Quaternary structure depends on primary structure for subunit assembly

Steps:

• Define quaternary structure as the arrangement of multiple polypeptide subunits into a functional protein.
• Examine each option against this definition to identify the accurate fact.
• Eliminate options that impose incorrect limitations or dependencies.
• Confirm the option linking quaternary to primary structure as correct.

Why C is correct:

• Primary structure (amino acid sequence) determines subunit interactions like hydrogen bonds and hydrophobic effects, dictating quaternary assembly per protein folding principles.

Why the others are wrong:

• A: Quaternary structure involves two or more polypeptides, not specifically four.
• B: Metal ions can stabilize some proteins but are not required for quaternary structure in general.
• D: Quaternary structure comprises various polypeptide types, not limited to alpha and beta chains.

Final answer: C