Cytosolic phosphofructokinase from spinach leaves : I. Purification, characteristics, and regulation

Cytosolic phosphofructokinases are important for sugar homeostasis in leaves of Arabidopsis thaliana

BACKGROUND AND AIMS

ATP-dependent phosphofructokinases (PFKs) catalyse phosphorylation of the carbon-1 position of fructose-6-phosphate, to form fructose-1,6-bisphosphate. In the cytosol, this is considered a key step in channelling carbon into glycolysis. Arabidopsis thaliana has seven genes encoding PFK isoforms, two chloroplastic and five cytosolic. This study focuses on the four major cytosolic isoforms of PFK in vegetative tissues of A. thaliana.

METHODS

We isolated homozygous knockout individual mutants (pfk1, pfk3, pfk6 and pfk7) and two double mutants (pfk1/7 and pfk3/6), and characterized their growth and metabolic phenotypes.

KEY RESULTS

In contrast to single mutants and the double mutant pfk3/6 for the hypoxia-responsive isoforms, the double mutant pfk1/7 had reduced PFK activity and showed a clear visual and metabolic phenotype with reduced shoot growth, early flowering and elevated hexose levels. This mutant also has an altered ratio of short/long aliphatic glucosinolates and an altered root-shoot distribution. Surprisingly, this mutant does not show any major changes in short-term carbon flux and in levels of hexose-phosphates.

CONCLUSIONS

We conclude that the two isoforms PFK1 and PFK7 are important for sugar homeostasis in leaf metabolism and apparently in source-sink relationships in A. thaliana, while PFK3 and PFK6 only play a minor role under normal growth conditions.

Biochemical characterization of phosphoenolpyruvate carboxykinases from Arabidopsis thaliana

ATP-dependent phosphoenolpyruvate carboxykinases (PEPCKs, EC 4.1.1.49) from C4 and CAM plants have been widely studied due to their crucial role in photosynthetic CO2 fixation. However, our knowledge on the structural, kinetic and regulatory properties of the enzymes from C3 species is still limited. In this work, we report the recombinant production and biochemical characterization of two PEPCKs identified in Arabidopsis thaliana: AthPEPCK1 and AthPEPCK2. We found that both enzymes exhibited high affinity for oxaloacetate and ATP, reinforcing their role as decarboxylases. We employed a high-throughput screening for putative allosteric regulators using differential scanning fluorometry and confirmed their effect on enzyme activity by performing enzyme kinetics. AthPEPCK1 and AthPEPCK2 are allosterically modulated by key intermediates of plant metabolism, namely succinate, fumarate, citrate and α-ketoglutarate. Interestingly, malate activated and glucose 6-phosphate inhibited AthPEPCK1 but had no effect on AthPEPCK2. Overall, our results demonstrate that the enzymes involved in the critical metabolic node constituted by phosphoenolpyruvate are targets of fine allosteric regulation.

Acclimation responses of Arabidopsis thaliana to sustained phosphite treatments

Phosphite (H₂PO⁻₃) induces a range of physiological and developmental responses in plants by disturbing the homeostasis of the macronutrient phosphate. Because of its close structural resemblance to phosphate, phosphite impairs the sensing, membrane transport, and subcellular compartmentation of phosphate. In addition, phosphite induces plant defence responses by an as yet unknown mode of action. In this study, the acclimation of Arabidopsis thaliana plants to a sustained phosphite supply in the growth medium was investigated and compared with plants growing under varying phosphate supplies. Unlike phosphate, phosphite did not suppress the formation of lateral roots in several Arabidopsis accessions. In addition, the expression of well-documented phosphate-starvation-induced genes, such as miRNA399d and At4, was not repressed by phosphite accumulation, whilst the induction of PHT1;1 and PAP1 was accentuated. Thus, a mimicking of phosphate by phosphite was not observed for these classical phosphate-starvation responses. Metabolomic analysis of phosphite-treated plants showed changes in several metabolite pools, most prominently those of aspartate, asparagine, glutamate, and serine. These alterations in amino acid pools provide novel insights for the understanding of phosphite-induced pathogen resistance.

