title: Mutations in the charged residues of the amino terminus of rat liver fructose 6-phosphate,2-kinase:Fructose 2,6-bisphosphatase: effects on regulation.
abstract: Amino and carboxyl termini of the bifunctional enzyme Fru 6-P, 2-kinase:Fru 2,6-bisphosphatase regulate the relative activities of the kinase/phosphatase. The N-terminus of the rat liver bifunctional enzyme is highly basic, containing a protein kinase A phosphorylation site that regulates these enzyme activities in a reciprocal manner. To determine the role of charged residues in the N-terminal peptide, mutant enzymes were constructed in which these residues were altered to residues carrying opposite charges, and the effect on the catalytic properties, thermal lability, and susceptibility to trypsin digestion and phosphorylation by protein kinase A was determined. Most of these mutations decreased k(cat)/K(ATP) and/or k(cat)/K(Fru) (6-P) of the kinase and increased k(cat)/K(Fru 2,6-P2) of the phosphatase. These mutant enzymes were more susceptible to trypsin digestion, phosphorylation by protein kinase A, and thermal inactivation. In general, the effect was greater with amino acid residues located more distant from the N-terminus. The resulting changes were not as large as observed with the phosphorylated enzyme. Mutation of Ser22 to Pro produced large changes in the kinetic properties comparable to those of phosphorylation, suggesting that the flexible region of the N-terminus containing five serines (Ser20 to S24) is essential for the enzyme activities. These results indicated that the charged residues as well as Ser20-Ser24 in the N-terminus of the liver Fru 6-P,2-kinase:Fru 2,6-Pase are essential in the allosteric regulation and probably involved in interactions with the catalytic domains that induce a conformation that has high Fru 6-P,2-kinase and low Fru 2,6-Pase activities. Any disruption of this N-terminal interaction results in inhibition of the kinase and activation of the phosphatase.
authors.affiliation: Research Service, Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, 4500 South Lancaster Road, Dallas, Texas 75216, USA.
authors.forename: R F
authors.initials: RF
authors.lastname: Wu
authors.forename: K
authors.initials: K
authors.lastname: Uyeda
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D003001:Cloning, Molecular; D017931:DNA Primers; D004926:Escherichia coli; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D016297:Mutagenesis, Site-Directed; D025481:Phosphofructokinase-2; D010744:Phosphoric Monoester Hydrolases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D017931:DNA Primers; D011994:Recombinant Proteins; D025481:Phosphofructokinase-2; D010744:Phosphoric Monoester Hydrolases