Regulation and roles for alternative pathways of hexose metabolism in plants

Plant cells have two cytoplasmic pathways of glycolysis and gluconeogenesis for the reversible interconversion of fructose 6-phosphate (F-6-P) and fructose 1,6-bisphosphate (F-1,6-P₂). One pathway is described as a maintenance pathway that is catalyzed by a nucleotide triphosphate-dependent phosphofructokinase (EC 2.7.1.11; ATP-PFK) glycolytically and a F-1,6 bisphosphatase (EC 3.1.3.11) gluconeogenically. These are non-equilibrium reactions that are energy consuming. The second pathway, described as an adaptive pathway, is catalyzed by a readily reversible pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90; PP-PFK) in an equilibrium reaction that conserves energy through the utilization and the synthesis of pyrophosphate. A constitutive regulator cycle is also present for the synthesis and hydrolysis of fructose 2,6-bisphosphate (F-2,6-P₂) via a 2-kinase and a 2-phosphatase, respectively. The pathway catalyzed by the ATP-PFK and F-1,6-bisphosphatase, the maintenance pathway, is fairly constant in maximum activity in various plant tissues and shows less regulation by F-2,6-P₂. Plants use F-2,6-P₂ initially to regulate the adaptive pathway at the reversible PP_i-PFK step. The adaptive pathway, catalyzed by PP_i-PFK, varies in maximum activity with a variety of phenomena such as plant development or changing biological and physical environments. Plants can change F-2,6-P₂ levels rapidly, in less than 1 min when subjected to rapid environmental change, or change levels slowly over periods of hours and days as tissues develop. Both types of change enable plants to cope with the environmental and developmental changes that occur during their lifetimes. The two pathways of sugar metabolism can be efficiently linked by the cycling of uridylates and pyrophosphate required for sucrose breakdown via a proposed sucrose synthase pathway. The breakdown of sucrose via the sucrose synthase pathway requires half the net energy of breakdown via the invertase pathway. Pyrophosphate occurs in plant tissues as a substrate pool for biosynthetic reactions such as the PP_i-PFK or uridine diphosphate glucose pyrophosphorylase (EC 2.7.7.9; UDPG pyrophosphorylase) that function in the breakdown of imported sucrose. Also, pyrophosphate links the two glycolytic/gluco-neogenic pathways; and in a reciprocal manner pyrophosphate is produced as an energy source during gluconeogenic carbon flow from F-1,6-P₂ toward sucrose synthesis.     

Biphasic effect of fructose 2,6-bisphosphate on the liver fructose-1,6-bisphosphatase: mechanistic and physiological implications

Fructose 2,6-bisphosphate has been claimed to be both a substrate analogue and an allosteric inhibitor of fructose-1,6-bisphosphatase. The results reported here show that fructose 2,6-bisphosphate can be both an inhibitor and an activator of the enzyme, depending on the substrate concentration. This biphasic behaviour at saturating concentrations of substrate can only be due to an allosteric effect. In addition to the mechanistic implication it is possible that this finding may have physiological meaning.     

Active glycolysis and glycogenolysis in early stages of primary cultured hepatocytes. Role of AMP and fructose 2.6-bisphosphate

Summary This study examines the factors involved in the rapid glycolysis and glycogenolysis that occur during the first stages of hepatocyte culture: a) Shortly after seeding glycolysis, estimated as lactate released to culture medium, increased 10 times in comparison to that reported in vivo. By 8 to 9 h of culture, hepatocytes were nearly glycogen-depleted even in the presence of insulin. b) 6-Phosphofructo-2-kinase remained 100% active during this period. The proportion of the initial active phosphorylase (87%) decreased to 57% by 7 h of culture. c) Fructose 2,6-bisphosphate content was initially similar to that found in liver of fed animals, decreased after seeding and increased thereafter up to four times the initial concentration. In spite of changes in the concentration of this activator, the glycolytic rate remained high and constant. d) ADP and AMP increased sharply after cell plating, reaching values 1.7 and 3.5 times higher. The rise in AMP levels may be involved in the activation of glycolysis and glycogenolysis, because this metabolite is known to act as an allosteric activator of phosphofrucktokinase and glycogen phosphorylase. This metabolic situation resembles that of cells under hypoxia. Part of this work was presented at the 38th Annual Meeting of the Tissue Culture Association, Washington, DC, May 1987.     

