What is the Difference Between Induced Fit and Lock and Key

The main difference between induced fit and lock and key model is that in the induced fit model, the active site of the enzyme does not completely fit to the substrate whereas in the lock and key model, the active site of the enzyme is the complement of the substrate and hence, it precisely fits to the substrate. Furthermore, in the induced fit model, the active site of the enzyme has to undergo a conformational change to improve binding while the lock and key model describes the specificity of the active site of the enzyme to a particular substrate. 

Induced fit and lock and key model are the two models of enzyme-substrate interactions. Generally, they describe how the enzymes interact with the substrate. 

Key Areas Covered 

1. What is Induced Fit Model
     – Definition, Mechanism of Action, Significance
2. What is Lock and Key Model
     – Definition, Mechanism of Action, Significance
3. What are the Similarities Between Induced Fit and Lock and Key Model
     – Outline of Common Features
4. What is the Difference Between Induced Fit and Lock and Key Model
     – Comparison of Key Differences

Key Terms 

Active Site, Enzyme, Induced Fit Model, Lock and Key Model, Substrate

What is Induced Fit Model 

The induced fit model is one of the main models, describing the enzyme-substrate interaction. Also, Daniel Koshland suggested this model in 1958. Basically, according to the hypothesis, the active site of the enzyme does not have a rigid conformation. Therefore, the substrate does not completely fit into the active site of the enzyme. Hence, the active site of the enzyme modifies its shape upon the binding of the substrate, becoming complementary to the shape of the substrate. Significantly, this conformational change is possible due to the flexibility of the protein molecule, which serves as the ellnzyme.

Figure 1: Induced Fit Model of Hexokinase

Furthermore, the active site of the enzyme is not static and it requires a separate catalytic group for the action of the enzyme. However, the binding of the catalytic group weakens the bonds formed by the substrate with the active site. Thereby, the induced fit model describes the mechanism of nonaction over competitive inhibitors. 

What is Lock and Key Model  

Lock and key model is the second model, which describes the enzyme-substrate interaction. However, Emil Fischer suggested this model in 1894. Therefore, it is also called Fisher’s theory. According to the lock and key model, the active site of the enzymes serves as the ‘lock’ while its substrate serves as the ‘key’. On that account, the shape of the active site of the enzyme is complementary to the shape of the substrate. Thereby, the active site of the enzyme can hold the substrate closer to the enzyme by forming an unusable intermediate compound, which is the enzyme-substrate complex.

Figure 2: Induced Fit and Lock and Key Models

Moreover, the close proximity allows the biological reaction to proceed. Therefore, the subsequent dissociation of the enzyme-substrate complex results in the enzyme and the products. Also, the lock and key model does not need a separate catalytic group for the action of the enzyme. In addition to these, the static active site of the enzyme consists of a single entity in the lock and key model. 

Similarities Between Induced Fit and Lock and Key  Model

• Induced fit and lock and key are the two models, which describe the mechanism of action of the enzyme.  
• Both models depend on the degree of precise binding of the substrate to the active site of the enzyme. 
• They are important in describing how enzymes increase the rate of a  biological reaction through catalysis.  
• Both models reduce the activation energy of a specific biochemical reaction. 

Difference Between Induced Fit and Lock and Key Model 

Definition 

The induced-fit model refers to a model for enzyme-substrate interaction in which the active site of the enzyme does not completely fit to the substrate. On the other hand, the lock and key model refers to a second model for enzyme-substrate interaction in which the active site of the enzyme completely fits with the substrate. 

Suggested by 

The induced fit model was suggested by Daniel Koshland in 1958 while the lock and key model was suggested by Emil Fischer in 1894. 

Fitting of the Active Site of the Enzyme to the Substrate 

The active site of the enzyme does not completely fit with the substrate in the induced fit model, while the active site of the enzyme precisely fits with the substrate in the lock and key model. 

Significance of the Active Site 

In the induced fit model, the active site of the enzyme has to undergo a conformational change to improve binding, while the lock and key model describes the specificity of the active site of the enzyme to a particular substrate. 

Active Site Composition 

The active site of the enzyme contains two components in the induced fit model, while the active site of the enzyme contains a single entity in the lock and key model. 

Catalytic Groups 

There is a separate catalytic group in the enzyme in the induced fit model while there is no separate catalytic group in the enzyme in the lock and key model. 

Properties of the Active Site 

The active site of the enzyme is not static in the induced fit model, while the active site of the enzyme is static in the lock and key model. 

Development of a Transition State 

A transition state develops before the reactants undergo changes in the induced fit model, while a transition state does not develop before the reactants undergo changes in the lock and key model. 

The weakening of the Catalytic Bonds 

Catalytic group weakens the substrate bonds either by the nucleophilic or electrophilic attack in the induced fit model, while the catalytic group does not weaken the substrate bonds in the lock and key model. 

Nonaction over Competitive Inhibitors 

The induced-fit model describes the mechanism of nonaction over competitive inhibitors, while the lock and key model describes the specificity of the active site of the enzyme to a particular substrate.  

Conclusion 

In brief, the induced fit model is a model for enzyme-substrate interactions in which the substrate does not completely fit into the active site of the enzyme. Hence, the active site of the enzyme has to undergo a conformational change while binding to the substrate. In comparison, the lock and key model is a second model for enzyme-substrate interaction in which the substrate completely fits into the active site of the enzyme. Therefore, it describes the specificity of binding of the active site of the enzyme towards a particular substrate. Therefore, the main difference between induced fit and lock and key model is the mechanism of substrate binding and importance. 

References:

1. Cornell, Brent. “Models of Action.” BioNinja, Available Here.

Image Courtesy:

1. “Hexokinase induced fit” By Thomas Shafee – Own work (CC BY 4.0) via Commons Wikimedia    
2. “CNX Chem 12 07 Enzyme” By OpenStax (CC BY 4.0) via Commons Wikimedia