Hemoglobin and Iron Metabolism Practice Questions

Q: Hemoglobin is present in which type of pockets?

Hydrophobic pockets. ### Explanation **1. Why Hydrophobic Pockets are Correct:** The heme group in hemoglobin is situated in a **non-polar, hydrophobic pocket** located between the E and F helices of the globin chain. This environment is crucial for the function of hemoglobin. If the pocket were hydrophilic (aqueous), the ferrous iron ($Fe^{2+}$) in the heme would be easily oxidized to ferric iron ($Fe^{3+}$), forming **methemoglobin**, which cannot bind oxygen. The hydrophobic nature of the pocket excludes water, thereby protecting the iron from oxidation and allowing for the reversible binding of $O_2$. **2. Analysis of Incorrect Options:** * **Option A (Hydrophilic pockets):** As mentioned, a water-friendly environment would lead to the permanent oxidation of iron, rendering the hemoglobin non-functional for oxygen transport. * **Option C (Pyrrole rings):** This is a structural component, not the "pocket" itself. Heme consists of a protoporphyrin IX ring made of four pyrrole rings linked by methenyl bridges. The heme *is* the prosthetic group, while the pocket is the *space* within the globin protein that holds it. * **Option D (Cationic ring):** This is a distractor term. While the iron atom at the center is a cation ($Fe^{2+}$), the surrounding porphyrin ring is an organic planar structure, not a "cationic ring." **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Gating" Mechanism:** The only hydrophilic residues near the heme are two **Histidines**: **Proximal Histidine (F8)**, which binds directly to the iron, and **Distal Histidine (E7)**, which helps stabilize the $O_2$ binding and prevents CO poisoning. * **Methemoglobinemia:** If the hydrophobic pocket is compromised (e.g., in **HbM disease**), iron stays in the $Fe^{3+}$ state, leading to "chocolate-colored blood" and central cyanosis. * **Myoglobin** also utilizes a hydrophobic pocket to store oxygen in muscles.

Q: Hemoglobin does not bind with:

HCN. **Explanation:** The correct answer is **HCN (Hydrogen Cyanide)**. Hemoglobin (Hb) is a globular protein designed to transport gases by binding to the ferrous iron ($Fe^{2+}$) in its heme group or to specific amino acid residues. **1. Why HCN is the correct answer:** Hydrogen cyanide (HCN) does not bind to hemoglobin. Instead, the cyanide ion ($CN^-$) has a high affinity for the **ferric iron ($Fe^{3+}$)** found in **cytochrome oxidase a3** (part of the mitochondrial electron transport chain). By inhibiting this enzyme, cyanide halts cellular respiration, leading to "histotoxic hypoxia." While cyanide can bind to **methemoglobin** (which contains $Fe^{3+}$), it does not bind to normal functional hemoglobin ($Fe^{2+}$). **2. Why the other options are incorrect:** * **Oxygen (A):** Hemoglobin’s primary function is the reversible binding of $O_2$ to the $Fe^{2+}$ of heme to form oxyhemoglobin. * **Carbon dioxide (B):** $CO_2$ binds to the N-terminal amino groups of the globin chains (not the heme) to form **carbaminohemoglobin**. This accounts for about 15-20% of $CO_2$ transport. * **Carbon monoxide (C):** CO binds to the $Fe^{2+}$ of heme with an affinity **200–250 times greater** than oxygen, forming carboxyhemoglobin and causing CO poisoning. **Clinical Pearls for NEET-PG:** * **Cyanide Poisoning Treatment:** We intentionally induce methemoglobinemia (using nitrites) because methemoglobin ($Fe^{3+}$) acts as a "sink" to pull cyanide away from cytochrome oxidase. * **Bohr Effect:** High $CO_2$ and low pH decrease Hb's affinity for $O_2$, shifting the dissociation curve to the **right**. * **2,3-BPG:** An important regulator that stabilizes the "T" (Tense) state of Hb, promoting oxygen unloading.

Q: Along with succinyl CoA, which of the following amino acids serves as a starting material in heme synthesis?

