Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 141: What is the number of prosthetic groups in a hemoglobin molecule?

A. 1
B. 2
C. 3
D. 4 (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Hemoglobin is a **tetrameric** protein, meaning it consists of four polypeptide globin chains (in adults, typically $2\alpha$ and $2\beta$). Each individual globin chain is non-covalently bound to one **Heme** group. Since a prosthetic group is defined as a non-protein cofactor tightly bound to a protein, and each hemoglobin molecule contains **four Heme groups**, the correct answer is 4. Each Heme group contains one ferrous iron ($Fe^{2+}$) atom at its center, allowing one molecule of hemoglobin to bind up to four molecules of oxygen ($O_2$). **2. Why Other Options are Incorrect:** * **Option A (1):** This describes **Myoglobin**, which is a monomeric protein found in muscles containing only one globin chain and one heme group. * **Option B (2):** This might be confused with the number of *types* of globin chains (e.g., $\alpha$ and $\beta$), but it does not represent the total count of prosthetic groups. * **Option C (3):** There is no physiological form of hemoglobin that functions as a trimer. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Structure:** Heme is a **Protoporphyrin IX** ring complexed with $Fe^{2+}$. If iron is oxidized to $Fe^{3+}$ (ferric state), it forms **Methemoglobin**, which cannot bind oxygen. * **Cooperativity:** The binding of $O_2$ to one heme group increases the affinity of the remaining heme groups for $O_2$ (Sigmoid curve). * **2,3-BPG:** This molecule binds to the central cavity of the hemoglobin tetramer (specifically the $\beta$-chains), stabilizing the "T" (Tense) state and promoting oxygen unloading. * **Fetal Hemoglobin ($HbF$):** Composed of $2\alpha$ and $2\gamma$ chains; it has a higher affinity for $O_2$ because it binds 2,3-BPG less strongly.

Question 142: Which amino acid is primarily responsible for the buffering action of hemoglobin?

A. Histidine (Correct Answer)
B. Arginine
C. Valine
D. Lysine

Explanation: **Explanation:** The buffering capacity of a protein is determined by the **pKa of the ionizable side chains** of its constituent amino acids. For a molecule to act as an effective buffer, its pKa must be close to the physiological pH of the environment (approximately 7.4). **Why Histidine is Correct:** Histidine is the only amino acid with an imidazole side chain that has a **pKa of approximately 6.0 to 7.0**. In the hemoglobin molecule, specific histidine residues (notably the C-terminal histidines) have their pKa shifted closer to 7.4 due to the local protein environment. This allows histidine to readily accept or donate protons at physiological pH, making it the primary contributor to the **Bohr effect** and the overall buffering capacity of blood. Hemoglobin is rich in histidine, containing 38 histidine residues per tetramer. **Why Incorrect Options are Wrong:** * **Arginine (B) and Lysine (D):** These are basic amino acids, but their side chains have very high pKa values (~12.5 and ~10.5, respectively). At physiological pH, they are almost entirely protonated and cannot effectively act as buffers. * **Valine (C):** Valine is a non-polar, hydrophobic amino acid with no ionizable side chain. It plays a role in the structure of hemoglobin (and its mutation to valine causes Sickle Cell Anemia), but it has no buffering capacity. **High-Yield Clinical Pearls for NEET-PG:** * **Deoxyhemoglobin** is a better buffer than oxyhemoglobin because it has a higher affinity for protons (H+). * **The Bohr Effect:** Describes how an increase in H+ (acidity) or CO₂ decreases hemoglobin's affinity for oxygen, shifting the dissociation curve to the **right**. * **Proximal vs. Distal Histidine:** In the heme pocket, the **Proximal Histidine (F8)** binds directly to the iron atom, while the **Distal Histidine (E7)** helps stabilize the oxygen binding site and prevents carbon monoxide toxicity.

Question 143: What percentage of normal transferrin is saturated with iron?

