Question

Why is the induced-fit model a more likely model than the lock-and-key model?

Short answer

The induced-fit model is more likely because it accounts for enzyme conformation changes upon substrate binding, supported by X-ray crystallography evidence.

Step-by-step solution

Understanding the Lock-and-Key Model

The lock-and-key model suggests that the active site of an enzyme is precisely shaped to fit a specific substrate, like a key fitting into a lock. This model portrays enzymes as rigid structures.

Explaining the Induced-Fit Model

The induced-fit model proposes that while active sites have a specific shape, they are somewhat flexible. When the substrate binds, the enzyme adjusts its shape slightly for a better fit, enhancing catalytic action.

Comparison of Models

The induced-fit model accounts for the slight conformational changes that occur when an enzyme binds to a substrate, explaining the flexibility and adaptability of enzymes. The lock-and-key model does not accommodate structural changes.

Evaluating Experimental Evidence

Experimental evidence supports the induced-fit model by demonstrating changes in enzyme conformation upon substrate binding, observed through techniques like X-ray crystallography. Such evidence is inconsistent with the rigid nature proposed by the lock-and-key model.

Key concepts

Enzyme Active Site

The enzyme active site is the region on an enzyme where the substrate molecules bind and undergo a chemical reaction. It is highly specific, often determining the enzyme's selectivity for its substrate. The active site is usually a groove or pocket formed by the enzyme's unique amino acid sequence. This specific area not only holds the substrate in place but also provides the exact environment needed for the reaction to occur.
Additionally, the active site contains residues that help stabilize the transition state, which lowers the activation energy required for the reaction to proceed. This specificity and catalytic ability make the active site crucial for an enzyme's function.

Substrate Binding

Substrate binding is the initial step in the enzyme-mediated reaction. It describes the process by which a substrate attaches to the active site of an enzyme. In the induced-fit model, substrate binding is a dynamic event where the initial interaction slightly alters the enzyme's conformation.
During substrate binding, the enzyme's shape is modified to create the best possible fit with the substrate, enhancing interaction through mild adjustments. The induced-fit model explains this as an adaptive process that enhances specificity and catalytic efficiency.
Key points about substrate binding include:

• The enzyme and substrate interaction is specific and dynamic.
• Slight shifts in enzyme structure enhance binding efficiency and reactivity.
• Enzyme conformation adapts to optimize the catalytic environment.

Conformational Change

A conformational change refers to the alteration in the shape of an enzyme after the substrate binds. This phenomenon is central to the induced-fit model, which suggests that enzymes are flexible and able to adjust to accommodate their substrate.
Upon substrate binding, the enzyme undergoes a change in shape, leading to an even tighter fit and optimal positioning for the chemical reaction. This increases the rate of the reaction by stabilizing the transition state.
Important aspects of conformational change include:

• Flexibility allows precise molecular recognition and binding.
• Energy from substrate binding can be used to assist in the conformational shift.
• These changes enhance the enzyme's ability to lower activation energy.

X-ray Crystallography

X-ray crystallography is a technique used to determine the three-dimensional structure of molecules, including enzymes. It provides detailed insights into enzyme conformations before and after substrate binding, supporting the induced-fit model.
By analyzing the diffraction patterns of X-rays passing through crystallized molecules, scientists can obtain atom-by-atom maps of the enzyme's structure. This allows observation of structural changes, providing evidence of conformational shifts essential for enzyme function.
Notable points about X-ray crystallography involve:

• It delivers precise structural details of enzyme-substrate complexes.
• Supports understanding of flexible changes in enzyme structures.
• Confirms the dynamic nature of the induced-fit model compared to the rigid lock-and-key model.