Mutations in the charged residues of the amino terminus of rat liver fructose 6-phosphate,2-kinase:Fructose 2,6-bisphosphatase: effects on regulation

N-terminal truncation affects the kinetics and structure of fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana

The enzyme fructose-6-phosphate 2-kinase (F6P,2K; 6-phosphofructo-2-kinase)/fructose-2,6-bisphosphatase (F26BPase) catalyses the formation and degradation of the regulatory metabolite fructose 2,6-bisphosphate. A cDNA encoding the bifunctional plant enzyme isolated from Arabidopsis thaliana (AtF2KP) was expressed in yeast, and the substrate affinities and allosteric properties of the affinity-purified enzyme were characterized. In addition to the known regulators 3-phosphoglycerate, dihydroxyacetone phosphate, fructose 6-phosphate and P(i), several metabolites were identified as important new effectors. PP(i), phosphoenolpyruvate and 2-phosphoglycerate strongly inhibited F6P,2K activity, whereas fructose 1,6-bisphosphate and 6-phosphogluconate inhibited F26BPase activity. Furthermore, pyruvate was an activator of F6P,2K and an inhibitor of F26BPase. Both kinase and phosphatase activities were rapidly inactivated by mild heat treatment (42 degrees C, 10 min), but the presence of phosphate protected both enzyme activities from inactivation. In addition to the catalytic regions, the Arabidopsis enzyme comprises a 345-amino-acid N-terminus of unknown function. The role of this region was examined by the expression of a series of N-terminally truncated enzymes. The full-length and truncated enzymes were analysed by gel-filtration chromatography. The full-length enzyme was eluted as a homotetramer, whereas the truncated enzymes were eluted as monomers. Deletion of the N-terminus decreased the kinase/phosphatase activity ratio by 4-fold, and decreased the affinity for the substrate fructose 6-phosphate. The data show that the N-terminus is important both for subunit assembly and for defining the kinetic properties of the enzyme.

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.

The difference in the carboxy-terminal sequence is responsible for the difference in the activity of chicken and rat liver fructose-2,6-bisphosphatase

The fructose-2,6-bisphosphatase domain of the bifunctional chicken liver enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase shares approximately 95% amino acid sequence homology with that of the rat enzyme. However, these two enzymes are significantly different in their phosphatase activities. In this report, we show that the COOH-terminal 25 amino acids of the two enzymes are responsible for the different enzymatic activities. Although these 25 amino acids are not required for the phosphatase activity, their removal diminishes the differences in the activities between the two enzymes. In addition, two chimeric molecules (one consisting of the catalytic core of the chicken bisphosphatase domain and the rat COOH-terminal 25 amino acids, and the other consisting of most of the intact chicken enzyme and the rat COOH-terminal 25 amino acids) showed the same kinetic properties as the rat enzyme. Furthermore, substitution of the residues Pro456Pro457Ala458 of the chicken enzyme with GluAlaGlu, the corresponding sequence in the rat liver enzyme, yields a chicken enzyme that behaves like the rat enzyme. These results demonstrate that the different bisphosphatase activities of the chicken and rat liver bifunctional enzymes can be attributed to the differences in their COOH-terminal amino acid sequences, particularly the three residues.