Polypeptide chains are cross-linked together by these side bonds.

• A. Hydrogen bonds
• B. Salt bonds
• C. Disulfide bonds
• D. Hydrogen, salt, and disulfide bonds

Answer: (D)

Proteins are stabilized by multiple kinds of side-chain interactions that can link one polypeptide chain to another. Hydrogen bonds form between polar groups on nearby chains, helping hold chains in close proximity and contributing to overall stability. Salt bridges, or ionic interactions, occur between oppositely charged side chains (acidic and basic residues) and can cross-link chains, with their strength and presence influenced by the environment and pH. Disulfide bonds are covalent links between cysteine residues, creating strong cross-links that can connect two different chains or segments of the same chain, often providing durable stabilization, especially in extracellular proteins. Because cross-linking of polypeptide chains can involve hydrogen bonding, ionic interactions, and covalent disulfide links, the combination of all three types best describes how these chains are joined together.