Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 151: Which of the following conditions is associated with decreased total iron-binding capacity?

A. Iron deficiency
B. Thalassemia
C. Sideroblastic anemia
D. Anemia of chronic inflammation (Correct Answer)

Explanation: **Explanation:** The **Total Iron-Binding Capacity (TIBC)** is a functional measurement of the amount of **Transferrin** available in the blood. In the body, Transferrin and Iron levels usually have an inverse relationship to maintain homeostasis. **Why Anemia of Chronic Inflammation (ACD) is correct:** In chronic inflammation, the liver produces high levels of **Hepcidin** (an acute-phase reactant). Hepcidin degrades ferroportin, trapping iron inside macrophages and hepatocytes. Simultaneously, the body downregulates the production of Transferrin to "starve" potential pathogens of iron. Since TIBC is a surrogate marker for Transferrin, a decrease in Transferrin leads to a **decreased TIBC**. **Analysis of Incorrect Options:** * **Iron Deficiency Anemia (IDA):** This is the classic condition where **TIBC is increased**. The liver compensates for low serum iron by synthesizing more Transferrin to maximize iron transport efficiency. * **Thalassemia:** This is a quantitative defect in globin chain synthesis, not a primary disorder of iron metabolism. Iron studies are typically normal, though TIBC may decrease only if the patient develops secondary iron overload from multiple transfusions. * **Sideroblastic Anemia:** This involves a defect in protoporphyrin synthesis leading to iron overload. Here, Transferrin becomes saturated, leading to a **decreased or normal TIBC**, but it is less characteristic than in ACD. **NEET-PG High-Yield Pearls:** * **Hepcidin** is the "Master Regulator" of iron metabolism; it is increased in ACD and decreased in IDA. * **Ferritin** is an acute-phase reactant; it is **increased** in ACD (iron is trapped) but **decreased** in IDA (iron stores are exhausted). * **Soluble Transferrin Receptor (sTfR) test:** This is the best test to differentiate IDA (increased sTfR) from ACD (normal sTfR).

Question 152: Which of the following enzymes of heme synthesis is located in the mitochondria?

A. ALA synthase (Correct Answer)
B. ALA dehydratase
C. Uroporphyrinogen I synthase
D. Uroporphyrinogen decarboxylase

Explanation: ### Explanation Heme synthesis is a compartmentalized process that occurs partly in the **mitochondria** and partly in the **cytosol**. Understanding this distribution is high-yield for NEET-PG. **1. Why ALA Synthase is correct:** The first and the last three steps of heme synthesis occur in the mitochondria. **ALA Synthase (ALAS)** catalyzes the rate-limiting step—the condensation of Succinyl CoA and Glycine to form $\delta$-Aminolevulinic acid (ALA). Since Succinyl CoA is an intermediate of the TCA cycle (which occurs in the mitochondria), ALAS must be located within the mitochondrial matrix to access its substrate. **2. Why the other options are incorrect:** The intermediate steps (from ALA to Coproporphyrinogen III) occur in the **cytosol**: * **ALA Dehydratase (Porphobilinogen Synthase):** Converts ALA to Porphobilinogen in the cytosol. It is highly sensitive to lead poisoning. * **Uroporphyrinogen I Synthase (PBG Deaminase):** Converts Porphobilinogen to Hydroxymethylbilane in the cytosol. Deficiency leads to Acute Intermittent Porphyria. * **Uroporphyrinogen Decarboxylase:** Converts Uroporphyrinogen III to Coproporphyrinogen III in the cytosol. Deficiency causes Porphyria Cutanea Tarda. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Mitochondrial Enzymes:** Remember "**S**it **F**ast **H**ere" (**S**-ALA **S**ynthase, **F**-Ferrochelatase, **H**-Heme Oxygenase/Heme synthase steps). * **Rate-Limiting Step:** ALA Synthase 1 (liver) is inhibited by heme (feedback inhibition), while ALA Synthase 2 (erythroid cells) is regulated by iron levels. * **Lead Poisoning:** Inhibits two enzymes—**ALA Dehydratase** and **Ferrochelatase**. * **Vitamin Link:** Pyridoxal Phosphate (**Vitamin B6**) is a mandatory cofactor for ALA Synthase. B6 deficiency can lead to Sideroblastic Anemia.

