The substrate has to diffuse towards the 'active site.'. There is thus, an ____formation of a 'ES' complex.

Answer

To solve the question, we need to understand the process of enzyme-substrate interaction and the formation of the enzyme-substrate (ES) complex. Here’s a step-by-step breakdown:

### Step-by-Step Solution:

1. **Understanding the Role of Enzymes**: Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required for the reaction to occur.

**Hint**: Recall that enzymes are proteins that facilitate biochemical reactions.

2. **Substrate Diffusion**: The substrate is the molecule upon which an enzyme acts. For the reaction to occur, the substrate must diffuse towards the active site of the enzyme.

**Hint**: Think about how molecules move in solution and the importance of proximity for reactions.

3. **Active Site Binding**: The active site is a specific region on the enzyme where the substrate binds. This binding is crucial for the formation of the enzyme-substrate complex.

**Hint**: Remember that the active site has a specific shape that complements the substrate.

4. **Formation of the ES Complex**: Once the substrate binds to the active site, an enzyme-substrate (ES) complex is formed. This complex is essential for the reaction to proceed.

**Hint**: Consider the transient nature of this complex and its role in the reaction mechanism.

5. **Transient Nature of the ES Complex**: The formation of the ES complex is temporary (transient), meaning it does not last long. It is necessary for the reaction to occur, as the substrate must first bind to the enzyme.

**Hint**: Reflect on the importance of temporary interactions in biochemical processes.

6. **Conclusion**: Given that the formation of the ES complex is necessary for the reaction to proceed, we can fill in the blank in the question. The correct term to describe this necessary formation is "obligatory."

**Final Answer**: The substrate has to diffuse towards the 'active site.' There is thus an **obligatory** formation of an 'ES' complex.

### Final Answer:
The substrate has to diffuse towards the 'active site.' There is thus an **obligatory** formation of an 'ES' complex.