Which group is at the N-terminus of a polypeptide?

• A. Amino group
• B. Carboxyl group
• C. Sulfhydryl group
• D. Phosphate group

Answer: (A)

Proteins have directionality: one end starts with an amino group and the other ends with a carboxyl group. Each amino acid has both an amino group (NH2) and a carboxyl group (COOH). When amino acids link to form a polypeptide, peptide bonds form between the carboxyl group of the growing chain and the amino group of the next amino acid, which leaves the first amino acid’s amino group free. That free amino group marks the N-terminus, while the free carboxyl group at the opposite end marks the C-terminus. So the group at the N-terminus is the amino group.