Fructose 6-phosphate metabolism in plants

The kinetic and regulatory properties of the ATP-dependent phosphofructokinase from various plant tissues are reviewed. Particular attention is given to the differences in properties between the plastid and cytosolic isozymes of this enzyme. A model for fructose 6-phosphate utilization in plants is presented which incorporates a role for the pyrophosphate-dependent phosphofructokinase.     

Low levels of pyrophosphate in transgenic potato plants expressing E. coli pyrophosphatase lead to decreased vitality under oxygen deficiency

BACKGROUND AND AIMS: The aim of this study was to investigate the importance of pyrophosphate (PPi) for plant metabolism and survival under low oxygen stress. Responses of roots of wild-type potato plants were compared with roots of transgenic plants containing decreased amounts of PPi as a result of the constitutive expression of Escherichia coli pyrophosphatase in the cytosol. METHODS: For the experiments, roots of young wild-type and transgenic potato plants growing in nutrient solution were flushed for 4 d with nitrogen, and subsequently metabolite contents as well as enzyme activities of the glycolytic pathway were determined. KEY RESULTS AND CONCLUSIONS: In roots of transgenic plants containing 40% less PPi, UDPglucose accumulated while the concentrations of hexose-6-phosphate, other glycolytic intermediates and ATP were decreased, leading to a growth retardation in aerated conditions. Apart from metabolic alterations, the activity of sucrose synthase was increased to a lower extent in the transgenic line than in wild type during hypoxia. These data suggest that sucrose cleavage was inhibited due to PPi deficiency already under aerated conditions, which has severe consequences for plant vitality under low oxygen. This is indicated by a reduction in the glycolytic activity, lower ATP levels and an impaired ability to resume growth after 4 d of hypoxia. Interestingly, the phosphorylation of fructose-6-phosphate via PPi-dependent phosphofructokinase was not altered in roots of transgenic plants. Nevertheless, our data provide some evidence for the importance of PPi to maintain plant growth and metabolism under oxygen deprivation.     

Carbohydrate metabolism during fruit development in sweet pepper (Capsicum annuum) plants

Growth, accumulation of sugars and starch, and the activity of enzymes involved in sucrose mobilization were determined throughout the development of sweet pepper fruits. Fruit development was roughly divided into three phases: (1) an initial phase with high relative growth rate and hexose accumulation, (2) a phase with declining growth rate and accumulation of sucrose and starch, and (3) a ripening phase with no further fresh weight increase and with accumulation of hexoses, while sucrose and starch were degraded. Acid and neutral invertase (EC 3.2.1.26) were closely correlated to relative growth rate until ripening and inversly correlated to the accumulation of sucrose. Acid invertase specifically increased during ripening, concurrently with the accumulation of hexoses. Sucrose synthase (EC 2.4.1.13) showed little correlation to fruit development, and in periods of rapid growth the activity of sucrose synthase was low compared to the invertases. However, during late fruit growth sucose synthase was more active than the invertases. We conclude that invertase activities determine the accumulation of assimilates in the very young fruits, and a reactivation of acid invertase is responsible for the accumulation of hexoses during ripening. During late fruit growth, before ripening, sucrose synthase is transiently responsible for the sucrose breakdown in the fruit tissue. Results also indicate that pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) and its activator fructose-2,6-bisphosphate (Fru2,6bisP) are involved in the regulation of the sink metabolism of the fruit tissue.     

