Kinetic properties of ATP-dependent phosphofructokinase from grapefruit juice sacs: effect of TCA cycle intermediates.

Grapefruit juice sac ATP-PFK was studied kinetically for its substrates ATP and Fru-6-P at pH = 7.5. The Km for ATP is equal to 39.8 +/- 4.6 microM. ATP becomes inhibitory at concentrations above 80 microM. The Km for ATP is not affected by the addition of citrate (10 mM). For Fru-6-P, the saturation curve is sigmoidal, with an S0.5 equal to 0.17 +/- 0.03 mM, in the presence of Mg++ (2.5 mM) and ATP (1 mM). ATP-PFK shows a negative cooperativity at lower concentrations of Fru-6-P (h = 0.5), while higher concentrations of the substrate induce a positive cooperation (h = 1.5). The presence of citrate affects the S0.5 affinity value, but not the Vmax. The presence of citrate (10 mM) removes the cooperative effect at higher concentrations of the substrate, as h = 1.0. A theoretical Ki for citrate was calculated and equals 1.30 mM.