What is the role of the carboxyl group on an amino acid?

The carboxyl group on an amino acid plays a critical role in the formation of peptide bonds, which are essential for creating proteins. Each amino acid is characterized by two functional groups: an amino group (-NH2) and a carboxyl group (-COOH). The carboxyl group, which contains a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH), is crucial for linking amino acids together.

During the formation of a peptide bond, the carboxyl group of one amino acid reacts with the amino group of another, resulting in the release of a water molecule (a process known as dehydration synthesis). This reaction results in a covalent bond between the two amino acids, effectively forming a dipeptide. Therefore, the carboxyl group is directly involved in establishing the backbone of protein structures through peptide bond formation.

In contrast, the other choices focus on different aspects of amino acid properties or functions that are not primarily related to the role of the carboxyl group in forming peptide bonds.