A. 20%
B. 35% (Correct Answer)
C. 50%
D. 70%

Explanation: **Explanation:** **Core Concept:** Transferrin is the primary plasma protein responsible for the transport of ferric iron ($Fe^{3+}$). Under normal physiological conditions, the total iron-binding capacity (TIBC) of transferrin is not fully utilized. Only about **one-third (approximately 33–35%)** of the available iron-binding sites on transferrin are saturated with iron. This is known as the **Transferrin Saturation (TSAT)**. The remaining two-thirds represent the "Unsaturated Iron Binding Capacity" (UIBC), which acts as a buffer to bind additional iron entering the plasma. **Analysis of Options:** * **Option B (35%) is Correct:** This aligns with the physiological norm where serum iron is roughly 1/3 of the TIBC. * **Option A (20%):** This value is subnormal. A saturation below 20% (specifically <16%) is a diagnostic hallmark of **Iron Deficiency Anemia (IDA)**. * **Option C (50%):** This is higher than normal. Saturation levels start rising in conditions of iron overload or chronic hemolysis. * **Option D (70%):** This indicates severe **Iron Overload**. TSAT >45–50% is often used as a screening cutoff for **Hereditary Hemochromatosis**. **NEET-PG High-Yield Pearls:** 1. **Formula:** $TSAT (\%) = (\text{Serum Iron} / \text{TIBC}) \times 100$. 2. **Diurnal Variation:** Serum iron levels are highest in the morning; therefore, TSAT should ideally be measured on a fasting morning sample. 3. **Negative Acute Phase Reactant:** Transferrin levels *decrease* during inflammation, while Ferritin (storage iron) *increases*. 4. **Diagnostic Trend:** In Iron Deficiency Anemia, Serum Iron ↓, Ferritin ↓, but **TIBC ↑** and **TSAT ↓**.

Question 144: Sickle cell disease (SCD) is caused by a recessive, single-nucleotide mutation in the beta globin gene of Hgb. All of the following are TRUE about Sickle cell disease, EXCEPT:

A. A single nucleotide change results in the substitution of Glutamic acid with Valine.
B. A sticky patch is generated as a result of the replacement of a nonpolar residue with a polar residue. (Correct Answer)
C. HbS confers resistance against malaria in heterozygotes.
D. Restriction Fragment Length Polymorphism (RFLP) results from a single base change.

Explanation: ### Explanation **1. Why Option B is the Correct Answer (The False Statement):** In Sickle Cell Disease (SCD), the mutation occurs at the 6th position of the beta-globin chain. **Glutamic acid** (a polar, negatively charged amino acid) is replaced by **Valine** (a nonpolar, hydrophobic amino acid). This is the exact opposite of what Option B states. The presence of the nonpolar Valine on the surface of the hemoglobin molecule creates a "sticky patch." In deoxygenated states, these hydrophobic patches interact with complementary sites on other hemoglobin molecules, leading to polymerization and the characteristic "sickling" of RBCs. **2. Analysis of Other Options:** * **Option A:** This is a true statement. The mutation is a **missense point mutation** (GAG → GTG) resulting in the substitution of Glutamic acid with Valine at the 6th position of the $\beta$-chain. * **Option C:** This is true. The "Heterozygote Advantage" explains that individuals with the Sickle Cell Trait (HbAS) have a survival advantage against *Plasmodium falciparum* malaria, as the parasite cannot thrive in cells that sickle and are prematurely cleared by the spleen. * **Option D:** This is true. The mutation (A to T) destroys a specific recognition site for the restriction enzyme **MstII**. This change in DNA fragment length can be detected via Southern Blotting, making RFLP a valid diagnostic tool. **3. NEET-PG High-Yield Clinical Pearls:** * **Molecular Basis:** It is a **transversion** mutation (Purine to Pyrimidine). * **Precipitating Factors for Sickling:** Hypoxia, acidosis, dehydration, and increased 2,3-BPG. * **Electrophoresis:** On alkaline electrophoresis (pH 8.6), the order of mobility toward the anode (+) is **A > F > S > C** (Mnemonic: **A** Fat **S**low **C**at). HbS moves slower than HbA because it loses the negative charge of glutamic acid. * **Diagnosis:** Solubility test (Screening) and Hb Electrophoresis/HPLC (Confirmatory).

