Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 21: What is the key regulatory enzyme in hepatic biosynthesis of heme?

A. Decarboxylase
B. ALA synthase (Correct Answer)
C. Coproporphyrinogen oxidase
D. Protoporphyrinogen oxidase

Explanation: **Explanation:** **1. Why ALA Synthase is Correct:** Aminolevulinate synthase (**ALA synthase**) is the **rate-limiting and key regulatory enzyme** of heme biosynthesis in the liver. It catalyzes the condensation of Succinyl CoA and Glycine to form $\delta$-aminolevulinic acid (ALA), requiring **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. In the liver, this enzyme is strictly regulated by **heme** via feedback inhibition (repression of enzyme synthesis and inhibition of mitochondrial transport). **2. Why Other Options are Incorrect:** * **Decarboxylase (Uroporphyrinogen Decarboxylase):** This enzyme converts Uroporphyrinogen III to Coproporphyrinogen III. While its deficiency leads to *Porphyria Cutanea Tarda*, it is not the rate-limiting step. * **Coproporphyrinogen Oxidase:** This mitochondrial enzyme converts Coproporphyrinogen III to Protoporphyrinogen IX. Deficiency causes *Hereditary Coproporphyria*. * **Protoporphyrinogen Oxidase:** This enzyme converts Protoporphyrinogen IX to Protoporphyrin IX. Deficiency leads to *Variegate Porphyria*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoforms:** There are two isoforms: **ALAS1** (ubiquitous/hepatic) and **ALAS2** (erythroid-specific). ALAS2 is regulated by **iron** levels, not heme. * **Cofactor:** B6 deficiency can lead to **Sideroblastic Anemia** because ALA synthase cannot function. * **Inducers:** Drugs like barbiturates and griseofulvin induce ALAS1, which can precipitate acute attacks of porphyria. * **Inhibitors:** Hematin (a heme analog) is used to treat porphyria attacks because it inhibits ALA synthase.

Question 22: An increase in the concentration of 2,3-bisphosphoglycerate (2,3-DPG) may be seen in all of the following conditions except?

A. Anemia
B. Hypoxia
C. Inosine
D. Hypoxanthine (Correct Answer)

Explanation: **Explanation:** The concentration of **2,3-bisphosphoglycerate (2,3-BPG/DPG)** in erythrocytes is a critical regulator of hemoglobin’s oxygen affinity. It binds to the central cavity of the deoxyhemoglobin tetramer, stabilizing the **T (Tense) state** and shifting the oxygen dissociation curve to the **right**, facilitating oxygen unloading to tissues. **Why Hypoxanthine is the Correct Answer:** Hypoxanthine is a purine derivative and a breakdown product of adenosine monophosphate (AMP). It does not enter the glycolytic pathway or the **Rapoport-Luebering shunt** (the pathway responsible for 2,3-BPG synthesis). Therefore, it has no direct role in increasing 2,3-BPG levels. **Analysis of Incorrect Options:** * **Anemia & Hypoxia:** These conditions trigger a compensatory increase in 2,3-BPG. In anemia (low hemoglobin) and hypoxia (low arterial $PO_2$), the body increases 2,3-BPG to enhance oxygen delivery to peripheral tissues by decreasing hemoglobin's affinity for $O_2$. * **Inosine:** In blood banking, inosine is added to stored blood as a "rejuvenation" fluid. Inosine is a nucleoside that can be converted into **Ribose-5-phosphate**, which then enters the pentose phosphate pathway and subsequently the glycolytic pathway, eventually leading to the synthesis of 2,3-BPG. This helps restore the oxygen-carrying efficiency of stored blood. **Clinical Pearls for NEET-PG:** * **Rapoport-Luebering Shunt:** Uses the enzyme *2,3-BPG synthase/phosphatase* to bypass the phosphoglycerate kinase step of glycolysis. * **Stored Blood:** 2,3-BPG levels **decrease** in stored blood, causing a "left shift" (increased $O_2$ affinity). This is why inosine is added. * **Fetal Hemoglobin (HbF):** Has a lower affinity for 2,3-BPG compared to HbA, allowing HbF to have a higher affinity for $O_2$, facilitating oxygen transfer from mother to fetus. * **High Altitude:** Chronic exposure leads to an increase in 2,3-BPG as an adaptive mechanism.

