Amino Acid Substitution Affects All Levels of Protein Structure

Steps:

• A single amino acid change in the β chain directly alters the primary structure, the linear sequence of amino acids.
• This substitution can disrupt hydrogen bonds or other interactions, affecting secondary structure like alpha helices in hemoglobin.
• The altered sequence influences side-chain packing and folding, impacting tertiary structure and overall stability.
• In hemoglobin's tetrameric quaternary structure, the change disrupts subunit interfaces, affecting assembly and function.

Why D is correct:

• Primary structure defines the amino acid sequence, which hierarchically determines secondary (local folding), tertiary (3D shape), and quaternary (multi-subunit interactions) levels per protein folding principles.

Why the others are wrong:

• A: Excludes higher levels, but the change propagates beyond the sequence to disrupt folding and stability.
• B: Omits secondary and tertiary, ignoring local and global conformational effects.
• C: Ignores primary, but the mutation is a direct sequence alteration.

Final answer: D