Glycine. ### Explanation **Correct Option: C (Glycine)** Heme synthesis begins in the mitochondria with the condensation of **Succinyl CoA** (from the TCA cycle) and the amino acid **Glycine**. This reaction is catalyzed by the enzyme **ALA Synthase (ALAS)**, which requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. This is the **rate-limiting and committed step** of heme biosynthesis. Together, they form $\delta$-aminolevulinic acid (ALA). Glycine is the only amino acid that provides the carbon and nitrogen atoms required for the porphyrin ring structure. **Why Incorrect Options are Wrong:** * **A (Lysine):** An essential, purely ketogenic amino acid. It is involved in carnitine synthesis and collagen cross-linking but plays no role in the porphyrin pathway. * **B (Leucine):** A branched-chain amino acid (BCAA) and purely ketogenic. It is primarily used for protein synthesis and energy metabolism. * **D (Alanine):** A key glucogenic amino acid involved in the Cahill cycle (glucose-alanine cycle) for nitrogen transport from muscle to liver, but it is not a substrate for heme. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Enzyme:** ALA Synthase (ALAS1 in liver; ALAS2 in erythroid cells). * **Cofactor Alert:** Deficiency of **Vitamin B6** leads to Sideroblastic Anemia because ALA synthase cannot function, causing iron to accumulate in mitochondria (forming ringed sideroblasts). * **Inhibitors:** Heme acts as a feedback inhibitor of ALAS. Lead poisoning inhibits the subsequent enzymes (ALA Dehydratase and Ferrochelatase). * **Location:** Heme synthesis occurs partly in the **mitochondria** (first and last three steps) and partly in the **cytosol**. Memory aid: "The first and the last are in the furnace (mitochondria)."

Q: A 34-year-old female has a history of intermittent episodes of severe abdominal pain. She has had multiple abdominal surgeries and exploratory procedures with no abnormal findings. Her urine appears dark during an attack and gets even darker if exposed to sunlight. The attacks seem to peak after she takes erythromycin, because of her penicillin allergy. This patient most likely has difficulty in synthesizing which one of the following?

Heme. **Explanation:** The clinical presentation describes a classic case of **Acute Intermittent Porphyria (AIP)**. AIP is an autosomal dominant disorder caused by a deficiency of the enzyme **Porphobilinogen (PBG) deaminase**, a key enzyme in the **Heme synthesis** pathway. **Why Heme is the Correct Answer:** 1. **Clinical Triad:** The patient presents with the "3 Ps": **P**ainful abdomen (neurovisceral symptoms leading to unnecessary surgeries), **P**ort-wine colored urine (due to oxidation of PBG to porphobilin), and **P**sychological disturbances (though not mentioned here, common in AIP). 2. **Urine Findings:** PBG and ALA (delta-aminolevulinic acid) accumulate in the urine. Upon exposure to light/air, PBG polymerizes into porphobilin, causing the urine to darken. 3. **Precipitating Factors:** Attacks are triggered by drugs that induce **Cytochrome P450** (like Erythromycin), alcohol, or fasting. These triggers increase the demand for heme, upregulating ALA synthase and leading to the accumulation of toxic intermediates. **Why Other Options are Wrong:** * **Creatine phosphate:** Synthesized from glycine, arginine, and methionine; not related to porphyrin metabolism. * **Cysteine:** A non-essential amino acid synthesized from methionine and serine; its deficiency causes homocystinuria, not abdominal pain or dark urine. * **Thymine:** A pyrimidine base; defects in its synthesis or salvage lead to conditions like Orotic Aciduria (presenting with megaloblastic anemia and growth failure). **NEET-PG High-Yield Pearls:** * **Enzyme Deficient in AIP:** PBG Deaminase (also known as HMB Synthase). * **Key Feature:** AIP is a "non-photosensitive" porphyria (unlike Porphyria Cutanea Tarda) because the accumulation occurs *before* the formation of porphyrin rings. * **Management:** Treatment involves **Hematin/Heme arginate** and **Glucose** infusion, which inhibit ALA synthase (the rate-limiting enzyme) via feedback inhibition.

Q: Iron absorption is decreased in which of the following?

Tea. ***Tea*** - Contains **tannins and polyphenols** that bind to non-heme iron in the gastrointestinal tract - Forms **insoluble iron complexes** that cannot be absorbed - Can reduce iron absorption by **60-70%** when consumed with meals - Most significant dietary inhibitor of iron absorption *Incorrect: Amla* - Indian gooseberry is rich in **vitamin C (ascorbic acid)** - Vitamin C **enhances iron absorption** by reducing ferric (Fe³⁺) to ferrous (Fe²⁺) form - Also prevents formation of insoluble iron compounds *Incorrect: Lemon* - Rich in **vitamin C and citric acid** - Both compounds **enhance iron absorption** significantly - Citric acid chelates iron, keeping it soluble and bioavailable *Incorrect: Sprouting* - **Reduces phytate content** in grains and legumes - Phytates are iron absorption inhibitors - Sprouting therefore **enhances iron bioavailability**

Q: A patient who was in excruciating pain all over his body was taken to the hospital. In recent years, he has experienced these episodes frequently. When exercising vigorously, the pain begins. Anemia was detected on blood examination along with sickled RBCs as opposed to normal biconcave ones. It was determined that he had sickle cell anemia. What substitution takes place in sickle cell anemia?

