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Enzymes and activation energy

Video transcript

today we're going to talk about how enzymes can influence reactions activation energy but first let's review the idea that enzymes make biochemical reactions go faster and in order to do that they use a bunch of different catalytic strategies now there are lots of different catalytic strategies that enzymes use but a couple of the key ones are acid-base catalysis where enzymes use their acidic or basic properties to make reactions go faster by helping it with proton transfer there's also covalent catalysis where enzymes covalently bind reacting molecule to help with electron transfer there's electrostatic catalysis where enzymes use charged molecules or metal ions to stabilize big positive or negative charges and we also have proximity and orientation effects where enzymes make collisions between reacting molecules happen a little more often so what effect these catalytic strategies actually have on a reaction well let's look at a sample reaction where we're having molecule a being converted to molecule B now we can look at the process of this reaction using something called reaction coordinate diagram and here we'll plot the energy state of our molecules against the progress of the reaction so essentially using this graph we'll follow the energy level of molecule a as its converted to molecule B remember the molecules energy level is related to its stability and something that is a lower energy state is more stable and for something to transform to a more unstable form it needs an input of energy to get there so looking at this graph you will notice that the energy of molecule a will rise up pretty high and then drop all the way down to the energy of molecule B and we can actually define a couple of values from this graph the transition state of a reaction which is represented by this double dagger symbol is the highest energy point on the path from A to B and it's where you'll find the most instability throughout the entire reaction now the difference between the energy level where we start and the top of our graph at our transition state is what we call the Delta G double dagger or the free energy of activation and this is the amount of energy that a needs to have in order to break the reaction barrier to ultimately get to point B you'll also notice that there is a difference in energy between point a and point and we call this the standard free energy change for the entire reaction and it represents the net change in energy levels between our reactants and our product and it's also the energy that is released into the environment once the reaction is over reactions you typically look at will have their products at a lower energy state than their reactants since that makes the reaction spontaneous now it's important to recognize that it is the free energy of activation energy value which is the difference between point a and the transition state that usually determines how quickly a reaction will go and usually this energy value is much higher than the free energy change for the reaction which is why enzymes speed up a reaction by lowering a reactions activation energy now I want to quickly point out that you may see Delta G double dagger written out as e a in some textbooks and you may see the standard free energy change for the reaction written out as e reaction and I'm just letting you know that you might see both sets of terms used from time to time now let's look at an analogy to get a closer look at how this all works and let's it is a giant hill that you're trying to climb and it's a pretty steep hill that goes up really high but you need to get to the other side of the hill now this would be a pretty scary thing on its own since you would need to go all the way up and then all the way down the mountain to get to the finish line but if I were to give you a shovel then now you could dig your way through the mountain and not have to climb up so high in this example the shovel represents an enzyme and the hill represents the activation energy barrier that prevents you from getting to start to finish by using the shovel you're able to lower the height of the hill you have to climb but in both cases it's important to recognize that you still started and finish at the same points so let's go back to our example from before with our reaction coordinate diagram but now let's say that the reaction has a catalyst so with the catalyst the activation energy barrier that molecule a has to overcome in order to get to point B is much smaller and this will mean that your reaction will have a transition state with a much lower energy meaning that it's more stable with the enzyme and also that the reaction as a whole will have a much lower activation energy now it's really important to recognize that like our example where you're trying to climb the hill the enzyme will not be changing starting and ending points of the reaction it doesn't change molecule a or molecule B your starting and ending points are always the same and the only thing that changes is the path that you take to get from A to B now since our starting and ending points aren't changing it follows that the enzymes are not used up when they catalyze a reaction and there is no permanent change to the enzyme following a reaction so what did we learn well first we learned that enzymes work by lowering the free energy of activation of a reaction making it much easier for the reactants to transition and form products and we also learned that the free energy of the reaction doesn't really change when you use an enzyme and when you don't second we learned that despite the change in pathway to get from A to B the reactants and products do not change when using enzyme vs. when not using an enzyme and finally we learned that enzymes are not consumed when they catalyze a reaction and the same enzyme can catalyze reactions over and over again