Evolving concepts in plant glycolysis: two centuries of progress

Glycolysis, the process responsible for the conversion of monosaccharides to pyruvic acid, is a ubiquitous feature of cellular metabolism and was the first major biochemical pathway to be well characterized. Although the majority of glycolytic enzymes are common to all organisms, the past quarter of a century has revealed that glycolysis in higher plants possesses numerous distinctive features. Research in the nineteenth century established convincingly that plants carry out alcoholic fermentation under anaerobic conditions. In 1878, Wilhelm Pfeffer asserted that a non-oxygen-requiring ‘intramolecular respiration’ was involved in the aerobic respiration of plants. Between 1900 and 1950 it was demonstrated that plants metabolize sugar and starch by a glycolytic pathway broadly similar to that of yeasts and muscle tissue. In 1948, the first purification and characterization of a plant glycolytic enzyme, aldolase, was published by Paul Stumpf. By 1960 the presence of each of the 10 enzymes of glycolysis, presumed at the time to be located in the cytosol, had been confirmed in higher plants. Shortly after 1960 it was shown that the mechanism of glycolytic regulation in plants had features in common with that of animals and yeasts, especially as regards the important role played by the enzyme phosphofructokinase; but important regulatory properties peculiar to plants were soon demonstrated. In the last 30 years, higher-plant glycolysis has been found to exhibit a number of additional characteristics peculiar to plant systems. One conspicuous feature of plant glycolysis, discovered in the 1970s, is the presence of a complete or nearly complete sequence of glycolytic enzymes in plastids, distinct and spatially separated from the glycolytic enzymes located in the cytosol. Plastidic and cytosolic isoenzymes of glycolysis have been shown to differ in their kinetic and regulatory properties, suggesting that the two pathways are independently regulated. Since about 1980 it has become increasingly clear that the cytosolic glycolysis of plants may make use of several enzymes other than the conventional ones found in yeasts, muscle tissue and plant plastids: these enzymes include a pyrophosphate-dependent phosphofructokinase, a non-reversible and nonphosphorylating glyceraldehyde-3-phosphate dehydrogenase, a phosphoenolpyruvate phosphatase (vacuolar location) and a three-enzyme sequence able to produce pyruvate from phosphoenolpyruvate avoiding the pyruvate-kinase step. These non-conventional enzymes may catalyze glycolysis in the plant cytosol especially under conditions of metabolic stress. Experiments on transgenic plants possessing significantly elevated or reduced (reduced to virtually nil in some cases) levels of glycolytic enzymes are currently playing an important part in improving our understanding of the regulation of plant glycolysis; such experiments illustrate an impressive degree of flexibility in the pathway's operation. Plant cells are able to make use of enzymes bypassing or substituting for several of the conventional enzymic steps in the glycolytic pathway; the extent and conditions under which these bypasses operate are the subject of current research. The duplication of the glycolytic pathway in plants and the flexible nature of the pathway have possibly evolved in relation to the crucial biosynthetic role played by plant glycolysis beyond its function in energy generation; both functions must proceed if a plant is to survive under varying and often stressful environmental or nutritional conditions.     

Metabolic effects of benzoate and sorbate in the yeast Saccharomyces cerevisiae at neutral pH

Preincubation of yeast cells in the presence of benzoate or sorbate at an extracellular pH value of 6.8 elicited a set of metabolic effects on sugar metabolism, which became apparent after the subsequent glucose addition. They can be summarized as follows: a) reduced glucose consumption; b) inhibition of glucose- and fructose-phosphorylating activities; c) supression of glucose-triggered peak of hexoses monophosphates; d) substantial reduction of glucose-triggered peak of fructose 2,6-bisphosphate; e) block of catabolite inactivation of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase, but not of cytoplamic malate dehydrogenase. On the whole this pattern resulted in prevention of glucose-induced switch of metabolism from a gluconeogenetic to a glycolytic state. Our data also show that, unlike former assumptions, intracellular acidification is not likely to mediate the bulk of metabolic effects of benzoate and sorbate, since under our working conditions intracellular pH kept close to neutrality.     

