Here's why:

* Amino acids are the building blocks of proteins. They are organic molecules that contain both an amino group (-NH2) and a carboxyl group (-COOH).

* The amino group is essential for the structure and function of proteins. It plays a role in:

* Peptide bond formation: The amino group of one amino acid reacts with the carboxyl group of another amino acid to form a peptide bond, linking amino acids together to create protein chains.

* Chemical reactions: The amino group can participate in a variety of chemical reactions, including acid-base reactions and the formation of hydrogen bonds.

* Protein folding: The interactions between amino groups and other parts of the protein molecule help determine the three-dimensional structure of proteins.

While the carboxyl group is also present in every amino acid, it's the amino group that's specifically designated as the characteristic feature defining the amino acid structure.