Regulation of pea seed pyrophosphate-dependent phosphofructokinase: Evidence for interconversion of two molecular forms as a glycolytic regulatory mechanism

Molecular and kinetic characterisation of sugarcane pyrophosphate: fructose-6-phosphate 1-phosphotransferase and its possible role in the sucrose accumulation phenotype

The amount of pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) activity in sugarcane internodal tissue is inversely correlated with sucrose content. To help elucidate this apparent role of PFP in sucrose accumulation in sugarcane we have determined its molecular and kinetic properties. Sugarcane PFP was purified 285-fold to a final specific activity of 4.23 µmol min^-1 mg^-1 protein. It contained two polypeptides of 63.2 and 58.0 kDa respectively, at near equal amounts that cross-reacted with potato PFP-α and -β antiserum. In gel filtration analyses the native enzyme eluted in three peaks of 129, 245 and 511 kDa, corresponding to dimeric, tetrameric and octameric forms, respectively and fructose 2,6-bisphosphate (Fru 2,6-P₂) influenced this aggregation state. Both the glycolytic (forward) and gluconeogenic (reverse) reactions had relative broad pH optima between pH 6.7 and 8.0. The Fru 2,6-P₂ saturation curves were hyperbolic with approximate K_a values of 69 and 82 nm for the forward and reverse reactions, respectively. The enzyme showed hyperbolic saturation curves for all its substrates with K_m values comparable with that of other plant PFP, i.e. 150, 37, 39 and 460 µM for fructose 6-phosphate, inorganic pyrophosphate, fructose 1,6-bisphosphate and inorganic phosphate, respectively. Sugarcane PFP's molecular and kinetic characteristics differed slightly from that of other plant PFP in that: (i) Fru 2,6-P₂ directly induced the octameric state from the dimeric state; (ii) Fru 2,6-P₂ shifted the pH optimum for the forward reaction to a slightly more basic pH; and (iii) Fru 2,6-P₂ increased the V_max for the forward and reverse reactions by similar amounts.

Lepidimoic acid increases fructose 2,6-bisphosphate in Amaranthus seedlings

This study examines the influence of the growth promoter, lepidimoic acid, on the level of an important cytosolic signal metabolite, fructose 2,6-bisphosphate (Fru-2,6-P2), which can activate pyrophosphatedependent:phosphofructokinase (PFP, EC 2.7.1.90), and on glycolytic metabolism in Amaranthus caudatus seedlings. Fru-2,6-P2 concentrations were respectively increased by approximately 2-, 3- and 4-fold when the seedlings were treated with 0.3, 3 and 30 mM lepidimoic acid. Exogenous lepidimoic acid also affected levels of glycolytic intermediates in the seedlings. The increase in fructose 1,6-bisphosphate and decreases in fructose 6-phosphate and glucose 6-phosphate were found in response to the elevated concentration of lepidimoic acid. These results suggest that lepidimoic acid may affect glycolytic metabolism in the Amaranthus seedlings by increasing the activity of PFP due to increasing level of Fru-2,6-P2.

Pyrophosphate Dependent Phosphofructokinase of Citrullus lanatus: Molecular Forms and Expression of Subunits

During germination and seedling establishment, the total pyrophosphate-dependent phosphofructokinase (PFP) activity in the cotyledons increases. Two types of subunits with molecular weights of 68 (alpha-subunit) and 65 (beta-subunit) kilodaltons are present. The increase in activity coincides with an approximately 10-fold increase in beta-subunit and twofold increase in alpha-subunit content. Different isoforms of PFP are present at all stages of incubation, but the ratio between the isoforms significantly changes. A linear relationship exists between the ratio of the two PFP subunits and the ratio of the two isoforms of the enzyme. The more anionic (peak 2) isoform of the enzyme apparently is favored by a high ratio of total beta-subunit to alpha-subunit content. The beta- to alpha-subunit ratio of the peak 2 isoform is also approximately fivefold higher than that of the peak 1 (less anionic) isoform. It is evident that the two subunits are not coordinately expressed and the level of expression of each subunit appears to be the primary factor determining the molecular form in which the enzyme is present. In some tissues, only the 65 kilodalton polypeptide is expressed in large amounts. The peak 1 isoform has a higher affinity for pyrophosphate than the peak 2 isoform, while the affinity for fructose-6-phosphate is similar. Both molecular forms are activated by fructose-2,6-bisphosphate.

