Hemoglobin and Iron Metabolism — MCQs

Hemoglobin and Iron Metabolism Indian Medical PG Practice Questions and MCQs

Question 121: What is the rate-limiting enzyme of heme synthesis in erythroid tissue?

A. PBG deaminase
B. Uroporphyrinogen III synthase
C. ALA synthase I
D. ALA synthase II (Correct Answer)

Explanation: ### Explanation The rate-limiting step of heme synthesis is the condensation of glycine and succinyl-CoA to form **$\delta$-aminolevulinic acid (ALA)**. This reaction is catalyzed by the enzyme **ALA Synthase (ALAS)**, which exists in two distinct isoforms: 1. **ALA Synthase II (ALAS2):** This isoform is specific to **erythroid tissue** (bone marrow). Its activity is primarily regulated by the availability of **intracellular iron**. Since the question specifies erythroid tissue, ALAS2 is the correct answer. 2. **ALA Synthase I (ALAS1):** This isoform is found in the **liver** and other non-erythroid tissues. It is regulated via feedback inhibition by **hemin** (the end product). #### Analysis of Incorrect Options: * **ALA Synthase I (Option C):** While it is the rate-limiting enzyme for heme synthesis in the **liver**, it is not the primary isoform in erythroid cells. * **PBG Deaminase (Option A):** This enzyme (also known as HMB synthase) catalyzes the third step of heme synthesis. A deficiency here leads to **Acute Intermittent Porphyria (AIP)**, but it is not the rate-limiting step. * **Uroporphyrinogen III Synthase (Option B):** This enzyme catalyzes the fourth step. Its deficiency causes **Congenital Erythropoietic Porphyria (Gunther’s disease)**. #### High-Yield Clinical Pearls for NEET-PG: * **Cofactor:** Both ALAS1 and ALAS2 require **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. Sideroblastic anemia can occur if B6 is deficient or inhibited (e.g., by Isoniazid). * **Location:** The ALAS reaction occurs in the **mitochondria**, whereas the subsequent steps (until Ferrochelatase) occur in the cytosol. * **Genetics:** Mutations in the *ALAS2* gene are the most common cause of **X-linked Sideroblastic Anemia**. * **Regulation:** Unlike ALAS1, ALAS2 is **not** inhibited by hemin; this allows erythroid cells to produce the massive amounts of hemoglobin required for RBC maturation.

Question 122: Which property of hemoglobin is affected in sickle cell anemia?

A. Stability
B. Function
C. Affinity
D. Solubility (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Solubility** Sickle cell anemia (SCA) is caused by a point mutation in the $\beta$-globin gene, where **glutamic acid** (polar/hydrophilic) is replaced by **valine** (non-polar/hydrophobic) at the **6th position**. * **The Mechanism:** In the deoxygenated state (T-state), this hydrophobic valine residue is exposed on the surface of the hemoglobin molecule (HbS). To minimize contact with water, these hydrophobic patches interact with complementary sites on adjacent HbS molecules. This leads to the **polymerization** of hemoglobin into long, insoluble fibers. * **The Result:** These fibers distort the red blood cell into a "sickle" shape. Therefore, the primary biochemical defect is a **marked decrease in the solubility of deoxygenated hemoglobin.** **Why other options are incorrect:** * **A. Stability:** While the RBC membrane becomes fragile, the HbS molecule itself is relatively stable compared to unstable hemoglobins (like Hb Koln) which precipitate as Heinz bodies. * **B. Function:** HbS can still bind and release oxygen; the pathology arises from the physical state of the molecule after oxygen release, not its inherent ability to function as a gas transporter. * **C. Affinity:** While the P50 of sickle cells may be slightly shifted due to 2,3-BPG levels, the primary defect defining the disease is solubility, not a change in oxygen affinity. **High-Yield Clinical Pearls for NEET-PG:** * **Mutation:** $\beta^6 \text{ Glu} \rightarrow \text{Val}$ (GAG to GTG). * **Electrophoresis:** On alkaline electrophoresis (pH 8.6), HbS moves **slower** than HbA toward the anode because it has lost two negative charges (one per $\beta$ chain). * **Protective Factor:** HbF (Fetal Hemoglobin) inhibits polymerization, which is why symptoms appear only after 6 months of age and why **Hydroxyurea** (which increases HbF) is used in treatment. * **T-state vs. R-state:** Sickling occurs exclusively in the **deoxygenated (T) state**. Factors like acidosis, dehydration, and increased 2,3-BPG promote sickling.

