Lock and Key and Induced Fit Models of Enzyme Activity

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  • 0:01 Enzymes
  • 1:20 How Enzymes Work
  • 3:04 Lock and Key Hypothesis
  • 3:44 Induced Fit Model
  • 5:21 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Cells are full of chemical reactions, but nature doesn't like waiting any more than we do. Enzymes help speed up these reactions, and over time, scientists have modified their theories on how this works. In this lesson, we'll explore the two main theories that explain this.


See this piece of wood? If I place it here on the ground and leave it alone, eventually it will completely disintegrate until nothing but the basic carbon molecules are left. But, this could take a very, very long time, and I am rather impatient. So instead of waiting, I'll just set it on fire and achieve the same result in a much shorter time period. Apart from satisfying my pyromania, do you see what I did there? I used fire as a catalyst, which is just a substance to increase the rate of a chemical reaction. The wood would have disintegrated in time, but the fire sped up that process.

Well, as it turns out, nature is just as impatient as I am. These sorts of catalysts are all around us, and even within us. Nearly all cells contain large molecules that catalyze chemical reactions within the cell, called enzymes. Enzymes are important. The cell could wait for nutrients to dissolve into energy, but frankly the cell could die waiting for this to happen. Enzymes speed up the process. Sometimes it pays to be a little impatient.

How Enzymes Work

We know that enzymes are catalysts, but how exactly do they work? Well, this here is glucosidase, an enzyme that processes specific kinds of sugars. It starts when the cell ingests a complex sugar molecule, like this one here (see video). The complex molecule in this process is called the substrate. Over time, this complex sugar molecule will decompose into smaller molecules called products. In this case, the complex sugar becomes individual glucose molecules, which is a much simpler compound that the cell can use for energy. But, the cell can't just wait for the substrate to naturally dissolve into the products, so that's where the enzyme comes in. Glucosidase turns the substrate, the complex sugar, into two smaller products, simple glucose molecules.

See how that works? Now, it's important to note that this specific reaction can only be produced by glucosidase. Another enzyme cannot break complex sugars into glucose, and glucosidase cannot break down other substrates. Each chemical reaction can only be catalyzed by a specific kind of enzyme. Considering that there are roughly 5,000 chemical reactions that enzymes are known to catalyze, we've got a lot of enzymes with very specific focuses. So how do they know which molecules they can catalyze and which ones they can't? After all, real enzymes don't actually have hands and eyes like our cute little cartoons here. This is a major scientific debate, and there are two leading theories to explain this complex, but very important, process.

The Lock and Key Hypothesis

Scientists have long wondered exactly how enzymes know which substrates to process and which to ignore. In 1894, German chemist Emil Fischer proposed the lock and key theory, which states that enzymes have a specific shape that directly correlates to the shape of the substrate. Basically, substrates fit into an enzyme the way a key fits into a lock. If the substrate is not the correct shape, it won't fit into the enzyme, and no chemical reaction can occur. Only those substrates that exactly fit into the enzyme can be catalyzed.

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