Pyrophosphate Fructose-6-P 1-Phosphotransferase from Tomato Fruit : Evidence for Change during Ripening

Three forms of pyrophosphate fructose-6-phosphate 1-phosphotransferase (PFP) were purified from both green and red tomato (Lycopersicon esculentum) fruit: (a) a classical form (designated Q(2)) containing alpha- (66 kilodalton) and beta- (60 kilodalton) subunits; (b) a form (Q(1)) containing a beta-doublet subunit; and (c) a form (Q(0)) that appeared to contain a beta-singlet subunit. Several lines of evidence suggested that the different forms occur under physiological conditions. Q(2) was purified to apparent electrophoretic homogeneity; Q(1) and Q(0) were highly purified, but not to homogeneity. The distribution of the PFP forms from red (versus green) tomato was: Q(2), 29% (90%); Q(1), 47% (6%); and Q(0), 24% (4%). The major difference distinguishing the red from the green tomato enzymes was the fructose-2,6-bisphosphate (Fru-2,6-P(2))-induced change in K(m) for fructose-6-phosphate (Fru-6-P), the ;green forms' showing markedly enhanced affinity on activation (K(m) decrease of 7-9-fold) and the ;red forms' showing either little change (Q(0), Q(1)) or a relatively small (2.5-fold) affinity increase (Q(2)). The results extend our earlier findings with carrot root to another tissue and indicate that forms of PFP showing low or no affinity increase for Fru 6-P on activation by Fru-2,6-P(2) (here Q(1) and Q(0)) are associated with sugar storage, whereas the classical form (Q(2)), which shows a pronounced affinity increase, is more important for starch storage.

Banana ripening: implications of changes in glycolytic intermediate concentrations, glycolytic and gluconeogenic carbon flux, and fructose 2,6-bisphosphate concentration

In ripening banana (Musa sp. [AAA group, Cavendish subgroup] cv Valery) fruit, the concentration of glycolytic intermediates increased in response to the rapid conversion of starch to sugars and CO(2). Glucose 6-phosphate (G-6-P), fructose 6-phosphate (Fru 6-P), and pyruvate (Pyr) levels changed in synchrony, increasing to a maximum one day past the peak in ethylene synthesis and declining rapidly thereafter. Fructose 1,6-bisphosphate (Fru 1,6-P(2)) and phosphoenolpyruvate (PEP) levels underwent changes dissimilar to those of G 6-P, Fru 6-P, and Pyr, indicating that carbon was regulated at the PEP/Pyr and Fru 6-P/Fru 1,6-P(2) interconversion sites. During the climacteric respiratory rise, gluconeogenic carbon flux increased 50- to 100-fold while glycolytic carbon flux increased only 4- to 5-fold. After the climacteric peak in CO(2) production, gluconeogenic carbon flux dropped dramatically while glycolytic carbon flux remained elevated. The steady-state fructose 2,6-bisphosphate (Fru 2,6-P(2)) concentration decreased to (1/2) that of preclimacteric fruit during the period coinciding with the rapid increase in gluconeogenesis. Fru 2,6-P(2) concentration increased thereafter as glycolytic carbon flux increased relative to gluconeogenic carbon flux. It appears likely that the initial increase in respiration in ripening banana fruit is due to the rapid influx of carbon into the cytosol as starch is degraded. As starch reserves are depleted and the levels of intermediates decline, the continued enhancement of respiration may, in part, be maintained by an increased steady-state Fru 2,6-P(2) concentration acting to promote glycolytic carbon flux at the step responsible for the interconversion of Fru 6-P and Fru 1,6-P(2).