Question 123: Which hemoglobin appears first in the fetus?

A. Hb A
B. Hb A2
C. Hb F
D. Hb Gowers (Correct Answer)

Explanation: **Explanation:** The synthesis of hemoglobin undergoes a sequential transition during embryonic, fetal, and neonatal life, reflecting the changing sites of erythropoiesis. **1. Why Hb Gowers is Correct:** Hemoglobin synthesis begins in the **yolk sac** during the first few weeks of gestation (mesoblastic stage). The very first hemoglobins to appear are the **embryonic hemoglobins**, which include **Hb Gower-1** ($\zeta_2\epsilon_2$), **Hb Gower-2** ($\alpha_2\epsilon_2$), and **Hb Portland** ($\zeta_2\gamma_2$). Among these, Hb Gower-1 is typically the earliest to be detected. By the end of the first trimester (around 10–12 weeks), as erythropoiesis shifts to the liver, these embryonic forms are replaced by fetal hemoglobin (HbF). **2. Why the other options are incorrect:** * **Hb F ($\alpha_2\gamma_2$):** Known as fetal hemoglobin, it is the predominant hemoglobin from the 12th week of gestation until birth. While it is the "major" hemoglobin of the fetus, it is **not the first** to appear. * **Hb A ($\alpha_2\beta_2$):** This is adult hemoglobin. Synthesis begins in small amounts around the 20th week of gestation but only becomes the dominant form approximately 6 months after birth. * **Hb A2 ($\alpha_2\delta_2$):** A minor adult hemoglobin. It appears in very small quantities late in fetal life and normally comprises <3.5% of total hemoglobin in adults. **High-Yield Clinical Pearls for NEET-PG:** * **Globin Chain Switch:** The transition follows the order: $\zeta \rightarrow \alpha$ (alpha-like) and $\epsilon \rightarrow \gamma \rightarrow \beta$ (beta-like). * **Site of Erythropoiesis:** Yolk sac (3–8 weeks) $\rightarrow$ Liver (6–30 weeks) $\rightarrow$ Bone Marrow (from 20 weeks onwards). * **HbF Affinity:** HbF has a higher affinity for oxygen than HbA because it binds **2,3-BPG** less strongly, facilitating oxygen transfer from mother to fetus across the placenta.

Question 124: Which of the following molecules has a strong affinity for hemoglobin and acts in the blood as a "mop-up" molecule to bind free hemoglobin?

A. Ferritin
B. Transferrin
C. Haptoglobin (Correct Answer)
D. Albumin

Explanation: **Explanation:** **Haptoglobin** is an acute-phase reactant protein synthesized by the liver. Its primary physiological role is to bind free hemoglobin (Hb) released into the plasma during intravascular hemolysis. This binding forms a large **Haptoglobin-Hemoglobin (Hp-Hb) complex**, which is too large to be filtered by the renal glomeruli, thereby preventing iron loss through urine and protecting the kidneys from hemoglobin-induced oxidative damage (tubular necrosis). The complex is rapidly cleared from circulation by the reticuloendothelial system (specifically via CD163 receptors on macrophages). **Analysis of Incorrect Options:** * **Ferritin:** This is the primary **intracellular storage form** of iron, found mainly in the liver and spleen. While small amounts circulate in the blood (reflecting total body iron stores), it does not bind free hemoglobin. * **Transferrin:** This is the primary **transport protein for iron** ($Fe^{3+}$) in the plasma. It delivers iron to bone marrow and other tissues but does not interact with intact hemoglobin molecules. * **Albumin:** While it is a versatile transport protein, it does not bind free hemoglobin. However, it can bind **Heme** (forming methemalbumin) once haptoglobin is saturated. **High-Yield Clinical Pearls for NEET-PG:** * **Hemolysis Marker:** A **decreased serum haptoglobin level** is a highly specific marker for **intravascular hemolysis** (e.g., G6PD deficiency, HUS, or AIHA), as the protein is consumed faster than the liver can synthesize it. * **Hemopexin:** If haptoglobin is depleted, **Hemopexin** acts as a secondary backup to bind free Heme. * **Acute Phase Reactant:** Since haptoglobin levels rise during inflammation, a "normal" level in an inflammatory state might mask underlying hemolysis.

Question 125: The four pyrrole rings in a hemoglobin molecule are joined together by which type of bond?

