Bonds in Fibrous Protein Chains

Steps:

• Fibrous proteins consist of linear amino acid chains linked by peptide bonds forming the backbone.
• Peptide bonds (Y) covalently connect the carboxyl group of one amino acid to the amino group of the next.
• Hydrophobic interactions (X) occur between non-polar side chains, stabilizing the protein's elongated structure.
• Analyze the diagram: Y links sequential residues, while X aligns side chains in a non-covalent manner.

Why B is correct:

• Peptide bonds define the primary structure by amide linkages (–CO–NH–) between amino acids, while hydrophobic interactions drive folding of non-polar residues away from water, as per protein folding principles.

Why the others are wrong:

• A: Disulfide bonds are covalent S–S links between cysteines, not hydrophobic; hydrophobic interactions are non-covalent.
• C: Hydrogen bonds stabilize secondary structures like alpha helices, but ionic bonds involve charged side chains, not matching chain linkages.
• D: Ionic bonds form between oppositely charged residues, but Y is specifically the covalent peptide bond, not ionic.

Final answer: B