Haemoglobin's oxygen-binding site

Steps:

• Identify haemoglobin's structure: a protein with four polypeptide chains, each containing a haem group.
• Recall haem group's composition: a porphyrin ring with a central iron atom.
• Understand oxygen transport: the iron in haem reversibly binds one oxygen molecule per haem group.
• Confirm: four haem groups allow haemoglobin to carry up to four oxygen molecules.

Why C is correct:

• Haem groups contain iron ions that directly bind and transport oxygen, as per haemoglobin's molecular function in red blood cells.

Why the others are wrong:

• A: Polypeptide chains form the protein scaffold but do not bind oxygen.
• B: Hydrogen bonds stabilize the structure but play no role in oxygen carrying.
• D: Hydrophilic groups aid solubility in blood but do not transport oxygen.

Final answer: C