Substitution of glutamic acid by valine at the 6th position. ***Substitution of glutamic acid by valine at the 6th position***- This is the defining molecular defect in **sickle cell anemia** (HbS), resulting from a point mutation (GAG $\rightarrow$ GTG) in the $\beta$-globin gene.- The replacement of the hydrophilic amino acid **glutamic acid** by the hydrophobic amino acid **valine** at the **6th position** facilitates the polymerization of **deoxy-HbS** under low oxygen tension, causing RBC sickling.*Substitution of valine by glutamic acid at the 6th position*- This change is the **opposite** of the mutation causing HbS; HbS involves the substitution of a hydrophilic residue (Glutamic acid) with a **hydrophobic** one (Valine).- This particular reverse substitution would not lead to the formation of the **deoxy-HbS polymers** responsible for the sickling phenomenon.*Substitution of valine by glutamic acid at the 5th position*- The amino acid at the 5th position of the $\beta$-globin chain is typically **proline**, and the crucial molecular abnormality in HbS involves the **6th position**, not the 5th.- The defining mutation involves Glutamic acid being replaced by Valine, not Valine being replaced by Glutamic acid.*Substitution of glutamic acid by valine at the 5th position*- While the amino acid change (Glutamic acid $\rightarrow$ Valine) is correct for the type of substitution, the characteristic mutation of **sickle cell anemia** occurs specifically at the **6th position**.- Substitutions at different positions (like the 5th) usually result in other, distinct hemoglobin variants.

Q: Which of the following correctly represents the effect of the mutation causing sickle cell anemia?

Glutamate by valine at the 6th position. ***Glutamate by valine at the 6th position***- This mutation involves a single nucleotide substitution (A to T) in the $\beta$-globin gene, resulting in the replacement of hydrophilic **glutamate** with hydrophobic **valine** at the sixth position.- This change (E6V) creates a sticky patch on the **hemoglobin S (HbS)** molecule, leading to polymerization and sickling of red blood cells under conditions of low oxygen tension.*Valine by glutamate at the 6th position*- This represents the reverse substitution: replacing **valine** with **glutamate** at the 6th position.- This reversal of the substitution direction would describe the change from **HbS** back to the normal $\beta$-globin chain structure (**HbA**), not the cause of sickle cell anemia.*Glutamate by valine at the 5th position*- Although the amino acid substitution (**Glutamate** replaced by **Valine**) is correct, the position specified is inaccurate.- The critical substitution causing **sickle cell disease** is precisely at the **6th position** of the $\beta$-globin chain, not the 5th.*Valine by glutamate at the 5th position*- This option fails on two counts: the substitution direction is reversed (Valine $\rightarrow$ Glutamate), and the position of the mutation (**5th**) is incorrect.- The pathogenesis of sickle cell anemia depends on the replacement of the charged hydrophilic amino acid (**glutamate**) with the uncharged hydrophobic amino acid (**valine**) at position 6.

Q: Hemoglobin with iron in ferric form is

Methemoglobin. ***Methemoglobin***- **Methemoglobin** is characterized by the presence of iron in the **ferric state ($\text{Fe}^{3+}$)** in the heme group, which **cannot reversibly bind oxygen**, unlike the ferrous form. - This oxidized form of hemoglobin cannot function as an **oxygen carrier** and its accumulation leads to clinical conditions like **methemoglobinemia**. *HbA*- **HbA** (Adult hemoglobin) is the predominant form of hemoglobin in adults, and its iron must be in the **ferrous state ($\text{Fe}^{2+}$)** to bind and transport oxygen effectively.- The conversion of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ in normal HbA is constantly prevented by the **methemoglobin reductase system** (especially **NADH-cytochrome $b_5$ reductase**). *Fetal hemoglobin*- **Fetal hemoglobin (HbF)**, like HbA, uses iron in the **ferrous state ($\text{Fe}^{2+}$)** for reversible oxygen binding, reflecting its normal physiological function in the fetus. - HbF's unique feature is its structure ($\text{alpha}_2\text{gamma}_2$) which confers a **higher affinity for oxygen** by binding **2,3-BPG** less avidly than HbA, but this does not involve ferric iron. *HbS*- **HbS** (Sickle hemoglobin) is defective due to a mutation in the **beta-globin chain** (E6V), but the iron in its heme group remains in the **functional ferrous state ($\text{Fe}^{2+}$)**.- The abnormality in HbS relates to its **polymerization** upon deoxygenation, causing red blood cell sickling, not an oxidation state change of the heme iron.