Question 145: Bart hemoglobin is a tetramer of which of the following globin chains?

A. γ globin (Correct Answer)
B. β globin
C. α globin
D. δ globin

Explanation: **Explanation:** **Hemoglobin Bart (Hb Bart)** is a pathological hemoglobin tetramer consisting of **four gamma (γ) chains (γ₄)**. The underlying medical concept involves **Alpha-thalassemia**. In normal physiology, alpha (α) globin chains are essential to pair with other globin chains (γ in fetuses, β in adults). When there is a total or near-total deficiency of α-globin chains (as seen in *Hydrops Fetalis*, where all four α-genes are deleted), the excess γ-globin chains in the fetus cannot find α-partners. Consequently, they aggregate to form stable tetramers of γ₄. Hb Bart has an extremely high affinity for oxygen, meaning it does not release oxygen to tissues, leading to severe intrauterine hypoxia and fetal demise. **Analysis of Incorrect Options:** * **Option B (β globin):** A tetramer of four beta chains (**β₄**) is known as **Hemoglobin H (HbH)**. This occurs in α-thalassemia when three out of four α-genes are deleted (HbH disease). * **Option C (α globin):** Free α-chains do not form stable tetramers; they are highly unstable and precipitate, causing damage to the red cell membrane (ineffective erythropoiesis). * **Option D (δ globin):** Delta chains pair with alpha chains to form **HbA₂ (α₂δ₂)**, which normally comprises <3% of adult hemoglobin. **High-Yield Facts for NEET-PG:** * **Hb Bart:** γ₄ (Associated with Hydrops Fetalis/4-gene deletion). * **HbH:** β₄ (Associated with HbH disease/3-gene deletion). * **Normal Fetal Hemoglobin (HbF):** α₂γ₂. * **Normal Adult Hemoglobin (HbA):** α₂β₂. * **Electrophoresis:** Hb Bart is "fast-moving" on alkaline electrophoresis, migrating further than HbA.

Question 146: Which of the following is downregulated by Hepcidin?

A. Ferroportin (Correct Answer)
B. Transferrin
C. DMT1
D. Haphaestin

Explanation: **Explanation:** **Hepcidin** is the master regulator of iron homeostasis in the human body. It is a peptide hormone synthesized by the liver in response to high iron stores or inflammation. **Why Ferroportin is correct:** Ferroportin is the only known cellular iron exporter, found on the basolateral membrane of enterocytes and on macrophages. Hepcidin works by binding to **Ferroportin**, inducing its internalization and lysosomal degradation. By removing Ferroportin from the cell surface, Hepcidin prevents iron from entering the plasma, effectively "locking" iron inside enterocytes and macrophages. This results in decreased serum iron levels. **Why the other options are incorrect:** * **Transferrin:** This is the primary protein responsible for transporting iron in the blood. While its saturation changes based on iron levels, it is not directly downregulated by Hepcidin. * **DMT1 (Divalent Metal Transporter 1):** This transporter facilitates iron uptake from the intestinal lumen into the enterocyte. While its expression can be modulated by iron levels, it is not the primary target of Hepcidin. * **Hephaestin:** This is a ferroxidase that converts $Fe^{2+}$ to $Fe^{3+}$ to facilitate iron binding to transferrin. It works in conjunction with Ferroportin but is not directly degraded by Hepcidin. **High-Yield Clinical Pearls for NEET-PG:** * **Anemia of Chronic Disease (ACD):** Inflammatory cytokines (specifically **IL-6**) increase Hepcidin production. This leads to iron sequestration in macrophages, causing the low serum iron and high ferritin characteristic of ACD. * **Hemochromatosis:** Mutations leading to Hepcidin deficiency result in uncontrolled Ferroportin activity, leading to systemic iron overload. * **Stimuli for Hepcidin:** Increased by high iron and inflammation; decreased by hypoxia and increased erythropoietic activity.

