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Lock-and-key model

Lock-and-key model definition example

strong>Lock-and-key model
n., [lɑk ænd ki ˈmɑdl̩]
Definition: a model for enzyme-substrate interaction

Lock-and-key model Definition

Lock-and-key model is a model for enzyme-substrate interaction suggesting that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. In this model, enzymes are depicted as highly specific. They must bind to specific substrates before they catalyze chemical reactions. The term is a pivotal concept in enzymology to elucidate the intricate interaction between enzymes and substrates at the molecular level. In the lock-and-key model, the enzyme-substrate interaction suggests that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. Like a key into a lock, only the correct size and shape of the substrate (the key) would fit into the active site (the keyhole) of the enzyme (the lock).

Compare: Induced fit model  See also: enzyme, active site, substrate

Lock-and-key vs. Induced Fit Model

At present, two models attempt to explain enzyme-substrate specificity; one of which is the lock-and-key model, and the other is the Induced fit model. The lock and key model theory was first postulated by Emil Fischer in 1894. The lock-and-key enzyme action proposes the high specificity of enzymes. However, it does not explain the stabilization of the transition state that the enzymes achieve. The induced fit model (proposed by Daniel Koshland in 1958) suggests that the active site continues to change until the substrate is completely bound to the active site of the enzyme, at which point the final shape and charge are determined. Unlike the lock-and-key model, the induced fit model shows that enzymes are rather flexible structures. Nevertheless, Fischer’s Lock and Key theory laid an important foundation for subsequent research, such as during the refinement of the enzyme-substrate complex mechanism, as ascribed in the induced fit model. The lock-and-key hypothesis has opened ideas where enzyme action is not merely catalytic but incorporates a rather complex process in how they interact with the correct substrates with precision.

Lock and key model definition and example
Figure 1: Lock and key model. Image Source: Maria Victoria Gonzaga of Biology Online.


induced fit model definition and example
Figure 2: Induced Fit Model. Image Source: Maria Victoria Gonzaga of Biology Online.


Enzyme-Substrate Complex
Figure 3: The enzyme binds to the substrate and forms the enzyme-substrate complex. This illustration represents how enzymes bind substrates via their binding sites. Image Source: from the diagram of Khan Academy and OpenStax College, CC BY-SA 4.0, modified by Maria Victoria Gonzaga of Biology Online.

Key Components

Components of the lock and key model:

  • Enzyme: the enzyme structure is a three-dimensional protein configuration, with an active site from where the substrate binds.
  • Substrate: often an organic molecule, a substrate possesses a structural feature that complements the geometry of the enzyme’s active site.

In the lock and key model, both the enzymes and the substrates facilitate the formation of a complex that lowers the activation energy needed for a chemical transformation to occur. Such reduction in the activation energy allows the chemical reaction to proceed at a relatively faster rate, making enzymes crucial in various biological and molecular processes.



Lock-and-key Model Examples

Some of the common examples that are often discussed in the context of the Lock and Key Model are as follows:

  • Enzyme lactate dehydrogenase with a specific active site for its substrates, pyruvate and lactate. The complex facilitates the interconversion of pyruvate and lactate during anaerobic respiration
  • Enzyme carbonic anhydrase with a specific active site for the substrates carbon dioxide and water. The complex facilitates the hydration of carbon dioxide, forming bicarbonate
  • Enzyme lysozyme binding with a bacterial cell wall peptidoglycan, which is a vital immune function
Take note though that these examples may have been described in the context of the lock and key model but the distinction between the lock and key model and the induced fit model is not as absolute as it seems. Many enzymes may exhibit characteristics of both models to varying extents.

Answer the quiz below to check what you have learned so far about the lock-and-key model.


Choose the best answer. 

1. Enzyme-substrate interaction

2. Specific complementary geometric shapes that fit exactly into one another

3. Changes until the substrate is bound to the enzyme

4. First postulated by Emil Fischer

5. Which of the following depicts Lock-and-key model?

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  • Aryal, S. and Karki, P. (2023).  “Lock and Key Model- Mode of Action of Enzymes”. Microbenotes.com. https://microbenotes.com/lock-and-key-model-mode-of-action-of-enzymes/
  • Farhana, A., & Lappin, S. L. (2023, May). Biochemistry, Lactate Dehydrogenase. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK557536/


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