Properties of Pyrophosphate:Fructose-6-Phosphate Phosphotransferase from Endosperm of Developing Wheat (Triticum aestivum L.) Grains

Pyrophosphate:fructose-6-phosphate phosphotransferase (PFP, EC 2.7.1.90) from endosperm of developing wheat (Triticum aestivum L.) grains was purified to apparent homogeneity with about 52% recovery using ammonium sulfate fractionation, ion exchange chromatography on DEAE-cellulose and gel filtration through Sepharose-CL-6B. The purified enzyme, having a molecular weight of about 170,000, was a dimer with subunit molecular weights of 90,000 and 80,000, respectively. The enzyme exhibited maximum activity at pH 7.5 and was highly specific for pyrophosphate (PPi). None of the nucleoside mono-, di- or triphosphate could replace PPi as a source of energy and inorganic phosphate (Pi). Similarly, the enzyme was highly specific for fructose-6-phosphate. It had a requirement for Mg(2+) and exhibited hyperbolic kinetics with all substrates including Mg(2+). K(m) values as determined by Lineweaver-Burk plots were 322, 31, 139, and 129 micromolar, respectively, for fructose-6-phosphate, PPi, fructose-1,6-bisphosphate and Pi. Kinetic constants were determined in the presence of fructose-2,6-bisphosphate, which stimulated activity about 20-fold and increased the affinity of the enzyme for its substrates. Initial velocity studies indicated kinetic mechanism to be sequential. At saturating concentrations of fructose-2,6-bisphosphate (1 micromolar), Pi strongly inhibited PFP; the inhibition being mixed with respect to both fructose-6-phosphate and PPi, with K(i) values of 0.78 and 1.2 millimolar, respectively. The inhibition pattern further confirmed the mechanism to be sequential with random binding of the substrates. Probable role of PFP in endosperm of developing wheat grains (sink tissues) is discussed.

Characterization of Sucrolysis via the Uridine Diphosphate and Pyrophosphate-Dependent Sucrose Synthase Pathway

The breakdown of sucrose to feed both hexoses into glycolytic carbon flow can occur by the sucrose synthase pathway. This uridine diphosphate (UDP) and pyrophosphate (PPi)-dependent pathway was biochemically characterized using soluble extracts from several plants. The sucrolysis process required the simultaneous presence of sucrose, UDP, and PPi with their respective K(m) values being about 40 millimolar, 23 micromolar, and 29 micromolar. UDP was the only active nucleotide diphosphate. Slightly alkaline pH optima were observed for sucrose breakdown either to glucose 1-phosphate or to triose phosphate. Sucrolysis incrased with increasing temperature to near 50 degrees C and then a sharp drop occurred between 55 and 60 degrees C. The breakdown of sucrose to triose-P was activated by fructose 2,6-P(2) which had a K(m) value near 0.2 micromolar. The cytoplasmic phosphofructokinase and fructokinase in plants were fairly nonselective for nucleotide triphosphates (NTP) but glucokinase definitely favored ATP. A predicted stoichiometric relationship of unity for UDP and PPi was measured when one also measured competing UDPase and pyrophosphatase activity. The cycling of uridylates, UDP to UTP to UDP, was demonstrated both with phosphofructokinase and with fructokinase. Enzyme activity measurements indicated that the sucrose synthase pathway has a major role in plant sucrose sink tissues. In the cytoplasmic sucrose synthase breakdown pathway, a role for the PPi-phosphofructokinase was to produce PPi while a role for the NTP-phosphofructokinase and for the fructokinase was to produce UDP.

Sucrose metabolism in lima bean seeds

Developing and germinating lima bean (Phaseolus lunatus var Cangreen) seeds were used for testing the sucrose synthase pathway, to examine the competition for uridine diphosphate (UDP) and pyrophosphate (PPi), and to identify adaptive and maintenance-type enzymes in glycolysis and gluconeogenesis. In developing seeds, sucrose breakdown was dominated by the sucrose synthase pathway; but in the seedling embryos, both the sucrose synthase pathway and acid invertase were active. UDPase activity was low and seemingly insufficient to compete for UDP during sucrose metabolism in seed development or germination. In contrast, both an acid and alkaline pyrophosphatase were active in seed development and germination. The set of adaptive enzymes identified in developing seeds were sucrose synthase, PPi-dependent phosphofructokinase, plus acid and alkaline pyrophosphatase; and, the adaptive enzymes identified in germinating seeds included the same set of enzymes plus acid invertase. The set of maintenance enzymes identified during development, in the dry seed, and during germination were UDP-glucopyrophosphorylase, neutral invertase, ATP and UTP-dependent fructokinase, glucokinase, phosphoglucomutase, ATP and UTP-dependent phosphofructokinase and sucrose-P synthase.