Characterisation of the ATP-dependent phosphofructokinase gene family from Arabidopsis thaliana

Plants possess two different types of phosphofructokinases, an ATP-dependent (PFK) and a pyrophosphate-dependent form (PFP). While plant PFPs have been investigated in detail, cDNA clones coding for PFK have not been identified in Arabidopsis thaliana. Searching the A. thaliana genome revealed 11 putative members of a phosphofructokinase gene family. Among those, four sequences showed high homology to the alpha- or beta-subunits of plant PFPs. Seven cDNAs resulted in elevated PFK, but not PFP activity after transient expression in tobacco leaves suggesting that they encode Arabidopsis PFKs. RT-PCR revealed different tissue-specific expression of the individual forms. Furthermore, analysis of GFP fusion proteins indicated their presence in different sub-cellular compartments.

Purification, microsequencing and cloning of spinach ATP-dependent phosphofructokinase link sequence and function for the plant enzyme

Despite its importance in plant metabolism, no sequences of higher plant ATP-dependent phosphofructokinase (EC 2.7.1.11) are annotated in the databases. We have purified the enzyme from spinach leaves 309-fold to electrophoretic homogeneity. The purified enzyme was a homotetramer of approximately 52 kDa subunits with a specific activity of 600 mU x mg(-1) and a Km value for ATP of 81 microm. The purified enzyme was not activated by phosphate, but slightly inhibited instead, suggesting that it was the chloroplast isoform. The inclusion of adenosine 5'-(beta,gamma-imido)triphosphate was conducive to enzyme activity during the purification protocol. The sequences of eight tryptic peptides from the final protein preparation, which did not utilize pyrophosphate as a phosphoryl donor, were determined and an exactly corresponding cDNA was cloned. The sequence of enzymatically active spinach ATP-dependent phosphofructokinase suggests that a large family of genomics-derived higher plant sequences currently annotated in the databases as putative pyrophosphate-dependent phosphofructokinases according to sequence similarity is misannotated with respect to the cosubstrate.

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.

The role of inorganic phosphate in the regulation of C4 photosynthesis

Inorganic phosphate participates in many fundamental processes within the plant cell. Its broad influence on plant metabolism is related to such key operations as metabolite transport, enzyme regulation and carbohydrate metabolism in general. This review discusses these topics with special emphasis on the role assigned to this ubiquitous anion within the C4 pathway of photosynthesis.

Cytosolic Phosphofructokinase from Spinach Leaves : II. Affinity for Mg and Nucleoside Phosphates

Cytosolic ATP-phosphofructokinase (PFK) from spinach leaves (Spinacia oleracea L.) was inhibited by submillimolar concentrations of free Mg(2+). The free Mg(2+) concentration required for 50% inhibition of PFK activity was 0.22 millimolar. Inhibition by free Mg(2+) was independent of the MgATP(2-) concentration. Inorganic phosphate (Pi) reduces the inhibition of PFK activity by Mg(2+). Free ATP (ATP(4-)) also inhibits PFK activity. For free ATP the inhibition of PFK activity was dependent on the MgATP(2-) concentration. Fifty percent inhibition of PFK activity requires 1.2 and 3.7 millimolar free ATP at 0.1 and 0.5 millimolar MgATP(2-), respectively. It was proposed that free ATP competes for the MgATP(2-) binding site, whereas free Mg(2+) does not. Pi diminished the inhibitory effect of free ATP on PFK activity. Free ATP and Pi had substantial effects on the MgATP(2-) requirement of cytosolic PFK. For half-maximum saturation of PFK activity 3 and 76 micromolar MgATP(2-) was required at 0.007 and 0.8 millimolar free ATP in the absence of Pi. At 5 and 25 millimolar Pi, half-maximum saturation was achieved at 9 and 14 micromolar MgATP(2-). PFK activity was inhibited by Ca(2+). The inhibition by Ca(2+) depends upon the total Mg(2+) concentration. Fifty percent inhibition of PFK activity required 22 and 32 micromolar Ca(2+) at 0.1 and 0.2 millimolar Mg(2+), respectively. At physiological concentrations of about 0.5 millimolar free Mg(2+), Ca(2+) would have little effect on cytosolic PFK activity from spinach leaves. PFK is not absolutely specific for the nucleoside 5'-triphosphate substrate. Besides MgATP(2-), MgUTP(2-), MgCTP(2-), and MgGTP(2-) could be used as a substrate. All four free nucleotides inhibit PFK activity. The physiological consequences of the regulatory properties of cytosolic PFK from spinach leaves will be discussed. A model will be introduced, in an attempt to describe the complex interaction of PFK with substrates and the effectors Mg(2+) and Pi.