Overexpression of ubiquitous 6-phosphofructo-2-kinase in the liver of transgenic mice results in weight gain

Fructose 2,6-bisphosphate (Fru-2,6-P₂) is an important metabolite that controls glycolytic and gluconeogenic pathways in several cell types. Its synthesis and degradation are catalyzed by the bifunctional enzyme 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase (PFK-2). Four genes, designated Pfkfb1-4, codify the different PFK-2 isozymes. The Pfkfb3 gene product, ubiquitous PFK-2 (uPFK-2), has the highest kinase/bisphosphatase activity ratio and is associated with proliferation and tumor metabolism. A transgenic mouse model that overexpresses uPFK-2 under the control of the phosphoenolpyruvate carboxykinase promoter was designed to promote sustained and elevated Fru-2,6-P₂ levels in the liver. Our results demonstrate that in diet-induced obesity, high Fru-2,6-P₂ levels in transgenic livers caused changes in hepatic gene expression profiles for key gluconeogenic and lipogenic enzymes, as well as an accumulation of lipids in periportal cells, and weight gain.     

Phloem-specific expression of pyrophosphatase inhibits long distance transport of carbohydrates and amino acids in tobacco plants

As reported in a previous paper [Lerchl et al. (1995) Plant Cell, 7, 259–270], expression of Escherichia coli inorganic pyrophosphatase in the cytosol under the control of the phloem-specific rolC promoter from, Agobacterium rhizogenes results in decreased growth of transgenic tobacco plants. In this paper we investigate the effect of the phloem-specific expression of pyrophosphatase on phloem metabolism, and on plant growth and allocation. A small decrease in the hexose phosphate/UDP-glucose ratio, the ATP/ADP ratio and the respiration rate in the midribs of the transformants provides evidence Hint mobilization of sucrose via pyrophosphate-dependent reactions is necessary for phloem energy metabolism. The source leaves of the transformants had higher levels of carbohydrates and amino acids and a much higher glutamine/glutamate ratio than the wild type, showing that export was inhibited and that the growth inhibition was not due to a lack of photoas-similates or organic nitrogen in the leaves. The accumulation of photoassimilates was paralleled by a decrease in photosynthesis, chlorophyll content and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity, a small increase in hexose phosphates and triose phosphates and a decrease in glycerate 3-phosphate in the source leaves. There was a decrease of soluble sugars and amino acids in sink leaves of the transformants. In sink leaves amino acids decreased more than carbohydrates and a decrease in the glutamine/ glutamate ratio was observed. This was accompanied by a large decrease of nitrate. Sugars and amino acids were also reduced in the root tips of the transformants. The carbohydrate /amino acid ratio decreased 5-fold in the root tips, indicating a particularly smile shortage of carbohydrates. Relatively high levels of sugars and amino acids in the basal regions of the root and the increase in sugars in the midrib indicate that there is also increased leakage of assimilates out of the phloem during long-distance transport. Metabolism is required to maintain phloem function along the transport route, as well as for the initial step of loading. The transformants showed decreased stem and root growth. The growth inhibition was largest in conditions allowing rapid growth of the wild type (high light and nitrogen supply).     

Effects of vanadate on 6-phosphofructo 2-kinase activity and fructose 2,6-bisphosphate levels in cultured rat hepatocytes

The presence of vanadate in primary cultures of rat hepatocytes produced a significant increase in the concentration of fructose 2,6-bisphosphate and in the activity of 6-phosphofructo 2-kinase. Compared with insulin, vanadate had a more potent action on the metabolite increase, but a similar effect on the 6-phosphofructo 2-kinase activity. Both the insulin- and the vanadate-dependent enhancements of 6-phosphofructo 2-kinase were inhibited by cycloheximide which specifically blocks protein synthesis on the translational level, suggesting that the increase of the enzyme activity was due to induction rather than to a change in the catalytic activity.     

