Here's why:

* Amino Acids: The general structure of an amino acid includes a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a side chain (R group). The carboxyl group is the acidic part of the molecule.

* Fatty Acids: Fatty acids are long chains of hydrocarbons with a carboxyl group at one end. The carboxyl group makes them acidic.

Therefore, both amino acids and fatty acids share the carboxyl group as a common structural feature.