A metal ion required for normal functioning of an enzyme is

Options

A. Holoenzyme
B. Coenzyme
C. Cofactor
D. Prosthetic group.

Answer

To answer the question "A metal ion required for normal functioning of an enzyme is," we need to analyze the options provided: holoenzyme, coenzyme, cofactor, and prosthetic group.

### Step-by-Step Solution:

1. **Understanding Enzyme Structure**:
- Enzymes are biological catalysts that speed up chemical reactions in the body. They are typically proteins but may require additional components to function properly.

2. **Identifying Components of Enzymes**:
- **Apoenzyme**: This is the protein part of the enzyme that is inactive without its non-protein components.
- **Holoenzyme**: This is the complete, active form of the enzyme, which includes the apoenzyme and its necessary cofactors.

3. **Defining Cofactors**:
- **Cofactor**: This term refers to non-protein components that assist in enzyme activity. They can be metal ions (like zinc, magnesium, or iron) or organic molecules (like vitamins).
- **Coenzyme**: This is a specific type of cofactor that is organic and often derived from vitamins. Coenzymes are usually loosely bound to the enzyme.
- **Prosthetic Group**: This is a type of cofactor that is tightly bound to the enzyme, often forming a permanent part of the enzyme structure.

4. **Identifying the Correct Answer**:
- Since the question specifically asks for a metal ion required for the normal functioning of an enzyme, the correct answer is **cofactor**. Metal ions fall under the category of cofactors, as they are essential for the activity of many enzymes.

### Final Answer:
The metal ion required for normal functioning of an enzyme is **cofactor**.