Examkin

Question

Difficulty: HardComparing and Contrasting Models

### Models of Enzyme-Substrate Binding

Enzymes are biological catalysts that speed up chemical reactions by binding to specific reactant molecules called substrates. Scientists have proposed different models to explain the physical and structural dynamics of this binding process at the enzyme's active site.

* Model 1 (Lock-and-Key Model): The enzyme's active site possesses a rigid, pre-determined shape that is exactly complementary to the shape of the substrate. The substrate fits into the active site like a key into a lock. No conformational (structural) changes occur in either the enzyme or the substrate during the binding process.
* Model 2 (Induced-Fit Model): The enzyme's active site is flexible and not initially fully complementary to the substrate. As the substrate approaches and begins to interact with the active site, the physical contact induces a conformational change in the enzyme. This change molds the active site around the substrate to form a tight, complementary fit.
* Model 3 (Conformational Selection Model): The enzyme is highly dynamic and spontaneously fluctuates between multiple conformations (shapes), including active (complementary to the substrate) and inactive shapes, even in the complete absence of the substrate. The substrate does not induce a shape change; instead, it selectively binds only to the enzyme when the enzyme happens to fluctuate into the complementary active conformation.

Based on the models presented, which of the following statements identifies a key difference between Model 2 and Model 3 regarding the interaction between the enzyme and the substrate?

  1. Model 2 describes the substrate actively causing a change in the enzyme's shape during binding, whereas Model 3 describes the substrate selecting a pre-existing shape without causing a conformational change.Answer
  2. B
    Model 2 assumes the enzyme is completely rigid, whereas Model 3 assumes the enzyme changes shape only after the substrate binds.
  3. C
    Model 2 states that the enzyme exists in multiple conformations in the absence of the substrate, whereas Model 3 states that the enzyme only changes shape when the substrate is present.
  4. D
    Both Model 2 and Model 3 propose that the enzyme and substrate must have static, perfectly complementary shapes before any interaction begins.

Answer

Model 2 describes the substrate actively causing a change in the enzyme's shape during binding, whereas Model 3 describes the substrate selecting a pre-existing shape without causing a conformational change.
The correct option correctly contrasts the models: Model 2 relies on the substrate to actively induce a conformational change in the flexible enzyme, while Model 3 states that the enzyme spontaneously fluctuates among shapes on its own, and the substrate simply binds to the pre-existing active shape.

Step-by-Step Solution

1
Analyze the mechanism of conformational change in Model 2.
In Model 2, the enzyme's active site is flexible and changes shape directly due to physical interaction with the incoming substrate.
To understand how the substrate drives changes in the enzyme under this model.
2
Analyze the mechanism of conformational change in Model 3.
In Model 3, the enzyme fluctuates among conformations independently of the substrate, and the substrate binds only when the active conformation occurs spontaneously.
To identify how shape variation occurs without substrate-induced forces.
3
Compare the two mechanisms to find the key distinction.
Model 2 involves an active induction of change by the substrate, whereas Model 3 involves selective binding to a pre-existing state without induction.
To match the correct contrast with the options provided.

Key Concept

Comparing and Contrasting Models
Rate this question

Topics

Comparing and Contrasting Models

More questions from this topic