What is an Allosteric Site of the Enzyme? - Definition & Biology

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  • 0:03 What Are Enzymes?
  • 1:01 Enzyme Activity:…
  • 2:27 Examples
  • 5:06 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
In this lesson, we'll review what enzymes are and how they work. Then, we'll learn what the allosteric site is on an enzyme and how it influences enzyme activity. We'll also go over some examples of allosteric regulation in the body.

What Are Enzymes?

Have you ever played with Legos or building blocks as a kid? You have a bunch of pieces in a plastic container, and you take out the shapes you need. You can put them together to build structures, or take them apart and hopefully put them back in the bucket so your parents don't trip on them.

Although this is a simple analogy, small molecules inside the cells of your body, called enzymes, serve a similar function by making and breaking apart other molecules. Enzymes are protein catalysts. A catalyst is something that increases the speed of a reaction. Enzymes speed up chemical reactions that are used to build structures in your cells or break them down. The substrate is the name for the substance an enzyme works on. The substrate binds to the active site, or the place on the enzyme that actually does the work. In our analogy, you are like the enzyme and the Legos are like the substrate. You grab the substrate and change it, either putting pieces together or breaking them apart.

Enzyme Activity: Allosteric Sites

Enzymes work at different speeds depending on their environment. The rate at which the enzyme does its job is called enzyme activity. How hot or cold the environment is, the pH, the location in the body, and what other substances are around all influence enzyme activity.

Some substances bind the enzyme at a site other than the active site. This other site is called the allosteric site. The allosteric site allows molecules to either activate or inhibit, or turn off, enzyme activity. These molecules bind the allosteric site and change the confirmation, or shape, of the enzyme.

Molecules that turn off enzymes are called allosteric inhibitors. Allosteric inhibitors change how the active site is shaped and prevents it from binding, or attaching, to the substrate. If the enzyme can't attach to the substrate, it can't do its job! These molecules and the allosteric site to which they bind are like the 'off switch' for the enzyme.

Allosteric activators on the other hand, make the enzyme more efficient. They change the shape of the enzyme, like allosteric inhibitors, but they make the enzyme better able to bind the substrate, instead of worse. This makes the enzyme do its job better. These molecules and their allosteric sites are like the 'on switch' for the enzyme.

Next, let's look at a few examples of how allosteric inhibitors and activators affect the allosteric site and enzyme activity.


First, let's look at some examples of some different kinds of allosteric inhibitors. If you go outside and run a mile, you'll feel your pulse racing. Your heart is beating fast, trying to send blood all over your body to get your cells oxygen, which we need to make energy. An enzyme in your blood called hemoglobin is the worker that hauls the oxygen through your blood to your tissues.

There are two forms of hemoglobin, the T state (or tense state), which makes hemoglobin bind oxygen less tightly, and the R state (or relaxed state), which makes hemoglobin bind oxygen more tightly. Now, this might give you the impression that the T state is bad since it prevents hemoglobin from binding oxygen. However, when hemoglobin gets to the cells in our body, it's got to let go of that oxygen so the cells can have it. Think of giving someone a present. It's great to pick up the present at the store, like hemoglobin picks up oxygen at the lungs, but you have to actually hand it over to the recipient for it to do what you intended.

When you are making energy, your body makes a product called 2,3 bisphosphoglycerate (better known as 2,3 BPG), an allosteric inhibitor of hemoglobin. If you're making more energy, you need more oxygen to do so. 2,3 BPG binds the allosteric site in hemoglobin and puts it into the T state when it gets to your cells. This lets the hemoglobin release the oxygen for your tissues to use. Without it, hemoglobin wouldn't release oxygen as effectively and you would get less oxygen delivered to those cells that need oxygen to make energy. Since 2,3 BPG makes hemoglobin less likely to bind oxygen, it's considered an allosteric inhibitor, even though it does good things for our body.

Now, let's look at some examples of some allosteric activators. Other molecules turn on enzymes, making them bind their substrate better. When we make energy, an important enzyme called phosphofructokinase (or PFK) controls if the first step of glycolysis, the body's process of making energy, is going to start or not. It's like the gatekeeper for making energy.

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