Asian Journal of Physical and Chemical Sciences

Background: The equation of the difference between reverse and forward Gibbs free energy of activation (ΔΔG_ES^#) reflects Michaelis-Menten constant (K_M) in both directions; this may not be applicable to all enzymes even if the reverse reaction is speculatively Michaelian. Arrhenius activation energy, E_a and (E_a - ΔG_ES^#)/RT) are considered = ΔG_ES^# and K_M respectively. The equations are considered unlikely.

Objectives: The objectives of this research are: 1) To derive what is considered as an appropriate equation for the determination of the difference in ΔG_ES^# between the reverse and forward directions, 2) calculate the difference between the reverse and total forward ΔG_ES^#, and 3) show reasons why E_a ≠ ΔG_ES^#  in all cases.

Methods: A major theoretical research and experimentation using Bernfeld method.

Results and Discussion: A dimensionless equilibrium constant K_ES is given. Expectedly, the rate constants were higher at higher temperatures and the free energy of activation with salt was < the Arrhenius activation energy, E_a; ΔΔG_ES^#ranges between 67 - 68 kJ/mol.

Conclusion: The equations for the calculation of the difference in free energy of activation (ΔΔG_ES^#) between the forward and reverse directions and a dimensionless equilibrium constant for the formation of enzyme-substrate (ES) were derivable. The large positive value of the ΔΔG_ES^# shows that the forward reaction is not substantially spontaneous; this is due perhaps, to the nature of substrate. The equality of Arrhenius activation energy (E_a) and ΔG_ES^# may not be ruled out completely but it must not always be the case; the presence of additive like salt can increase the magnitude of E_a well above the values of the ΔG_ES^#. A dimensionless equilibrium constant for the net yield of ES seems to be a better alternative than K_M. The E_a unlike ΔG_ES^#  requires at least two different temperatures for its calculation.