Analysis of the allosteric properties of rabbit muscle phosphofructokinase by means of affinity labeling with a reactive ATP analog.

A reactive ATP analog, N6-(6-bromoacetamidohexyl)-AMP-PCP, reacted specifically with the ATP inhibitory site of rabbit skeletal muscle phosphofructokinase without affecting the active site. Modification resulted in the incorporation of 1.01 mol of the reagent per mol of enzyme subunit. The modified enzyme was insensitive to allosteric inhibition by ATP and to activation by AMP at pH 7.2, where the native enzyme exhibits allosteric kinetic behavior. These observations demonstrate that we had succeeded in obtaining PFK fixed in the T state. Using the kinetic parameters of this modified enzyme, the kinetic properties of native enzyme can be quantitatively accounted for by the allosteric model of Monod-Wyman-Changeux. Further, the reagent was shown to have reacted with a specific cysteine residue near or at the ATP inhibitory site, and the sequence around the cysteine was determined as Cys-Lys-Asp-Phe-Arg.