Glycolytic activity in embryos of Pisum sativum and of non-dormant or dormant seeds of Avena sativa L. expressed through activities of PFK and PPi-PFK

A quantitative cytochemical assay for PPi-PFK activity in the presence of Fru-2,6-P2 is described along with its application to determine levels of activity in embryos of Pisum sativum and Avena sativa. The activity of ATP-PFK has also been studied in parallel as have PFK activities during the switch from dormant to non-dormant embryos in Avena sativa. PPi-PFK activity has been demonstrated in all tissues of Pisum sativum embryos and of Avena sativa embryos including the scutellum and the aleurone layers. The PPi-PFK activity was greater than that of ATP-PFK in both dormant and non-dormant seeds though with only marginally more activity in the dormant as opposed to the non-dormant state.

Reduced Cytosolic Fructose-1,6-Bisphosphatase Activity Leads to Loss of O(2) Sensitivity in a Flaveria linearis Mutant

The mutant plant of Flaveria linearis characterized by Brown et al. (Plant Physiol. 81: 212-215) was studied to determine the cause of the reduced sensitivity to O(2). Analysis of CO(2) assimilation metabolites of freeze clamped leaves revealed that both 3-phosphoglycerate and ribulose 1,5-bisphosphate were high in the mutant plant relative to F. linearis with normal O(2) sensitivity. The k(cat) of ribulose-1,5-bisphosphate carboxylase (RuBPCase) was equal in all plant material tested (range 18-22 s(-1)) indicating that no tight binding inhibitor was present. The degree of RuBPCase carbamylation was reduced in the mutant plant relative to the wild-type plant. Since 3-phosphoglycerate was high in the mutant plant and photosynthesis did not exhibit properties associated with RuBPCase limitations, we believe that the decarbamylation of RuBPCase was a consequence of another lesion in photosynthesis. Fructose 1,6-bisphosphate and its precursors, such as the triose phosphates, were in high concentration in the mutant plant relative to the wild type. The concentrations of the product of the fructose 1,6-bisphosphatase reaction, fructose 6-phosphate, and its isomer, glucose 6-phosphate, were the same in both plants. We found that the mutant plant had up to 75% less cytosolic fructose 1,6-bisphosphatase activity than the wild type but comparable levels of stromal fructose 1,6-bisphosphatase. We conclude that the reduced fructose-1,6-bisphosphatase activity restricts the mutant plant's capacity for sucrose synthesis and this leads to reduced or reversed O(2) sensitivity.

Day-night changes in the levels of adenine nucleotides, phosphoenolpyruvate and inorganic pyrophosphate in leaves of plants having Crassulacean acid metabolism

The levels of phosphorylated compounds studied during the dark period of Crassulacean acid metabolism (CAM) in Kalanchoë leaves showed increases for ATP and pyrophosphate and decreases for ADP, AMP and phosphenolpyruvate; levels of inorganic phosphate remained constant. Changes in adenylate levels and the correlated nocturnal increase in adenylate-energycharge were closely related to changes in malate levels. The increase in ATP levels was much inhibited in CO2-free air and stimulated after induction of CAM in short-day-treated plants of K. blossfeldiana cv. Tom Thumb. Changes in levels of phosphoenolpyruvate and pyrophosphate were independent of the presence of CO2. The results show the operation of complex regulatory mechanisms in the energy metabolism of CAM plants during nocturnal malic-acid accumulation.

