Which one of the following amino acids is not found in proteins ?

Options

A. Arginine
B. Ornithine
C. Aspartic acid
D. Tyrosine

Answer

To determine which amino acid is not found in proteins, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks which amino acid from the given options is not found in proteins.

2. **List the Options**: The options provided are:
- Arginine
- Ornithine
- Aspartic acid
- Tyrosine

3. **Identify Amino Acids in Proteins**:
- Amino acids can be classified into two categories: essential and non-essential.
- Essential amino acids cannot be synthesized by the body and must be obtained from food.
- Non-essential amino acids are synthesized by the body.

4. **Examples of Amino Acids**:
- **Essential Amino Acids**: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
- **Non-Essential Amino Acids**: Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glycine.

5. **Analyze Each Option**:
- **Arginine**: This is a non-essential amino acid and is found in proteins.
- **Ornithine**: This is not typically found in proteins; it is an intermediate in the urea cycle.
- **Aspartic Acid**: This is a non-essential amino acid and is found in proteins.
- **Tyrosine**: This is a non-essential amino acid and is found in proteins.

6. **Conclusion**: Based on the analysis, the amino acid that is not found in proteins is **Ornithine**.

### Final Answer:
**Ornithine is the amino acid that is not found in proteins.**

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