Kinetic and thermodynamic aspects of glucose-6-phosphate dehydrogenase activity and synthesis

The contribution of enzyme synthesis on the overall activity of glucose-6-phosphate dehydrogenase (G6PDH) in a genetically modified Saccharomyces cerevisiae strain is taken into consideration by a simple thermodynamic approach that describes it as an instantaneous equilibrium. The experimental data of batch cultures, expressed in terms of G6PDH activity, biomass growth and glucose consumption, have been used to check this model as well as to estimate the thermodynamic parameters involved in both activation and thermal inactivation. Cell growth exhibited the typical two-phase behaviour resulting from the dominating contribution of thermal inactivation (ΔH°i,x=220 kJ mol−1) over activation (ΔH#x=72.2 kJ mol−1) at T>30 °C, with no apparent dependence on G6PDH synthesis. On the contrary, enzyme synthesis appeared to be the limiting factor of G6PDH activity at T≥35 °C, but showed a standard variation of enthalpy (ΔH°s,a=380 kJ mol−1) which was slightly below that of reversible enzyme unfolding (ΔH°i,a=408 kJ mol−1) but quite higher than the one of enzyme activity (ΔH#a=135 kJ mol−1).