Carbon dioxide entering erythrocytes reacts with water to form carbonic acid. The enzyme is

Options

A. Carbonic anhydrase
B. Carboxypeptidase
C. Hydrolase
D. Oxidoreductase.

Answer

To solve the question regarding the enzyme that catalyzes the reaction of carbon dioxide with water to form carbonic acid, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction**: The question states that carbon dioxide (CO2) enters erythrocytes (red blood cells) and reacts with water (H2O) to form carbonic acid (H2CO3).

2. **Recognize the Role of Enzymes**: Enzymes are biological catalysts that speed up chemical reactions. In this case, we need to identify which enzyme facilitates the reaction between CO2 and H2O.

3. **Consider the Options**: The options provided are:
- Carbonic anhydrase
- Carbopeptidase
- Hydroxylase
- Oxidoreductase

4. **Recall the Function of Carbonic Anhydrase**: Carbonic anhydrase is an enzyme specifically known for catalyzing the reversible reaction between carbon dioxide and water to form carbonic acid. It is predominantly found in red blood cells.

5. **Eliminate Other Options**:
- Carbopeptidase is involved in protein digestion.
- Hydroxylase is involved in the addition of hydroxyl groups to substrates.
- Oxidoreductase is a general term for enzymes that catalyze oxidation-reduction reactions.

6. **Conclude the Answer**: Since carbonic anhydrase is the enzyme that catalyzes the formation of carbonic acid from carbon dioxide and water, the correct answer is **carbonic anhydrase**.

### Final Answer:
The enzyme that catalyzes the reaction of carbon dioxide with water to form carbonic acid is **carbonic anhydrase**.