Video: Enzyme Action

In this video, we will learn how to describe the properties of enzymes, and outline the lock and key analogy of enzyme action.

15:59

Video Transcript

In this video, we’ll learn how enzymes catalyze biological reactions, how lock and key is related to enzyme action, and what it means to denature an active site. We’ll also take a look at why enzymes are important for life and find out what they’re made of. Then, we’ll work some practice problems about enzymes as well.

So why is it that we study enzymes? Well, enzymes catalyze biological reactions. And if we take a look at the big picture first, we’ll gain an understanding of why this is important. We can use a human as an example of a biological or living thing because we, like all living things, are made of cells. Cells are the basic unit of life because they organize the chemical reactions that underlie biological processes, biological processes being things like respiration, excretion, moving, thinking, digestion, transporting things within cells between cells or across your body, sensory information processing, and many more.

Humans are made up of about 37 trillion cells, each busy doing many, many chemical reactions every second. And that means that chemical reactions aren’t just important for life; there’re the essence of life. And biological reactions, which are chemical reactions mediated by an enzyme, are really important. Let’s take a look at the term “catalyze” next. The term that’s synonymous with catalyze is speed up. And enzymes, amazingly, speed up chemical reactions that they catalyze. Comparing the speed of a typical biochemical reaction with an enzyme to one without an enzyme is certainly like comparing a rocket to a rock. So that’s why we study enzymes. They speed up really important chemical reactions that underlie our biological processes.

What is it about enzymes that enables them to speed up or catalyze chemical reactions? Let’s start by taking a look at energy. And this graph will help us make sense of what’s happening to the amount of energy during a chemical reaction with or without an enzyme. Okay, so energy is represented on the 𝑦-axis. The 𝑥-axis represents the progress of the reaction as time goes by. So we start with the reactant or reactants on the left and the product or products on the right. In our example, we have two reactants, A and B, and one product, C. You can see that our reactants have to go up an energy hill. Imagine the excitement at the top, so high up, and about to go screaming down, forming the lower-energy product C.

Let’s see what happens to the shape of the graph when we add a catalyst such as an enzyme to the reaction. The roller coaster car actually helps to bond the reactants together, allowing for the product to form without having to go over such a high energy hill. And we use the term substrate instead of reactant in reactions involving enzymes. The smaller amount of energy required in enzyme-mediated reactions allows them to happen where they otherwise wouldn’t, such as a reaction in a test tube that requires a large input of energy. That happens in a cell with an enzyme rather than a flame. So now we know that enzymes reduce the amount of energy required for a reaction to occur.

Next, let’s look at how a roller coaster car can change its own tracks or, rather, how enzyme speed the production of products from substrates. Okay, so we just learned that enzymes reduce the amount of energy that’s required for a reaction to take place. But now we’re gonna learn how they do it. Let’s take a look at a very simplified cell with just two reactants floating in it, A and B, and no enzyme. In order for a reaction to occur between the reactants A and B, they need to collide with enough force and be oriented in the right way. So if this particular reactant A moves towards B while this particular reactant B moves towards A and they collide, there still may not be a reaction.

So, let’s try again with more force, which, of course, requires more energy. Okay, so we’ve got A hurtling towards B and B hurtling towards A with more force this time. And they still may not react. So next time, let’s try orienting them as if they were actual molecules that need to face each other in a certain way before they can form a bond. Now, reactants A and B hurtle towards each other with sufficient force and oriented so the legs of the A puncture into the loops of the B, producing a chemical reaction that forms a product which, in this case, is C.

The low probability of specific chemicals colliding with enough force and the correct orientation seems to allow reaction rates that are just fine for general universe building. But life requires mind-boggling numbers of chemical reactions per second without added energy from things like Bunsen burners or hot plates. So, without enzymes, reactions just require too much energy and have to slow a reaction rate to support life. Now, let’s take a look at how this reaction might happen with an enzyme, which I have shown here in green except for the pink portion, which is called the active site. And that’s where the substrates A and B will bind.

Now the substrates are bound to the enzyme in a very particular way that’s determined by how their shapes fit together. The closely matching fit between the enzyme and the substrates is called complementary, since they tend to complete each other. The interaction within the complex of enzyme and aligned substrates reduces the energy or force otherwise required for the chemical reaction to occur, producing the product C, which is then released, leaving the enzyme free to catalyze more reactions. So reactions catalyzed by an enzyme require less energy and have an increased reaction rate due to the complementary fit in interaction between enzyme and substrate.

