In proteins, how is a peptide bond formed between two amino acids?

• A. By a hydrolysis reaction that consumes $H_{2}O$ to join the carboxyl and amino groups
• B. By forming a disulfide bond between the side chains of two cysteine residues
• C. By an ionic attraction (salt bridge) between a protonated amino group and a deprotonated carboxyl group
• D. By a condensation (dehydration) reaction in which the carboxyl group of one amino acid reacts with the amino group of another, releasing $H_{2}O$ and forming an amide linkage

Answer: (D)