Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 61: Which of the following is not a haemoprotein?

A. Albumin (Correct Answer)
B. Myoglobin
C. Cytochrome P450
D. Cytochrome C

Explanation: **Explanation:** The core concept tested here is the definition of a **haemoprotein**: a protein that contains a **heme prosthetic group** (iron-protoporphyrin IX) covalently or non-covalently bound to its structure. **Why Albumin is the correct answer:** Albumin is a simple globular protein synthesized by the liver. It lacks a heme group. Its primary functions include maintaining **plasma oncotic pressure** and acting as a non-specific carrier for various substances like bilirubin, fatty acids, calcium, and certain drugs. While it can bind to free heme (forming methaemalbumin) to prevent oxidative damage, it is not classified as a haemoprotein because heme is not an intrinsic part of its functional structure. **Why the other options are incorrect:** * **Myoglobin:** A classic haemoprotein found in muscle tissue. It consists of a single polypeptide chain and one heme group, serving as a reservoir for oxygen. * **Cytochrome P450:** A large family of heme-containing enzymes primarily located in the endoplasmic reticulum of hepatocytes. They are crucial for the detoxification of xenobiotics and steroid synthesis. * **Cytochrome C:** A small haemoprotein found in the inner mitochondrial membrane. It plays a vital role in the **Electron Transport Chain (ETC)** by carrying electrons between Complex III and Complex IV. **High-Yield Clinical Pearls for NEET-PG:** * **Other Haemoproteins to remember:** Hemoglobin, Catalase, Peroxidase, Tryptophan pyrrolase, and Nitric Oxide Synthase (NOS). * **Heme Synthesis:** The rate-limiting step is catalyzed by **ALA Synthase**, requiring Vitamin B6 (Pyridoxine) as a cofactor. * **Lead Poisoning:** Inhibits ALA Dehydratase and Ferrochelatase, leading to impaired heme synthesis.

Question 62: In sickle cell hemoglobin (HbS), glutamic acid is replaced by valine. How does this substitution affect its electrophoretic mobility compared to normal hemoglobin?

A. Increased mobility
B. Decreased mobility (Correct Answer)
C. No change in mobility
D. Mobility depends on the concentration of HbS

Explanation: ### Explanation **1. Why the Correct Answer is Right (Decreased Mobility)** Electrophoresis separates proteins based on their **net electrical charge**. Normal Hemoglobin (HbA) has **Glutamic acid** at the 6th position of the beta-globin chain. Glutamic acid is a dicarboxylic amino acid that carries a **negative charge** at physiological pH. In Sickle Cell Hemoglobin (HbS), this is replaced by **Valine**, which is a non-polar, neutral amino acid. By losing a negative charge, the HbS molecule becomes **more positive (less negative)** than HbA. During alkaline electrophoresis (pH 8.6), hemoglobin molecules migrate toward the positive electrode (Anode). Since HbS is less negative than HbA, it moves more slowly toward the anode, resulting in **decreased electrophoretic mobility**. **2. Why the Incorrect Options are Wrong** * **Option A (Increased mobility):** This would occur if the substitution added a negative charge (e.g., if a neutral amino acid were replaced by Aspartate). Since HbS loses a negative charge, it cannot move faster toward the anode. * **Option C (No change):** This would only occur if the substitution involved amino acids with the same charge (e.g., Valine to Leucine). * **Option D (Concentration-dependent):** Electrophoretic mobility is a property of the molecule's charge-to-mass ratio, not its concentration. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Mnemonic for Mobility (Fastest to Slowest):** **A** Fat **S**anta **C**laus (**A** > **F** > **S** > **C**). HbA moves the fastest, HbC the slowest. * **HbC Mutation:** Glutamic acid is replaced by **Lysine** (a positively charged amino acid). HbC is even less negative than HbS, thus it moves even slower. * **Molecular Basis:** The mutation is a **point mutation (missense)**: GAG (Glu) → GTG (Val). * **Sticky Patches:** The substitution of Valine (hydrophobic) creates "sticky patches," leading to polymerization of deoxygenated HbS.

Question 63: A young girl presents with neuropsychiatric symptoms and a history of licking paint from walls. These symptoms are likely due to the inhibition of which enzyme?

