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Enzymes and their local environment

Video transcript

So today, we're going to talk about the effects of the environment on enzymes and how a changing environment can affect an enzyme's ability to catalyze a reaction. But first, let's review the idea that enzymes make reactions go faster. And looking at a reaction coordinate diagram, you'd notice that enzymes speed things up by lowering a reaction's activation energy. Now, it's important to recognize that enzymes work best in specific environments. And when I say environment, I can be really referring to many different aspects of an enzyme's surroundings. But right now, we're really only going to be focusing on pH and temperature values. So let's take another look at this by imagining that we have this person over here. And he's hungry, so he's eating some food. Now, there are a bunch of different digestive enzymes in this guy's body that are going to help him break down all the food he's eating into tiny usable parts. So first the food will be in his mouth. And one of the enzymes found inside a human's mouth is called alpha amylase, which is responsible for breaking down complex carbohydrates like starches into small simple carbohydrates like individual sugars. And alpha amylase is able to work well in your mouth since it functions best at a pH of around 7, which is about the same pH as a human's mouth. Now moving along, the food that our guy ate is going to go all the way down to his stomach, where a whole different group of enzymes will start breaking down the food. Now, one enzyme that humans have in their stomachs is called pepsin, which breaks down big proteins into smaller peptides. Now, pepsin will be most active at a pH of around 2, which is also the pH of your stomach, which is so low because of all the stomach acid that you'd find there. Now in terms of temperature, both of these enzymes typically work at a temperature of around 37 degrees, which is the same as body temperature. But you can see that these two different enzymes are functioning at different environmental conditions. So what would happen if we took an enzyme and moved it into a different environment? Well, let's first look at the effects of changing the pH of an enzyme's environment and jump right in with an example. So remember that DNA is a very negatively charged molecule because of all the negatively charged phosphate groups that you'd find on DNA. And in order for the enzyme DNA polymerase to help out with DNA replication, it binds a magnesium ion cofactor, which it uses to stabilize all the negative charge on DNA. Now under normal pH conditions, the DNA polymerase hold onto that magnesium ion through an electrostatic interaction between magnesium and one of its aspartate residues, which would be deprotonated and thus negatively charged at neutral pH values. Now, if we were to take DNA polymerase and put it into an environment with a reduced pH, then that aspartate residue will become protonated since the pH has dropped so much. And in its protonated form, aspartate no longer has a negative charge and can't hold on to that magnesium ion anymore. And overall, this means that DNA polymerase won't be able to do its job properly in a low pH environment. And keeping this enzyme at an appropriate pH is essential to its normal function. So now, let's take a look at the effects of temperature changes on enzyme function. So remember that proteins need to fold into their secondary, tertiary, and possibly quaternary structures in order to be in their functional form. And significant changes to a protein's temperature can disrupt a protein's folded geometry and cause it to lose its functionality. If we have our same person from before, who was really hungry and really wants to eat, but now this person get sick with a fever, his temperature will rise. And a bunch of the digestive enzymes in his body will get all jumbled up and won't be properly folded anymore. And this is why you might have a hard time eating and digesting food when you have a fever. And this can sometimes lead you to throwing up anything you'll eat since all of the digestive enzymes in your body won't work anymore because of the increase in your body's temperature. And the food you eat will just sit there, sit there in your body and make you feel sick. So what did we learn? Well, first we learned that enzymes generally function only under very specific environmental conditions. And different enzymes will often function ideally in different environments from other enzymes. And next, we learned that changes to an enzyme's environment, like changes to the surrounding pH or temperature, can lead to a loss of enzyme functionality.