Question 23: A polypeptide of hemoglobin closely resembles which of the following?

A. Collagen
B. Albumin
C. Myoglobin (Correct Answer)
D. Transferrin

Explanation: **Explanation:** The correct answer is **Myoglobin (Option C)**. **Why Myoglobin is correct:** Hemoglobin is a tetrameric protein ($α_2β_2$), whereas myoglobin is a monomeric protein. Despite this difference in quaternary structure, the individual polypeptide chains (subunits) of hemoglobin are structurally very similar to myoglobin. Both share a common evolutionary origin and possess the **"globin fold"**—a characteristic tertiary structure consisting of eight α-helices (labeled A through H) that create a hydrophobic pocket to house the heme group. This structural homology allows both proteins to bind oxygen reversibly. **Why other options are incorrect:** * **Collagen:** A fibrous structural protein characterized by a triple helix (not α-helices) and a repeating Gly-X-Y amino acid sequence. It does not bind oxygen or contain heme. * **Albumin:** The most abundant plasma protein, primarily responsible for maintaining oncotic pressure and transporting various ligands (bilirubin, fatty acids, drugs). Its structure is globular but lacks the globin fold and heme moiety. * **Transferrin:** A plasma glycoprotein specifically designed to transport iron ($Fe^{3+}$) in the blood. While involved in iron metabolism, its structure and function are distinct from the oxygen-binding globin family. **High-Yield Clinical Pearls for NEET-PG:** * **Oxygen Dissociation Curve:** Myoglobin exhibits a **hyperbolic** curve (high affinity, storage function), while Hemoglobin exhibits a **sigmoidal** curve (cooperative binding, transport function). * **P50 Value:** The $P_{50}$ of myoglobin is much lower (~1–2 mmHg) than that of hemoglobin (~26 mmHg), reflecting myoglobin's higher affinity for $O_2$. * **Homology:** While the primary amino acid sequences of myoglobin and hemoglobin subunits differ significantly, their **tertiary structures** are nearly identical.

Question 24: Glucose is linked to haemoglobin through which type of linkage?

A. N-linkage (Correct Answer)
B. O-linkage
C. C-C linkage
D. O-H linkage

Explanation: **Explanation:** The formation of glycated hemoglobin (HbA1c) is a non-enzymatic process known as **glycation**. This process involves the attachment of glucose to the hemoglobin molecule, specifically at the **N-terminal valine** of the beta-globin chain. 1. **Why N-linkage is correct:** The aldehyde group of glucose reacts with the free amino group (-NH₂) of the N-terminal valine. This initially forms an unstable aldimine (Schiff base), which then undergoes an **Amadori rearrangement** to form a stable ketoamine. Because the bond is formed with a nitrogen atom of the amino group, it is classified as an **N-linkage**. 2. **Why other options are incorrect:** * **O-linkage:** This typically refers to O-glycosylation, where sugars attach to the oxygen atom of hydroxyl groups (e.g., on Serine or Threonine residues). This is an enzymatic process common in glycoproteins, not HbA1c. * **C-C linkage:** Carbon-carbon bonds are extremely stable and do not form spontaneously between glucose and proteins under physiological conditions. * **O-H linkage:** This refers to a hydroxyl group or hydrogen bonding, not a covalent structural linkage between a sugar and a protein backbone. **Clinical Pearls for NEET-PG:** * **HbA1c reflects glycemic control** over the preceding **8–12 weeks** (the average lifespan of an RBC). * The reaction is **non-enzymatic**; its rate is directly proportional to the blood glucose concentration. * **False Lows:** Seen in conditions with high RBC turnover (e.g., Hemolytic anemia, recent hemorrhage, or treatment with Erythropoietin). * **False Highs:** Seen in Iron deficiency anemia (due to increased RBC lifespan and altered glycation rates).