Question 153: Hepcidin acts on which receptor to inhibit iron absorption?

A. DMT-1
B. Ferroportin (Correct Answer)
C. Hephaestin
D. Transferrin

Explanation: **Explanation:** **1. Why Ferroportin is correct:** Hepcidin is the master regulator of iron homeostasis. It is a peptide hormone synthesized by the liver in response to high iron stores or inflammation. Hepcidin acts by binding to **Ferroportin**, the only known cellular iron exporter found on the basolateral membrane of enterocytes and on macrophages. Upon binding, Hepcidin induces the internalization and lysosomal degradation of Ferroportin. This effectively "locks" iron inside the cells, preventing its absorption into the bloodstream and its release from macrophage stores. **2. Why the other options are incorrect:** * **DMT-1 (Divalent Metal Transporter 1):** This protein is located on the apical membrane of enterocytes and is responsible for the initial uptake of ferrous iron ($Fe^{2+}$) from the intestinal lumen into the cell. It is not the direct target of Hepcidin. * **Hephaestin:** This is a ferroxidase enzyme that converts $Fe^{2+}$ to $Fe^{3+}$ to facilitate its binding to transferrin. It works in conjunction with Ferroportin but is not its receptor. * **Transferrin:** This is the primary transport protein for iron in the plasma. It carries iron to various tissues but does not act as a receptor for Hepcidin. **Clinical Pearls for NEET-PG:** * **Anemia of Chronic Disease (ACD):** Inflammatory cytokines (specifically **IL-6**) increase Hepcidin levels, leading to iron sequestration and the characteristic low serum iron despite adequate stores (high Ferritin). * **Hereditary Hemochromatosis:** Often caused by a deficiency in Hepcidin or its signaling pathway, leading to uncontrolled Ferroportin activity and systemic iron overload. * **Synthesis:** Hepcidin synthesis is **increased** by iron overload and inflammation, and **decreased** by hypoxia and increased erythropoietic activity.

Question 154: Porphyrins are synthesized mainly in which organs?

A. Spleen
B. Liver and spleen
C. Bone marrow and spleen
D. Liver and Bone marrow (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Liver and Bone Marrow** Porphyrin synthesis is the metabolic pathway leading to the production of **Heme**. The synthesis occurs in almost all mammalian cells, but the primary sites are the **liver** and the **erythroid precursor cells of the bone marrow**. 1. **Bone Marrow (~85%):** Here, heme is synthesized for the production of **hemoglobin**, which is essential for oxygen transport in developing red blood cells. 2. **Liver (~15%):** In hepatocytes, heme is primarily used for the synthesis of **Cytochrome P450 enzymes**, which are crucial for drug metabolism and detoxification. **Why other options are incorrect:** * **Spleen (Options A, B, and C):** The spleen is primarily the site of **heme degradation** (catabolism), not synthesis. In the spleen, senescent red blood cells are broken down, and heme is converted into bilirubin via the action of heme oxygenase. It does not contribute significantly to the de novo synthesis of porphyrins. **High-Yield Clinical Pearls for NEET-PG:** * **Subcellular Localization:** Porphyrin synthesis is unique because it occurs compartmentalized between the **mitochondria** and the **cytosol**. * **Rate-Limiting Step:** The first step, catalyzed by **ALA Synthase (ALAS)**, is the rate-limiting step. * **ALAS-1** is found in the liver (inhibited by heme/hemin). * **ALAS-2** is erythroid-specific (regulated by iron availability). * **Lead Poisoning:** Lead inhibits two key enzymes in this pathway: **ALA Dehydratase** and **Ferrochelatase**, leading to sideroblastic anemia and elevated ALA levels. * **Porphyrias:** Genetic defects in the enzymes of this pathway lead to Porphyrias, characterized by either neuropsychiatric symptoms or cutaneous photosensitivity.

Question 155: Which of the following statements is false regarding the Bohr effect?

A. It is the reciprocal coupling of protons and O2.
B. It depends on the cooperative interactions between the hemes of the hemoglobin tetramer.
C. It is also exhibited by myoglobin. (Correct Answer)
D. Protons responsible for the effect arise from the rupture of salt bridges.

