Fructose-2,6-bisphosphate counteracts guanidinium chloride-, thermal-, and ATP-induced dissociation of skeletal muscle key glycolytic enzyme 6-phosphofructo-1-kinase: A structural mechanism for PFK allosteric regulation

Rabbit muscle 6-phosphofructo-1-kinase (PFK) is the key glycolytic enzyme being regulated by diverse molecules and signals. This enzyme may undergo a reversible dissociation from a fully active homotetramer to a quite inactive dimer. There are evidences that some positive and negative modulators of PFK, such as ADP and citrate, may interfere with the enzyme oligomeric structure shifting the tetramer–dimer equilibrium towards opposite orientations, where the negative modulators favor the dissociation of tetramers into dimers and vice versa. PFK is allosterically inhibited by ATP at its physiological range of concentration, an effect counteracted by fructose-2,6-bisphosphate (F2,6BP). However, the structural molecular mechanism by which ATP and F2,6BP regulate PFK is hitherto demonstrated. The present paper aimed at demonstrating that either the ATP-induced inhibition of PFK and the reversion of this inhibition by F2,6BP occur through the same molecular mechanism, i.e., the displacement of the oligomeric equilibrium of the enzyme. This conclusion is arrived assessing the effects of ATP and F2,6BP on PFK inactivation through two distinct ways to dissociate the enzyme: (a) upon incubation at 50 °C, or (b) incubating the enzyme with guanidinium hydrochloride (GdmCl). Our results reveal that temperature- and GdmCl-induced inactivation of PFK prove remarkably more effective in the presence 5 mM ATP than in the absence of additives. On the other hand, the presence of 100 nM F2,6BP attenuate the effects of both high-temperature exposition and GdmCl on PFK, even in the simultaneous presence of 5 mM ATP. These data support the hypothesis that ATP shifts the oligomeric equilibrium of PFK towards the smaller conformations, while F2,6BP acts in the opposite direction. This conclusion leads to important information about the molecular mechanism by which PFK is regulated by these modulators.