A. Disulfide bridges
B. Methylene bridges (Correct Answer)
C. Hydrogen bonds
D. Alpha bonds

Explanation: **Explanation:** The structure of heme, the prosthetic group of hemoglobin, consists of a **protoporphyrin IX** ring coordinated with a central ferrous iron ($Fe^{2+}$) atom. The protoporphyrin ring is composed of four pyrrole rings (labeled A, B, C, and D). These pyrrole rings are linked together by **methenyl (methine) bridges** ($–CH=$). In the biosynthetic pathway of heme, these bridges originate from the alpha-carbon of glycine and are initially formed as **methylene bridges** ($–CH_2–$) before oxidation. Therefore, the structural integrity of the porphyrin macrocycle relies on these carbon-based bridges. **Analysis of Incorrect Options:** * **A. Disulfide bridges:** These are covalent bonds between sulfur atoms of cysteine residues, primarily responsible for stabilizing the tertiary and quaternary structures of proteins (like the insulin molecule), not the porphyrin ring. * **C. Hydrogen bonds:** These are weak non-covalent interactions. While they stabilize the alpha-helices of the globin chains and the binding of oxygen to iron, they do not link the pyrrole rings. * **D. Alpha bonds:** This is a non-specific term in this context. While "alpha-methenyl" is sometimes used, "alpha bond" is not a recognized chemical linkage in porphyrin chemistry. **NEET-PG High-Yield Pearls:** * **Heme Synthesis:** Occurs partly in the mitochondria (first and last three steps) and partly in the cytosol. * **Rate-limiting step:** Catalyzed by **ALA Synthase**, which requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Lead Poisoning:** Inhibits ALA Dehydratase and Ferrochelatase, leading to increased ALA levels and protoporphyrin. * **Porphyrin Color:** The conjugated system of double bonds in the methenyl bridges is responsible for the characteristic red color of hemoglobin.

Question 126: When does the switchover from fetal to adult hemoglobin synthesis begin?

A. 14 weeks gestation
B. 30 weeks gestation
C. 36 weeks gestation (Correct Answer)
D. 7-10 days postnatal

Explanation: ### Explanation **Concept:** The transition from fetal hemoglobin (HbF, $\alpha_2\gamma_2$) to adult hemoglobin (HbA, $\alpha_2\beta_2$) is a genetically programmed process known as **hemoglobin switching**. While HbF is the predominant hemoglobin during intrauterine life due to its high oxygen affinity, the synthesis of the $\beta$-globin chain begins to increase while $\gamma$-globin synthesis declines late in the third trimester. **Why 36 weeks gestation is correct:** The definitive "switchover" or the significant crossover point where HbA synthesis begins to accelerate and HbF synthesis sharply declines occurs at approximately **36 weeks of gestation**. By the time of birth, a full-term neonate typically has about 60–80% HbF and 20–40% HbA. **Analysis of Incorrect Options:** * **A. 14 weeks gestation:** At this stage, the liver is the primary site of hematopoiesis, and HbF is the dominant hemoglobin. $\beta$-chain synthesis is negligible. * **B. 30 weeks gestation:** While trace amounts of $\beta$-globin are detectable, the major transition has not yet gained momentum. * **D. 7-10 days postnatal:** This is too late. The switch begins *in utero*. However, the process continues after birth, with HbA becoming the dominant hemoglobin ( >95%) by 6–12 months of age. **High-Yield Clinical Pearls for NEET-PG:** * **HbF Composition:** $\alpha_2\gamma_2$. It does not bind 2,3-BPG effectively, resulting in a **left-shift** of the oxygen dissociation curve (higher $O_2$ affinity). * **Site of Synthesis:** The switch from HbF to HbA coincides with the shift of hematopoiesis from the **liver to the bone marrow**. * **Clinical Correlation:** Conditions like $\beta$-Thalassemia and Sickle Cell Anemia do not manifest at birth because HbF levels are still high; symptoms appear only after 6 months when the HbF-to-HbA switch is nearly complete. * **Inducing HbF:** Hydroxyurea is used in Sickle Cell Anemia to pharmacologically increase HbF levels, which inhibits the polymerization of HbS.