Question 25: What causes a decrease in 2,3-DPG?

A. Decrease in binding of O2 and hemoglobin (Correct Answer)
B. Increased acidity in RBC
C. Decreased acidity in RBC
D. Lysis of RBC

Explanation: ### Explanation **Correct Option: A. Decrease in binding of O2 and hemoglobin** The relationship between 2,3-Bisphosphoglycerate (2,3-DPG) and hemoglobin is defined by an inverse allosteric interaction. 2,3-DPG binds to the central cavity of the deoxyhemoglobin (T-state), stabilizing it and **decreasing the affinity** of hemoglobin for oxygen. Therefore, a **decrease** in 2,3-DPG levels results in an **increased affinity** of hemoglobin for oxygen (shifting the Oxygen Dissociation Curve to the left). Conversely, when the binding of O2 to hemoglobin decreases (favoring the T-state), the body typically responds by increasing 2,3-DPG to facilitate oxygen unloading. In the context of this question, the physiological consequence of decreased 2,3-DPG is a shift toward higher O2 affinity, meaning O2 remains bound to hemoglobin more tightly. **Analysis of Incorrect Options:** * **B & C (Acidity in RBC):** The enzyme **Phosphofructokinase (PFK)**, which is the rate-limiting step of glycolysis, is highly sensitive to pH. **Increased acidity (low pH)** inhibits PFK, leading to decreased glycolysis and a subsequent **decrease in 2,3-DPG**. Conversely, decreased acidity (alkalosis) stimulates 2,3-DPG production. * **D (Lysis of RBC):** While lysis destroys the cell, it is not a physiological regulator of 2,3-DPG concentration within the metabolic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift (Reduced Affinity):** Increased 2,3-DPG, Increased H+ (Bohr Effect), Increased CO2, Increased Temperature. (Mnemonic: **CADET**, face Right! — **C**O2, **A**cid, **D**PG, **E**xercise, **T**emp). * **Left Shift (Increased Affinity):** Decreased 2,3-DPG, Fetal Hemoglobin (HbF), Carbon Monoxide (CO), Hypothermia. * **Stored Blood:** 2,3-DPG levels drop in stored blood; hence, massive transfusions can lead to impaired oxygen delivery to tissues. * **Rapoport-Luebering Shunt:** The specific pathway in RBCs that generates 2,3-DPG.

Question 26: Mucosal transfer of iron in the gastrointestinal tract is mediated by which protein?

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

Explanation: **Explanation:** The absorption of iron in the gastrointestinal tract is a highly regulated process. Iron enters the enterocyte (mucosal cell) via the **Divalent Metal Transporter 1 (DMT-1)**. Once inside the cell, iron must be transported across the cytoplasm to the basolateral membrane for systemic absorption. This mucosal transfer is mediated by **Apoferritin**. Apoferritin binds with free ferrous iron ($Fe^{2+}$) to form **Ferritin**. This complex acts as a temporary storage form within the mucosal cell. If the body requires iron, it is released and transported into the blood; if not, the iron remains trapped as ferritin and is lost when the mucosal cell is sloughed off (the "mucosal block" phenomenon). **Analysis of Options:** * **A. Transferrin:** This is the primary protein responsible for transporting iron in the **plasma**, not within the mucosal cell. * **C. Apotransferrin:** This is the iron-free form of transferrin found in the blood. It becomes transferrin once it binds two molecules of ferric iron ($Fe^{3+}$). * **D. Ferritin:** While ferritin is the storage form, **Apoferritin** is the specific protein shell that mediates the binding and transfer process within the mucosa. **High-Yield Clinical Pearls for NEET-PG:** * **Hepcidin:** The master regulator of iron metabolism; it inhibits **Ferroportin**, preventing iron exit from enterocytes into the blood. * **Hephaestin:** A ferroxidase that converts $Fe^{2+}$ to $Fe^{3+}$ at the basolateral membrane to allow binding to plasma transferrin. * **Vitamin C:** Enhances iron absorption by maintaining iron in the more soluble ferrous ($Fe^{2+}$) state.

