Cooperative Binding of Oxygen to Hemoglobin

Steps:

• Hemoglobin has four subunits, each with a heme group that binds one oxygen molecule.
• Oxygen binding is cooperative: the first oxygen binds to the tense (T) state, shifting hemoglobin to the relaxed (R) state with higher affinity.
• This shape change makes subsequent oxygen bindings easier, as affinity increases progressively.
• Option D matches this by stating the second oxygen facilitates the third's binding.

Why D is correct:

• In cooperative binding, each successive oxygen molecule increases hemoglobin's affinity for the next, following the sigmoidal oxygen dissociation curve.

Why the others are wrong:

• A: Oxygen binding induces shape changes, rather than stopping them.
• B: Each heme group binds one oxygen molecule, for a total of four per hemoglobin.
• C: The first oxygen binding triggers the T-to-R conformational shift.

Final answer: D