Pyrophosphate and fructose 2,6-bisphosphate effects on glycolysis in pea seed extracts

The participation of pyrophosphate-dependent phosphofructokinase (PPi-PFK) in plant glycolysis was examined using extracts from pea seeds (Pisum sativum L. cv Alaska). Glycolysis starting with fructose 6-phosphate was measured under aerobic conditions as the accumulation of pyruvate. Pyruvate accumulated in a medium containing PPi and adenosine diphosphate at about two-thirds of the rate in a medium containing adenosine diphosphate and adenosine triphosphate (ATP). The PPi-dependent pyruvate accumulation had the same reactant requirements and sensitivity to glycolysis inhibitors, sodium fluoride, and iodoacetamide, as the well-established ATP-dependent glycolysis. Added fructose 2,6-bisphosphate stimulated both the PPi-dependent pyruvate accumulation and PPi-PFK activity whereas this modulator had no effect on ATP-dependent glycolysis or ATP-PFK. Collectively these results demonstrate a PPi-dependent glycolytic pathway in plants which is responsive to fructose 2,6-bisphosphate.     

Amino acid sequence similarity between spinach chloroplast and mammalian gluconeogenic fructose-1,6-bisphosphatase

Chloroplast fructose-1,6-bisphosphatase is an essential enzyme in the photosynthetic pathway of carbon dioxide fixation into sugars and the properties of this enzyme are clearly distinct from cytosolic gluconeogenic fructose-1,6-bisphosphatase. Light-dependent activation via a ferredoxin/thioredoxin system and insensitivity to inhibition by AMP are unique characteristics of the chloroplast enzyme. In the present study, purified spinach chloroplast fructose-1,6-bisphosphatase was reduced, S-carboxymethylated with iodoacetic acid, and cleaved with either cyanogen bromide or trypsin. The resulting peptides were purified by reversed-phase high performance liquid chromatography. Automated Edman degradation of some of the purified peptides showed amino acid sequences highly homologous to residues 72-86, 180-199, and 277-319 of pig kidney fructose-1,6-bisphosphatase. These findings suggest a common evolutionary origin for mammalian gluconeogenic and chloroplast fructose-1,6-bisphosphatase, enzymes catalyzing the same reaction but having different functions and modes of regulation.     

Regulation of Carbon Partitioning to Respiration during Dark Ammonium Assimilation by the Green Alga Selenastrum minutum

The assimilation of NH₄⁺ causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoenolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH₄⁺ assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.     

Pyrophosphate-dependent phosphofructo-1-kinase complements fructose 1,6-bisphosphatase but not phosphofructokinase deficiency in Escherichia coli.

The gene from Propionibacterium freudenreichii for PPi-dependent phosphofructo-1-kinase, an enzyme that is found in some bacteria, in a number of anaerobic protists, and in plants, complements the absence of fructose 1,6-bisphosphatase in Escherichia coli but does not complement the deficiency of the ATP-dependent phosphofructokinase.     

Specific, reversible inactivation of phosphofructokinase by fructose-1,6-bisphosphatase. Involvement of adenosine 5'-triphosphate, oleate, and 3-phosphoglycerate.

