A major regulatory mechanism for multistep reaction is

Options

A. Presence of inhibitors
B. Availability of activators
C. Feed-back mechanism
D. All the above

Answer

**Step-by-Step Solution:**

1. **Understanding Multi-step Reactions**:
- Multi-step reactions involve a series of chemical reactions where the product of one reaction becomes the reactant for the next. For instance, a product P can be converted into intermediate products R1, R2, R3, and so on, until a final product is formed.

2. **Identifying Regulatory Mechanisms**:
- In biochemical pathways, various mechanisms regulate the rate of reactions. These include:
- Presence of inhibitors: Substances that decrease the rate of reactions.
- Availability of activators: Substances that increase the rate of reactions.
- Feedback mechanisms: A process where the output of a reaction influences the rate of the reaction itself.

3. **Focus on Feedback Mechanism**:
- In multi-step reactions, feedback mechanisms play a crucial role. For example, if the concentration of the final product (e.g., R4) increases, it can signal the pathway to slow down, thereby reducing the concentration of earlier products (e.g., P3, P2). This regulation helps maintain homeostasis within the cell.

4. **Evaluating the Options**:
- The question provides four options:
- Presence of inhibitors
- Availability of activators
- Feedback mechanism
- All of the above
- While inhibitors and activators can influence reactions, they are more relevant to single-step reactions. In contrast, feedback mechanisms are specifically designed to regulate multi-step reactions.

5. **Conclusion**:
- Therefore, the major regulatory mechanism for multi-step reactions is the feedback mechanism. The correct answer is **option number three**.

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