A. ALA dehydratase (Correct Answer)
B. ALA synthase
C. Heme oxygenase
D. CPG oxidase

Explanation: **Explanation:** The clinical presentation of neuropsychiatric symptoms combined with pica (licking paint) in a child is a classic indicator of **Lead Poisoning**. Old wall paint often contains lead, which interferes with heme synthesis by inhibiting specific enzymes. **1. Why ALA Dehydratase is correct:** Lead is a heavy metal that inhibits two key enzymes in the heme biosynthetic pathway by displacing zinc from their active sites: **ALA Dehydratase (Porphobilinogen Synthase)** and **Ferrochelatase**. Inhibition of ALA dehydratase leads to an accumulation of delta-aminolevulinic acid (δ-ALA), which is neurotoxic and responsible for the neuropsychiatric symptoms (irritability, abdominal pain, and encephalopathy). **2. Why the other options are incorrect:** * **ALA Synthase:** This is the rate-limiting enzyme of heme synthesis, regulated by heme levels via feedback inhibition. It is not the primary target of lead. * **Heme Oxygenase:** This enzyme is involved in heme **degradation** (converting heme to biliverdin), not synthesis. * **CPG (Coproporphyrinogen) Oxidase:** This enzyme converts Coproporphyrinogen III to Protoporphyrinogen IX. While lead can cause secondary coproporphyrinuria, it is not the primary site of inhibition associated with the initial toxic symptoms. **Clinical Pearls for NEET-PG:** * **Diagnosis:** Elevated blood lead levels and increased urinary δ-ALA. * **Hematology:** Look for **Basophilic Stippling** on a peripheral smear (due to inhibition of pyrimidine 5'-nucleotidase causing RNA degradation products to clump). * **Radiology:** "Lead lines" (increased metaphyseal density) on X-rays of long bones. * **Treatment:** Chelation therapy with Succimer (oral), CaEDTA, or British Anti-Lewisite (BAL/Dimercaprol).

Question 64: All of the following enzymes involved in heme synthesis are found in the mitochondria, EXCEPT:

A. Ferrochelatase
B. Protoporphyrinogen oxidase
C. Coproporphyrinogen oxidase
D. Uroporphyrinogen decarboxylase (Correct Answer)

Explanation: **Explanation:** Heme synthesis is a compartmentalized process that occurs partly in the **mitochondria** and partly in the **cytosol**. A high-yield rule for NEET-PG is that the **first step** and the **last three steps** occur in the mitochondria, while the intermediate steps occur in the cytosol. **Why Uroporphyrinogen Decarboxylase is the correct answer:** Uroporphyrinogen decarboxylase (UROD) is the enzyme responsible for converting Uroporphyrinogen III to Coproporphyrinogen III. This reaction occurs entirely within the **cytosol**. A deficiency of this enzyme leads to **Porphyria Cutanea Tarda (PCT)**, the most common porphyria. **Analysis of Incorrect Options:** * **Coproporphyrinogen oxidase (Option C):** This enzyme catalyzes the conversion of Coproporphyrinogen III to Protoporphyrinogen IX. This step marks the reentry of the pathway into the **mitochondria**. * **Protoporphyrinogen oxidase (Option B):** This enzyme oxidizes Protoporphyrinogen IX to Protoporphyrin IX within the **mitochondria**. * **Ferrochelatase (Option A):** Also known as Heme Synthase, this is the final enzyme of the pathway. It incorporates ferrous iron ($Fe^{2+}$) into Protoporphyrin IX to form Heme inside the **mitochondria**. **NEET-PG High-Yield Pearls:** * **Mnemonic for Mitochondrial Enzymes:** "**F**irst and **L**ast" (ALA Synthase, Coproporphyrinogen oxidase, Protoporphyrinogen oxidase, and Ferrochelatase). * **Rate-limiting step:** ALA Synthase (requires Vitamin B6/Pyridoxal Phosphate as a cofactor). * **Lead Poisoning:** Inhibits **ALA Dehydratase** (cytosolic) and **Ferrochelatase** (mitochondrial), leading to anemia and elevated erythrocyte protoporphyrin. * **Site of Synthesis:** Primarily occurs in the Liver (for Cytochrome P450) and Erythroid precursor cells (for Hemoglobin).