Banana Ripening: Implications of Changes in Internal Ethylene and CO(2) Concentrations, Pulp Fructose 2,6-Bisphosphate Concentration, and Activity of Some Glycolytic Enzymes

In ripening banana (Musa acuminata L. [AAA group, Cavandish subgroup] cv. Valery) fruit, the steady state concentration of the glycolytic regulator fructose 2,6-bisphosphate (Fru 2,6-P(2)) underwent a transient increase 2 to 3 hours before the respiratory rise, but coincident with the increase in ethylene synthesis. Fru 2,6-P(2) concentration subsequently decreased, but increased again approximately one day after initiation of the respiratory climacteric. This second rise in Fru 2,6-P(2) continued as ripening proceeded, reaching approximately five times preclimacteric concentration. Pyrophosphate-dependent phosphofructokinase glycolytic activity exhibited a transitory rise during the early stages of the respiratory climacteric, then declined slightly with further ripening. Cytosolic fructose 1,6-bisphosphatase activity did not change appreciably during ripening. The activity of ATP-dependent phosphofructokinase increased approximately 1.6-fold concurrent with the respiratory rise. A balance in the simultaneous glycolytic and gluconeogenic carbon flow in ripening banana fruit appears to be maintained through changes in substrate levels, relative activities of glycolytic enzymes and steady state levels of Fru 2,6-P(2).

Comparison of the Activities and Some Properties of Pyrophosphate and ATP Dependent Fructose-6-Phosphate 1-Phosphotransferases of Phaseolus vulgaris Seeds

THE DISTRIBUTION OF PYROPHOSPHATE: fructose 6-phosphate phosphotransferase (PFP) and ATP: fructose-6-phosphate 1-phosphotransferase (PFK) was studied in germinating bean (Phaseolus vulgaris cv Top Crop) seeds. In the cotyledons the PFP activity was comparable with that of PFK. However, in the plumule and radicle plus hypocotyl, PFP activity exceeds that of PFK. Approximately 70 to 90%, depending on the stage of germination, of the total PFP and PFK activities were present in the cotyledons. Highest specific activity of both enzymes, however, occurred in the radicle plus hypocotyl (64-90 nanomoles.min.milligram protein). Fractionation studies indicate that 40% of the total PFK activity was associated with the plastids while PFP is apparently confined to the cytoplasm. The cytosolic isozyme of PFK exhibits hyperbolic kinetics with respect to fructose 6-P and ATP with K(m) values of 320 and 46 micromolar, respectively. PFP also exhibits hyperbolic kinetics both in the presence and absence of the activator fructose-2,6-P(2). The activation is caused by lowering the K(m) for fructose 6-P from 18 to 1.1 millimolar and that for pyrophosphate (PPi) from 40 to 25 micromolar, respectively. Levels of fructose 2,6-P(2) and PPi in the seeds are sufficient to activate PFP and thereby enable a glycolytic role for PFP during germination. However, the fructose 6-P content appears to be well below the K(m) of PFP for this compound and would therefore preferentially bind to the catalytic site of PFK, which has a lower K(m) for fructose 6-P. The ATP content appears to be at saturating levels for PFK.

A novel sucrose synthase pathway for sucrose degradation in cultured sycamore cells

Enzymes of sucrose degradation and glycolysis in cultured sycamore (Acer pseudoplatanus L.) cells were assayed and characterized in crude extracts and after partial purification, in an attempt to identify pathways for sucrose catabolism. Desalted cell extracts contained similar activities (20-40 nanomoles per milligram protein per minute) of sucrose synthase, neutral invertase, glucokinase, fructokinase, phosphofructokinase, and UDPglucose pyrophosphorylase (assayed with 2 micromolar pyrophosphate (PPi). PPi-linked phosphofructokinase activity was virtually dependent upon fructose 2,6-bisphosphate, and the maximum activity exceeded that of ATP-linked phosphofructokinase. Hexokinase activity, with glucose as substrate, was highly specific for ATP, whereas fructokinase activity was relatively nonspecific. At 1 millimolar nucleoside triphosphate, fructokinase activity decreased in the order: UTP > ATP > CTP > GTP. We propose two pathways for sucrose degradation. One involves invertase action, followed by classical glycolysis of hexose sugars, and the other is a novel pathway initiated by sucrose synthase. The K(m) for sucrose of sucrose synthase was severalfold lower than that of neutral invertase (15 versus 65 millimolar), which may determine carbon partitioning between the two pathways. The sucrose synthase pathway proposed involves cycling of uridylates and PPi. UDPglucose pyrophosphorylase, which is shown to be an effective ;PPi-scavenger,' would consume PPi and form UTP. The UTP could be then utilized in the UTP-linked fructokinase reaction, thereby forming UDP for sucrose synthase. The source of PPi is postulated to arise from the back reaction of PPi-linked phosphofructokinase. Sycamore cells contained a substantial endogenous pool of PPi (about 3 nanomoles per gram fresh weight, roughly 1/10 the amount of ATP in these cells), and sufficient fructose 2,6-bisphosphate (0.09 nanomole per gram fresh weight) to activate the PPi-linked phosphofructokinase. Possible regulation and energetic differences between the sucrose synthase and invertase pathways are discussed.