The fit between the active site of the enzyme is so close that we use a lock and a key as an analogy. Here’s an analogy problem for a page title. A lock is to a key as a blank is to a blank. And our word choices are either enzyme or substrate. Here’s a locking door knob and a couple of keys. Will either key open the door? No, of course not. That’s the whole point of keys. You need the right key to get in the door. Here’s tyrosine hydroxylase, and its name says a lot about what it does. First, most all enzymes end in this suffix, A-S-E. So we have a pretty good idea it’s an enzyme. And the rest of its name comes from its substrates, tyrosine and a hydroxyl group.

Now that the substrates are bonded to the enzyme, you can probably tell which is represented by the lock and which by the key. A lock is to a key as an enzyme is to a substrate. And here’s another question: what is the name for this closely matching fit? It’s called a complementary fit, since their shapes tend to complete each other. But to continue the story about our enzyme here, the reaction is catalyzed by the well-aligned complex of enzyme and substrates, producing the product, which in this case is L-dopa. And after a couple more enzyme-mediated reactions, L-dopa will become dopamine, a neurotransmitter that can make you feel like you’ve just gotten a reward. And the enzyme will continue to catalyze more of the same reaction.

Enzymes can have problems catalyzing reactions though, especially if something happens to their active site. If the complementary fit between enzyme and substrate is lost, the reaction won’t be catalyzed, and the enzyme is said to be denatured. Since enzymes are usually proteins and the shape of a protein results from a DNA gene, mutations can affect the shape of an enzyme, causing it to be denatured. Up next, let’s work a couple example problems.

Which of the following statements correctly defines an enzyme? (A) An enzyme is a product of digestion. (B) An enzyme is a molecule that has been broken down. (C) An enzyme is a biological catalyst. (D) An enzyme is an inorganic catalyst. (E) An enzyme is a fast reaction.

Key knowledge required to select the correct solution option is knowing what enzymes are and what they do. Let’s review by taking a look at the enzyme ATP synthase while we review key terms in the question. Like all enzymes, ATP synthase catalyzes or speeds up a chemical reaction. And that makes it a catalyst. Here’s the reaction that ATP synthase catalyzes. ADP plus a phosphate groups yields ATP. The two parts of a chemical reaction that you’re probably used to seeing are reactant or reactants that are converted through the chemical reaction into a product or products. But during a reaction with an enzyme, we say that the substrate or substrates are converted into products or a product. So that means we know that a product is a chemical that’s made in a reaction.

Now let’s take a look at what ATP synthase is gonna do with the substrates, ADP and a phosphate group. The substrates bond to the active site of the enzyme. And that’s where the chemical reaction takes place, forming the product ATP. And the active site of the enzyme is now free to catalyze more reactions. Okay, just a few key terms left. Let’s take a look at “broken down.” if we have a larger molecule that we separate into smaller parts, we say we’re breaking it down. If we go in the opposite direction, we take some smaller molecules and put them together into a larger molecule, we call that building up.

And, of course, the term biological means “from a living system.” And enzymes are proteins produced in living systems. And since proteins are organic molecules, we know that enzymes are not inorganic. And we’re ready to answer our question. Which of the following correctly defines an enzyme? Let’s start with option (A). An enzyme is a product of digestion. Well, we saw that enzymes are catalysts. They’re not products, so option (A) can’t be correct. Option (B) says an enzyme is a molecule that has been broken down, but we saw that enzymes are not broken down. They have to stay intact because they’re reused over and over and over again.

Option (C) says an enzyme is a biological catalyst, and that sounds pretty good. Option (D), an enzyme is an inorganic catalyst, but we already know that can’t be true because proteins are not inorganic. Option (E) says an enzyme is a fast reaction, but we know that enzymes speed up or catalyze chemical reactions. They’re not the reactions themselves. So option (E) is also incorrect. Therefore, option (C), an enzyme is a biological catalyst, is the answer to our question.

We’re running a little bit short on time. So instead of doing another practice problem, let’s go to reviewing the key points from the lesson. First, an enzyme is a biological molecule and usually a protein that speeds up chemical reactions. The active site is the part of the enzyme where the substrate will bind. And substrate is another word for reactant in an enzyme-mediated reaction. The closely matching shapes of active site and the substrate is called a complementary fit. The complementary fit lowers the energy requirement for the chemical reaction to take place. The resulting products are released from the enzyme. And active site is open for more and more reactions.

Catalysts speed up chemical reactions, and enzymes are biological catalysts. When an enzyme doesn’t work anymore because the shape of its active site is changed, it’s said to be denatured. Here’s just a couple more facts about enzymes: they lower the energy requirement of chemical reactions, and the complementary fit between the enzyme’s active site and the substrate is the basis for the lock and key analogy we discussed.

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