A substance that unrelated to substrate reversibly changes the activity of an enzyme. It is

Options

A. Competitive inhibitor
B. Allosteric subunit
C. Allosteric modulator
D. None of the above.

Answer

To solve the question regarding the substance that is unrelated to the substrate and reversibly changes the activity of an enzyme, we can follow these steps:

### Step 1: Understand the Question
The question asks for a substance that is not related to the substrate of an enzyme but can still change the enzyme's activity in a reversible manner.

**Hint:** Focus on the terms "unrelated to substrate" and "reversibly changes the activity."

### Step 2: Identify Key Terms
- **Enzyme Activity**: Refers to the rate at which an enzyme catalyzes a reaction.
- **Substrate**: The specific reactant that an enzyme acts upon.
- **Reversible Change**: Indicates that the change in enzyme activity can be undone.

**Hint:** Think about how enzymes can be regulated by substances that do not directly compete with the substrate.

### Step 3: Consider Types of Enzyme Regulation
1. **Competitive Inhibitor**: A substance that competes with the substrate for the active site of the enzyme. This is related to the substrate.
2. **Allosteric Regulation**: Involves a modulator that binds to a site other than the active site, causing a conformational change in the enzyme, which can increase or decrease its activity.
3. **Allosteric Modulator**: A specific type of allosteric regulator that can enhance or inhibit enzyme activity.
4. **Allosteric Subunit**: Refers to a component of an allosteric enzyme that may not be directly involved in the catalytic activity.

**Hint:** Eliminate options that directly relate to the substrate or do not fit the criteria of being unrelated.

### Step 4: Analyze the Options
- **Competitive Inhibitor**: Incorrect, as it is related to the substrate.
- **Allosteric Subunit**: Incorrect, as it refers to a part of the enzyme rather than a modulating substance.
- **Allosteric Modulator**: Correct, as it is a substance that binds to a different site on the enzyme and can change its activity without being related to the substrate.
- **None of the Above**: Incorrect, as we have identified a valid option.

**Hint:** Identify which option fits the description of being unrelated to the substrate and capable of reversible modulation.

### Step 5: Conclusion
The correct answer is **Allosteric Modulator**. This substance binds to an enzyme at a site other than the active site and can change the enzyme's activity in a reversible manner.

**Final Answer:** Allosteric Modulator