FMGE 2023

Question 11: Hemoglobin with iron in ferric form is

• A. Methemoglobin (Correct Answer)
• B. HbA
• C. Fetal hemoglobin
• D. HbS

Explanation: ***Methemoglobin***- **Methemoglobin** is characterized by the presence of iron in the **ferric state ($\text{Fe}^{3+}$)** in the heme group, which **cannot reversibly bind oxygen**, unlike the ferrous form. - This oxidized form of hemoglobin cannot function as an **oxygen carrier** and its accumulation leads to clinical conditions like **methemoglobinemia**. *HbA*- **HbA** (Adult hemoglobin) is the predominant form of hemoglobin in adults, and its iron must be in the **ferrous state ($\text{Fe}^{2+}$)** to bind and transport oxygen effectively.- The conversion of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ in normal HbA is constantly prevented by the **methemoglobin reductase system** (especially **NADH-cytochrome $b_5$ reductase**). *Fetal hemoglobin*- **Fetal hemoglobin (HbF)**, like HbA, uses iron in the **ferrous state ($\text{Fe}^{2+}$)** for reversible oxygen binding, reflecting its normal physiological function in the fetus. - HbF's unique feature is its structure ($\text{alpha}_2\text{gamma}_2$) which confers a **higher affinity for oxygen** by binding **2,3-BPG** less avidly than HbA, but this does not involve ferric iron. *HbS*- **HbS** (Sickle hemoglobin) is defective due to a mutation in the **beta-globin chain** (E6V), but the iron in its heme group remains in the **functional ferrous state ($\text{Fe}^{2+}$)**.- The abnormality in HbS relates to its **polymerization** upon deoxygenation, causing red blood cell sickling, not an oxidation state change of the heme iron.