Mechanism of modulation of rat liver fructose-2,6-bisphosphatase by nucleoside triphosphates

Enzymatic preparation of high-specific-activity beta-D-[6,6'-3H]fructose-2,6-bisphosphate: Application to a sensitive assay for fructose-2,6-bisphosphatase

beta-D-Fructose-2,6-bisphosphate (Fru-2,6-P(2)) is an important regulator of eukaryotic glucose homeostasis, functioning as a potent activator of 6-phosphofructo-1-kinase and inhibitor of fructose-1,6-bisphosphatase. Pharmaceutical manipulation of intracellular Fru-2,6-P(2) levels, therefore, is of interest for the treatment of certain diseases, including diabetes and cancer. [2-(32)P]Fru-2,6-P(2) has been the reagent of choice for studying the metabolism of this effector molecule; however, its short half-life necessitates frequent preparation. Here we describe a convenient, economical, one-pot enzymatic preparation of high-specific-activity tritium-labeled Fru-2,6-P(2). The preparation involves conversion of readily available, carrier-free d-[6,6'-(3)H]glucose to [6,6'-(3)H]Fru-2,6-P(2) using hexokinase, glucose-6-phosphate isomerase, and 6-phosphofructo-2-kinase. The key reagent in this preparation, bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from human liver, was produced recombinantly in Escherichia coli and purified in a single step using an appendant C-terminal hexa-His affinity tag. Following purification by anion exchange chromatography using triethylammonium bicarbonate as eluant, radiochemically pure [6,6'-(3)H]Fru-2,6-P(2) having a specific activity of 50 Ci/mmol was obtained in yields averaging 35%. [6,6'-(3)H]Fru-2,6-P(2) serves as a stable, high-specific-activity substrate in a facile assay capable of detecting fructose-2,6-bisphosphatase in the range of 10(-14) to 10(-15) mol, and it should prove to be useful in many studies of the metabolism of this important biofactor.

Binding of ATP to the fructose-2,6-bisphosphatase domain of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase leads to activation of its 6-phosphofructo-2-kinase

To understand the mechanism by which the activity of the 6-phosphofructo-2-kinase (6PF-2K) of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is stimulated by its substrate ATP, we studied two mutants of the enzyme. Mutation of either Arg-279, the penultimate basic residue within the Walker A nucleotide-binding fold in the bisphosphatase domain, or Arg-359 to Ala eliminated the activation of the chicken 6PF-2K by ATP. Binding analysis by fluorescence spectroscopy using 2'(3')-O-(N-methylanthraniloyl)-ATP revealed that the kinase domains of these two mutants, unlike that of the wild type enzyme, showed no cooperativity in ATP binding and that the mutant enzymes possess only the high affinity ATP binding site, suggesting that the ATP binding site on the bisphosphatase domain represents the low affinity site. This conclusion was supported by the result that the affinity of ATP for the isolated bisphosphatase domain is similar to that for the low affinity site in the wild type enzyme. In addition, we found that the 6PF-2K of a chimeric enzyme, in which the last 25 residues of chicken enzyme were replaced with those of the rat enzyme, could not be activated by ATP, despite the fact that the ATP-binding properties of this chimeric enzyme were not different from those of the wild type chicken enzyme. These results demonstrate that activation of the chicken 6PF-2K by ATP may result from allosteric binding of ATP to the bisphosphatase domain where residues Arg-279 and Arg-359 are critically involved and require specific C-terminal sequences.

PFK-2/FBPase-2: maker and breaker of the essential biofactor fructose-2,6-bisphosphate

Fructose-2,6-bisphosphate is responsible for mediating glucagon-stimulated gluconeogenesis in the liver. This discovery has led to the realization that this compound plays a significant role in directing carbohydrate fluxes in all eukaryotes. Biophysical studies of the enzyme that both synthesizes and degrades this biofactor have yielded insight into its molecular enzymology. Moreover, the metabolic role of fructose-2,6-bisphosphate has great potential in the treatment of diabetes.

