Comparison of the Inhibition by Phospho(enol)Pyruvate and Phosphoglycolate of Phosphofructokinase from B. stearothermophilus

A comparison between the inhibition by phospho(enol)pyruvate (PEP) versus the inhibition by phosphoglycolate (PG) of phosphofructokinase (PFK) from Bacillus stearothermophilus is presented. Both inhibitors act by decreasing the apparent affinity displayed by the enzyme for its substrate fructose 6-phosphate (Fru-6-P) while having little effect on Vmax. However, the two ligands differ in both their affinity for the enzyme and their effectiveness at antagonizing the subsequent binding of Fru-6-P. Although PG binds with approximately 10-fold lower affinity, it antagonizes the binding of Fru-6-P 3.5-fold more strongly than does PEP. Moreover, the enthalpy and entropy contributions to the coupling free energy between inhibitor and Fru-6-P, from which these antagonisms derive, reveal even greater differences between the ligands. These data indicate, therefore, that the changes in the structure of PFK from B. stearothermophilus that result from PG binding, which have been determined by X-ray crystallography (T. Schirmer and P. R. Evans, 1990 Nature 343, 140-145), may not be comparable to those that result from PEP binding and consequently do not represent the generic "T-state," as has been presumed.