Description: This passage is mainly about what is activation energy. In this passage, the writer tells us the enzymes’ effect on activation energy and free energy, which allows equilibrium to be achieved more quickly by increasing the rate and by decreasing that activation energy.
When we once understand the way that a chemical reaction takes place, we usually study the thermodynamics and the kinetics of that chemical reaction, and that involves studying things such as Gibbs free energy which involves enthalpy and entropy, as well as studying things such as activation energy of that chemical reaction.
Now because enzymes act on chemical reactions, if we are to understand how enzymes behave and act on those chemical reactions, we also have to study the Gibbs free energy and the activation energy of that chemical reaction, so let’s begin by discussing Gibbs free energy, then we’ll look at activation energy, and we’ll finish off with how the enzyme effects these two quantities.
So let’s begin by supposing that we have the following hypothetical reaction, so we have reactants being transformed into products, now we’re going to assume that the reaction has not reached equilibrium, and what that means is that the reaction can either have a negative Gibbs free energy or a positive Gibbs free energy.
So what is Gibbs free energy? The Gibbs free energy loosely speaking describes how much energy can be used in that chemical reaction, so let’s suppose that we have the following graph, so the y-axis is the energy value and the x-axis is the reaction progress, so these are the reactants here and the energy value of the reactant is somewhere here.
Now the products have a free energy value that is equal to somewhere here, and notice that the products have a lower free energy than the reactants, now to calculate mathematically the Gibbs free energy of this reaction, all we have to do is to take the free energy of the products and subtract the free energy of the reactants, and that gives us the Gibbs free energy given by Delta G.
So this quantity here is how much energy is going to be released in this reaction, and it’s how much we can use in some processes, now for this particular case, this reaction describes an exergonic reaction and exergonic reactions which always have a negative Delta G, which means energies are released in this reaction and the reaction is set to be spontaneous.
So a chemical reaction is said to be exergonic and spontaneous, if the Delta G is negative, and one example of a spontaneous reaction in nature is combustion, so combustion reactions are examples of exergonic reactions where the Delta G value is negative, now what about the opposite? If we read this reaction going backwards, if this is the reactant and this is the product, if we subtract a high free energy from a low free energy, we’re going to get a positive Delta G.
The positive Delta G means that the reaction is endergonic and non spontaneous, and that means that it will not take place unless we input a certain amount of energy, and one example of an endergonic reaction is not spontaneous, it is the synthesis of ATP molecules inside our body, so to synthesize ATP, we have to input energy and the ATP molecules.
When they break down, that is an exergonic reaction and energy is released, and every time we break down ATP molecules inside our body, energy is released and we can use that energy to power different types of processes that take place inside our body that require those ATP molecules.
So on the other hand, a chemical reaction is set to be endergonic and non spontaneous, if the Delta G is positive and ATP synthesis is an example of such an endergonic reaction, so we can see that if we know what the Gibbs free energy value is of some particular reaction, we know whether or not that reaction is spontaneous.
Now another important fact that you have to know about this quantity Gibbs free energy is that Gibbs free energy only depends on the energy, the free energy value of the products and the free energy value of the reactants, so if we know what the free energy of the products is and the free energy of the reactants, all we have to do is to subtract two to find that Gibbs free energy.
So the pathway that we take when we go from the reactant to the products does not determine, it does not change what the Gibbs free energy is, it doesn’t matter if we take pathway one two or three, when we go from the reactants to products, Gibbs free energy will not change, so if we compare a reaction that has an enzyme and that same reaction that is on catalyzed and does not have an enzyme the Gibbs free energy in, those two reactions will be exactly the same.
So a catalyzed and an uncannily reaction will have the same exact Gibbs free energy value and that leads us to a very important point enzymes, when they act on chemical reactions, they do not affect the Gibbs free energy value, they do not change the energy of the reactants nor they change the energy of the products, and that’s exactly the reason.
The Gibbs free energy will remain exactly the same when an enzyme is used or when an enzyme is not used, now the final thing that I’d like to mention about Gibbs free energy is what happens if Gibbs free energy is equal to zero, if Gibbs free energy is zero, then no energy is being produced in that reaction, that can be used in any useful way.
In fact when Gibbs free energy is zero, that reaction is said to have reached equilibrium, and in that moment the rate of the for reaction is equal to the rate of the reverse reaction, so if the Gibbs free energy is zero, the reaction has achieved equilibrium and is said to be neither spontaneous nor non-spontaneous.
In such a case, the rate of the four reaction going from reactants to products is equal to the rate of the reverse reaction going from products back to reactants, now let’s move on to activation energy, so what exactly is the activation energy? Any reaction has some activation energy, and this is simply the amount of energy that we have input for the reaction to take place to convert the reactants to the products or in reverse.
Now let’s suppose that we go from reactants to products, in this case our activation energy is simply this quantity here, it’s the difference between the energy of the molecule found on this topmost portion of the hill and the energy of that reactant, this is the Gibbs free energy given by Delta G with the symbol on top, or simply Delta e a where a stand for activation.
Now this topmost apex of the hill describes the energy of the transition state of this chemical reaction, and if you recall from organic chemistry, the transition state is not something that exists for a very long time, and that’s because it has a very high energy value, it can be seen by the following diagram.
This apex has the highest energy value in that reaction and that’s precisely why the transition state does not exist for a very long time, and in fact because it doesn’t exist for a very long time, it’s unstable and we can’t study how the transition state looks like, we can’t isolate it and we can’t examine it, because it quickly converts into the products.
So the activation energy Delta G with that symbol describes the amount of energy that must be supplied to any reaction in order to get it, now the activation energy describes how quickly a reaction takes place, so a reaction can be spontaneous, it can have a negative Delta G value, but it can take place very slowly.
If a reaction takes place very slowly, it means that it has a very high activation energy, so activation energy is not the same thing as Gibbs free energy, Gibbs free energy describes the difference between the energy of the reactants and the products, but activation energy describes how quickly a reaction takes place.
So Gibbs free energy talks about where that equilibrium will be achieved, while activation energy talks about how quickly that equilibrium will be achieved, and we’ll see in more details in a future lecture, the apex of this curve describes the energy of the transition state, now what exactly does the enzyme do and how does the enzyme affect the activation energy?
So we have said previously that the enzyme does not change the Gibbs free energy of the reaction, it has no effect on the energy of the reactants and the products, so the Delta G is exactly the same, it remains unchanged when the enzyme acts on that chemical reaction, but the enzyme has an effect under activation energy.
In fact what the enzyme typically does is that it lowers the energy of that transition state and ball and by lowering the energy of the transition state, it makes this mountain smaller, so this height will be smaller and the Delta G and the Gibbs free on the activation energy of that reaction will become smaller.
If we decrease the activation energy by essentially stabilizing that transition state, we will speed up the react, because ultimately it’s the activation energy, it’s the energy barrier that determines the kinetics the speed and the rate of that chemical reaction, so enzymes do not affect the equilibrium, they have no effect on the Gibbs free energy of that reaction.
The thermal free energy of the reactants and the free energy of products remain unchanged in any catalyzed reaction, however what the enzymes do is that they stabilize the transition state and lower its energy, they lower the energy of that transition state, and they decrease the activation energy and that speeds up that chemical reaction.
So when enzymes act on chemical reactions, they do not change the Gibbs free energy, and that means they do not increase or decrease how much products is formed at the end of that reaction, but they allow equilibrium to be achieved more quickly by increasing the rate and by decreasing that activation energy.