Question 127: Barbiturates precipitate symptoms of porphyria because:

A. they induce ALA synthase (Correct Answer)
B. they inhibit ALA synthase
C. they induce heme oxygenase
D. they inhibit heme oxygenase

Explanation: ### Explanation **Why Option A is Correct:** The rate-limiting step of heme synthesis is catalyzed by the enzyme **ALA Synthase (ALAS1)**. Barbiturates are potent inducers of the **Cytochrome P450 (CYP450)** system in the liver. Since CYP450 is a hemeprotein, its increased production consumes the intracellular "free heme pool." Heme normally exerts **negative feedback inhibition** on ALA Synthase. When the heme pool is depleted (due to its utilization for CYP450), this feedback inhibition is lifted, leading to the induction of ALA Synthase. In patients with underlying porphyrias (like Acute Intermittent Porphyria), this induction causes a massive overproduction of toxic porphyrin precursors (ALA and PBG), precipitating an acute attack. **Why Other Options are Incorrect:** * **Option B:** Inhibiting ALA synthase would actually decrease the production of porphyrin precursors and is the goal of treatment (e.g., using Hemin), not the cause of an attack. * **Options C & D:** Heme oxygenase is the rate-limiting enzyme in **heme degradation** (converting heme to biliverdin). While its induction might decrease heme levels, the primary mechanism of barbiturate-induced porphyria is specifically linked to the demand for CYP450 and the subsequent induction of ALA Synthase. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** ALA Synthase (requires **Pyridoxal Phosphate/Vitamin B6** as a cofactor). * **Acute Intermittent Porphyria (AIP):** Most common acute porphyria; characterized by the "5 Ps": **P**ainful abdomen, **P**ort-wine urine, **P**olyneuropathy, **P**sychological disturbances, and **P**recipitated by drugs (Barbiturates, Sulfonamides, Phenytoin). * **Treatment of Acute Attack:** Intravenous **Hemin** or Glucose (both inhibit ALA Synthase). * **Enzyme Deficient in AIP:** Porphobilinogen (PBG) Deaminase.

Question 128: One hemoglobin molecule can carry how many oxygen molecules?

A. 1
B. 2
C. 3
D. 4 (Correct Answer)

Explanation: ### Explanation **Correct Answer: D (4)** **The Underlying Concept:** Hemoglobin (Hb) is a tetrameric protein composed of four polypeptide subunits (in adults, typically two alpha and two beta chains). Each subunit contains one **heme group**, and at the center of each heme group is an **iron atom in the ferrous state ($Fe^{2+}$)**. Since one molecule of oxygen ($O_2$) binds reversibly to one ferrous iron atom, a single hemoglobin molecule—possessing four heme groups—can bind a total of **four oxygen molecules** (or eight oxygen atoms). **Why the other options are incorrect:** * **Option A (1):** This describes the oxygen-binding capacity of **Myoglobin**, which is a monomer containing only one heme group. * **Options B & C (2 & 3):** These represent intermediate stages of oxygenation. Due to **cooperative binding** (the "Haldane effect" in reverse/sigmoidal kinetics), once the first $O_2$ binds, the affinity for subsequent molecules increases, rapidly progressing until all four sites are occupied. **NEET-PG High-Yield Pearls:** * **State of Iron:** Iron must be in the **Ferrous ($Fe^{2+}$)** state to bind oxygen. If oxidized to the **Ferric ($Fe^{3+}$)** state, it forms **Methemoglobin**, which cannot bind $O_2$. * **Binding Curve:** Hemoglobin exhibits a **sigmoidal** (S-shaped) oxygen dissociation curve due to cooperativity, whereas myoglobin exhibits a **hyperbolic** curve. * **1 gram of Hb** can carry approximately **1.34 ml** of oxygen (Hüfner's constant). * **Allosteric Effectors:** 2,3-BPG, $H^+$ (low pH), and $CO_2$ decrease Hb's affinity for oxygen, shifting the curve to the **right** (facilitating unloading at tissues).

Question 129: What is the rate-limiting step in porphyrin synthesis?

