Kinetic studies on the reaction catalyzed by phosphoglycerate kinase

1. 1.|The Michaelis constants for the two substrates of phosphoglycerate kinase (ATP: d -3-phosphoglycerate i -phosphotransferase, EC 2.7.2.3), MgATP2− and 3-phosphoglycerate, are each independent of the concentration of the second substrate. Three prevalent mechanisms can describe this situation. 2. 2.|The Mg2+ complex of 3-phosphoglycerate does not appear to be in an active substrate form. 3. 3.|Mg2+ at high concentrations inhibits the enzyme non-competitively with respect to 3-phosphoglycerate. 4. 4.|High concentrations of Mg2+ change the kinetic relationships and non-linear Lineweaver-Burk plots are obtained for the two substrates. The curves can be approximated to two straight lines. The two intersection points with the abciss axis are independent of the concentration of the second substrate. 5. 5.|The data are interpreted in terms of two different binding sites for each substrate, the second set of sites being involved only at high Mg2+ concentrations. Various mechanisms for this effect are considered and it is suggested that these could also be important for other enzymes involving reaction with ATP.