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.
Van Praag
E
E
Kennedy-Leigh Centre for Horticultural Research, Hebrew University of Jerusalem, Rehovot, Israel. vanpraag@botinst.unizh.ch
Zehavi
U
U
Goren
R
R
D000255:Adenosine Triphosphate / Q000737:chemistry; D001628:Beverages; D002952:Citric Acid Cycle; D002957:Citrus / Q000201:enzymology*; D004305:Dose-Response Relationship, Drug; D005636:Fructosephosphates / Q000737:chemistry; D007700:Kinetics; D010732:Phosphofructokinase-1 / Q000737:chemistry*
D005636:Fructosephosphates; C027618:fructose-6-phosphate; D000255:Adenosine Triphosphate; D010732:Phosphofructokinase-1; C068652:1-phosphofructokinase