A. ALA dehydratase
B. ALA synthase (Correct Answer)
C. UPG decarboxylase
D. Ferrochelatase

Explanation: **Explanation:** The synthesis of heme (porphyrin) begins in the mitochondria. The **correct answer is B: ALA synthase (ALAS)**. This enzyme catalyzes the condensation of Succinyl CoA and Glycine to form $\delta$-aminolevulinic acid (ALA). It is the **committed and rate-limiting step** because it is strictly regulated by the end-product, heme, via feedback inhibition and repression of enzyme synthesis. **Analysis of Options:** * **ALA dehydratase (Option A):** Also known as Porphobilinogen synthase, it catalyzes the second step (ALA to Porphobilinogen). While important, it is not the primary rate-limiting enzyme. It is, however, highly sensitive to **lead poisoning**. * **UPG decarboxylase (Option C):** This enzyme converts Uroporphyrinogen III to Coproporphyrinogen III. Deficiency of this enzyme leads to **Porphyria Cutanea Tarda**, the most common porphyria. * **Ferrochelatase (Option D):** This is the final mitochondrial enzyme that inserts ferrous iron ($Fe^{2+}$) into Protoporphyrin IX to form Heme. Like ALA dehydratase, it is inhibited by lead. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cofactor:** ALA synthase requires **Pyridoxal Phosphate (Vitamin B6)**. B6 deficiency can lead to Sideroblastic Anemia. 2. **Isoforms:** There are two forms: **ALAS1** (liver, regulated by heme) and **ALAS2** (erythroid precursors, regulated by iron). 3. **Inducers:** Drugs like Barbiturates and Griseofulvin induce ALAS1 by decreasing the heme pool (via Cytochrome P450 induction), which can precipitate an attack of Acute Intermittent Porphyria (AIP). 4. **Lead Poisoning:** Specifically inhibits ALA dehydratase and Ferrochelatase, leading to elevated ALA levels in urine.

Question 130: Which of the following statements about Hemoglobin S (HbS) is not true?

A. Hemoglobin HbS differs from hemoglobin HbA by the substitution of Val for Glu in position 6 of the beta chain.
B. One altered peptide of HbS migrates faster towards the cathode (-) than the corresponding peptide of HbA.
C. Binding of HbS to the deoxygenated HbA can extend the polymer and cause sickling of the red blood cells. (Correct Answer)
D. Lowering the concentration of deoxygenated HbS can prevent sickling.

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The "Not True" Statement)** Sickle cell polymerization is a process unique to **deoxygenated HbS**. While HbS molecules readily polymerize with each other, **HbA acts as a "polymer chain breaker."** HbA does not have the necessary hydrophobic "sticky patch" to facilitate long-chain polymerization. Therefore, the presence of HbA actually **inhibits** the sickling process. This is why individuals with Sickle Cell Trait (HbAS) are generally asymptomatic—the HbA interferes with the polymerization of HbS. **2. Analysis of Incorrect Options (True Statements)** * **Option A:** This is the classic molecular definition of HbS. A point mutation (GAG → GTG) results in the substitution of **Glutamate (polar) with Valine (non-polar)** at the 6th position of the $\beta$-globin chain. * **Option B:** Glutamate is negatively charged, while Valine is neutral. Losing a negative charge makes HbS **more positive** than HbA. On electrophoresis (pH 8.6), HbS moves slower toward the anode (+) and, by extension, would migrate faster toward the **cathode (-)** compared to HbA. * **Option D:** Sickling is a concentration-dependent phenomenon. Reducing the concentration of deoxy-HbS (either by increasing oxygenation or by increasing the concentration of other hemoglobins like HbF) prevents the formation of the rigid tactical polymers that cause RBC distortion. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **HbF Effect:** HbF (Fetal Hemoglobin) is the most potent inhibitor of HbS polymerization. This is the rationale behind using **Hydroxyurea**, which increases HbF levels. * **Electrophoresis Mnemonic:** On alkaline electrophoresis, the speed of migration from Anode (+) to Cathode (-) follows the order: **A** Fat **S**anta **C**laus (**A** > **F** > **S** > **C**). * **Sickling Trigger:** Factors that shift the oxygen dissociation curve to the **right** (increased 2,3-BPG, low pH/acidosis, high $CO_2$, and hypoxia) promote the "T" (Tense) state of hemoglobin, which triggers sickling.

Practice by Chapter

Hemoglobin Structure and Function

Practice Questions

Oxygen Transport and Oxygen-Hemoglobin Dissociation Curve

Practice Questions

Hemoglobin Variants and Hemoglobinopathies

Practice Questions

Thalassemias

Practice Questions

Methemoglobin and Abnormal Hemoglobins

Practice Questions

Hemoglobin Synthesis

Practice Questions

Heme Synthesis and Porphyrias

Practice Questions

Iron Absorption and Transport

Practice Questions

Iron Storage and Recycling

Practice Questions

Disorders of Iron Metabolism

Practice Questions

Anemia: Biochemical Aspects

Practice Questions

Biochemistry of Hemostasis

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free