Optimal conditions necessary for the reversible inactivation of crystalline rabbit muscle phosphofructokinase by homogeneous rabbit liver fructose-1,6-bisphosphatase have been studied. At higher enzyme levels (to 530 mug/ml of phosphofructokinase) the two proteins were mixed and incubated in a pH 7.5 buffer composed of 50 mM Tris-HC1, 2 mM potassium phosphate, and 0.2 mM dithiothreitol. Aliquots were removed at various times and assayed for enzyme activity. A time dependent inactivation of phosphofructokinase caused by 1-2.3 times its weight of fructose-1,6-bisphosphatase was observed at 30, 23, and 0 degree C. This inactivation did not require the presence of adenosine 5'-triphosphate or Mg2+ in the incubation mixture, but an adenosine 5'-triphosphate concentration of 2.7 mM or greater was required in the assay to keep phosphofructokinase in an inactive form. A mixture of activators (inorganic phosphate, (NH4)2SO4, and adenosine 5'-monophosphate), when added to the assay cuvette, restored nearly all of the expected enzyme activity. Incubations with other proteins, including aldolase, at concentrations equal to or greater than the effective quantity of fructose-1,6-bisphosphatase had no inhibitory effect on phosphofructokinase activity. Removal of tightly bound fructose 1,6-bisphosphate from phosphofructokinase could not explain this inactivation, since several analyses of crystalline phosphofructokinase averaged less than 0.1 mol of fructose 1,6-bisphosphate/320 000 g of enzyme. Furthermore, the inactivation occurred in the absence of Mg2+ where the complete lack of fructose-1-6-bisphosphatase activity was confirmed directly. At lower phosphofructokinase concentrations (0.2-2 mug/ml) the inactivation was studied directly in the assay cuvette. Higher ratios of fructose-1,6-bisphosphatase to phosphofructokinase were necessary in these cases, but oleate and 3-phosphoglycerate acted synergistically with lower amounts of fructose-1,6-bisphosphatase to cause inactivation. The inactivation did not occur when high concentrations of fructose 6-phosphate were present in the assay, or when the level of adenosine 5'-triphosphate was decreased. However, the inactivation was found at pH 8, where the effects of allosteric regulators on phosphofructokinase are greatly reduced. Experiments with rat liver phosphofructokinase showed that this enzyme was also subject to inhibition by rabbit liver fructose 1,6-bisphosphatase under conditions similar to those used in the muscle enzyme studies. Attempts to demonstrate direct interaction between phosphofructokinase and fructose-1,6-bisphosphate by physical methods were unsuccessful. Nevertheless, our results suggest that, under conditions which approximate the physiological state, the presence of fructose-1,6bisphosphatase can cause phosphofructokinase to assume an inactive conformation. This interaction may have a significant role in vivo in controlling the interrelationship between glycolysis and gluconeogenesis.     

Fructose-1,6-bisphosphatase from rat liver. A comparison of the kinetics of the unphosphorylated enzyme and the enzyme phosphorylated by cyclic AMP-dependent protein kinase

A purification procedure for rat hepatic fructose-1,6-bisphosphatase, described earlier, has been improved, resulting in an enzyme preparation with a neutral pH optimum and with both phosphorylatable serine residues present. The subunit Mr was 40,000. Phosphorylation in vitro with cyclic AMP-dependent protein kinase resulted in the incorporation of 1.4 mol of phosphate/mol of subunit and led to an almost 2-fold decrease in apparent Km for fructose-1,6-bisphosphate. In contrast to yeast fructose-1,6-bisphosphatase, fructose-2,6-bisphosphate had no effect on the rate of phosphorylation or dephosphorylation of the intact enzyme. The effects of the composition of the assay medium, with regard to buffering substance and Mg2+ concentration, on the apparent Km values of phosphorylated and unphosphorylated enzyme were investigated. The kinetics of phosphorylated and unphosphorylated fructose-1,6-bisphosphatase were studied with special reference to the inhibitory effects of adenine nucleotides and fructose-2,6-bisphosphate. Unphosphorylated fructose-1,6-bisphosphatase was more susceptible to inhibition by both AMP and fructose 2,6-bisphosphate than phosphorylated enzyme, at high and low substrate concentrations. Both ATP and ADP had a similar effect on the two enzyme forms, ADP being the more potent inhibitor. Finally, the combined effect of several inhibitors at physiological concentrations was studied. Under conditions resembling the gluconeogenic state, phosphorylated fructose-1,6-bisphosphatase was found to have twice the activity of the unphosphorylated enzyme.     

Regulation by glucagon of hepatic pyruvate kinase, 6-phosphofructo 1-kinase, and fructose-1,6-bisphosphatase

