Induced Fit Enzyme Model: Definition & Theory

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.

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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.

The Induced-Fit Model.
Diagram illustrating the Induced-Fit Model.

In this model, the shape change is regarded as the beginning of the catalysis (rather than the reaction occurring after the bond has been completed, as in the lock and Key model). During the shape change, the most reactive molecules and atoms move closer together, encouraging the quickest reaction possible. Furthermore, this model explains why some molecules are able to bind to the enzyme but are unable to produce a reaction. If the enzyme is distorted too much in the process, or if the binding substrate is not the correct size, the reaction cannot occur. Only the proper substrate can induce the exact change that initiates the reaction.

Lesson Summary

Let’s review. Enzymes work by lowering the activation energy of the biochemical reactions in your body. Without them, the reactions would not occur quickly enough to sustain life. However, each enzyme only works on a particular substrate. The induced-fit model, proposed by Daniel Koshland in 1958, attempts to explain how this is accomplished. His theory asserts that when the active site on the enzymes makes contact with the proper substrate, the enzyme molds itself to the shape of the molecule. This allows the necessary atoms in the enzyme and the substrate to contact each other so that the chemical reaction can begin. Only the correct substrate can induce the change required to start a reaction. Once the reaction is complete, the enzyme releases the product and is ready to bind with another substrate.

Key Terms

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Enzymes – biological substances which speed up the biochemical reactions required for life

Substrate – the molecule or molecules with which the enzymes bind

Active site – a small section of the enzyme to which the substrate binds

Product – the molecule or molecules produced at the end of an enzymatic reaction

Induced-fit model – an offshoot of the earlier lock-and-key model; states that the substrate induces a change of shape in the enzyme

Lock and key model – theory proposed by Emil Fischer in 1894; states that the substrate acts as a ‘key’ to the ‘lock’ of the active site

Learning Outcomes

After completion, see if this lesson helped to prepare you to:

  • Describe the importance of enzymes in our bodily systems
  • Express understanding of the relationship between enzymes and substrates
  • Interpret the induced-fit model
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