Question 147: What is the transport form of iron in the body?

A. Transferrin (Correct Answer)
B. Ferritin
C. Apoferritin
D. Lactoferrin

Explanation: **Explanation:** Iron metabolism is a high-yield topic in Biochemistry, centered on the movement and storage of iron in different states. **1. Why Transferrin is Correct:** Iron is transported in the blood in the **ferric state (Fe³⁺)**. Because free iron is toxic and can generate free radicals via the Fenton reaction, it must be bound to a carrier protein. **Transferrin** is a glycoprotein synthesized in the liver that specifically binds two molecules of Fe³⁺ for safe transport through the plasma to various tissues (like bone marrow for erythropoiesis). **2. Why the Other Options are Incorrect:** * **Ferritin:** This is the primary **intracellular storage form** of iron. It is found mainly in the liver, spleen, and bone marrow. Serum ferritin levels are the most sensitive indicator of body iron stores. * **Apoferritin:** This is the protein shell of ferritin **without** the iron core. Once iron enters the cell and binds to apoferritin, it becomes ferritin. * **Lactoferrin:** This is an iron-binding protein found in exocrine secretions (milk, saliva, tears) and neutrophil granules. It has antimicrobial properties by sequestering iron away from bacteria but is not the systemic transport form. **Clinical Pearls for NEET-PG:** * **State of Iron:** Iron is absorbed in the **Ferrous (Fe²⁺)** state ("Fe-2 goes in-too") but transported and stored in the **Ferric (Fe³⁺)** state. * **Hepcidin:** The "Master Regulator" of iron metabolism. It inhibits **Ferroportin**, preventing iron release from enterocytes and macrophages. * **TIBC (Total Iron Binding Capacity):** This is an indirect measure of transferrin levels. In Iron Deficiency Anemia (IDA), TIBC increases while Ferritin decreases.

Question 148: Which type of hemoglobin is not normally found within human erythrocytes?

A. HbA
B. HbA2
C. HbCO (Correct Answer)
D. HbO2

Explanation: **Explanation:** The correct answer is **HbCO (Carboxyhemoglobin)**. **Why HbCO is the correct answer:** Hemoglobin (Hb) is the primary oxygen-carrying protein in red blood cells. While HbA, HbA2, and HbF are physiological variants, **Carboxyhemoglobin (HbCO)** is a pathological form. It is formed when carbon monoxide (CO) binds to the heme iron. CO has an affinity for hemoglobin that is **200–250 times greater** than that of oxygen. Under normal physiological conditions, HbCO is not a constituent of human erythrocytes; its presence indicates carbon monoxide poisoning, which shifts the oxygen-dissociation curve to the left, leading to tissue hypoxia. **Analysis of incorrect options:** * **HbA ($\alpha_2\beta_2$):** This is the major adult hemoglobin, comprising approximately 95–97% of total hemoglobin in a healthy adult. * **HbA2 ($\alpha_2\delta_2$):** This is a minor adult hemoglobin, normally comprising 1.5–3.5% of total hemoglobin. Elevated levels are a diagnostic marker for $\beta$-thalassemia trait. * **HbO2 (Oxyhemoglobin):** This is the normal, physiological form of hemoglobin bound to oxygen. It is the primary state of hemoglobin as it leaves the pulmonary circulation. **High-Yield Clinical Pearls for NEET-PG:** * **Methemoglobin (metHb):** Another pathological form where iron is in the ferric ($Fe^{3+}$) state rather than the ferrous ($Fe^{2+}$) state, preventing oxygen binding. * **HbA1c:** A glycosylated form of HbA used to monitor long-term glycemic control (reflects average blood glucose over 90–120 days). * **CO Poisoning Treatment:** Managed with 100% oxygen or hyperbaric oxygen to displace CO from the hemoglobin molecule.

Question 149: Bilirubin is synthesized from which of the following?