Explanation: The **Bohr effect** describes the decrease in hemoglobin's oxygen affinity in response to increased acidity (low pH) and high $CO_2$ concentration. This mechanism ensures that oxygen is released efficiently to metabolically active tissues. ### Why Option C is the Correct Answer (False Statement) The Bohr effect is an **allosteric property** that requires a quaternary structure (multiple subunits). **Myoglobin** is a monomeric protein (single polypeptide chain). Because it lacks multiple subunits and the ability to shift between T (tense) and R (relaxed) states, it does not show cooperativity or the Bohr effect. Its oxygen dissociation curve is hyperbolic, not sigmoidal. ### Analysis of Other Options * **A. Reciprocal coupling of protons and $O_2$:** This is true. As $H^+$ binds to hemoglobin, $O_2$ is released (in tissues). Conversely, when $O_2$ binds to hemoglobin, $H^+$ is released (in lungs). * **B. Cooperative interactions:** The Bohr effect depends on the transition between the T-state (low affinity) and R-state (high affinity). This transition is the hallmark of the hemoglobin tetramer’s cooperative nature. * **D. Rupture of salt bridges:** In the T-state (deoxygenated), specific amino acids (like His146) form salt bridges that are stabilized by protons. When $O_2$ binds, these salt bridges rupture, releasing the protons. ### High-Yield Clinical Pearls for NEET-PG * **Right Shift Factors:** Factors that shift the Oxygen Dissociation Curve (ODC) to the **Right** (releasing $O_2$) include: **C**ADET, face **Right** (**C**O2, **A**cidosis/H+, **D**PG/2,3-BPG, **E**xercise, **T**emperature). * **Haldane Effect:** While the Bohr effect describes how $H^+/CO_2$ affect $O_2$ binding, the Haldane effect describes how **$O_2$ levels** affect hemoglobin's affinity for $CO_2$. * **Key Residue:** The most important amino acid involved in the Bohr effect is **Histidine 146** of the $\beta$-chain.

Question 156: What is the role of 2,3-diphosphoglycerate (2,3-DPG) in hemoglobin?

A. Facilitates oxygen unloading to tissues (Correct Answer)
B. Increases hemoglobin's affinity for oxygen
C. Contributes to buffering capacity
D. Affects osmotic fragility of red blood cells

Explanation: ### Explanation **1. Why Option A is Correct:** 2,3-Diphosphoglycerate (2,3-DPG) is a byproduct of the Rapoport-Luebering shunt in glycolysis. It acts as an **allosteric effector** that binds to the central cavity of the deoxyhemoglobin (T-state) tetramer. By forming salt bridges between the beta chains, it stabilizes the **T-state (Tense state)**, which has a lower affinity for oxygen. This stabilization shifts the oxygen-dissociation curve to the **right**, promoting the release (unloading) of oxygen to the peripheral tissues. **2. Why Other Options are Incorrect:** * **Option B:** 2,3-DPG *decreases* hemoglobin’s affinity for oxygen. A decrease in 2,3-DPG (as seen in stored blood) would increase affinity, making it harder for oxygen to be released. * **Option C:** While hemoglobin itself acts as a buffer (Bohr effect), 2,3-DPG’s primary physiological role is the modulation of oxygen affinity, not acid-base buffering. * **Option D:** Osmotic fragility is primarily determined by the surface-area-to-volume ratio of the RBC and membrane integrity (e.g., Spectrin/Ankyrin defects), not by 2,3-DPG levels. **3. NEET-PG High-Yield Clinical Pearls:** * **Fetal Hemoglobin (HbF):** HbF has a lower affinity for 2,3-DPG because its $\gamma$-chains lack certain positively charged amino acids found in $\beta$-chains. This results in HbF having a **higher oxygen affinity** than HbA, allowing oxygen transfer from mother to fetus. * **Adaptation to Altitude:** 2,3-DPG levels **increase** at high altitudes and in chronic hypoxia/anemia to facilitate better oxygen delivery to tissues. * **Stored Blood:** 2,3-DPG levels drop in stored blood. Transfusing large amounts of "old" blood can cause a "left shift," temporarily impairing oxygen delivery to the recipient's tissues.