Question 65: Bilirubin is absent in urine because it is:

A. Distributed in body fat
B. Conjugated with glucuronide
C. Not filterable by the glomerulus
D. Lipophilic (Correct Answer)

Explanation: **Explanation:** The question refers to **unconjugated bilirubin (UCB)**. In normal physiological conditions, bilirubin is absent in urine because it is **lipophilic** (hydrophobic) and insoluble in water. **Why the correct answer is right:** Unconjugated bilirubin is produced from the breakdown of heme. Due to its lipophilic nature, it cannot travel freely in the blood; it must bind tightly to **albumin**. This large bilirubin-albumin complex is **not filterable by the glomerulus**. Because it cannot pass into the renal tubules, it does not appear in the urine. Only water-soluble substances can be excreted by the kidneys. **Analysis of Incorrect Options:** * **A. Distributed in body fat:** While UCB is fat-soluble and can deposit in brain lipids (causing kernicterus), its absence in urine is primarily due to its inability to be filtered by the kidneys, not its storage in adipose tissue. * **B. Conjugated with glucuronide:** This is incorrect because **conjugated bilirubin** is water-soluble. If bilirubin were conjugated, it would be filtered and *present* in urine (as seen in obstructive jaundice). * **C. Not filterable by the glomerulus:** While this statement is technically true, it is a *consequence* of the bilirubin being lipophilic and bound to albumin. In NEET-PG, when both a physical property (lipophilic) and a physiological result (non-filterable) are present, the **underlying chemical property (Lipophilicity)** is often the preferred answer. **High-Yield Clinical Pearls for NEET-PG:** * **Acholuric Jaundice:** This term refers to hemolytic jaundice where there is an increase in UCB. Since UCB cannot enter urine, the urine remains normal in color (no "bilirubinuria"). * **Dark Urine:** Seen in obstructive jaundice or hepatitis due to **conjugated bilirubin**, which is water-soluble and can pass through the glomerulus. * **Van den Bergh Reaction:** UCB gives an **indirect** positive result, while conjugated bilirubin gives a **direct** positive result.

Question 66: The polypeptide chains of hemoglobin A are composed of which subunits?

A. 1 alpha, 3 beta
B. 2 alpha
C. 2 alpha, 2 beta (Correct Answer)
D. 1 alpha, 2 beta, 1 delta

Explanation: ### Explanation **Concept Overview** Hemoglobin (Hb) is a tetrameric protein responsible for oxygen transport. In adults, the predominant form is **Hemoglobin A (HbA1)**, which constitutes approximately 95–97% of total hemoglobin. A functional hemoglobin molecule must consist of four polypeptide chains (a tetramer) arranged in two identical dimers (αβ1 and αβ2). **Why Option C is Correct** The structure of **HbA1** specifically consists of **two alpha (α) chains and two beta (β) chains**. * The alpha chains contain 141 amino acids each. * The beta chains contain 146 amino acids each. * Each chain is associated with a heme group, allowing one HbA molecule to carry four molecules of oxygen. **Analysis of Incorrect Options** * **Option A & D:** Hemoglobin is always a **tetramer** (4 subunits). These options suggest irregular stoichiometry or combinations that do not form stable, functional adult hemoglobin. * **Option B:** While alpha chains are a component, a functional hemoglobin molecule requires four subunits (two pairs) to exhibit cooperative binding (the sigmoid oxygen dissociation curve). **High-Yield NEET-PG Pearls** 1. **Hemoglobin Variants:** * **HbA2:** 2 alpha, 2 delta (α2δ2) — Normal variant (2–3% in adults). * **HbF (Fetal):** 2 alpha, 2 gamma (α2γ2) — Has higher oxygen affinity than HbA. * **HbH:** 4 beta (β4) — Seen in Alpha-thalassemia (3-gene deletion). * **Hb Barts:** 4 gamma (γ4) — Seen in Hydrops fetalis (4-gene deletion). 2. **Genetics:** Alpha chains are coded on **Chromosome 16**, while Beta, Delta, and Gamma chains are coded on **Chromosome 11**. 3. **2,3-BPG:** It binds to the central cavity of the deoxy-Hb tetramer, specifically interacting with the **beta chains**, shifting the curve to the right (promoting O2 release).

