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Competitive Inhibition of Enzymes: Definition & Examples

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  • 0:00 Enzymes and Enzyme Function
  • 1:25 Regulation of Enzyme Function
  • 1:54 Competitive Inhibition
  • 3:42 Examples of…
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson gives a brief overview of enzymes and enzyme function. It further defines competitive inhibition and provides real-life examples of competitive inhibitors.

Enzymes and Enzyme Function

What would be the quickest way to completely destroy a piece of paper? You could tear it up or shred it, but you'd still have pieces of it. You could dissolve it in water, but that might take a while. The fastest option (other than eating it) would probably be to burn it.

Cells like to have the fastest option when it comes to destroying, building, or changing molecules. Just as you would use fire to change the paper into ash as fast as possible, cells use enzymes to speed up the chemical reactions that change molecules.

Let's take a look at how this works and get some basic terminology down. Using our example of burning paper, the thing that is being changed (the paper) is called a substrate. The place on the enzyme where the substrate attaches to and gets changed (the flame) is called the active site, and the final result (ash) is called a product. A real-life example of this is pepsin, which is an enzyme that helps us digest the protein in food. Pepsin (the enzyme) breaks down proteins (the substrate) into peptide groups called polypeptides (the product). Without the work of enzymes, such as pepsin, we wouldn't be able to live.

Enzymes use substrates to make products

In the image, you can see that substrates fit into an active site kind of like a puzzle piece, and they need to fit into the right spot. If you are holding a candle, putting a piece of paper on the wax or under the candle won't burn it. It will only burn if you put it where the flame is.

Regulation of Enzyme Function

Now like the fire in our example above, enzymes need to be regulated because just as an out-of-control fire is dangerous to a house, out-of-control enzyme activity is dangerous to cells. Cells can regulate enzyme activity by activating or inhibiting their functions. Cells can inhibit enzyme activity by changing the form of the active site to stop substrate binding, stopping the formation of product after the substrate is bound, or allowing fake substrates to compete with real substrates.

Competitive Inhibition

This last option, allowing fake substrates to compete with real ones, is called competitive inhibition. Fake substrates competing with real substrates for the opportunity to bind at an active site (hence the name competitive) slows or stops enzyme function. Fake substrates look like real substrates to an enzyme's active site; they are, in essence, 'look-a-likes.' When a fake substrate binds to the active site of an enzyme, it can't be processed in the same way and it won't turn into a product. A fake substrate is called a competitive inhibitor. Competitive inhibitors bind the active site of an enzyme, preventing a real substrate from binding and a product from being formed.

Competitive inhibition can be overcome by addition of substrate, which increases an enzyme's chance of finding real substrate. Let's look at how this might work. Say you're trapped in a dark pen with a lion. Next to you is a tree trunk that has the same shape as you. If the lion mistakes the tree trunk for you and gets it stuck in its mouth, you are not turned into its next meal. The lion (enzyme) is inhibited from eating you (substrate) because the tree trunk (competitive inhibitor) is stuck in its mouth (active site). But, if we trap ten more people in with you, the lion (enzyme) has a much better chance of finding a person (substrate) instead of a tree trunk (competitive inhibitor).

Most competitive inhibition is reversible. Competitive inhibitors can be removed from the active site without changing enzyme activity. Often, competitive inhibitors will leave, or dissociate from, an active site on their own, leaving them both free to bind again. When the tree is removed from the lion's mouth, the lion is free to go after you again. Once removed, the enzyme is free to bind substrate and form products.

Examples of Competitive Inhibition

Competitive inhibition can be a very useful tool for researchers and doctors. While competitive inhibitors can cause health issues, such as seizures, they can also be used to treat and/or prevent diseases. And sometimes they can be helpful and harmful at the same time.

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