Collagen's triple helical quaternary structure

Steps:

• Quaternary structure involves the arrangement of multiple polypeptide chains in a protein.
• Collagen's basic unit, tropocollagen, consists of three polypeptide chains (two alpha-1, one alpha-2 in type I).
• These chains each adopt a left-handed helical conformation and supercoil into a right-handed triple helix.
• The structure is stabilized primarily by interchain hydrogen bonds, matching the key features in option C.

Why C is correct:

• Standard biochemistry defines collagen's quaternary structure as three polypeptides assembling into a helix, as described in texts like Lehninger Principles of Biochemistry.

Why the others are wrong:

• A: Describes alpha-keratin, not collagen, which uses polyproline II helices without interchain alpha-helices.
• B: Mimics DNA's double helix with incorrect covalent bonding; collagen uses hydrogen bonds.
• D: Incorrectly states four polypeptides; collagen has three.

Final answer: C