PPi-dependent phosphofructotransferase (phosphofructokinase) activity in the mollicutes (mycoplasma) Acholeplasma laidlawii

A PPi-dependent phosphofructotransferase (PPi-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.90) which catalyzes the conversion of fructose 6 phosphate (F-6-P) to fructose 1,6-bisphosphate (F-1, 6-P2) was isolated from a cytoplasmic fraction of Acholeplasma laidlawii B-PG9 and partially purified (430-fold). PPi was required as the phosphate donor. ATP, dATP, CTP, dCTP, GTP, dGTP, UTP, dUTP, ITP, TTP, ADP, or Pi could not substitute for PPi. The PPi-dependent reaction (2.0 mM PPi) was not altered in the presence of any of these nucleotides (2.0 mM) or in the presence of smaller (less than or equal to 300 microM) amounts of fructose 2,6-bisphosphate, (NH4)2SO4, AMP, citrate, GDP, or phosphoenolpyruvate. Mg2+ and a pH of 7.4 were required for maximum activity. The partially purified enzyme in sucrose density gradient experiments had an approximate molecular weight of 74,000 and a sedimentation coefficient of 6.7. A second form of the enzyme (molecular weight, 37,000) was detected, although in relatively smaller amounts, by using Blue Sepharose matrix when performing electrophoresis experiments. The back reaction, F-1, 6-P2 to F-6-P, required Pi; arsenate could substitute for Pi, but not PPi or any other nucleotide tested. The computer-derived kinetic constants (+/- standard deviation) for the reaction in the PPi-driven direction of F-1, 6-P2 were as follows: v, 38.9 +/- 0.48 mM min-1; Ka(PPi), 0.11 +/- 0.04 mM; Kb(F-6-P), 0.65 +/- 0.15 mM; and Kia(PPi), 0.39 +/- 0.11 mM. A. laidlawii B-PG9 required PPi not only for the PPi-phosphofructotransferase reaction which we describe but also for purine nucleoside kinase activity. a dependency unknown in any other organism. In A. laidlawii B-PG9, the PPi requirement may be met by reactions in this organism already known to synthesize PPi (e.g., dUTPase and purine nucleobase phosphoribosyltransferases). In almost all other cells, the conversion of F-6-P to F-1,6-P2 is ATP dependent, and the reaction is generally considered to be the rate-limiting step of glycolysis. The ability of A. laidlawii B-PG9 and one other acholeplasma to use PPi instead of ATP as an energy source may offer these cytochrome-deficient organisms some metabolic advantage and may represent a conserved metabolic remnant of an earlier evolutionary process.

Rapid oscillations in fructose 2,6-bisphosphate levels in plant tissues

The fructose 2,6-bisphosphate (Fru 2,6-P(2)) content of pea, Pisum sativum, roots and leaves were measured following flooding with water and found to change in times of minutes and to exhibit oscillatory-type changes. Each organ changes its Fru 2,6-P(2) content in a unique pattern in response to environmental disturbances such as flooding or light. For example, when roots of intact illuminated pea plants are flooded, roots decrease their Fru 2,6-P(2) content while simultaneously leaves increase their Fru 2,6-P(2) content; but both organs exhibit oscillatory-type patterns within flooding time of about 30 minutes. Half-change times can be as rapid as 2 to 3 minutes. The endogenous extractable activity of the root pyrophosphate-dependent phosphofructokinase also exhibits an oscillatory pattern upon root immersion slightly after Fru 2,6-P(2) changes occur. We postulate from these results that Fru 2,6-P(2) is a primary signal molecule which enables plants to regulate their metabolism to cope with changing environments.