Hexose phosphate binding sites of fructose 6-phosphate,2-kinase:fructose 2,6-bisphosphatase

A previous chemical modification study [Kitamura et al. (1989) J. Biol. Chem. 264, 6344-6438] has shown that N-bromoacetylethanolamine phosphate labeled specifically Cys107 of rat liver Fru 6-P,2-kinase:Fru 2.6-Pase and the corresponding Cys of the bovine heart enzyme, leading to inactivation of kinase activity. Since Fru 6-P provided protection against the inactivation, this region of the enzyme was thought to be a Fru 6-P binding site of the kinase enzyme. To examine this possibility, oligonucleotide-directed mutagenesis has been used to alter several residues in expressed rat testis Fru 6-P,2-kinase:Fru 2,6-Pase. The change of Lys100, Lys103, and Asp112 caused at most a 2-fold increase in KmF6P and a 2-3-fold increase in KmATP, suggesting that these residues are not involved in the direct binding of Fru 6-P. However, change of Arg102 to Leu and to Lys resulted in a 325x and 22x, respectively, increase in KmF6P, and change of Arg102 to Glu resulted in nearly complete loss of the kinase activity. Change of Cys105 to Ala or Ser increased KmF6P about 10x. The Vmax of all these mutated enzymes except the one that changed Arg102 to Glu (R102E) was increased 10% to 85%. The kinetic parameters of Fru 2,6-Pase were not altered by these changes. R102E formed several polymeric forms of the enzyme, including a tetramer. Both R102E and an additional derivative that substituted Lys for Arg102 (R102K) were slightly more susceptible to guanidine inactivation than the wild-type enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)