Question 157: Carbon monoxide (CO) is released in a reaction catalyzed by which of the following enzymes?

A. Decarboxylases
B. Carboxylases
C. Heme oxygenase (Correct Answer)
D. Pyruvate Dehydrogenase

Explanation: **Explanation:** **Correct Answer: C. Heme oxygenase** The correct answer is **Heme oxygenase** because it is the rate-limiting enzyme in the degradation of heme. This reaction occurs primarily in the reticuloendothelial system (spleen and liver). Heme oxygenase acts on the heme molecule, cleaving the α-methene bridge to produce three specific products: 1. **Biliverdin** (which is later reduced to bilirubin). 2. **Ferrous iron (Fe²⁺)** (which is recycled). 3. **Carbon Monoxide (CO)**. This is the **only endogenous source of carbon monoxide** in the human body. CO acts as a signaling molecule and vasodilator at physiological levels, but its production is a hallmark of heme catabolism. **Why the other options are incorrect:** * **Decarboxylases (A):** These enzymes catalyze the removal of a carboxyl group, releasing **Carbon Dioxide (CO₂)**, not CO (e.g., Histidine to Histamine). * **Carboxylases (B):** These enzymes add CO₂ to a substrate (usually requiring Biotin as a cofactor), such as in the conversion of Pyruvate to Oxaloacetate. * **Pyruvate Dehydrogenase (D):** This multi-enzyme complex converts Pyruvate to Acetyl-CoA via oxidative decarboxylation, releasing **CO₂** and NADH. **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous CO:** Since CO is produced in a 1:1 molar ratio with bilirubin, measuring the **Carboxyhemoglobin (COHb)** levels or CO in exhaled breath can be used as an index of the rate of hemolysis. * **Heme Oxygenase Isoforms:** HO-1 is inducible (stress-response), while HO-2 is constitutive (found in the brain and testes). * **Substrate/Cofactors:** The reaction requires **Molecular Oxygen (O₂)** and **NADPH**. * **Bilirubin formation:** Biliverdin is converted to Bilirubin by *Biliverdin Reductase*.

Question 158: Hemochromatosis is a defect in the metabolism of which element?

A. Iron (Correct Answer)
B. Copper
C. Magnesium
D. Calcium

Explanation: **Explanation:** **Hemochromatosis** is a clinical condition characterized by excessive accumulation of **Iron** in the body, leading to multi-organ damage. **1. Why Iron is Correct:** The most common form is **Hereditary Hemochromatosis**, an autosomal recessive disorder typically caused by a mutation in the **HFE gene** (C282Y mutation). This defect leads to the downregulation of **Hepcidin**, the master regulator of iron homeostasis. Low hepcidin levels result in uncontrolled iron absorption from the duodenum and excessive release from macrophages. This "iron overload" deposits in organs like the liver (cirrhosis), pancreas (diabetes), and skin (hyperpigmentation), classically known as **"Bronze Diabetes."** **2. Why Other Options are Incorrect:** * **Copper:** Defects in copper metabolism lead to **Wilson’s Disease** (accumulation in liver and brain) or **Menkes Disease** (deficiency due to impaired absorption). * **Magnesium:** Disorders of magnesium are usually related to renal handling or GI absorption (e.g., Gitelman syndrome), but no primary "chromatosis" exists for magnesium. * **Calcium:** Abnormalities in calcium metabolism are typically linked to Parathyroid Hormone (PTH) or Vitamin D defects (e.g., Hyperparathyroidism). **3. High-Yield Clinical Pearls for NEET-PG:** * **Triad of Hemochromatosis:** Cirrhosis, Diabetes Mellitus, and Skin Pigmentation. * **Diagnosis:** Best initial test is **Transferrin Saturation** (>45%); Gold standard is **Liver Biopsy** (Prussian Blue staining shows hemosiderin). * **Treatment:** Therapeutic phlebotomy is the treatment of choice. * **Cardiac Involvement:** Can lead to Restrictive or Dilated Cardiomyopathy.

Question 159: Which of the following is involved in iron absorption, synthesized by the liver, and secreted through bile into the intestine?