Q: The shown pattern in electrophoresis is due to:

Charges. ***Charges*** - Electrophoresis separates molecules, such as **hemoglobin variants**, based on their **electrical charge** when placed in an electric field. - Different amino acid substitutions in hemoglobin lead to changes in net charge, causing them to migrate at different rates in the gel, as seen by the distinct bands for Hb A, Hb S/D, and Hb A2. *Molecular weight* - While molecular weight can influence migration in some electrophoretic techniques (e.g., SDS-PAGE), standard hemoglobin electrophoresis primarily separates based on **charge differences**, not molecular size. - All common hemoglobin variants have a very similar **molecular weight** (approximately 64,500 Da), so this factor would not effectively separate them into distinct bands. *Bound oxygen* - The amount of **bound oxygen** to hemoglobin does not significantly alter its overall electrical charge or molecular weight to cause distinct bands in electrophoresis. - Oxygen binding is a dynamic process and does not account for the **structural differences** in hemoglobin variants that lead to their electrophoretic separation. *Hydrophobicity* - Hydrophobicity is a characteristic of molecules, but it is not the primary principle by which standard **gel electrophoresis** separates hemoglobin variants. - Techniques like **hydrophobic interaction chromatography** would exploit hydrophobicity, not the electrophoretic gel shown.

Question 1

Hemoglobin is present in which type of pockets?

Accepted Answer

Hydrophobic pockets

Answer

Hydrophilic pockets

Answer

Pyrrole rings

Answer

Cationic ring

Question 2

Hemoglobin does not bind with:

Accepted Answer

HCN

Answer

Oxygen

Answer

Carbon dioxide

Answer

Carbon monoxide

Question 3

The heme portion of the hemoglobin molecule consists of:

Accepted Answer

Four porphyrin rings, each containing a molecule of Fe in the center

Answer

Porphyrin ring with a molecule of Fe in the center

Answer

A polypeptide chain containing Fe

Answer

A pyrole ring with four molecules of Fe in the center

Question 4

Along with succinyl CoA, which of the following amino acids serves as a starting material in heme synthesis?

Accepted Answer

Glycine

Answer

Lysine

Answer

Leucine

Answer

Alanine

Question 5

A 34-year-old female has a history of intermittent episodes of severe abdominal pain. She has had multiple abdominal surgeries and exploratory procedures with no abnormal findings. Her urine appears dark during an attack and gets even darker if exposed to sunlight. The attacks seem to peak after she takes erythromycin, because of her penicillin allergy. This patient most likely has difficulty in synthesizing which one of the following?

Accepted Answer

Heme

Answer

Creatine phosphate

Answer

Cysteine

Answer

Thymine

Question 6

Iron absorption is decreased in which of the following?

Accepted Answer

Tea

Answer

Amla

Answer

Lemon

Answer

Sprouting

Question 7

A patient who was in excruciating pain all over his body was taken to the hospital. In recent years, he has experienced these episodes frequently. When exercising vigorously, the pain begins. Anemia was detected on blood examination along with sickled RBCs as opposed to normal biconcave ones. It was determined that he had sickle cell anemia. What substitution takes place in sickle cell anemia?

Accepted Answer

Substitution of glutamic acid by valine at the 6th position

Answer

Substitution of valine by glutamic acid at the 5th position

Answer

Substitution of glutamic acid by valine at the 5th position

Answer

Substitution of valine by glutamic acid at the 6th position

Question 8

Which of the following correctly represents the effect of the mutation causing sickle cell anemia?

Accepted Answer

Glutamate by valine at the 6th position

Answer

Glutamate by valine at the 5th position

Answer

Valine by glutamate at the 5th position

Answer

Valine by glutamate at the 6th position

Question 9

Hemoglobin with iron in ferric form is

Accepted Answer

Methemoglobin

Answer

HbA

Answer

Fetal hemoglobin

Answer

HbS

Question 10

The shown pattern in electrophoresis is due to:

Accepted Answer

Charges

Answer

Molecular weight

Answer

Bound oxygen

Answer

Hydrophobicity

Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?