Glucagon stimulates gluconeogenesis in part by decreasing the rate of phosphoenolpyruvate disposal by pyruvate kinase. Glucagon, via cyclic AMP (cAMP) and the cAMP-dependent protein kinase, enhances phosphorylation of pyruvate kinase, phosphofructokinase, and fructose-1,6-bisphosphatase. Phosphorylation of pyruvate kinase results in enzyme inhibition and decreased recycling of phosphoenolpyruvate to pyruvate and enhanced glucose synthesis. Although phosphorylation of 6-phosphofructo 1-kinase and fructose-1,6-bisphosphatase is catalyzed in vitro by the cAMP-dependent protein kinase, the role of phosphorylation in regulating the activity of and flux through these enzymes in intact cells is uncertain. Glucagon regulation of these two enzyme activities is brought about primarily by changes in the level of a novel sugar diphosphate, fructose 2,6-bisphosphate. This compound is an activator of phosphofructokinase and an inhibitor of fructose-1,6-bisphosphatase; it also potentiates the effect of AMP on both enzymes. Glucagon addition to isolated liver systems results in a greater than 90% decrease in the level of this compound. This effect explains in large part the effect of glucagon to enhance flux through fructose-1,6-bisphosphatase and to suppress flux through phosphofructokinase. The discovery of fructose 2,6-bisphosphate has greatly furthered our understanding of regulation at the fructose 6-phosphate/fructose 1,6-bisphosphate substrate cycle.     

Fructose-1, 6-bisphosphatase in human fetal brain and liver during development

Activity of fructose-1,6-bisphosphatase (EC 3.1.3.11), one of the key gluconeogenic enzymes, was measured in human fetal brain and liver during development. Fructose-1,6-bisphosphatase was distributed throughout the different regions of the brain. In contrast to the partially purified enzyme from the brain, the liver enzyme was dependent on Mg²⁺ for maximal activity, EDTA, citrate, oleate and linoleate were stimulatory, whereas 5′-AMP inhibited the activity of the liver enzyme.     

The yeast GID complex, a novel ubiquitin ligase (E3) involved in the regulation of carbohydrate metabolism

Glucose-dependent regulation of carbon metabolism is a subject of intensive studies. We have previously shown that the switch from gluconeogenesis to glycolysis is associated with ubiquitin-proteasome linked elimination of the key enzyme fructose-1,6-bisphosphatase. Seven glucose induced degradation deficient (Gid)-proteins found previously in a genomic screen were shown to form a complex that binds FBPase. One of the subunits, Gid2/Rmd5, contains a degenerated RING finger domain. In an in vitro assay, heterologous expression of GST-Gid2 leads to polyubiquitination of proteins. In addition, we show that a mutation in the degenerated RING domain of Gid2/Rmd5 abolishes fructose-1,6-bisphosphatase polyubiquitination and elimination in vivo. Six Gid proteins are present in gluconeogenic cells. A seventh protein, Gid4/Vid24, occurs upon glucose addition to gluconeogenic cells and is afterwards eliminated. Forcing abnormal expression of Gid4/Vid24 in gluconeogenic cells leads to fructose-1,6-bisphosphatase degradation. This suggests that Gid4/Vid24 initiates fructose-1,6-bisphosphatase polyubiquitination by the Gid complex and its subsequent elimination by the proteasome. We also show that an additional gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, is subject to Gid complex-dependent degradation. Our study uncovers a new type of ubiquitin ligase complex composed of novel subunits involved in carbohydrate metabolism and identifies Gid4/Vid24 as a major regulator of this E3.     

Mechanism of action of 2,5-anhydro-D-mannitol in hepatocytes. Effects of phosphorylated metabolites on enzymes of carbohydrate metabolism

