Haemoglobin's haem groups and oxygen binding

Steps:

• Haemoglobin consists of four subunits, each containing one haem group.
• Each haem group has an iron atom that binds one oxygen molecule.
• Therefore, one oxygen molecule binds per haem group, allowing four total.
• This directly supports option B as the true statement.

Why B is correct:

• Haemoglobin's structure includes four haem groups, each binding one O2 molecule via the iron (Fe2+) in the porphyrin ring.

Why the others are wrong:

• A: Binding oxygen induces cooperative conformational changes, altering haemoglobin's shape from tense (T) to relaxed (R) state.
• C: The first oxygen binding initiates the shape change, facilitating subsequent bindings.
• D: Cooperative binding makes the fourth oxygen easier to bind, not more difficult, due to increased affinity in the R state.

Final answer: B