Ligand Binding — For myoglobin, when pO2 equals the dissociation constant (Kd), what is the fractional saturation (YO2)?

Introduction:
Ligand-binding curves relate ligand concentration (or partial pressure for gases) to the fraction of occupied binding sites. For simple one-site binding (like myoglobin), the dissociation constant Kd provides a direct benchmark of occupancy.

Given Data / Assumptions:

• Myoglobin behaves approximately as a single-site, noncooperative binder.
• YO2 denotes fractional saturation (occupied fraction of binding sites).
• pO2 is analogous to ligand concentration for oxygen.

Concept / Approach:
Use the standard binding relation: YO2 = pO2 / (pO2 + Kd). When pO2 = Kd, the expression simplifies immediately.

Step-by-Step Solution:

Verification / Alternative check:
The definition of Kd ensures that at ligand concentration equal to Kd, half the binding sites are occupied. This is analogous to enzymology where Km marks half-maximal velocity.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing Kd with the concentration for near-saturation; misapplying cooperative (hemoglobin) vs noncooperative (myoglobin) models.

Final Answer:
0.5.