Overexpression of fructose 2,6-bisphosphatase decreases glycolysis and delays cell cycle progression

The ability to overexpress 6-phosphofructo-2-kinase/fructose 2, 6-bisphosphatase (PFK-2)/(FBPase-2) or a truncated form of the enzyme with only the bisphosphatase domain allowed us to analyze the relative role of the kinase and the bisphosphatase activities in regulating fructose 2,6-bisphosphate (Fru-2,6-P(2)) concentration and to elucidate their differential metabolic impact in epithelial Mv1Lu cells. The effect of overexpressing PFK-2/FBPase-2 resulted in a small increase in the kinase activity and in the activity ratio of the bifunctional enzyme, increasing Fru-2,6-P(2) levels, but these changes had no major effects on cell metabolism. In contrast, expression of the bisphosphatase domain increased the bisphosphatase activity, producing a significant decrease in Fru-2,6-P(2) concentration. The fall in the bisphosphorylated metabolite correlated with a decrease in lactate production and ATP concentration, as well as a delay in cell cycle. These results provide support for Fru-2,6-P(2) as a regulator of glycolytic flux and point out the role of glycolysis in cell cycle progression.

Labeling of recombinant protein for NMR spectroscopy: global and specific labeling of the rat liver fructose 2,6-bisphosphatase domain

Methods for the efficient use of the 13C-labeled nutrients, glucose and histidine, in the production of recombinant protein were developed to provide the large amount of sample required for NMR studies. The nutrient requirements were reduced by determining the minimum amount of these metabolites needed during both the growth and the induction phases of the BL21(DE3) and newly constructed BL21(DE3) histidine auxotrophic Escherichia coli cultures. These methods were developed using the separate bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/ fructose-2,6-bisphosphatase, which is expressed to high levels in the pET3a/BL21 (DE3) bacterial system. Use of the optimized expression methods reduced the requirements for the labeled nutrients, glucose and histidine, by 90 and 93.8%, respectively. The savings realized by use of the minimized media and modified induction protocols were obtained without significant reduction of the yield of purified protein. Comprehensive study of the bisphosphatase domain by NMR spectroscopy requires large amounts of protein because of its low solubility and the short lifetime (2-3 days) of the NMR samples. The significant reduction in the costs of labeled protein samples realized by the optimized expression methods can meet these sample requirements in a cost-effective way, and thereby, allow NMR studies of the bisphosphatase domain to proceed.

Crystal structure of a trapped phosphoenzyme during a catalytic reaction

The crystal structure of the fructose-2,6-bisphosphatase domain trapped during the reaction reveal a phosphorylated His 258, and a water molecule immobilized by the product, fructose-6-phosphate. The geometry suggests that the dephosphorylation step requires prior removal of the product for an 'associative in-line' phosphoryl transfer to the catalytic water.

The crystal structure of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase reveals distinct domain homologies

BACKGROUND

Glucose homeostasis is maintained by the processes of glycolysis and gluconeogenesis. The importance of these pathways is demonstrated by the severe and life threatening effects observed in various forms of diabetes. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase catalyzes both the synthesis and degradation of fructose-2,6-bisphosphate, a potent regulator of glycolysis. Thus this bifunctional enzyme plays an indirect yet key role in the regulation of glucose metabolism.

RESULTS

We have determined the 2.0 A crystal structure of the rat testis isozyme of this bifunctional enzyme. The enzyme is a homodimer of 55 kDa subunits arranged in a head-to-head fashion, with each monomer consisting of independent kinase and phosphatase domains. The location of ATPgammaS and inorganic phosphate in the kinase and phosphatase domains, respectively, allow us to locate and describe the active sites of both domains.

CONCLUSIONS

The kinase domain is clearly related to the superfamily of mononucleotide binding proteins, with a particularly close relationship to the adenylate kinases and the nucleotide-binding portion of the G proteins. This is in disagreement with the broad speculation that this domain would resemble phosphofructokinase. The phosphatase domain is structurally related to a family of proteins which includes the cofactor independent phosphoglycerate mutases and acid phosphatases.