3.4.4 Pyruvate Kinase
The mechanism of pyruvate kinase is similar to that of hexokinase—except that the reaction proceeds the other way, producing ATP rather than consuming it. Yet, both reactions are irreversible. How come?
The intermediate product of the pyruvate kinase reaction, after transfer of the phosphate group from position 2 to ADP, is enolpyruvate. Getting rid of the phosphate group enables the enol group to rearrange itself into a keto group (Figure 3.4.4-1, bottom). This second step of the reaction is strongly exergonic, and it thus pulls the overall equilibrium of the reaction over to its side. No such exergonic step occurs with hexokinase, which explains the difference in ΔG.