The Induced Fit Model: Unlike the rigid 'Lock and Key' model, the induced fit model suggests that the active site is flexible. As the substrate enters, the enzyme undergoes a conformational change to wrap more tightly around the substrate.
Maximizing Catalysis: This slight change in shape ensures an ideal binding arrangement. The pressure exerted by the enzyme's changing shape helps to stress the substrate's bonds, making the reaction more likely to happen.
Reversibility and Recycling: Once the reaction is complete, the products no longer fit the active site and are released. The enzyme then returns to its original state, ready to catalyse another reaction with a new substrate molecule.
Lock and Key vs. Induced Fit: The Lock and Key model views the active site as a fixed, rigid shape, whereas the Induced Fit model recognizes the dynamic nature of protein structures that adjust upon binding.
Specificity vs. Denaturation: Specificity is the ability to select a substrate, while denaturation is the loss of that ability due to structural collapse. Denaturation occurs when external factors like high temperature or extreme pH break the bonds maintaining the tertiary structure.
| Feature | Competitive Inhibition | Non-Competitive Inhibition |
|---|---|---|
| Binding Site | Active Site | Allosteric Site (Alternative site) |
| Effect on Shape | No change to active site shape | Distorts active site shape |
| Substrate Fit | Blocked by inhibitor | Substrate no longer fits |
| Reversibility | Can be overcome by increasing substrate concentration | Cannot be overcome by increasing substrate concentration |
Use Precise Terminology: When describing specificity, always use the word 'complementary' rather than 'the same' or 'matching'. The active site is complementary to the substrate's shape.
Link Structure to Function: Always connect the 'specific shape' of the active site back to the 'tertiary structure' of the protein. Examiners look for this hierarchical link from amino acid sequence to 3D shape.
Explain Denaturation Correctly: If a question asks about the effect of pH or temperature, explain that these factors break specific bonds (hydrogen/ionic), which changes the tertiary structure and therefore the shape of the active site, preventing the formation of enzyme-substrate complexes.
'Enzymes are killed': Enzymes are molecules, not living organisms. Use the term 'denatured' to describe the loss of functional shape, never 'killed'.
Induced Fit is not Random: Students often think the enzyme changes into any shape. In reality, the conformational change is a specific, evolved response to the presence of the correct substrate.
Active Site vs. Whole Enzyme: Remember that while the whole enzyme is a large protein, specificity is determined by the relatively tiny active site. However, the rest of the protein is necessary to maintain the scaffold that holds the active site in its correct orientation.
