I hope those two videos were helpful in getting a solid introductory idea about how enzymes work. But there are some key points that we want to take away from those videos. And that is that enzymes are catalysts. So they allow chemical reactions to proceed in cells that would not proceed under the kinds of temperatures and pressures and pHs that are normally found in cells. And cells are fairly delicate.
They... They are sensitive to temperature changes. They need to have a very neutral pH. They exist under atmospheric pressure conditions.
And so without enzymes, most of the reactions that your cells need to survive wouldn't take place. Now, enzymes as catalysts are specific. They bind to very specific substrates and not other molecules.
We'll talk about that. Enzymes also reduce the energy needed for chemical reactions to occur. So this is a key idea. And it's one of the reasons why you're able to live at normal body temperature. We don't have to heat you up all the time to make your chemical reactions happen because enzymes allow chemical reactions to happen at much more life-sustaining temperatures.
One of the ways they do that is they place substrates in the right orientation for reactions to occur. So they probably place two molecules that are going to react with each other. They place them right next to each other in such a way that they're set up to react much more easily than if they had to randomly collide and run into each other at just the right angle.
This is a nice diagram illustrating enzyme specificity and the idea here is you've got your globular protein and your substrate and you can see how there's a little binding pocket specifically for this substrate. So the NH group is going to fit right here. This hydrogen will fit right here.
Okay. Actually, I think they're trying to show a hydrophobic interaction with that H. So it kind of looks like a bottom of a benzene ring, doesn't it? So that's going to fit right here.
The idea here is, though, is that any molecule that doesn't have this structure won't fit into this enzyme. And that allows this particular enzyme to be very specific for this substrate. This is showing something similar here, just the idea that the enzyme itself has binding pockets that allow specific shapes to fit into the binding pocket.
And that's what's going to give the enzyme the ability to react with some molecules but not others. And it gives the cell exquisite control over the kinds of chemical reactions that are happening. One of the key things that enzymes do is they reduce the amount of energy that the cell needs in order to cause a chemical reaction to occur.
Without an enzyme, you may have a reactant and products that are favorable in terms of a chemical reaction because the reactants that are at a higher energy level than the products. But because it takes so much energy to actually initiate the reaction, this never really happens. So if I took a beaker of glucose and I put it on the lab bench and I sat there and waited, it wouldn't spontaneously turn into carbon dioxide and water.
But we know that CO2 and water have a much lower energy level than glucose. So that's an example that many chemical reactions that are favorable in terms of their energy don't happen spontaneously because they require too much activation energy in order to occur. You'd have to add heat or pressure or change the pH or something major. With enzymes, however, your cells can take that glucose. And without ever raising your body temperature, they can convert it to carbon dioxide and water and produce a ton of energy in the process.
So enzymes, one of their roles is to reduce the amount of energy that's necessary in order for chemical reactions to occur. One of the ways enzymes make chemical reactions so much more favorable is they actually put substrates and products in the right orientation. So in the case where an enzyme is building a molecule, you can see these two substrates, right? And they are binding to the enzyme. And they need to interact here and here.
This is where the chemical bond is being formed. You can see the enzyme puts them in exactly the right position to bind and form a new covalent bond and form products. Without the enzyme, this end might crash into this end.
And that wouldn't cause a product that the cell needs. So the enzyme is controlling exactly. how these molecules are interacting to form just the product that it needs. And of course enzymes break down molecules as well. So here's a substrate and it's going to fit into this enzyme and the enzyme is going to have the job of breaking it apart.
So enzymes build molecules, enzymes also break down molecules. function to break them down as well. Hopefully at this point you have a sense of just how critical enzymes are in cells. And what happens when you make the temperature go up or make the temperature go down?
Well, it turns out that it can be pretty significant for cells. When you look at enzyme activity that's on this axis, as you increase the temperature, typically, it depends on the enzyme, the enzymatic activity increases. This is true of all chemical reactions.
As you increase temperature, reactions increase. However, the difference here is that while it goes up and up and up to a point, when you get beyond a temperature that's favorable for the life of that cell, you'll notice that the enzymatic activity drops off really significantly. So what's happening here?
Well, it turns out that when you heat an enzyme, you take your beautiful act... folded protein here some alpha helix here maybe some beta sheet here and some random coil beautiful it's all folded and as you increase the temperature you're going to cause this protein to unfold you're gonna put so much energy into the system that the interactions that are holding these amino acids into just the right three-dimensional structure get disrupted and for the vast majority of proteins when things cool down again they do not refold There are exceptions, but the vast majority of proteins become permanently denatured and they'll never work again when you heat them up. So this is kind of another illustration.
Here's our beautiful active enzyme. And after you denature it, you can see the shape no longer matches the substrate. Now, as you get colder and colder and colder, it turns out that this doesn't happen. So a cold enzyme, you can often store proteins in the cold. So And keeping a protein on an ice bath will slow the enzymatic activity way down, but it usually is protective.
So it won't cause it to denature. And so all you need to do is warm it up and it'll work again. But that's not the same. You can't go back the other way with heat.
Because once you unfold it, it's essentially permanently unfolded in every case. So what's one of the major ways we kill bacteria? Well, you flame your lupin lab and incinerate them. It's going to... it's going to convert them all into ash, but it's also going to start by denaturing proteins.
Autoclaves, you know, all of the media that we use in laboratory has been through an autoclave. And that moist heat in the autoclave, the number one thing it does to kill all the living things that are present is to denature the proteins. Moist heat is very good at denaturing proteins. All right, another common...... change that can occur in solutions is the pH and proteins are also quite sensitive to pH. So we're going to look at a protein here that has an optimal pH around 7. Actually, this one has an optimal pH around 5, doesn't it?
And we're going to look to see what happens when you lower the pH or increase the pH. And what happens actually is it's a little bit like heating the protein. As you increase the pH you disrupt charged and polar interactions inside. the protein.
And the same thing happens actually when you decrease the pH. So you're going to change the negative and positive charges that are present in this protein. And what happens is that will result in permanent unfolding, so denaturation of the enzyme. And again, we're just looking at the same idea here, that once you denature the enzyme, it no longer will bind the substrate.
So pH, whether it becomes too low or too high, is associated with permanent denaturation. enzymes and this is one of the reasons why when a bacterial cell produces a lot of acid we start to see a reduction in its ability to keep growing because that low pH really hinders its ability to survive by the same token we use buffers in media to control pH and try to keep it at a pH that's suitable for life and that allows us to grow bacteria to higher concentrations all right so bacteria all cells are very sensitive to temperature and pH. And the reason they are is because of the effect that extremes in pH and extremely high temperature has on permanent denaturation of proteins. All right, our next topic is going to be cofactors and coenzymes.