Considering that O₂ triggers hemoglobin to switch from its low affinity (T) state to its high affinity (R) state to bind more O₂, what kind of allosteric effector is O ₂ relative to hemoglobin?

• A. Heterotropic; activator.
• B. Homotropic; inhibitor.
• C. Heterotropic; inhibitor.
• D. Homotropic; activator.

Answer: (D)

Verified step by step guidance

1. Understand the concept of allosteric effectors: Allosteric effectors are molecules that bind to a protein at a site other than the active site, causing a change in its activity. They can be classified as activators or inhibitors.
2. Identify the role of O2 in hemoglobin: Oxygen (O2) binds to hemoglobin and increases its affinity for more oxygen molecules. This is a classic example of cooperative binding, where the binding of one molecule facilitates the binding of additional molecules.
3. Differentiate between homotropic and heterotropic effectors: Homotropic effectors are molecules that are the same as the substrate, while heterotropic effectors are different from the substrate. In this case, O2 is the substrate for hemoglobin.
4. Determine the type of effector: Since O2 is the substrate and it enhances the binding of more O2, it acts as a homotropic effector.
5. Classify the effector as an activator or inhibitor: O2 increases the affinity of hemoglobin for more O2, thus it acts as an activator. Therefore, O2 is a homotropic activator for hemoglobin.