Induced Fit Enzyme Model: Definition & Theory

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  • 0:00 Why Are Enzymes Important?
  • 1:35 How Do Enzymes Work?
  • 2:15 The Induced-Fit Model
  • 3:55 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Enzymes bind with substrates to keep all the necessary biochemical reactions in your body going at the proper pace. But each enzyme can only bind with one particular substrate. So how do the enzymes know which substrate is meant for them? In this lesson, learn about the Induced-Fit model and find out!

Why Are Enzymes Important?

Enzymes are found almost everywhere in your body and do everything from copying your DNA to digesting your food. They are essential for pretty much all your life processes. You can better understand the importance of enzymes when you recognize the effects of NOT having a particular enzyme: a missing or malfunctioning enzymatic protein characterizes many diseases and disorders, such as albinism, Hunter syndrome, and Tay-Sachs disease.

Enzymes catalyze, or speed up the biochemical reactions required for life. They do so by lowering the activation energy, the energy required for the chemical reaction to start, of all the necessary reactions in your cells. This is similar to changing a high wall into a low fence. A lot more energy is required to climb a wall than to hop over a fence. If all the walls in front of you are converted into fences, you will get through your obstacle course in a much shorter time, so to speak. Basically, everything moves along much quicker. In fact, without enzymes, the essential chemical reactions would not be fast enough to sustain life.

Enzymes lower the activation energies of chemical reactions.
Diagram showing activation energy with and without an enzyme.

Consider this example. When you eat a candy bar, your body immediately goes to work breaking down the sugar into water, carbon dioxide, and energy. In some cases, you can feel the effects within seconds. However, sugar can sit in your kitchen pantry for years and remain unchanged. A series of enzymes kick-starts the breakdown of that sugar, since there is no way that we would be able to wait years for the sugar to degrade on its own.

How Do Enzymes Work?

The molecule (or molecules) with which the enzymes bind is referred to as the substrate. The substrate binds to a small section of the enzyme referred to as the active site. The molecule (or molecules) produced at the end of the reaction is referred to as the product. Once the reaction is complete, the enzyme releases the product and is ready to bind with another substrate.

The substrate binds with the active site of the enzyme to start the reaction. Once the reaction is complete, the enzyme releases the product and is ready to bind with another substrate.
Diagram illustrating the bonding of a substrate with an enzyme and the subsequent release of the product.

Enzymes are extremely particular, and each enzyme only binds with one particular substrate. So how does the enzyme know whether or not the molecule next to it is the one that it wants? Daniel Koshland offered a solution to this puzzle in 1958.

The Induced-Fit Model

The induced-fit model is actually an offshoot of an earlier theory proposed by Emil Fischer in 1894, the lock-and-key model. The lock-and-key model states that the substrate acts as a 'key' to the 'lock' of the active site. The active site and substrate are exact matches for each other, similar to puzzle pieces fitting together. In this model, only a single substrate is the precise match for the enzyme. Once the enzyme finds its exact counterpart, the chemical reaction can begin.

The Lock and Key Model.
Diagram illustrating the Lock and Key Model.

The induced-fit model is generally considered the more correct version. This theory maintains that the active site and the substrate are, initially, not perfect matches for each other. Rather, the substrate induces a change of shape in the enzyme. This is similar to placing your hand in a glove. Getting the first finger in may be difficult, but, once you complete this initial step, the glove slides on easily (read, 'with much less energy') because it is now properly aligned for your hand.

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