Page 30 - Enzymes in Tropical Soils

18 Enzymes In Tropical Soils

where k is the rate constant, A is pre-exponential factor, E is Energy of Activation, R
0
is the universal gas constant, and T is absolute temperature in K. This equation
can also be expressed in logarithmic form in Eq. 2.3 as follows.

....... Eq. 2.3

By using a plot of log k versus 1/T, A can be calculated from the intercept while E
can be calculated from the value of the slope because R is a constant and T is an
absolute temperature value.
The rate of a biochemical reaction is also controlled by pH. The rate of an
enzymatic biochemical reaction is usually highest at an optimum value and is lower
at pH values lower or higher than the optimum value. The depencence of the rate
of an enzymatic reaction on pH is related to several factors. First is the stability of
enzyme proteins that is highest at optimum pH. Higher or lower pH denatures the
enzyme proteins. Second, pH may affect the ionization and deionization of
enzymes active sites. Third, pH may influence the solubility of the substrate and
also the adsorption-desorption of enzyme molecules on soil active sites.

2.3 Energy of Activation

The Arrhenius equations (Eq. 2.2 and Eq. 2.3) suggest that the rate of any
biochemical reaction that transforms the reactants into products is controlled by
the Energy of Activation. Energy of Activation is a measure on the magnitude of all
forces that must be surmounted during a reaction process (Sparks, 1989). Sparks
(1989) stated that a higher Energy of Activation results in a lower rate of reaction.
Castellan (1983) stated that this energy barrier separates the reactant state from
the product state. To form the products, the reactants must have sufficient energy
to overcome this energy barrier. Without enough energy to surmount the energy
barrier, the reactants will stay at their orgininal states and the products will never
exist.
The Activation Energy in Arrhenius equation can be described by Fig. 2.2
0
(Castellan, 1983). As shown, the state of Enthalpy of Reactants with Enthalpy H R
0
are separated from those of Products with Enthalpy H P . The quantity of energy (E)
is as shown in Eq. 2.4 as follows.

Abdul Kadir Salam - 2014