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 analyze the options provided:

1. **Arginine**:
- Arginine is an amino acid that is classified as non-essential. This means that our body can synthesize it, and it is indeed found in proteins.

2. **Ornithine**:
- Ornithine is an amino acid that plays a crucial role in the urea cycle (also known as the ornithine cycle). However, it is classified as a non-proteinogenic amino acid, meaning it does not incorporate into proteins. Therefore, ornithine is not found in proteins.

3. **Aspartic Acid**:
- Aspartic acid is a non-essential amino acid that is commonly found in proteins. It plays a role in various metabolic processes and is integrated into protein structures.

4. **Tyrosine**:
- Tyrosine is another non-essential amino acid that is incorporated into proteins. It is derived from phenylalanine and is important for the synthesis of neurotransmitters.

Based on the analysis of the options, the amino acid that is not found in proteins is **Ornithine**.

### Final Answer:
The amino acid that is not found in proteins is **Ornithine** (Option 2).