Stabilization of glucoso-6-phosphate dehydrogenase by its substrate and cofactor in an ultrasonic field

ISSN： 1068-1620

Source： Russian Journal of Bioorganic Chemistry, Vol.32, Iss.5, 2006-09, pp. : 436-443

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Abstract

The inactivation kinetics of glucoso-6-phosphate dehydrogenase (GPDH) and its complexes with glucoso-6-phosphate and NADP⁺ was characterized in aqueous solutions at 36–47°C under treatment with low frequency (27 kHz, 60 W/cm²) and high frequency ultrasound (880 kHz, 1 W/cm²). To this end, we measured three effective first-order inactivation rate constants: thermal k _in^* , total (thermal and ultrasonic) k _in, and ultrasonic k _in(US). The values of the constants were found to be higher for the free enzyme than for its complexes GPDH-GP and GPDH-NADP⁺ at all temperatures, which confirms the enzyme stabilization by its substrate and cofactor under both thermal and ultrasonic inactivation. Effective values of the activation energies (E _a) were determined and the preexponential factors of the rate constants and thermodynamic activation parameters of inactivation processes (∆H*, ∆S*, and ∆G*) were calculated from the temperature dependences of the inactivation rate constants of GPDH and its complexes. The sonication of aqueous solutions of free GPDH and its complexes was accompanied by a reduction of E _a and ∆H* values in comparison with the corresponding values for thermal inactivation. The E _a, ∆H*, and ∆S* inactivation values for GPDH are lower than the corresponding values for its complexes. A linear dependence between the growth of the ∆H* and ∆S* values was observed for all the inactivation processes for free GPDH and its complexes.