A. Apoferritin
B. Transferrin
C. Apotransferrin (Correct Answer)
D. All of the above

Explanation: ### Explanation The correct answer is **Apotransferrin**. **1. Why Apotransferrin is Correct:** Iron absorption in the small intestine involves a specific mechanism where the liver synthesizes **apotransferrin** and secretes it into the **bile**. This apotransferrin travels to the duodenum and jejunum, where it binds with free iron (forming transferrin) or iron compounds in the intestinal lumen. This complex then binds to receptors on the brush border of enterocytes and is internalized via pinocytosis. Once inside the cell, iron is released into the blood, and the apotransferrin is recycled back into the lumen to pick up more iron. **2. Why Other Options are Incorrect:** * **Apoferritin (A):** This is the protein shell that combines with iron *inside* cells (primarily in the liver and intestinal mucosa) to form **Ferritin**, the primary intracellular storage form of iron. It is not secreted into bile for absorption. * **Transferrin (B):** This is the plasma transport protein. While apotransferrin becomes transferrin once it binds iron, the specific substance *synthesized and secreted* by the liver into the bile to facilitate the initial uptake from the gut is apotransferrin. **3. NEET-PG High-Yield Clinical Pearls:** * **Hepcidin:** The "Master Regulator" of iron. It is a liver-derived peptide that inhibits iron absorption by causing the degradation of **Ferroportin** (the basal exporter). * **DMT-1 (Divalent Metal Transporter 1):** Responsible for the uptake of inorganic non-heme iron ($Fe^{2+}$) into the enterocyte. * **Ferroportin:** The only known cellular iron exporter; it moves iron from the enterocyte into the portal circulation. * **Ceruloplasmin/Hephaestin:** Ferrooxidases that convert $Fe^{2+}$ to $Fe^{3+}$ so it can bind to plasma transferrin.

Question 160: What is the primary protein responsible for iron homeostasis?

A. Ceruloplasmin
B. Metallothionein
C. Transferrin (Correct Answer)
D. Ferritin

Explanation: **Explanation:** **Correct Answer: C. Transferrin** Iron homeostasis is primarily regulated through the transport and distribution of iron in the plasma. **Transferrin** is the principal iron-transport protein. It binds ferric iron ($Fe^{3+}$) with high affinity, ensuring it remains soluble and non-toxic while being delivered to tissues (especially the bone marrow for erythropoiesis) via transferrin receptors. By controlling the flux of iron between storage sites and utilization sites, it serves as the central hub for systemic iron homeostasis. **Analysis of Incorrect Options:** * **A. Ceruloplasmin:** This is a copper-containing ferroxidase. Its role is to oxidize $Fe^{2+}$ to $Fe^{3+}$ so iron can bind to transferrin. While essential for iron mobilization, it is not the primary homeostasis protein. * **B. Metallothionein:** This protein is primarily involved in the homeostasis and detoxification of heavy metals like **zinc and copper**, not iron. * **C. Ferritin:** This is the primary **intracellular storage** form of iron. While it reflects total body iron stores, it is a "reservoir" rather than the active regulator of systemic iron distribution. **High-Yield Clinical Pearls for NEET-PG:** * **Hepcidin Connection:** Hepcidin is the "master regulator" of iron; it decreases iron levels by degrading ferroportin. If the question asks for the *hormonal* regulator, choose Hepcidin. * **TIBC:** Total Iron Binding Capacity is an indirect measure of Transferrin levels. In Iron Deficiency Anemia (IDA), TIBC increases as the body tries to maximize transport. * **Saturation:** Normal transferrin saturation is approximately **33%**. * **Apotransferrin:** Transferrin without iron; **Holotransferrin:** Transferrin bound to iron.

Practice by Chapter

Hemoglobin Structure and Function

Practice Questions

Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve

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Hemoglobin Variants and Hemoglobinopathies

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Thalassemias

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Methemoglobin and Abnormal Hemoglobins

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Hemoglobin Synthesis

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Heme Synthesis and Porphyrias

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Iron Absorption and Transport

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Iron Storage and Recycling

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Disorders of Iron Metabolism

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Anemia: Biochemical Aspects

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Biochemistry of Hemostasis

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