Video Transcript
Which of the following best explains what happens when an enzyme denatures? A change in the lipid structure of an enzyme causes a change in the shape of the active site. A change in the structure of an enzyme causes the active site to become permanently bound to a substrate. A change in the carbohydrate structure of an enzyme causes the change in the shape of the active site. Or a change in the protein structure of an enzyme causes a change in the shape of the active site.
Let’s start by defining some key terms in the question to better understand how to answer it. Enzymes are biological catalysts which speed up the rates of reaction without being used up themselves. They do this by lowering the activation energy needed for a reaction to occur as you can see in these two graphs. We call reactions like this enzyme-controlled reactions. The reaction on the left without an enzyme has a much higher activation energy than the one on the right, which does use an enzyme and has quite a low activation energy. Reducing the activation energy for a reaction means that more reactions can occur over a set period of time. This increases the overall rate of reaction.
Let’s look at the structure of an enzyme. The blue structure here shows a simple diagram of an enzyme, which is made up of protein molecules. Enzymes have a region on their surface called an active site, which is labeled here. Each different enzyme has a specifically shaped active site, which is complementary to the shape of a certain substrate molecule, such as this pink one here. Substrates are a molecule, or molecules, which bind to the active site of an enzyme, sometimes also called the reactants of a reaction. When a substrate binds to the active site of the enzyme, the whole structure is now called an enzyme substrate complex. When the enzyme has done its job, it releases the molecules from its active site, which are now called the products.
In chemical reactions, enzymes are not used up. This means that they can continue catalyzing reactions and producing products, even after several reactions have occurred. Their enzymes are reusable; they are often not indestructible. Enzymes all have a specific temperature and pH at which they function best, called their optimum temperature or their optimum pH. When enzymes are exposed to conditions such as a very high temperature, however, or when they’re exposed to a pH that is much higher or lower than the enzyme’s optimum pH, the enzyme’s protein structure changes, and it is said to denature.
When an enzyme denatures, its active site changes its shape, as you can see in this diagram. This is because the protein structure of the enzyme has changed. You might notice that the active site is no longer a complementary shape to a specific substrate molecule. This means that the substrate cannot bind to the active site anymore. And this denatured enzyme can no longer catalyze enzyme-controlled reactions. When an enzyme denatures, this change in the shape of the active site is usually irreversible. If lots of enzymes denature, very few enzyme-controlled reactions can occur, therefore, decreasing the overall rate of reaction.
Let’s review our options to see which we can eliminate. Enzymes are proteins. So, the options referring to its structure as a lipid and as a carbohydrate are incorrect. The option stating that a change in the structure of an enzyme causes the active site to become permanently bound to a substrate molecule is also incorrect, as a substrate cannot bind to the enzyme’s active site at all when an enzyme denatures, let alone be permanently bound. The correct explanation of what happens when an enzyme denatures is, therefore, that a change in the protein structure of an enzyme causes a change in the shape of the active site.
