protein and a corresponding decrease in the capacity of the alternative pathway of respiration. External feeding of pyruvate to tuber tissue or isolated mitochondria resulted in activation of the AOX pathway, both in the wild type and the PKc transgenic lines, providing direct evidence for the regulation of AOX by changes in pyruvate levels. Overall, these results provide evidence for a crucial role of PKc in the regulation of pyruvate levels as well as the level of the AOX in heterotrophic plant tissue, and furthermore reveal that these parameters are interlinked in vivo.

Glycolytic enzymes associate dynamically with mitochondria in response to respiratory demand and support substrate channeling

In Arabidopsis thaliana, enzymes of glycolysis are present on the surface of mitochondria and free in the cytosol. The functional significance of this dual localization has now been established by demonstrating that the extent of mitochondrial association is dependent on respiration rate in both Arabidopsis cells and potato (Solanum tuberosum) tubers. Thus, inhibition of respiration with KCN led to a proportional decrease in the degree of association, whereas stimulation of respiration by uncoupling, tissue ageing, or overexpression of invertase led to increased mitochondrial association. In all treatments, the total activity of the glycolytic enzymes in the cell was unaltered, indicating that the existing pools of each enzyme repartitioned between the cytosol and the mitochondria. Isotope dilution experiments on isolated mitochondria, using (13)C nuclear magnetic resonance spectroscopy to monitor the impact of unlabeled glycolytic intermediates on the production of downstream intermediates derived from (13)C-labeled precursors, provided direct evidence for the occurrence of variable levels of substrate channeling. Pull-down experiments suggest that interaction with the outer mitochondrial membrane protein, VDAC, anchors glycolytic enzymes to the mitochondrial surface. It appears that glycolytic enzymes associate dynamically with mitochondria to support respiration and that substrate channeling restricts the use of intermediates by competing metabolic pathways.

Pyruvate kinase: current status of regulatory and functional properties

Pyruvate kinase (PK) is a key enzyme for the glycolytic pathway and carbon metabolism in general. On the basis of the relevance and enormous diverse properties of this enzyme, this paper describes the results of a current and extensive review that determines the sites of conservation and/or difference in PK sequences, and the differences in the functional and regulatory properties of the enzymes. An alignment and analysis of 50 PK sequences from different sources and a phylogenetic tree are presented. This analysis was performed with reference to crystallographically characterized PK principally from E. coli, cat and rabbit muscle. A number of attributes of the enzyme that make it of particular interest in biomedicine and industry are also discussed.

Pyruvate kinase from Chlamydia trachomatis is activated by fructose-2,6-bisphosphate

Pyruvate kinase is the final regulatory point in the catabolic Embden-Meyerhoff-Parnas pathway, which controls the carbon flux of glycolytic intermediates and regulates the level of ATP in the cell. In a previous study, we identified, cloned and sequenced pyruvate kinase from the obligate intracellular bacterium Chlamydia trachomatis and demonstrated that the enzyme was active in crude extract. Here, we report the kinetic properties of highly purified C. trachomatis pyruvate kinase. The results indicate that C. trachomatis pyruvate kinase is 53.5 kDa with a pH optima of 7.3. Kinetic studies show that C. trachomatis pyruvate kinase requires both K+ and Mg2+ ions for activity, exhibits sigmoidal kinetics with respect to phosphoenolpyruvate and Michaelis-Menten kinetics with respect to ADP. In addition, C. trachomatis pyruvate kinase is able to use alternative nucleoside diphosphates as phosphate acceptors, although it shows the greatest activity with ADP. In contrast to other bacterial pyruvate kinases that are activated by AMP, our data show that AMP, in addition to ATP and GTP, inhibits C. trachomatis pyruvate kinase. Surprisingly, unlike any other known bacterial pyruvate kinase, C. trachomatis pyruvate kinase was allosterically activated by fructose-2,6-bisphosphate, an important regulatory metabolite that has only been reported in eukaryotes.

Molecular and regulatory properties of leucoplast pyruvate kinase from Brassica napus (rapeseed) suspension cells

Plastidic pyruvate kinase (PK(p)) from Brassica napus suspension cells was purified 431-fold to a final specific activity of 28 micromol phosphoenolpyruvate (PEP) utilized/min/mg protein. SDS-PAGE, immunoblot and gel filtration analyses indicated that this PK(p) exists as a 380-kDa heterohexamer composed of equal proportions of 64- (alpha-subunit) and 58-kDa (beta-subunit) polypeptides. The N-terminal sequence of the PK(p) alpha- and beta-subunits exhibited maximal identity with the corresponding regions deduced from putative PK genes of Arabidopsis thaliana and Methylobacterium extorquens, respectively. B. napus PK(p) displayed a sharp pH optimum of pH 8.0, and hyperbolic saturation kinetics with PEP and ADP (K(m) = 0.052 and 0.14 mM, respectively). 6-Phosphogluconate functioned as an activator (K(a) = 0.12 mM) by increasing V(max) by approximately 35% while decreasing the K(m)(PEP) and K(m)(ADP) values by 40 and 50%, respectively. 2-Oxoglutarate and oxalate were the most effective inhibitors (I(50) = 8.3 and 0.23 mM, respectively). A model is presented which highlights the role of 6-phosphogluconate in coordinating stromal NADPH and ATP production for anabolic processes of B. napus leucoplasts.