A. Amino acid
B. Hemoglobin (Correct Answer)
C. Myoglobin
D. WBCs

Explanation: **Explanation:** Bilirubin is the end product of **heme catabolism**. Approximately 80–85% of bilirubin is derived from the breakdown of **hemoglobin** from senescent (old) red blood cells in the Reticuloendothelial System (spleen, liver, and bone marrow). **Why Hemoglobin is Correct:** When RBCs reach the end of their 120-day lifespan, they are sequestered in the spleen. Hemoglobin is broken down into globin and heme. The enzyme **Heme Oxygenase** acts on heme to produce **Biliverdin**, which is then reduced by **Biliverdin Reductase** to form **unconjugated bilirubin**. This bilirubin is then transported to the liver bound to albumin. **Why Other Options are Incorrect:** * **Amino acids:** While amino acids are the building blocks of the *globin* chains, they are recycled into the body's protein pool and do not directly form bilirubin. * **Myoglobin:** Although myoglobin contains heme and its breakdown contributes to bilirubin production, it accounts for only a small fraction (along with cytochromes) compared to the massive turnover of hemoglobin. * **WBCs:** White blood cells do not contain hemoglobin or heme-based oxygen carriers; therefore, their degradation does not produce bilirubin. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Heme Oxygenase is the rate-limiting enzyme in bilirubin synthesis. * **Color changes:** Heme (purple) → Biliverdin (green) → Bilirubin (yellow). This sequence is visible in the changing colors of a healing bruise. * **Excretion:** Bilirubin must be conjugated with **Glucuronic acid** in the liver (by UDP-glucuronosyltransferase) to become water-soluble for excretion in bile. * **Van den Bergh Reaction:** Used to differentiate between conjugated (Direct) and unconjugated (Indirect) bilirubin.

Question 150: All of the following proteins are involved in the metabolism of iron, EXCEPT?

A. Transthyretin (Correct Answer)
B. Ceruloplasmin
C. Ferritin
D. Hepcidin

Explanation: **Explanation:** The correct answer is **Transthyretin (Option A)**. Transthyretin (formerly known as prealbumin) is a transport protein for **Thyroxine (T4)** and **Retinol (Vitamin A)** via its association with retinol-binding protein. It has no functional role in iron metabolism. **Analysis of other options:** * **Ceruloplasmin (Option B):** This is a copper-containing ferroxidase. It converts ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$), a necessary step for iron to bind to transferrin for systemic transport. A deficiency leads to iron accumulation in tissues (hemosiderosis). * **Ferritin (Option C):** This is the primary **intracellular storage form** of iron. It sequesters iron in a non-toxic, soluble form within the liver, spleen, and bone marrow. Serum ferritin levels are the most sensitive index for diagnosing iron deficiency anemia. * **Hepcidin (Option D):** Produced by the liver, hepcidin is the **master regulator** of iron homeostasis. It inhibits iron absorption from the gut and release from macrophages by causing the degradation of **ferroportin** (the only known cellular iron exporter). **NEET-PG High-Yield Pearls:** 1. **Hepcidin Regulation:** Levels increase during inflammation (via IL-6), leading to "Anemia of Chronic Disease" due to iron sequestration. 2. **Ferroportin:** The "exit door" for iron in enterocytes and macrophages. 3. **Transferrin:** The primary protein for transporting iron in the plasma. 4. **Hemosiderin:** An insoluble iron-storage complex used when iron levels exceed the storage capacity of ferritin.

Practice by Chapter

Hemoglobin Structure and Function

Practice Questions

Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve

Practice Questions

Hemoglobin Variants and Hemoglobinopathies

Practice Questions

Thalassemias

Practice Questions

Methemoglobin and Abnormal Hemoglobins

Practice Questions

Hemoglobin Synthesis

Practice Questions

Heme Synthesis and Porphyrias

Practice Questions

Iron Absorption and Transport

Practice Questions

Iron Storage and Recycling

Practice Questions

Disorders of Iron Metabolism

Practice Questions

Anemia: Biochemical Aspects

Practice Questions

Biochemistry of Hemostasis

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free