Incomplete and transitory decrease of glycolysis

During vessel sprouting, a migratory endothelial tip cell guides the sprout, while proliferating stalk cells elongate the branch. Tip and stalk cell phenotypes are not genetically predetermined fates, but are dynamically interchangeable to ensure that the fittest endothelial cell (EC) leads the vessel sprout. ECs increase glycolysis when forming new blood vessels. Genetic deficiency of the glycolytic activator PFKFB3 in ECs reduces vascular sprouting by impairing migration of tip cells and proliferation of stalk cells. PFKFB3-driven glycolysis promotes the tip cell phenotype during vessel sprouting, since PFKFB3 overexpression overrules the pro-stalk activity of Notch signaling. Furthermore, PFKFB3-deficient ECs cannot compete with wild-type neighbors to form new blood vessels in chimeric mosaic mice. In addition, pharmacological PFKFB3 blockade reduces pathological angiogenesis with modest systemic effects, likely because it decreases glycolysis only partially and transiently.     

Differential expression of genes related to glucose metabolism in domesticated pigs and wild boar

Glucose metabolism is a basic biological process that shows substantial variation within and between species. Using pig as a model organism, we investigated differences in glucose metabolic genes in seven tissues from domesticated pigs (Rongchang pig and Tibetan pig, meanwhile, the Tibetan pig just as a special case of the domesticated pig under plateau condition) and wild boar. We found large differences in the expression of genes involved in multiple aspects of glucose metabolism, including genes associated with glucose transport, gluconeogenesis, and glycolysis. In addition, we identified microRNAs (miRNAs) that may be involved in the divergence of glucose metabolism in pig. A combined analysis of mRNA and miRNA expression indicated that some miRNA:mRNA pairs showed ab facto function in it. Our results provide a valuable resource for further determination of miRNA regulatory roles in pig glucose metabolism and reveal the divergence of glucose metabolism in pigs under domestication.     

Modification of cys-128 of pig kidney fructose 1,6-bisphosphatase with different thiol reagents: Size dependent effect on the substrate and fructose-2,6-bisphosphate interaction

Treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide was shown to abolish the inhibition by fructose 2,6-bisphosphate, which also protected the enzyme against this chemical modification [Reyes, A., Burgos, M. E., Hubert, E., and Slebe, J. C. (1987),J. Biol. Chem. 262, 8451–8454]. On the basis of these results, it was suggested that a single reactive sulfhydryl group was essential for the inhibition. We have isolated a peptide bearing the N-ethylmaleimide target site and the modified residue has been identified as cysteine-128. We have further examined the reactivity of this group and demonstrated that when reagents with bulky groups are used to modify the protein at the reactive sulfhydryl [e.g., N-ethylmaleimide or 5,5-dithiobis-(2-nitrobenzoate)], most of the fructose 2,6-bisphosphate inhibition potential is lost. However, there is only partial or no loss of inhibition when smaller groups (e.g., cyanate or cyanide) are introduced. Kinetic and ultraviolet difference spectroscopy-binding studies show that the treatment of fructose 1,6-bisphosphatase with N-ethylmaleimide causes a considerable reduction in the affinity of the enzyme for fructose 2,6-bisphosphate while affinity for fructose 1,6-bisphosphate does not change. We can conclude that modification of this reactive sulfhydryl affects the enzyme sensitivity to fructose 2,6-bisphosphate inhibition by sterically interfering with the binding of this sugar bisphosphate, although this residue does not seem to be essential for the inhibition to occur. The results also suggest that fructose 1,6-bisphosphate and fructose 2,6-bisphosphate may interact with the enzyme in a different way.     

