Which of these best describes how the substrate fits with the enzyme ?

Options

A. The active site changes shape slightly to fit the substrate
B. It is exactly into the non-active site
C. Both (a) and (b)
D. None of these

Answer

To solve the question about how the substrate fits with the enzyme, we need to understand the relationship between enzymes and substrates, particularly focusing on the active site and the mechanisms involved in their interaction.

### Step-by-Step Solution:

1. **Understanding Enzymes and Substrates**: Enzymes are biological catalysts that speed up chemical reactions in the body. Each enzyme has a specific region called the active site, where the substrate (the molecule upon which the enzyme acts) binds.

**Hint**: Remember that enzymes are specific to their substrates due to the shape and chemical properties of the active site.

2. **Active Site Definition**: The active site is a specially shaped area of the enzyme that fits the substrate. The shape of the active site is complementary to the shape of the substrate, allowing for a precise fit.

**Hint**: Think of the active site as a lock and the substrate as a key; they must match for the reaction to occur.

3. **Induced Fit Model**: The interaction between an enzyme and its substrate can be described by the induced fit model. This model suggests that when the substrate binds to the active site, the enzyme undergoes a slight change in shape to better fit the substrate. This is different from the lock and key model, where the fit is perfect from the start.

**Hint**: Consider how a glove fits around a hand; it may adjust slightly to accommodate the shape of the hand.

4. **Evaluating the Options**: The question provides options regarding how the substrate fits with the enzyme. We need to identify which option accurately describes this interaction.

- The first option states that the active site changes shape slightly to fit the substrate. This is partially correct as the enzyme does adjust to some extent after the substrate binds.
- The second option incorrectly describes the interaction as occurring at a non-active site (allosteric site), which is not where the substrate binds.
- The final option states that none of the provided descriptions are correct.

5. **Conclusion**: Based on the understanding of the active site and the induced fit model, the best answer would be that the active site does change shape slightly to accommodate the substrate once it has bound. However, since the question asks for the best description and the options provided are misleading, the most accurate choice is "none of this."

**Final Answer**: The best description is "none of this."