Which of the following is not an `alpha`-amino acid?
Cysteine, Tyrosine, Trypsin, Proline, Serine

Explanation

To determine which of the following is not an alpha-amino acid among cysteine, tyrosine, trypsin, proline, and serine, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Alpha-Amino Acids**:
- Alpha-amino acids are organic compounds that contain both an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon atom (the alpha carbon). They are the building blocks of proteins.

2. **List the Given Options**:
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