in our discussion on the properties of enzymes we mentioned one very important property of enzymes namely the ability of the enzyme to catalyze to the biological reactions that take place inside our body and inside our cells now what that ultimately means is enzymes speed up the rate at which chemical reactions take place now by speeding up a chemical reaction enzymes essentially decrease the time that is needed for that particular chemical reaction to actually reach equilibrium now this is a very important thing to remember about enzymes enzymes decrease the time that is needed to reach equilibrium but enzymes do not actually change the equilibrium itself they do not change the energy of the products and reactants nor they actually change the amount of products or reactants that is formed at equilibrium so to see what we let's take a look at the following energy diagram so let's suppose we have a hypothetical Elementary single step reaction in which the reactants are these and the products are these so on the reactant side we have a bond between A and B and C Exist by itself on the product side we now have a bond between B and C and A Exist by itself now based on the following energy diagram so the Y AIS is the gives free energy and the xaxis is the reaction progress so we go from reactants to products notice that if we compare the y-coordinate value of the products to the y-coordinate value of the reactants this is lower in energy than this and what that means is if we take the free energy of the products and we subtracted from the free energy of the reactants we get a negative value and what that means is free energy will be produced will be released when this reaction takes place the Delta G is negative and so that implies this reaction is exergonic it is spontaneous and as long as we have enough energy to overcome the activation barrier this quantity here the reactants will spontaneously and naturally form these products because they are lower in energy and therefore more stable so once again enzymes do not affect the free energy value of the products nor will they affect the free energy value of the reactants and since the energy value of the products and reactants will not be affected that means the Delta G the difference between this and this will not be affected as well now because it's the energy of the products and reactants and specifically it's the difference between the energy of the products and reactants that determines the concentrations of products and reactants that will exist at equilibrium because the enzymes do not affect the energy values of the products and reactants they will not affect the concentration of the products and reactants that will exist at equilibrium so once again we know that enzymes do not affect the free energy of the reactants and products this implies that they will not change the equilibrium of the reaction that is the same concentration of products and reactants will be formed in the presence as in the absence of the enzyme so this is the case where we have our uncatalyzed reaction but if we add an enzyme the energy value of the reactants and products will not actually change now if the thermodynamics of the products and reactants is not Changed by an enzyme what is actually changed well recall that the kinetics of the chemical reaction is determined by the energy of the transition state and the transition state is this transient molecule transient stage that exists between the reactants and our products now what exactly will the transition state look like when we go from the reactants to the products well we have a single Elementary re a single step Elementary reaction in which on the reactant side we have a bond between a and b and on the product side we have a bond between B and C and what that implies is to actually go from the reactants to the products we have to break the bond between a and b and we have to form the bond between B and C so in that transition stage what we're going to see is a bond breaking between a and B so B will begin to move away from a and because the electron density will basically move away that bond between A and B will begin to break and that can be described by using a dashed line so this dashed Line Between A and B basically moves B is moving away and the bar and and that bond is being broken on the other hand because B is approaching C the electron densities of these two atoms or molecules is overlapping and so we begin to form we begin to form that Bond so we have a partially formed Bond here and a par and a partially broken bond here and because the electron densities aren't overlapping very well that will increase the energy of the transition state in fact according to the diagram the energy of the transition state represents the highest possible I free energy value on the following curve and so if we are to actually mark down on the curve where that transition state actually is this highest most Peak on the curve the Apex represents that transition state so we can basically write that this is in fact that transition state it represents the highest the maximum energy value in that particular chemical reaction and this daggered symbol describes the energy states so to calculate the free energy value of the transition stage of that molecule of that chemical reaction we simply take this y-coordinate value and we subtract the energy of the reactants so the energy of the transition State minus the energy of the reactants give gives us this quantity known as the free energy uh thee energy of activation or simply the activation energy the activation barrier of this particular chemical reaction now what happens is when the enzyme actually takes in these molecules so we have the enzyme in the enzyme we have this special location in that enzyme that we're going to discuss in much more detail in the next lecture but this special location in the enzyme is known as the active side and it's the active side that that creates a micro environment and binds to these molecules here so these reactants will move into the active side of that enzyme creating the enzyme substrate complex and what the enzyme actually does inside that active side is it stabilizes this partially broken Bond and this partially form Bond and by stabilizing these partially broken and form bonds it lowers the energy of activation it stabilizes the transition state lowering that free energy of activation and if we lower that free energy of activation we increase the rate at which that reaction takes place so once again enzymes bind specific substrates on regions called active sides to form the enzyme substrate complex by binding substrat to the active sides enzymes stabilize the energy of the transition state which in turn stimulates the breakage of the old bonds and the formation of the new bonds to form that product molecule as shown in this particular diagram so if we look at the following diagram once more when we go from the uncatalyzed to the catalyzed case we see that the energy of the products or reactants is not changed this energy is the same as this energ energy value and this energy is the same as this energy so the change in Gibs free energy between our products and reactants is not changed what is changed is the energy between the transition state and the reactants so here we see that there's a stabil a stabilization of the transition state and a lowering an energy and that means the difference between the energy of this transition state in the reactants will be smaller in this case than in this case and this is precisely what makes that reaction actually go quicker so by adding an enzyme we increase the or we decrease the time it takes for equilibrium to actually establish but once equilibrium is actually established the same concentrations of products and reactants are formed in the catalyzed case as that uncatalyzed case now let's discuss something called the maximum velocity of enzymes so the maximum velocity of enzymes basically describes the maximum activity at which the enzyme will actually operate so let's suppose in our mixture we have a total of 100 enzymes and each one of these enzymes will have its own active site now if we add 50 substrate molecules then only 50 enzymes will contain active sides that are filled and that means our entire mixture because only half of the enzymes are filled the entire activity of all the enzymes will be half of its maximum value but if we continue adding substrate molecules so that we add let's say 100 substrate molecules then all the active sides and all the enzymes will be filled and that means the entire activity of our mixture of enzy enes will be at a maximum velocity and that's exactly what this graph actually represents so the Y AIS is the enzyme activity also called the enzyme velocity and the xaxis is the substrate concentration so basically this dashed line represents the maximum velocity at which that enzyme or the mixture of enzymes will actually operate and notice we have this line and this Curve will never actually cross this line it will never actually go higher than the line because if we have for example 100 enzymes we only have 100 possible active sides so even if we add let's say 200 substrate molecules because we have an excess of substrate molecules and only 100 active sides we have a maximum activity at which only 100 active sides at a time can actually be filled so as we we increase our substrate concentration we see that that the enzyme activity increases up to a certain value known as the maximum enzyme activity or maximum enzyme velocity so we see that at a constant concentration of enzyme the enzyme activity will continue to rise until a certain maximum value is reached known as the maximum velocity and the maximum velocity represents the condition in which all the active sites this should be active not activate so let's change that so all the active sites act active so all the active sides are filled with the appropriate substrate so that basically represents the condition in which if let's say we have a thousand enzymes each enzyme has one active side if we have 1,000 substrate molecules then all the active sides will be filled and that is the point at which that enzyme is operating at a maximum enzyme velocity at a maximum enzyme activity so in the next lecture we're going to focus on how that active side actually stabilizes that transition state and we're going to focus much more on what that active side actually does and how it binds to its appropriate substrate molecule