Hemoglobin and Iron Metabolism Practice Questions

Q: Which of the following is not a haemoprotein?

Albumin. **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.

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

Decreased mobility. ### 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.

Q: 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?

ALA dehydratase. **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).

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

Uroporphyrinogen decarboxylase. **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).

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

2 alpha, 2 beta. ### 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).

Q: Iron is transported bound to:

Transferrin. **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 1

Which of the following is not a haemoprotein?

Accepted Answer

Albumin

Answer

Myoglobin

Answer

Cytochrome P450

Answer

Cytochrome C

Question 2

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

Accepted Answer

Decreased mobility

Answer

Increased mobility

Answer

No change in mobility

Answer

Mobility depends on the concentration of HbS

Question 3

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?

Accepted Answer

ALA dehydratase

Answer

ALA synthase

Answer

Heme oxygenase

Answer

CPG oxidase

Question 4

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

Accepted Answer

Uroporphyrinogen decarboxylase

Answer

Ferrochelatase

Answer

Protoporphyrinogen oxidase

Answer

Coproporphyrinogen oxidase

Question 5

Bilirubin is absent in urine because it is:

Accepted Answer

Lipophilic

Answer

Distributed in body fat

Answer

Conjugated with glucuronide

Answer

Not filterable by the glomerulus

Question 6

The polypeptide chains of hemoglobin A are composed of which subunits?

Accepted Answer

2 alpha, 2 beta

Answer

1 alpha, 3 beta

Answer

2 alpha

Answer

1 alpha, 2 beta, 1 delta

Question 7

One gram of hemoglobin liberates how many milligrams of bilirubin?

Accepted Answer

34

Answer

40

Answer

15

Answer

55

Question 8

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

Accepted Answer

Decarboxylation

Answer

Deamination

Answer

Hydrogenation

Answer

Dehydrogenation

Question 9

Iron is transported bound to:

Accepted Answer

Transferrin

Answer

Ferritin

Answer

Hemosiderin

Answer

Hemoglobin

Question 10

Unconjugated hyperbilirubinemia is seen in all EXCEPT?

Accepted Answer

Dubin-Johnson syndrome

Answer

Gilbert's syndrome

Answer

Crigler-Najjar syndrome

Answer

Neonatal physiological jaundice

Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism — MCQs

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