Question 27: What enzyme is deficient in erythropoietic porphyria?

A. PBG deaminase
B. Uroporphyrinogen III cosynthase
C. Coproporphyrinogen oxidase
D. Ferrochelatase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Ferrochelatase**. **Erythropoietic Protoporphyria (EPP)** is an autosomal dominant disorder caused by a deficiency in **Ferrochelatase**, the final enzyme in the heme biosynthetic pathway. This enzyme is responsible for inserting ferrous iron ($Fe^{2+}$) into Protoporphyrin IX to form Heme. When deficient, Protoporphyrin IX accumulates in erythrocytes, bone marrow, and plasma. Clinically, this manifests as **immediate cutaneous photosensitivity** (burning, itching, and edema upon sun exposure) rather than the blistering seen in other porphyrias. **Analysis of Incorrect Options:** * **A. PBG Deaminase:** Deficiency leads to **Acute Intermittent Porphyria (AIP)**. It presents with the "5 Ps": Painful abdomen, Polyneuropathy, Psychosis, Pink urine, and Precipitation by drugs (Barbiturates), but notably lacks cutaneous photosensitivity. * **B. Uroporphyrinogen III Cosynthase:** Deficiency causes **Congenital Erythropoietic Porphyria (Gunther’s disease)**. It is characterized by severe mutilating photosensitivity, erythrodontia (red teeth), and hemolytic anemia. * **C. Coproporphyrinogen Oxidase:** Deficiency leads to **Hereditary Coproporphyria (HCP)**, which presents with both neurovisceral symptoms and photosensitivity. **NEET-PG High-Yield Pearls:** * **Most common porphyria:** Porphyria Cutanea Tarda (Deficiency of Uroporphyrinogen decarboxylase). * **Key distinction:** Porphyrias occurring *before* the synthesis of tetrapyrroles (like AIP) do not show photosensitivity. Porphyrias occurring *after* (like EPP or PCT) result in photosensitivity due to the light-absorbing properties of porphyrin rings. * **Lead Poisoning:** Inhibits both **$\delta$-ALA dehydratase** and **Ferrochelatase**, mimicking some features of porphyria.

Question 28: In Sickle Cell Anaemia, the defect is in which globin chain?

A. Alpha chain
B. Beta chain (Correct Answer)
C. Both chains
D. None of these

Explanation: **Explanation:** **1. Why the Beta chain is correct:** Sickle Cell Anaemia (SCA) is an autosomal recessive disorder caused by a **point mutation** in the **$\beta$-globin gene** located on chromosome 11. Specifically, there is a substitution of **Glutamic acid** (a polar amino acid) by **Valine** (a non-polar, hydrophobic amino acid) at the **6th position** of the $\beta$-globin chain ($Glu6Val$). This substitution creates a "sticky" hydrophobic patch on the surface of the hemoglobin molecule (HbS). Under deoxygenated conditions, these patches cause HbS molecules to polymerize, distorting the red blood cell into a characteristic sickle shape. **2. Why other options are incorrect:** * **Alpha chain:** Mutations in the $\alpha$-globin chain (located on chromosome 16) are associated with **$\alpha$-Thalassemia**, not Sickle Cell Anaemia. * **Both chains:** SCA is strictly a $\beta$-chain pathology. While hemoglobin is a tetramer ($\alpha_2\beta_2$), the $\alpha$-chains in a patient with SCA are structurally normal. * **None of these:** Incorrect, as the molecular defect is well-established in the $\beta$-globin chain. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mutation type:** Transversion (GAG $\rightarrow$ GTG). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** towards the anode than HbA because Valine is neutral, whereas Glutamic acid is negatively charged. * **Protective effect:** Heterozygotes (Sickle cell trait) have a selective advantage against *Plasmodium falciparum* malaria. * **Metabolic trigger:** Sickling is precipitated by hypoxia, acidosis, dehydration, and increased 2,3-BPG.

