The function of the enzyme is lowering the activation energy by facilitating the formation of the transition state (see below sentence 5)
This function of enzymes results to
increasing the speed of the reaction. However it does
not change the equilibrium of the reaction,
nor the ΔG (energy consumed or produced)
It is important to understand that enzymes accelerates the forward and reverse reactions by the same factor.
More information:
8.2.3. Enzymes Alter Only the Reaction Rate and Not the Reaction Equilibrium
[...]
(1) An enzyme cannot alter the laws of thermodynamics and consequently cannot alter the equilibrium of a chemical reaction. This inability means that an enzyme accelerates the forward and reverse reactions by precisely the same factor. Consider the interconversion of A and B.
[...]
(2)Enzymes accelerate the attainment of equilibria but do not shift their positions. The equilibrium position is a function only of the free-energy difference between reactants and products.
8.3 Enzymes Accelerate Reactions by Facilitating the Formation of the Transition
State
[...]
(3) A chemical reaction of substrate S to form product P goes through a transition state that has a higher free energy than does either S or P.
[...]
(4) the energy of activation does not enter into the final ΔG calculation for the reaction, because the energy input required to reach the transition state is returned when the transition state forms the product.
[...]
(5) The activation energy barrier immediately suggests how enzymes enhance reaction rate without altering ΔG of the reaction: enzymes function to lower the activation energy, or, in other words, enzymes facilitate the formation of the transition state.[/b]
Example:
Consider the interconversion of A and B. Suppose that, in the absence of enzyme, the forward rate constant (k F) is 10-4 s-1 and the reverse rate constant (k R) is 10-6 s-1. The equilibrium constant K is given by the ratio of these rate constants:
(see the 1st attachment)
The equilibrium concentration of B is 100 times that of A, whether or not enzyme is present. However, it might take considerable time to approach this equilibrium without enzyme, whereas equilibrium would be attained rapidly in the presence of a suitable enzyme.
(Stryer Biochemistry)
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