A thermodynamic investigation of the glucose-6-phosphate isomerization.

In this work, Δ(R)g(+) 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, Δ(R)g(+) values were determined from activity-based equilibrium constants instead of molality-based apparent values. This yielded a Δ(R)g(+) value of 2.55±0.05 kJ mol(-1) at 37 °C, independent of the solution pH between 7.5 and 8.5. Furthermore, Δ(R)h(+) 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 Δ(R)g(+) 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.