Phosphofructokinase control in muscle: nature and reversal of pH-dependent ATP inhibition.

The kinetic and regulatory properties of rabbit muscle phosphofructokinase (PFK:EC 2.7.1.11) have been reexamined in an attempt to clarify how the enzyme could achieve significant catalytic rates over the physiological pH range (down to 6.4). At 5.0 mM ATP, the apparent Km for fructose 6-phosphate (fructose 6-P) increases by at least 50-fold as the pH is decreased from 7.67 to 6.8 in 50 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-KOH buffer at 25 degrees C (7.50 to 6.63 at 37 degrees C). This effect can be nearly completely abolished in the presence of 10 microM fructose 2,6-bisphosphate (fructose 2,6-P2), with the greatest percentage change seen at low pH. In this case, the rabbit enzyme behaves as if the ATP concentration was low (1.0 mM) at any given pH. Conversely, at high ATP levels and a low pH of 6.8 at 25 degrees C, PFK behaves in the presence of fructose 2,6-P2 as if the pH has been increased to approximately 7.15 or a 0.35 pH unit shift at any given fructose 6-P concentration. At physiological concentration of fructose 6-P (0.1 mM), the positive effectors glucose 1,6-bisphosphate (glucose 1,6-P2) and either AMP, inorganic phosphate, or NH4+ were found to be, respectively, 60 and 40% as effective as fructose 2,6-P2 in reversing this pH-dependent ATP inhibition over the physiological pH range. In combination, however, glucose 1,6-P2 plus AMP were as effective as fructose 2,6-P2.(ABSTRACT TRUNCATED AT 250 WORDS)