Mutagenesis of charged residues in a conserved sequence in the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Arg-136, Glu-137, Arg-138 and Arg-139 are conserved in all sequences of the 2-kinase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Their role was studied by site-directed mutagenesis. All the mutations had little, if any, effect on fructose-2,6-bisphosphatase activity. Mutations of Arg-136 and Glu-137 into Ala caused only minor modifications of phosphofructo-2-kinase activity. In contrast, mutation of Arg-138 into Ala increased 280-fold the K_m for fructose 6-phosphate of phosphofructo-2-kinase. Mutation of Arg-139 into Ala resulted in decreases in phosphofructo-2-kinase V_max/K_m for MgATP and fructose 6-phosphate 600-fold and 5000-fold respectively. Mutation of Arg-139 into Lys and Gln increased the K_m of phosphofructo-2-kinase for MgATP (20-fold and 25-fold respectively) and for fructose 6-phosphate (8-fold and 13-fold), and the IC₅₀ for MgADP (30-fold and 50-fold) and for magnesium citrate (7-fold and 25-fold). However, these two mutations did not affect nucleotide binding, as measured by quenching of intrinsic fluorescence. The changes in kinetic properties induced by mutations could not be attributed to structural changes. It is proposed that Arg-138 is involved in fructose 6-phosphate binding and that Arg-139 is probably involved in the stabilization of the transition state and so participates in catalysis.