The action of inhibitors (histidine and AMP) on the ATP phosphoribosyltransferase of E. coli.

The inhibitors histidine and AMP cause the enzyme ATP phosphoribosyltransferase of E. coli to associate into a hexamer from its initial dimeric form. The behaviour of these inhibitors has been studied by three different methods. I) Equilibrium dialysis studies have shown that one mole of dimeric enzyme (67,000 g) binds one mole of histidine. II) By kinetic inhibition of the reaction studied at 21, 25 and 38 degrees C the enthalpy changes in the process of histidine and of AMP inhibition have been deduced. The inhibition has also been studied in function of enzyme concentration and temperature. The inhibition appears to be slightly negatively cooperative for histidine and positively cooperative for AMP. In neither case is it possible to obtain 100% maximal inhibition. III) By microcalorimetric analysis the values obtained for the enthalpies of histidine and of AMP interaction with the enzyme are similar.