A thermodynamic investigation of the glucose-6-phosphate isomerization

- Equilibrium constant K strongly depends on G6P concentration for G6P ↔ F6P reaction.
- ΔRg+ upon using either Ka or Km differs by up to 30%.
- Activity-coefficient ratio of G6P and F6P strongly deviates from unity.
- Activity-coefficient ratio Kγ could be predicted with ePC-SAFT accurately.
- Influence of buffer and glutamate on Kγ could be predicted with ePC-SAFT accurately.

In this work, ΔRg+ values for the enzymatic G6P isomerization were determined as a function of the G6P equilibrium molality between 25 °C and 37 °C. The reaction mixtures were buffered at pH = 8.5. In contrast to standard literature work, ΔRg+ values were determined from activity-based equilibrium constants instead of molality-based apparent values. This yielded a ΔRg+ value of 2.55 ± 0.05 kJ mol− 1 at 37 °C, independent of the solution pH between 7.5 and 8.5. Furthermore, ΔRh+ was measured at pH = 8.5 and 25 °C yielding 12.05 ± 0.2 kJ mol− 1.Accounting for activity coefficients turned out to influence ΔRg+ up to 30% upon increasing the G6P molality. This result was confirmed by predictions using the thermodynamic model ePC-SAFT.Finally, the influence of the buffer and of potassium glutamate as an additive on the reaction equilibrium was measured and predicted with ePC-SAFT in good agreement.