Isolated rat hepatocytes convert 2,5-anhydromannitol to 2,5-anhydromannitol-1-P and 2,5-anhydromannitol-1,6-P2. Cellular concentrations of the monophosphate and bisphosphate are proportional to the concentration of 2,5-anhydromannitol and are decreased by gluconeogenic substrates but not by glucose. Rat liver phosphofructokinase-1 phosphorylates 2,5-anhydromannitol-1-P; the rate is less than that for fructose-6-P but is stimulated by fructose-2,6-P2. At 1 mM fructose-6-P, bisphosphate compounds activate rat liver phosphofructokinase-1 in the following order of effectiveness: fructose-2,6-P2 much greater than 2,5-anhydromannitol-1,6-P2 greater than fructose-1,6-P2 greater than 2,5-anhydroglucitol-1,6-P2. High concentrations of fructose-1,6-P2 or 2,5-anhydromannitol-1,6-P2 inhibit phosphofructokinase-1. Rat liver fructose 1,6-bisphosphatase is inhibited competitively by 2,5-anhydromannitol-1,6-P2 and noncompetitively by 2,5-anhydroglucitol-1,6-P2. The AMP inhibition of fructose 1,6-bisphosphatase is potentiated by 2,5-anhydroglucitol-1,6-P2 but not by 2,5-anhydromannitol-1,6-P2. Rat liver pyruvate kinase is stimulated by micromolar concentrations of 2,5-anhydromannitol-1,6-P2; the maximal activation is the same as for fructose-1,6-P2. 2,5-Anhydroglucitol-1,6-P2 is a weak activator. 2,5-Anhydromannitol-1-P stimulates pyruvate kinase more effectively than fructose-1-P. Effects of glucagon on pyruvate kinase are not altered by prior treatment of hepatocytes with 2,5-anhydromannitol. Pyruvate kinase from glucagon-treated hepatocytes has the same activity as the control pyruvate kinase at saturating concentrations of 2,5-anhydromannitol-1,6-P2 but has a decreased affinity for 2,5-anhydromannitol-1,6-P2 and is not stimulated by 2,5-anhydromannitol-1-P. The inhibition of gluconeogenesis and enhancement of glycolysis from gluconeogenic precursors in hepatocytes treated with 2,5-anhydromannitol can be explained by an inhibition of fructose 1,6-bisphosphatase, an activation of pyruvate kinase, and an abolition of the influence of phosphorylation on pyruvate kinase.     

The gluconeogenic enzyme fructose-1,6-bisphosphatase is dispensable for growth of the yeast Yarrowia lipolytica in gluconeogenic substrates

The genes encoding gluconeogenic enzymes in the nonconventional yeast Yarrowia lipolytica were found to be differentially regulated. The expression of Y. lipolytica FBP1 (YlFBP1) encoding the key enzyme fructose-1,6-bisphosphatase was not repressed by glucose in contrast with the situation in other yeasts; however, this sugar markedly repressed the expression of YlPCK1, encoding phosphoenolpyruvate carboxykinase, and YlICL1, encoding isocitrate lyase. We constructed Y. lipolytica strains with two different disrupted versions of YlFBP1 and found that they grew much slower than the wild type in gluconeogenic carbon sources but that growth was not abolished as happens in most microorganisms. We attribute this growth to the existence of an alternative phosphatase with a high K_m (2.3 mM) for fructose-1,6-bisphosphate. The gene YlFBP1 restored fructose-1,6-bisphosphatase activity and growth in gluconeogenic carbon sources to a Saccharomyces cerevisiae fbp1 mutant, but the introduction of the FBP1 gene from S. cerevisiae in the Ylfbp1 mutant did not produce fructose-1,6-bisphosphatase activity or growth complementation. Subcellular fractionation revealed the presence of fructose-1,6-bisphosphatase both in the cytoplasm and in the nucleus.     

Influence of fructose 2,6-bisphosphate on the phosphofructokinase/fructose 1,6-bisphosphatase cycle

In a reconstituted enzyme system multiple stationary states and oscillatory motions of the substrate cycle catalyzed by phosphofructokinase and fructose 1,6-bisphosphatase are significantly influenced by fructose 2,6-bisphosphate. Depending on the initial conditions, fructose 2,6-bisphosphate was found either to generate or to extinguish oscillatory motions between glycolytic and gluconeogenic states. In general, stable glycolytic modes are favored because of the efficient activation of phosphofructokinase by this effector. The complex effect of fructose 2,6-bisphosphate on the rate of substrate cycling correlates with its synergistic cooperation with AMP in the activation of phosphofructokinase and inhibition of fructose 1,6-bisphosphatase.     

Gid9, a second RING finger protein contributes to the ubiquitin ligase activity of the Gid complex required for catabolite degradation

The two major antagonistic pathways of carbon metabolism in cells, glycolysis and gluconeogenesis, are tightly regulated. In the eukaryotic model organism Saccharomyces cerevisiae the switch from gluconeogenesis to glycolysis is brought about by proteasomal degradation of the gluconeogenic enzyme fructose-1,6-bisphosphatase. The ubiquitin ligase responsible for polyubiquitylation of fructose-1,6-bisphosphatase is the Gid complex. This complex consists of seven subunits of which subunit Gid2/Rmd5 contains a RING finger domain providing E3 ligase activity. Here we identify an additional subunit containing a degenerated RING finger, Gid9/Fyv10. This subunit binds to Gid2/Rmd5. A mutation in the degenerated RING finger of Gid9/Fyv10 abolishes polyubiquitylation and degradation of three enzymes specific for gluconeogenesis.     

