Protein structural hierarchy: The four polypeptide subunits that assemble to form functional hemoglobin illustrate which level of protein structure?

Introduction / Context:
Proteins are described at multiple structural levels. Hemoglobin (Hb) is a classic example because it functions as a tetramer—two α and two β chains—making it ideal to test your grasp of quaternary structure.

Given Data / Assumptions:

• Hemoglobin comprises four separate polypeptide chains.
• Each chain folds into its own three-dimensional shape (tertiary structure).
• Functional Hb requires assembly of all four chains.

Concept / Approach:
Primary structure is the amino acid sequence. Secondary structure includes local motifs such as α helices and β sheets. Tertiary structure is the overall 3D fold of a single polypeptide. Quaternary structure describes the spatial arrangement and interactions of multiple polypeptide subunits in a complex.

Step-by-Step Solution:
Identify that hemoglobin contains multiple distinct chains.A functional protein formed by subunit assembly indicates quaternary structure.Therefore, choose “quaternary structure.”

Verification / Alternative check:
Dissociation of Hb into monomers disrupts cooperative oxygen binding, showing function depends on subunit interaction—definitive for quaternary structure.

Why Other Options Are Wrong:

• Primary: concerns sequence only, not subunit assembly.
• Secondary: local regular structures, not multi-chain association.
• Tertiary: single-chain fold, not multi-subunit complex.

Common Pitfalls:
Assuming “complex fold” means tertiary; the key is multiple polypeptide chains interacting.

Final Answer:
quaternary structure