Question 29: Plasma ferritin levels may be reduced in all of the following conditions, except?

A. Iron deficiency
B. Vitamin C deficiency
C. Liver disease (Correct Answer)
D. Hypothyroidism

Explanation: ### Explanation **Core Concept:** Serum ferritin is the primary storage form of iron. It is also an **acute-phase reactant**. This means its levels increase during inflammation, infection, or tissue damage, regardless of the body's actual iron stores. **Why Liver Disease is the Correct Answer:** In liver disease (such as hepatitis or cirrhosis), ferritin levels are characteristically **elevated**, not reduced. This occurs because the liver is the primary storage organ for ferritin. When hepatocytes are damaged or undergo necrosis, they leak their intracellular ferritin stores directly into the plasma. Therefore, liver disease is a classic cause of "falsely" high ferritin levels, even if a patient has a co-existing iron deficiency. **Analysis of Other Options:** * **A. Iron Deficiency:** This is the most common cause of low ferritin. Serum ferritin is the most sensitive and specific initial lab test for diagnosing iron deficiency anemia. * **B. Vitamin C Deficiency:** Vitamin C (Ascorbic acid) is essential for the stability of ferritin and the incorporation of iron into the ferritin shell. Deficiency can lead to impaired iron storage and reduced ferritin levels. * **C. Hypothyroidism:** Thyroid hormones influence the synthesis of ferritin. In hypothyroid states, the rate of ferritin synthesis decreases, leading to lower plasma levels. **High-Yield Clinical Pearls for NEET-PG:** * **Best Screening Test for IDA:** Serum Ferritin (it is the first parameter to decrease in iron deficiency). * **Ferritin as a Mask:** Because it is an acute-phase reactant, a "normal" ferritin level in a patient with chronic inflammation (e.g., Rheumatoid Arthritis) does not rule out iron deficiency. * **The "Rule of 30":** In the presence of inflammation/liver disease, a ferritin level <30 ng/mL still suggests iron deficiency, whereas in healthy individuals, the cutoff is usually <15 ng/mL.

Question 30: Which of the following molecules delivers iron to tissues by binding to specific cell surface receptors?

A. Ferritin
B. Ferredoxin
C. Hemosiderin
D. Transferrin (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Transferrin** Transferrin is the primary plasma glycoprotein responsible for the **transport** of iron in the blood. Iron is absorbed in the ferrous ($Fe^{2+}$) state but must be converted to the ferric ($Fe^{3+}$) state by ferroxidase (ceruloplasmin) to bind to Transferrin. Each Transferrin molecule can bind two atoms of $Fe^{3+}$. It delivers iron to tissues (especially the bone marrow and liver) by binding to specific **Transferrin Receptors (TfR)** on the cell surface. The complex is then internalized via receptor-mediated endocytosis. **Why the other options are incorrect:** * **A. Ferritin:** This is the primary **intracellular storage** form of iron. It is a hollow protein shell (apoferritin) that sequesters iron to prevent oxidative damage. While small amounts circulate in the serum (correlating with total body iron stores), it is not the transport vehicle for tissue delivery. * **B. Ferredoxin:** These are iron-sulfur proteins found in bacteria, algae, and plants (and within human mitochondria), primarily involved in **electron transfer** reactions, not systemic iron transport. * **C. Hemosiderin:** This is an insoluble, partially denatured form of ferritin found in states of **iron overload**. It represents long-term iron storage and is visible under light microscopy with Prussian blue staining. **High-Yield Clinical Pearls for NEET-PG:** * **Total Iron Binding Capacity (TIBC):** This is an indirect measure of serum transferrin levels. In **Iron Deficiency Anemia (IDA)**, Transferrin/TIBC increases while Ferritin decreases. * **Ferroportin:** The only known iron exporter that releases iron from enterocytes/macrophages into the blood. It is inhibited by **Hepcidin**. * **Atransferrinemia:** A rare genetic deficiency of transferrin leading to microcytic anemia and secondary iron overload.

Practice by Chapter

Hemoglobin Structure and Function

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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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