“Sink” regulation of photosynthetic metabolism in transgenic tobacco plants expressing yeast invertase in their cell wall involves a decrease of the Calvin-cycle enzymes and an increase of glycolytic enzymes

Leaves on transgenic tobacco plants expressing yeast-derived invertase in the apoplast develop clearly demarcated green and bleached sectors when they mature. The green areas contain low levels of soluble sugars and starch which are turned over on a daily basis, and have high rates of photosynthesis and low rates of respiration. The pale areas accumulate carbohydrate, photosynthesis is inhibited, and respiration increases. This provides a model system to investigate the sink regulation of photosynthetic metabolism by accumulating carbohydrate. The inhibition of photosynthesis is accompanied by a decrease of ribulose-1,5-bisphosphate and glycerate-3-phosphate, and an increase of triosephosphate and fructose-1,6-bisphosphate. The extracted activities of ribulose-1,5-bisphosphate carboxylase, fructose-1, 6-bisphosphatase and NADP-glyeraldehyde-3-phosphate dehydrogenase decreased. The activity of sucrose-phosphate synthase remained high or increased, an increased portion of the photosynthate was partitioned into soluble sugars rather than starch, and the pale areas showed few or no oscillations during transitions between darkness and saturating light in saturating CO₂. The increased rate of respiration was accompanied by an increased level of hexose-phosphates, triose-phosphates and fructose-1,6-bisphosphate while glycerate-3-phosphate and phosphoenolpyruvate decreased and pyruvate increased. The activities of pyruvate kinase, phosphofructokinase and pyrophosphate: fructose-6-phosphate phosphotransferase increased two- to four-fold. We conclude that an increased level of carbohydrate leads to a decreased level of Calvin-cycle enzymes and, thence, to an inhibition of photosynthesis. It also leads to an increased level of glycolytic enzymes and, thence, to a stimulation of respiration. These changes of enzymes are more important in middle- or long-term adjustments to high carbohydrate levels in the leaf than fine regulation due to depletion of inorganic phosphate or high levels of phosphorylated metabolites.Abbreviations Fru 1,6bisP fructose-1,6-bisphosphate - Fru 1,6bisPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc 1P glucose-1-phosphate - Glc6P glucose-6-phosphate - NADP-GAPDH NADP-dependent glyceraldehyde-3-phosphate dehydrogenase - PFK phosphofructokinase - PEP phosphoenolpyruvate - PFP pyrophosphate:fructose-6-phosphate phosphotransferase - PGA glycerate-3-phosphate - PK pyruvate kinase - Pi inorganic phosphate - Ru1,5bisP ribulose-1,5-bisphosphate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - SPS sucrose-phosphate synthase - triose-P triose-phosphates     

Essential role of pyrophosphate homeostasis mediated by the pyrophosphate-dependent phosphofructokinase in Toxoplasma gondii

Many biosynthetic pathways produce pyrophosphate (PPi) as a by-product, which is cytotoxic if accumulated at high levels. Pyrophosphatases play pivotal roles in PPi detoxification by converting PPi to inorganic phosphate. A number of apicomplexan parasites, including Toxoplasma gondii and Cryptosporidium parvum, express a PPi-dependent phosphofructokinase (PPi-PFK) that consumes PPi to power the phosphorylation of fructose-6-phosphate. However, the physiological roles of PPi-PFKs in these organisms are not known. Here, we report that Toxoplasma expresses both ATP- and PPi-dependent phosphofructokinases in the cytoplasm. Nonetheless, only PPi-PFK was indispensable for parasite growth, whereas the deletion of ATP-PFK did not affect parasite proliferation or virulence. The conditional depletion of PPi-PFK completely arrested parasite growth, but it did not affect the ATP level and only modestly reduced the flux of central carbon metabolism. However, PPi-PFK depletion caused a significant increase in cellular PPi and decreased the rates of nascent protein synthesis. The expression of a cytosolic pyrophosphatase in the PPi-PFK depletion mutant reduced its PPi level and increased the protein synthesis rate, therefore partially rescuing its growth. These results suggest that PPi-PFK has a major role in maintaining pyrophosphate homeostasis in T. gondii. This role may allow PPi-PFK to fine-tune the balance of catabolism and anabolism and maximize the utilization efficiency for carbon nutrients derived from host cells, increasing the success of parasitism. Moreover, PPi-PFK is essential for parasite propagation and virulence in vivo but it is not present in human hosts, making it a potential drug target to combat toxoplasmosis.     

