so when we want to 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 like Gibs free energy which involves enthalpy and entropy as well as studying things like activation energy of that chemical reaction now because enzymes act on chemical reactions if we are to actually 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 givs free energy then we'll look at activation energy and we'll finish off with how the enzyme actually affects 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 basically means is the reaction can either have a negative Gibs free energy or positive Gibs free energy so what is Gibs free energy well the Gibs free energy Loosely speaking describes how much energy can be used in that chemical reaction so let's suppose we have the following graph so the Y AIS is the energy value and the xaxis is 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 low free energy than the reactants now to calculate mathematically the Gibs free energy of this reaction all we have to do is take the free energy of the products and subtract the free energy of the reactants and that gives us the Gibs 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 basically how much energy we can use in some process now for this particular case this reaction describes an exergonic reaction and exergonic reactions always have a negative Delta G that means energy is released in this reaction and the reaction is set to be spontaneous so a chemical reaction is set 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 well if we read this reaction going back backwards if this is the reactant and this is the product then if we subtract a high free energy from a low free energy we're going to get a positive Delta G and the positive Delta g means the reaction is endergonic and nonspontaneous and that means it will not take place unless we input a certain amount of energy and one example of an endergonic reaction that is not spontaneous is the synthesis of ATP molecules inside our body so to synthesize ATP we have to actually 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 basically 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 gives free energy value is of some particular reaction we know whether or not that reaction is actually spontaneous now another important fact that you have to know about this quantity gies free energy is gies free energy only depends on the energy the free energy value of the products and the free energy value of the reactant 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 subtract the two to find that gives free energy so the pathway that we take when we go from the reactant to the products does not actually determine does not change what the Gibs free energy is it doesn't matter if we take pathway one two or three when we go from the reactants to products gibes free energy will not actually change so if we for example compare a reaction that has an enzyme and that same reaction that is uncatalyzed does not have an enzyme that gives free energy in those two reactions will be exactly the same so a catalyzed and an uncatalyzed 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 actually change the energy of the product and that's exactly why the difference namely the Delta G the givs free energy will remain exactly the same when an enzyme is used or when an enzyme is not used now the final thing I'd like to mention about Gibs free energy is what happens if Gibbs free energy is equal to zero well if Gibs free energy is zero then no energy is being produced in that reaction that actually can be used in any useful way in fact when Gibs free energy is zero that reaction is said to have reached equilibrium and in that moment in time the rate of the for reaction is equal to the rate of the reverse reaction so if the gibbes 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 for 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 well any reaction has some activation energy and this is simply the amount of energy that we have to input for the reaction to take place to convert the reactants to the products or in Reverse now let's suppose we go from reactants to products in 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 Gibs free energy given by Delta G with the symbol on top or simply Delta e uh a where the a stands for Activation now this topmost apex of the Hill describes the energy of the transition state of this chemical reaction and if you 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 as 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 actually study how the transition state looks like we can't isolated and we can't examine it because it it it 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 actually get it going now the activation energy describes how quick quickly a reaction actually takes place so a reaction can be spontaneous it can have a negative Delta G value but it can take place very very slowly and if a reaction takes place very slowly what that means is it has a very high activation energy so activation energy is not the same thing as Gibbs free energy Gibbs free energy basically describes the different between the energy of the reactants and the products but activation energy describes how quickly a reaction actually takes place so Gibs free energy talks about where that equilibrium will be achieved while activation energy talks about how quickly that equilibrium will actually be achieved and so once again as we'll see in more detail in a future lecture the Apex of this curve describes the energy of the transition States now what exactly does the enzyme do and how does the enzyme affect the activation energy so we 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 and so their difference the Delta G is exactly the same it remains unchanged when the enzyme acts on that chemical reaction but the enzyme does have an effect on the activation energy in fact what the enzyme typically does is it actually lowers the energy of that transition state and by and by lowering the energy of the transition state it makes this mountain smaller and so this height will be smaller and the Delta G that gives free um the activation energy of that reaction will become smaller and if we decrease the activation energy by essentially stabilizing that transition state we will speed up the reaction 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 uh the free energy of the reactants and the free energy of the products remains unchanged in any catalyzed reaction however what the enzymes do is they stabilize the transition state lower its energy they lower the energy of that transition state and so they decrease the activation energy and that speeds up that chemical reaction so once again enzymes when they act on chemical reactions they do not change the Gibs free energy and that means they do not increase or decrease how much much product is formed at the end of that reaction but they basically allow equilibrium to be achieved quicker by increasing the rate by decreasing that activation energy