Protein structure levels in multi-subunit molecules

Steps:

• Identify the molecule's components: multiple long polypeptides forming α-helices.
• Recall protein structure hierarchy: primary (sequence), secondary (α-helices), tertiary (folding of one chain), quaternary (multiple chains interacting).
• Analyze the description: long polypeptides indicate separate chains assembling via α-helices.
• Match to quaternary structure, as it involves multi-chain assembly beyond single-chain folding.

Why A is correct:

• Quaternary structure is defined as the arrangement of multiple polypeptide chains (subunits) into a functional complex, here via α-helices linking the chains.

Why the others are wrong:

• B: Tertiary involves folding within one chain; α-helices forming a fiber describes secondary/tertiary, not quaternary.
• C: Heads as globular proteins suggest tertiary folding, but ignores multi-chain quaternary assembly.
• D: Two polypeptides binding a head misattributes quaternary to a specific binding, not the overall α-helix polypeptide structure.

Final answer: A