In silico exploration of the fructose-6-phosphate phosphorylation step in glycolysis: genomic evidence of the coexistence of an atypical ATP-dependent along with a PPi-dependent phosphofructokinase in Propionibacterium freudenreichii subsp. shermanii

We performed a detailed bioinformatic study of the catalytic step of fructose-6-phosphate phosphorylation in glycolysis based on the raw genomic draft of Propionibacterium freudenreichii subsp. shermanii (P. shermanii) ATCC9614 [Meurice et al., 2004]. Our results provide the first in silico evidence of the coexistence of genes coding for an ATP-dependent phosphofructokinase (ATP-PFK) and a PPi-dependent phosphofructokinase (PPi-PFK), whereas the fructose-1,6-bisphosphatase (FBP) and ADP-dependent phosphofructokinase (ADP-PFK) are absent. The deduced amino acid sequence corresponding to the PPi-PFK (AJ508922) shares 100% similarity with the already characterised propionibacterial protein (P29495; Ladror et al., 1991]. The unexpected ATP-PFK gene (AJ509827) encodes a protein of 373 aa which is highly similar (50% positive residues) along at least 95% of its sequence length to different well-characterised ATP-PFKs. The characteristic PROSITE pattern important for the enzyme function of ATP-PFKs (PS00433) was conserved in the putative ATP-PFK sequence: 8 out of 9 amino acid residues. According to the recent evolutionary study of PFK proteins with different phosphate donors [Bapteste et al., 2003], the propionibacterial ATP-PFK harbours a G104-K124 residue combination, which strongly suggested that this enzyme belongs to the group of atypical ATP-PFKs. According to our phylogenetic analyses the amino acid sequence of the ATP-PFK is clustered with the atypical ATP-PFKs from group III of the Siebers classification [Siebers et al., 1998], whereas the expected PPi-PFK protein is closer to the PPi-PFKs from clade P [Müller et al., 2001]. The possible significance of the co-existence of these two PFKs and their importance for the regulation of glycolytic pathway flux in P. shermanii is discussed.     

Spinach chloroplast fructose-1,6-bisphosphatase: identification of the subtilisin-sensitive region and of conserved histidines

Chloroplast fructose-1,6-bisphosphatase (FbPase) is an essential enzyme in the photosynthetic pathway of carbon dioxide fixation into sugars. The properties of the chloroplast enzyme are clearly distinct from those of cytosolic gluconeogenic FbPases. Light-dependent activation via a ferredoxin/thioredoxin system and insensitivity to inhibition by AMP are unique characteristics of the chloroplast enzyme. However, preliminary amino acid sequence data (78 residues) have demonstrated that a significant degree of amino acid sequence similarity exists between spinach chloroplast and mammalian gluconeogenic fructose-1,6-bisphosphatase [Harrsch, P.B., Kim, Y., Fox, J.L., & Marcus, F. (1985) Biochem. Biophys. Res. Commun. 133, 520-526]. In the present study, we have identified two structural features of spinach chloroplast FbPase that appear to be common to all FbPases. These include (a) the presence of a protease-sensitive area located in a region equivalent to residues 51-71 of mammalian FbPases and (b) the recognition of two conserved histidine residues, equivalent to histidines-253 and -311 of the mammalian enzymes. In addition, we have obtained sequence information accounting for more than three-fourths of the primary structure of spinach chloroplast FbPase. The high degree of homology observed between the chloroplast enzyme and gluconeogenic FbPases suggests a common evolutionary origin for all fructose-1,6-bisphosphatases in spite of their different functions and modes of regulation.     

