Consequences of oxygen binding to hemoglobin Select the statement that best summarizes structural and affinity changes that occur as O2 binds.

Introduction / Context:
Hemoglobin is the textbook example of positive cooperativity. Binding of oxygen increases the affinity of remaining unoccupied sites and induces a substantial T to R conformational shift that optimizes oxygen transport from lungs to tissues.

Given Data / Assumptions:

• Hemoglobin has four O2 binding sites.
• We consider physiological conditions with typical allosteric effectors present.

Concept / Approach:
Positive cooperativity means that as one subunit binds O2, the others bind O2 more readily. Structural propagation from the heme to the tetramer produces a quaternary rearrangement. The apparent affinity difference between the first and last O2 can be very large (often cited on the order of 100-fold), reflecting cooperativity.

Step-by-Step Solution:
Acknowledge that O2 binding triggers T → R transition, a major conformational change.Recognize that affinity increases as saturation rises, so later binding events occur with higher affinity.Select the option capturing both increased affinity and structural change.

Verification / Alternative check:
Sigmoidal O2 saturation curves and Hill coefficients near 2.8 corroborate the affinity amplification. Structural studies show pronounced quaternary rearrangements between deoxy and oxy forms.

Why Other Options Are Wrong:
Lower affinity for later O2 would imply negative cooperativity; claiming no structural change contradicts extensive structural data.

Common Pitfalls:
Equating the numerical factor with an exact constant; the magnitude depends on conditions but the qualitative increase is robust.

Final Answer:
Both (a) and (b).