Question 67: One gram of hemoglobin liberates how many milligrams of bilirubin?

A. 40
B. 34 (Correct Answer)
C. 15
D. 55

Explanation: **Explanation:** The degradation of hemoglobin is a precisely regulated process occurring primarily in the reticuloendothelial system (spleen and liver). The conversion of hemoglobin to bilirubin follows a specific stoichiometric relationship based on the molecular weight of the components. **Why 34 mg is correct:** Hemoglobin is a tetramer with a molecular weight of approximately 64,450 Da. Each molecule of hemoglobin contains four heme groups. Through the action of **Heme Oxygenase**, each heme group is converted into one molecule of biliverdin, which is subsequently reduced to one molecule of bilirubin by **Biliverdin Reductase**. Mathematically, the breakdown of **1 gram of hemoglobin results in the production of approximately 34 mg of bilirubin**. This is a high-yield constant frequently tested in medical biochemistry. **Analysis of Incorrect Options:** * **A (40 mg):** This is an overestimation. While bilirubin production can increase in hemolytic states, the chemical yield from 1g of Hb remains constant at 34 mg. * **C (15 mg):** This value is too low. It does not account for the fact that all four heme groups in the hemoglobin tetramer contribute to bilirubin formation. * **D (55 mg):** This value is incorrect and does not correspond to any standard physiological measurement of heme catabolism. **Clinical Pearls for NEET-PG:** * **Daily Production:** A healthy adult produces roughly **250–350 mg** of bilirubin daily; 80% comes from senescent RBCs, while 20% comes from ineffective erythropoiesis or other hemeproteins (cytochromes, myoglobin). * **Rate-Limiting Step:** Heme Oxygenase is the rate-limiting enzyme in bilirubin synthesis. * **Iron Conservation:** During this process, iron is released in the **ferric state (Fe³⁺)** and is recycled via transferrin, while the globin chains are broken down into amino acids. * **Jaundice Threshold:** Clinical jaundice (icterus) usually becomes visible when serum bilirubin exceeds **2 mg/dL**.

Question 68: The conversion of uroporphyrinogen III to coproporphyrinogen III is an example of which type of reaction?

A. Deamination
B. Decarboxylation (Correct Answer)
C. Hydrogenation
D. Dehydrogenation

Explanation: ### Explanation **Correct Answer: B. Decarboxylation** In the heme synthesis pathway, the conversion of **uroporphyrinogen III to coproporphyrinogen III** is catalyzed by the enzyme **Uroporphyrinogen Decarboxylase (UROD)**. During this step, the four **acetate (A)** side chains of uroporphyrinogen III are converted into four **methyl (M)** groups. This process involves the removal of four molecules of carbon dioxide ($CO_2$), making it a classic **decarboxylation** reaction. This reaction occurs in the **cytosol** of the cell. --- ### Why the other options are incorrect: * **A. Deamination:** This involves the removal of an amino group ($-NH_2$). While deamination occurs during the formation of Porphobilinogen (PBG) from ALA (via ALA dehydratase), it is not the mechanism for side-chain modification in the later stages of heme synthesis. * **C. Hydrogenation:** This involves the addition of hydrogen. The heme pathway primarily involves oxidation/reduction and decarboxylation rather than simple hydrogenation. * **D. Dehydrogenation:** This involves the removal of hydrogen (oxidation). While the conversion of coproporphyrinogen III to protoporphyrinogen IX involves both decarboxylation and oxidation (dehydrogenation), the specific step from uroporphyrinogen to coproporphyrinogen is purely a decarboxylation. --- ### NEET-PG High-Yield Clinical Pearls: * **Enzyme Deficiency:** A deficiency of Uroporphyrinogen Decarboxylase leads to **Porphyria Cutanea Tarda (PCT)**, the most common type of porphyria. * **Clinical Presentation of PCT:** Patients present with photosensitivity, skin blistering, and hyperpigmentation. It is often associated with Hepatitis C, alcohol consumption, or iron overload. * **Pathway Location:** Remember the "Mnemonic": **"The first and last three steps are in the Mitochondria; the middle steps are in the Cytosol."** (Uroporphyrinogen → Coproporphyrinogen occurs in the cytosol). * **Lead Poisoning:** Lead inhibits ALA Dehydratase and Ferrochelatase, but **not** Uroporphyrinogen Decarboxylase.