Carbohydrate Metabolism and Activity of Pyrophosphate: Fructose-6-Phosphate Phosphotransferase in Photosynthetic Soybean (Glycine max, Merr.) Suspension Cells

Activity of pyrophosphate:fructose-6-phosphate phosphotransferase (PFP) was investigated in relation to carbohydrate metabolism and physiological growth stage in mixotrophic soybean (Glycine max Merr.) suspension cells. In the presence of exogenous sugars, log phase growth occurred and the cells displayed mixotrophic metabolism. During this stage, photosynthetic oxygen evolution was depressed and sugars were assimilated from the medium. Upon depletion of medium sugar, oxygen evolution and chlorophyll content increased, and cells entered stationary phase. Activities of various enzymes of glycolysis and sucrose metabolism, including PFP, sucrose synthase, fructokinase, glucokinase, UDP-glucose pyrophosphorylase, and fructose-1,6-bisphosphatase, changed as the cells went from log to stationary phases of growth. The largest change occurred in the activity of PFP, which was three-fold higher in log phase cells. PFP activity increased in cells grown on media initially containing sucrose, glucose, or fructose and began to decline when sugar in the medium was depleted. Western blots probed with antibody specific to the -subunit of potato PFP revealed a single 56 kilodalton immunoreactive band that changed in intensity during the growth cycle in association with changes in total PFP activity. The level of fructose-2,6-bisphosphate, an activator of the soybean PFP, increased during the first 24 hours after cell transfer and returned to the stationary phase level prior to the increase in PFP activity. Throughout the growth cycle, the calculated in vivo cytosolic concentration of fructose-2,6-bisphosphate exceeded by more than two orders of magnitude the previously reported activation coefficient (K_a) for soybean PFP. These results indicate that metabolism of exogenously supplied sugars by these cells involves a PFP-dependent step that is not coupled directly to sucrose utilization. Activity of this pathway appears to be controlled by changes in the level of PFP, rather than changes in the total cytosolic level of fructose-2,6-bisphosphate.     

High resolution phosphorus NMR spectroscopy of transfer ribonucleic acids

Summary A new activator of phosphofructokinase, which is bound to the enzyme and released during its purification, has been discovered. Its structure has been determined as -D Fructose-2,6-P₂ by chemical synthesis, analysis of various degradation products and NMR. D-Fructose-2,6-P₂ is the most potent activator of phosphofructokinase and relieves inhibition of the enzyme by ATP and citrate. It lowers the K_m for fructose-6-P from 6 mM to 0.1 mM.Fructose-6-P,2-kinase catalyzes the synthesis of fructose-2,6-P₂ from fructose-6-P and ATP, and the enzyme has been partially purified. The degradation of fructose-2,6-P₂ is catalyzed by fructose-2,6-bisphosphatase. Thus a metabolic cycle could occur between fructose-6-P and fructose-2,6-P₂, which are catalyzed by these two opposing enzymes. The activities of these enzymes can be controlled by phosphorylation. Fructose-6-P,2-kinase is inactivated by phosphorylation catalyzed by either cAMP dependent protein kinase or phosphorylase kinase. The inactive, phospho-fructose-6-P,2-kinase is activated by dephosphorylation catalyzed by phosphorylase phosphatase. On the other hand, fructose-2,6-bisphosphatase is activated by phosphorylation catalyzed by cAMP dependent protein kinase.Investigation into the hormonal regulation of phosphofructokinase reveals that glucagon stimulates phosphorylation of phosphofructokinase which results in decreased affinity for fructose-2,6-P₂, and decreases the fructose-2,6-P₂ levels. This decreased level in fructose-2,6-P₂ appears to be due to the decreased synthesis by inactivation of fructose-2,6-P₂,2-kinase and increased degradation as a result of activation of fructose-2,6-bisphosphatase. Such a reciprocal change in these two enzymes has been demonstrated in the hepatocytes treated by glucagon and epinephrine. The implications of these observations in respect to possible coordinated controls of glycolysis and glycogen metabolism are discussed.