Purification and characterization of cytosolic pyruvate kinase from Brassica napus (rapeseed) suspension cell cultures: implications for the integration of glycolysis with nitrogen assimilation

Cytosolic pyruvate kinase (PKc) from Brassica napus suspension cells was purified 201-fold to electrophoretic homogeneity and a final specific activity of 51 micromol phosphoenolpyruvate utilized per min per mg protein. SDS/PAGE and gel filtration analyses of the final preparation indicated that this PKc is a 220-kDa homotetramer composed of 56-kDa subunits. The enzyme was relatively heat-stable and displayed a broad pH optimum of pH 6.8. PKc activity was absolutely dependent upon the simultaneous presence of a bivalent and univalent cation, with Mg2+ and K+ fulfilling this requirement. Hyperbolic saturation kinetics were observed for phosphoenolpyruvate, ADP, Mg2+ and K+ (apparent Km values = 0.12, 0.075, 0.21 and 0.48 mM, respectively). Although the enzyme utilized UDP, CDP and IDP as alternative nucleotides, ADP was the preferred substrate. L-Glutamate, oxalate, and the flavonoids rutin and quercetin were the most effective inhibitors (I50 values = 4, 0.3, 0.07, and 0.10 mM, respectively). L-Aspartate functioned as an activator (Ka = 0.31 mM) by causing a 40% increase in Vmax while completely reversing the inhibition of PKc by L-glutamate. Reciprocal control by L-aspartate and L-glutamate is specific for these amino acids and provides a rationale for the in vivo activation of PKc that occurs during periods of enhanced NH +4-assimilation. Allosteric features of B. napus PKc are compared with those of B. napus phosphoenolpyruvate carboxylase. A model is presented that highlights the pivotal role of L-aspartate and L-glutamate in the coordinate regulation of these key phosphoenolpyruvate utilizing cytosolic enzymes.

Pyruvate kinase activity as an indicator of the level of K(+), Mg(2+), and Ca(2+) in leaves and fruits of the cucumber: the role of potassium fertilization

Little is known about the effects of K(+) fertilization on pyruvate kinase (PK) activities in cucumber (Cucumis sativus L. cv. Brunex) grown in the greenhouse on calcareous soils. Here, the effect of K rates on the concentrations of K(+), Mg(2+), and Ca(2+) and on the PK activity as a possible indicator of the levels of these cations in the leaves and fruits of cucumber plants has been studied. The treatments consisted of applications of three rates of K in the form of K(2)SO(4) (K1 = 0.075 mg mL(-)(1), K2 = 0.15 mg mL(-)(1), and K3 = 0.30 mg mL(-)(1)). In general, K(+) application in calcareous soils proved beneficial. The highest application of K(+) (K3) to the culture medium reduced, in both the leaves and fruits, the foliar concentrations of Mg(2+) and Ca(2+). These results are reflected by the lowest basal PK activities and the highest differences between the basal PK activity and activities stimulated by these two cations. The opposite effect resulted with K2, with maximal basal PK activity and minimal differences between this activity and activities stimulated by the cations, indicating a better balance in this treatment between the different cations under the experimental conditions. Finally, this trend might partly account for the highest commercial yield in plants treated with K2.

Differential genome analysis applied to the species-specific features of Helicobacter pylori

We introduce a simple and rapid strategy to identify genes that are responsible for species-specific phenotypes. The genome of a species that has a specific phenotype is compared with at least one, closely related, species that lacks this phenotype. Homologous genes that are shared among the species compared are identified and discarded from the list of candidates for species-specific genes. The process is automated and rapidly yields a small subset of the genome that likely contains genes responsible for the species-specific features. Functions are assigned to the genes, and dubious annotations are filtered out. Information is extracted not only from the presence of genes, but also from their absence with respect to known phenotypes. We have applied the technique to identify a set of species-specific genes in Helicobacter pylori by comparing it with its closest relatives for which complete genome sequences are available, Haemophilus influenzae and Escherichia coli. Of the genes of this set for which functional features can be obtained, a large fraction (63%, 123 proteins) is (potentially) involved in H. pylori's interaction with its host. We hypothesize that a family of outer membrane proteins is critical for the ability of H. pylori to colonize host cells in highly acidic environments.

THE ORGANIZATION AND REGULATION OF PLANT GLYCOLYSIS

This review discusses the organization and regulation of the glycolytic pathway in plants and compares and contrasts plant and nonplant glycolysis. Plant glycolysis exists both in the cytosol and plastid, and the parallel reactions are catalyzed by distinct nuclear-encoded isozymes. Cytosolic glycolysis is a complex network containing alternative enzymatic reactions. Two alternate cytosolic reactions enhance the pathway's ATP yield through the use of pyrophosphate in place of ATP. The cytosolic glycolytic network may provide an essential metabolic flexibility that facilitates plant development and acclimation to environmental stress. The regulation of plant glycolytic flux is assessed, with a focus on the fine control of enzymes involved in the metabolism of fructose-6-phosphate and phosphoenolpyruvate. Plant and nonplant glycolysis are regulated from the "bottom up" and "top down," respectively. Research on tissue- and developmental-specific isozymes of plant glycolytic enzymes is summarized. Potential pitfalls associated with studies of glycolytic enzymes are considered. Some glycolytic enzymes may be multifunctional proteins involved in processes other than carbohydrate metabolism.