Question 69: Iron is transported bound to:

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

Explanation: **Explanation:** Iron metabolism is a high-yield topic in biochemistry, focusing on the specific proteins responsible for storage, transport, and utilization. **1. Why Transferrin is correct:** Iron is highly reactive and can generate toxic free radicals (Fenton reaction) if left unbound. Therefore, in the plasma, ferric iron ($Fe^{3+}$) is bound to **Transferrin**, a glycoprotein synthesized in the liver. Each transferrin molecule can bind two atoms of ferric iron. It acts as the primary vehicle for delivering iron to bone marrow and other tissues via transferrin receptors. **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. While small amounts circulate in the serum (reflecting total body iron stores), it is not the transport protein. * **Hemosiderin:** This is an insoluble, partially degraded form of ferritin used for **long-term iron storage**, typically seen in states of iron overload. * **Hemoglobin:** This is a hemeprotein found in RBCs responsible for **oxygen transport**, not the transport of systemic iron. It contains iron, but its function is gas exchange. **Clinical Pearls for NEET-PG:** * **Total Iron Binding Capacity (TIBC):** This is an indirect measure of serum transferrin levels. In Iron Deficiency Anemia (IDA), TIBC increases as the body tries to capture more iron. * **Ferroxidase (Ceruloplasmin):** This enzyme is required to convert $Fe^{2+}$ to $Fe^{3+}$ so it can bind to transferrin. * **Hepcidin:** The "master regulator" of iron; it inhibits iron release into the plasma by degrading ferroportin. * **Apo-transferrin:** The protein without iron; **Holotransferrin:** The protein bound to iron.

Question 70: Unconjugated hyperbilirubinemia is seen in all EXCEPT?

A. Gilbert's syndrome
B. Dubin-Johnson syndrome (Correct Answer)
C. Crigler-Najjar syndrome
D. Neonatal physiological jaundice

Explanation: ### Explanation Hyperbilirubinemia is classified into **unconjugated (indirect)** and **conjugated (direct)** based on whether the pathology occurs before or after the bilirubin is processed by the liver enzyme UDP-glucuronosyltransferase (UGT1A1). **Why Dubin-Johnson Syndrome is the Correct Answer:** Dubin-Johnson syndrome is an autosomal recessive disorder characterized by a defect in the **MRP2 protein** (Multidrug Resistance-associated Protein 2). This protein is responsible for the transport of conjugated bilirubin from the hepatocytes into the bile canaliculi. Since the bilirubin has already been conjugated but cannot be excreted, it leaks back into the blood, leading to **conjugated hyperbilirubinemia**. A hallmark finding is a **grossly black liver** due to melanin-like pigment accumulation. **Analysis of Incorrect Options (Causes of Unconjugated Hyperbilirubinemia):** * **Gilbert’s Syndrome:** Caused by a mild reduction in UGT1A1 activity (approx. 30% of normal). It presents as transient, mild unconjugated jaundice triggered by stress, fasting, or illness. * **Crigler-Najjar Syndrome:** Involves a severe deficiency (Type II) or total absence (Type I) of UGT1A1, leading to significant unconjugated hyperbilirubinemia and risk of kernicterus. * **Neonatal Physiological Jaundice:** Occurs due to the immature liver's low UGT1A1 activity and increased RBC breakdown, resulting in a temporary rise in unconjugated bilirubin. **High-Yield Clinical Pearls for NEET-PG:** * **Rotor Syndrome:** Similar to Dubin-Johnson (conjugated hyperbilirubinemia) but **lacks** the black liver pigmentation and has different urinary coproporphyrin patterns. * **Crigler-Najjar Type I vs. II:** Type II (Arias Syndrome) responds to **Phenobarbital** (enzyme inducer), whereas Type I does not. * **Bilirubin Pathway:** Heme $\rightarrow$ Biliverdin $\rightarrow$ Unconjugated Bilirubin (bound to albumin) $\rightarrow$ Conjugated Bilirubin (via UGT1A1) $\rightarrow$ Excretion into bile.

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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