What type of bond links amino acids together in a protein, forming between the amine group of one and the carboxyl group of the next?

• A. Hydrogen bond
• B. Ionic bond
• C. Disulfide bond
• D. Peptide bond

Answer: (D)

Peptide bonds link amino acids in proteins. This bond forms when the amino group of one amino acid reacts with the carboxyl group of the next amino acid, releasing a molecule of water in a dehydration synthesis reaction. The result is a covalent amide linkage between the carbonyl carbon of one amino acid and the nitrogen of the next, creating the protein’s backbone and producing a polypeptide chain. Hydrogen bonds stabilize the folded structures that arise later (like alpha helices and beta sheets) but don’t join adjacent amino acids. Ionic bonds involve charged side chains and help stabilize folding through electrostatic interactions, not the main chain linkage. Disulfide bonds create crosslinks between cysteine residues, strengthening tertiary or quaternary structure rather than linking the amino acids in sequence. Hence, the key connection between amino acids in a protein is the peptide bond.