-COOH group of carboxylic acids
Releases a proton and becomes negatively charged
A carboxyl group is a carbon atom double-bonded to one oxygen atom and single-bonded to a hydroxyl group, usually written as -COOH. It defines the carboxylic acids: in water it releases a proton and becomes a negatively charged carboxylate group (-COO⁻). At the pH inside cells, most carboxyl groups are in this charged form. Every amino acid carries a carboxyl group, and fatty acids end in one.