JBC, Vol. 250, Issue 18, 7277-7279, Sep, 1975
Mechanistic implications of the pH independence of inhibition of phosphoglucose isomerase by neutral sugar phosphates
J. M. Chirgwin, T. F. Parsons and E. A. Noltmann
In contrast to the strongly pH-dependent inhibition of phosphoglucose
isomerase by substrate analogues with a free carboxyl group, inhibition of
this enzyme by neutral sugar phosphates is essentially invariant between pH
7 and 9. Competitive inhibition constants for glucitol 6-phosphate (40
muM), arabinose 5-phosphate (50 muM), and erythritol 4-phosphate (100 muM)
were found to be of the same order of magnitude as that reported previously
for substrate binding constants (50 to 240 muM). The unique exception is
erythrose 4-phosphate whose Ki (0.7 muM, independent of pH) reflects a
tightness of binding similar to that found at pH values near or below
neutrality for the transition state analogue 5-phosphorarabinonate. The pH
independence of inhibition by erythrose 4-phosphate and other neutral sugar
phosphates may reflect a mode and locus of binding to phosphoglucose
isomerase different from that of the aldonate inhibitors.