Cultivar differences in soluble sugar content of mature rice grain

Mature grain from 31 rice ( Oryza sativa L.) cultivars grown in the same location was analyzed for soluble sugar content to determine quantitative differences in this parameter. Cultivar variation in caryopsis sucrose content was 4-fold ranging from 15 to 59 μmol sucrose (g fresh weight)⁻¹. Soluble reducing sugar ranged from 7 to 15 μmol hexose (g fresh weight)⁻¹. Soluble sugar content was much greater in the outer part of the grain than in the endosperm. Caryopsis enzyme activities were measured in 8 cultivars having a range of grain sucrose content. No relationship between grain sucrose content of these cultivars and the level of enzymes of sucrose metabolism, glycolysis, and oxidative pentose pathway was established. Caryopsis respiration after 1 h of imbibition also was not related to the amount of soluble sugars available in the grain among these 8 cultivars. The results show that there are significant differences in the sugar-accumulating capacity of the caryopsis of different rice cultivars. The detection of a fructose 2,6-bisphosphate-sensitive, PP_i: fructose 6-phosphate phosphototransferase (EC 2.7.1.90) in the endosperm suggests the presence of a regulatory mechanism for sucrose/starch partitioning established in other plant tissues.     

Fructose 2,6-bisphosphate in relation with the resumption of metabolic activity in slices of Jerusalem artichoke tubers

When slices of Jerusalem artichoke tubers were incubated at 25°C, their concentration in fructose 2,6-bisphosphate increased up to 250-fold within 2 h. Fructose 2,6-bisphosphate was also formed, although at a slower rate, in slices incubated at 0°C. Its formation could not be explained by an increase in the concentration of fructose 6-phosphate or of ATP either by an activation of phosphofructo-2-kinase. Pyrophosphate—fructose-6-phosphate 1-phosphotransferase was the only enzyme present in a tuber extract which was found to be sensitive to fructose 2,6-bisphosphate. An improved procedure for the assay of fructose 2,6-bisphosphate is also reported.     

Cloning and expression of novel isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from bovine heart

Distinct 6-phosphofructo-2-kinase (PFK-2)/fructose 2,6-bisphosphatase (FBPase-2) cDNAs were cloned from bovine heart, showing that PFK-2/FBPase-2 gene B, which contains 16 exons, codes for at least five mRNAs. Three of them (B1, B2, B4) could encode the 58,000-M_r isozyme. In B2 mRNA, exon 15 encodes four more residues than in Bl. In B4 mRNA, exon 15 encodes six more residues than in B1, butexon 16 (20 residues) is missing. B3 mRNA corresponds to the 54,000-M_r isozyme. It lacks exon 15 and also differs from the other mRNAs in the 5' noncoding region. B5 mRNA encodes a truncated form. When expressed in E. coli, the recombinant isoforms corresponding to all these mRNAs except B5 exhibited PFK-2 activity.     

Ureide metabolism in higher plants

The synthesis, transport and assimilation of the ureides, allantoin and allantoic acid, in higher plants is reviewed. Evidence indicates that in nodulated legumes ureides are synthesized from products of N₂-fixation via purine synthesis and degradation. Their synthesis in other plants also appears to be via purine degradation but is dependent on the inorganic nitrogen source fed to the plant; greatest ureide production is associated with ammonium assimilation. The use of ureides rather than amides for N-transport from the root to the shoot via the xylem stream results in an improved carbon economy of the plant. Good evidence for the transport of ureides in the phloem is lacking for most species examined although it is assumed to be important, particularly in fruit and seed development. Ureides are stored and assimilated mainly in the shoot. The precise pathways, localization and regulation of ureide assimilation are poorly understood and require further investigation. Similarities exist between the properties of the enzymes involved in ureide assimilation in higher plants and in micro-organisms. However, the evidence that light appears to be involved in ureide assimilation in green tissues suggests that different regulatory mechanisms may exist in plants compared with micro-organisms. The economically important legume crops such as soybeans, cowpeas and Phaseolus sp. are all ureide producers. To aid our understanding of the productivity of these plants knowledge of how ureide-N is converted into seed protein is essential.