 Today, we're going to talk about enzymes. Enzymes are complex molecules usually made of proteins. And these guys act as a reaction catalyst. So they help reactions occur much more easily by lowering their energy of activation. But they are not used up in the reaction or changed by the reaction. So when they're done with one reaction, they can immediately go into the next reaction, and the next reaction, and the next reaction. So they can be used over and over again. Now what is energy of activation? Energy of activation is simply an energy hurdle that has to be overcome in order for a reaction to occur. Or another way of thinking about it is it's the minimum energy needed to start a reaction. So when you think about wooden gas, they're very flammable. But usually when they're just sitting there, they're not going to burst into flames. They need some kind of energy to be put into them, a little bit of heat to be applied before they can catch a fire. And it's the same kind of thing with other chemical reactions. Now rather than heat, cells use enzymes to help lower that energy of activation. So here's an example about what I'm talking about. So here we've got some substrates that have a certain amount of energy. And then there's an energy hurdle, an energy barrier that these guys have to overcome before a reaction can take place. And so you can see here that without an enzyme, we have a very high threshold, a very high energy hurdle that has to be overcome. But when you use an enzyme, that energy of activation can be reduced. So it's much more easy to take place. So the way that this can happen is that an enzyme can bring together two molecules in a very favorable arrangement so that these guys can be joined together. And so if they're brought together in that favorable arrangement, that lowers their energy of activation. Likewise, an enzyme can stress or bend a bond and that allows that bond to be stressed or bent. And so then we can break that molecule apart much more easily. So again, in that particular situation, again we can lower that energy of activation by using the enzyme one way or the other. So we start off with material that we're gonna be reacting together. And these guys are called substrates. So again, the beginning materials for a reaction are substrates and I've got two here, a hexagon and a circle. And then we've got our enzyme which is shown down here in blue. Now our enzyme has a special three-dimensional shape that allows it to bind the substrates. And so you can see that this enzyme has two different pockets that will fit these substrates in them. And also it has a special three-dimensional shape that lets these guys react together and sometimes interact with other proteins as well. But if that three-dimensional shape is unfolded or unwound or the term that we often use is denatured by either temperature or pH or something else, the enzyme won't work. So it has to have that right three-dimensional shape in order to bind these substrates and do this particular reaction. Now where these substrates bind directly, that's called the active site. So here, down here, these two spots are the active site for this particular enzyme. Now when the substrates bind to the active site, the protein changes its shape just a little bit. It's kind of like when you're reaching out to shake someone's hand, how they conform to your hand hopefully. And so they more snugly or more tightly hold those substrates together. And so that's called induced fit. Now induced fit can reduce the energy of activation and allow these molecules to react more easily. So you can see here we've got these two molecules who bring them together in close proximity. And then after induced fit, these guys are very, very tightly next to each other. So it's easier to have a reaction. And then after we've been able to have the chemical reaction, in this case, joining those two molecules together, those products are then released and then the enzyme is ready to go to do another reaction and another reaction and another reaction. Now again that word denaturation. So denaturation is the unfolding or the unwinding of a protein. So it no longer has its proper three-dimensional shape. So here for our particular protein, it needs to bind those two substrates. But if it's denatured or unfolded, now it can no longer bind those particular substrates. It has a different shape, a different form. And so because of that, the enzyme can no longer bind substrates and it can no longer catalyze that particular reaction. Now there's a lot of things that can cause a protein to denature. Temperature, if it gets too hot, can cause denaturation. pH, if it's too high or too low, can cause denaturation. And same thing with salt. Now the ideal temperature or the ideal pH or the ideal salt concentration depends on the enzyme. What job they do and where they work. So we have some enzymes that work in the stomach. The stomach is pH two. So we need to have an enzyme that doesn't denature, that doesn't unfold at that particular pH. Now we also have enzymes that work in our blood and our blood is closer to pH seven. So the ideal pH for those guys needs to be in the range of that pH or pH seven. If we took those blood proteins and put them in the stomach, that might cause them to unfold or denature. So again, each and every enzyme has an ideal condition where it's gonna work at. Now another term that's associated with enzymes is inhibition. And there's two kinds of inhibition that you can have. Competitive and non-competitive. So competitive inhibition is where you've got an inhibitor and it directly binds at the active site. And when it does that, it directly blocks the substrates from binding. So if you have an inhibitor that binds here or an inhibitor that binds here, it's gonna prevent those substrates from binding and you can't have a reaction. You've blocked that reaction. Or you can have non-competitive inhibition. This is where we have an inhibitor but it binds at a distant site, what it's often called an allosteric site. So you can see down here we've got another site where something can bind to and our inhibitor can bind here. And if it binds at that particular allosteric site, it can again cause the protein to change shape just a little bit. And when it does that, it prevents the substrates from binding at that active site. So again, if you change the shape of the protein, you change its ability to bind and interact with other things. So competitive inhibition, you directly block the active site by binding there. Non-competitive inhibition, you bind at a different site, a allosteric site, but you cause that protein to change shape so that the substrates can no longer bind to the active site. Now there's another kind of inhibition that's often associated with living systems. And this is called feedback inhibition. So you might have product A converted to product B by this enzyme, product B converted to product C by this enzyme, and product C converted to product D by this particular enzyme. Now as the cells going along, it might be producing a lot and even more truckloads, bucketloads, a ton of this product D. And if a cell is producing too much of a product, it's wasting its resources, it's wasting its time that it could be using for something else. And so cells have developed systems of stopping that pathway. So the fey have a lot of this end product. That end product can then inhibit or turn off the pathway. Again, it keeps the cell from wasting its resources. And so you can see here the end product D is able to feedback and stop that first enzyme in the entire pathway. And so when we're able to feedback and stop that enzyme, we're no longer making that final product. We're no longer making product D. And so the cell is able to use up and use up and use up this final product D. And so it's concentration or its values or its amounts drop. And when the concentration of D drops enough, then enzyme one is no longer inhibited and we can start making more of those final products. Again, when we have a buildup of product D, it's gonna inhibit enzyme one in this pathway, shut it down, shut it off to save resources. And then when the concentration of this final product drops, then we'll no longer be inhibiting it and we can have that pathway on. So turning it on, turning it off, turning it on, turning it off. So that we can regulate this pathway in the cell and again keep the cell from wasting its resources.