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Function of Enzymes: Substrate, Active Site & Activation Energy

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  • 0:06 Enzymes Lower the…
  • 2:45 Enzyme Structure
  • 4:49 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we'll learn how enzymes function to lower the activation energy of a chemical reaction. Enzymes bind to their substrates to perform all kinds of important and essential cellular processes, as well as processes that help you enjoy a slice of pizza!

Enzymes Lower the Activation Energy

Enzymes increase the rate of reaction by lowering activation energy.
Enzyme Activation Energy Graph

Making a snowman is always a big ball of fun - three snowballs full of fun, to be exact. And each snowball starts out small, getting bigger as you roll it in white fluffiness. Let's imagine that you go outside in a fresh snowfall to build one of these guys, but you don't have the best-suited yard for rolling snowballs. In fact, your backyard is one giant hill. This hill is fantastic for sledding, but not so much for making snowmen. Anyways, in order to make a giant snowball for the snowman's bottom, you'll need to actually push this snowball up over the hill and then down the other side. You do it because it's fun - but make no mistake, it takes a lot of energy to get started up this hill. This is hard work.

Right now, you share a quintessential problem with some chemical reactions within a cell. These chemical reactions might be a lot of fun when you're in a cell, but it takes a lot of energy to get them started. This is called the activation energy, or the energy required for a reaction to start. You can think of activation energy as that energy you needed to muster up in order to push that snowball up and over the hill in your backyard in order to maximize the size of your snowball and get it to the other side.

What your cells have to help overcome a problem of high activation energy are called enzymes. Enzymes are proteins that lower the activation energy of a reaction. In doing this, enzymes increase the rate of a reaction, helping it to occur faster. However, enzymes are not consumed in a reaction; they simply help it to occur. They do not add snow to your snowball; they just make it easier to make a snowball.

This makes enzymes reaction catalysts. The presence of enzymes is analogous to decreasing the size of the hill in your backyard. Enzymes make things easier for your cell and help chemical reactions occur. There are hundreds of different kinds of enzymes in your cells, which all participate in different types of reactions. Enzymes can break molecules apart, build or add molecules, and even rearrange them.

In lowering the activation energy of a reaction, enzymes decrease the barrier to starting a reaction. It's important to note, however, that the change in energy remains the same between the start and end of a chemical reaction. For example, if a reaction released 200 kJ of energy without an enzyme, the same reaction would still release 200 kJ of energy with some enzymatic aid. The difference would only be a lower activation energy and a faster rate of reaction. In other words, if enzymes were helping you build a snowman, you'd be done quicker, but you'd have just as much fun!

Enzyme Structure

The structure of enzymes includes an active site for chemical reactions.
Enzyme Active Site Diagram

Most of us are fortunate enough to be able to enjoy a grilled cheese sandwich and hot chocolate when we take a break from building our snowman outdoors in the cold. This is because our cells contain an enzyme called lactase. It's interesting to note and to remember that many enzyme names end in the suffix '-ase.' Lactase breaks down lactose disaccharide sugars into glucose and galactose. Lactose, of course, is the sugar found in dairy products. Some individuals are lactose-intolerant, and they lack this enzyme. Without lactase, lactose will break down on its own, but it is an extremely slow process, maybe even taking days. Lactase speeds up the rate of this reaction, allowing us to digest lactose. Let's talk about how it's done.

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