. Describe the lock-and-key model. What are the deficiencies and merits of the lock-and-key model for enzyme action? What is the role of the oxyanion hole in chymotrypsin? Describe the hole. 25. How does a low-barrier hydrogen bond promote catalysis? 26. What are the roles of proximity and orientation effects in enzyme catalysis? What is induced fit? Provide and describe an example of an induced fit.
The Correct Answer and Explanation is:
The lock-and-key model of enzyme action suggests that enzymes are highly specific in their interactions with substrates, much like a key fits into a lock. The enzyme’s active site has a precise shape that perfectly matches the substrate, ensuring only specific molecules can bind and undergo a reaction. This model was first proposed by Emil Fischer in 1894 and emphasizes the static, rigid nature of enzyme-substrate binding.
Merits:
- The model explains the high specificity of enzymes for their substrates.
- It shows how the enzyme’s active site can be a precise template for substrate recognition.
- It provides a simple and intuitive way to understand enzyme-substrate interactions.
Deficiencies:
- The model does not account for the flexibility of enzymes in their interactions with substrates.
- It overlooks the dynamic nature of enzyme-substrate binding, as enzymes undergo conformational changes during catalysis.
- The model fails to explain how enzymes lower the activation energy of reactions, as it assumes a perfect fit from the beginning.
The oxyanion hole in chymotrypsin is a key structural feature involved in catalysis. Chymotrypsin is a protease that breaks down proteins by cleaving peptide bonds. The oxyanion hole stabilizes the transition state during catalysis. It is formed by the enzyme’s active site and consists of a region that can form hydrogen bonds with the negatively charged oxygen atom in the tetrahedral intermediate formed during peptide bond cleavage. This stabilization lowers the activation energy for the reaction.
A low-barrier hydrogen bond promotes catalysis by stabilizing the transition state more effectively than conventional hydrogen bonds. These bonds are characterized by a very short bond length and occur when the proton is shared almost equally between the donor and acceptor atoms. This creates a stronger interaction, which helps lower the activation energy and facilitates the chemical reaction.
Proximity and orientation effects in enzyme catalysis refer to the way enzymes bring substrates into close proximity and orient them correctly for the reaction to occur. This reduces the entropy of the system, increasing the likelihood of the reaction happening. Induced fit is a model that suggests that the enzyme’s active site is flexible and can change shape upon substrate binding, enhancing the specificity and catalytic efficiency of the enzyme. An example of induced fit is seen in hexokinase, where the enzyme undergoes a conformational change upon binding to glucose, ensuring proper positioning for phosphorylation to occur.
