FMGE 2023 - Biochemistry PYQs Practice Questions

Q: In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

RNA editing. ***RNA editing (Correct Answer)*** - This post-transcription modification involves a specific **cytidine deaminase** enzyme (APOBEC-1) found primarily in the intestine. - This enzyme converts a **CAA codon** (coding for Glutamine) into a **UAA stop codon** within the *ApoB* mRNA, truncating the protein from ApoB-100 to **ApoB-48**. *Gene splicing* - Gene splicing, including **alternative splicing**, involves differential removal of **introns** and joining of **exons** to create various mRNA transcripts from a single gene. - However, gene splicing does not involve the direct **nucleotide change** (C to U) necessary to create the premature stop codon responsible for shortening ApoB. *Alternative polyadenylation* - This process selects different cleavage sites towards the 3' end of the mRNA, influencing the length of the **3' untranslated region** and mRNA stability. - While it affects mRNA processing, it does not involve a **base conversion** that fundamentally alters the coding sequence by introducing a stop codon. *Gene rearrangements* - Gene rearrangements involve physical changes to the **genomic DNA** sequence itself (e.g., V(D)J recombination in immunoglobulins) and are typically irreversible. - The distinction between ApoB-100 and ApoB-48 is purely a **post-transcriptional** event changing the mRNA, not an alteration of the ApoB gene structure. [Practice more Biochemistry PYQs on OnCourse]

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. [Practice more Biochemistry PYQs on OnCourse]

Q: Which of the following immunoglobulins shows the highest concentration in serum?

IgG. ***IgG*** - **IgG** accounts for approximately **75-80%** of the total immunoglobulins in human serum, making it the highest in concentration.- Its high levels reflect its crucial roles in providing **long-term humoral immunity**, neutralizing toxins, and being the only class able to cross the **placenta** to confer passive immunity to the fetus.*IgM* - **IgM** is present at much lower concentrations (about 5-10%); it is primarily known for being the first antibody produced during a **primary immune response**. - Although it is the largest immunoglobulin (a pentamer) and effective at complement activation, its total serum concentration is substantially less than IgG.*IgA* - **IgA** constitutes about 10-15% of serum immunoglobulins, ranking second to IgG in concentration. - While abundant in serum, its primary functional significance lies in its dimeric form (**Secretory IgA**) found in mucosal secretions, protecting epithelial surfaces.*IgE* - **IgE** is the immunoglobulin present at the lowest serum concentration (trace amounts). - It is mainly bound to **mast cells** and **basophils** and is responsible for mediating immediate **Type I hypersensitivity** reactions (allergies). [Practice more Biochemistry PYQs on OnCourse]

Q: Which among the following helps in the maturation of collagen?

Ascorbic acid. ***Ascorbic acid***- Ascorbic acid (Vitamin C) is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes, which hydroxylate proline and lysine residues in procollagen.- This **hydroxylation** is critical for stabilizing the collagen triple helix structure, a necessary step for proper collagen maturation and secretion.*Proline*- **Proline** (along with glycine and lysine) is a major amino acid component of the **collagen triple helix structure**.- While essential for collagen synthesis, proline itself is a substrate, not the **cofactor** required for the crucial hydroxylation steps necessary for maturation stabilization.*Copper and zinc*- **Copper** is a required cofactor for **lysyl oxidase**, the enzyme responsible for creating *covalent cross-links* between collagen molecules, which is a late-stage event for tensile strength.- Although cross-linking is part of the overall maturation process (extracellular), **ascorbic acid's** role in the intracellular hydroxylation of procollagen is a more fundamental step in maturation.*Phenylalanine*- **Phenylalanine** is an essential aromatic amino acid primarily used in protein synthesis and as a precursor for **tyrosine**.- It plays no direct role as a **cofactor** or required structural component in the specialized post-translational modification and subsequent maturation of collagen. [Practice more Biochemistry PYQs on OnCourse]

Q: Which of the following cell pathways occurs exclusively in cell cytoplasm?

Glycolysis. ***Glycolysis***- Glycolysis is the metabolic pathway converting **glucose** to **pyruvate** and occurs entirely within the **cytoplasm** (cytosol) [1].- This pathway is crucial for producing ATP both in the presence (aerobic) and absence (anaerobic) of oxygen [3].*Beta oxidation*- **Beta oxidation**, the breakdown of fatty acids, occurs primarily within the **mitochondrial matrix**.- Long-chain fatty acid activation occurs in the cytosol, but the subsequent oxidative steps are confined to the **mitochondria**.*TCA cycle*- The **TCA cycle** (Krebs cycle) is located exclusively within the **mitochondrial matrix** in eukaryotic cells.- It is responsible for the complete oxidation of acetyl-CoA, producing electron carriers like **NADH** and **FADH2**.*Urea cycle*- The **urea cycle** occurs across **two distinct cellular compartments**: reactions take place in both the **mitochondrial matrix** and the **cytosol** [2].- Specifically, the synthesis of **carbamoyl phosphate** and citrulline occurs in the mitochondria [2]. [Practice more Biochemistry PYQs on OnCourse]

Question 1

In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

Accepted Answer

RNA editing

Answer

Gene splicing

Answer

Alternative polyadenylation

Answer

Gene rearrangements

Question 2

Hemoglobin with iron in ferric form is

Accepted Answer

Methemoglobin

Answer

HbA

Answer

Fetal hemoglobin

Answer

HbS

Question 3

Which of the following immunoglobulins shows the highest concentration in serum?

Accepted Answer

IgG

Answer

IgM

Answer

IgA

Answer

IgE

Question 4

Which among the following helps in the maturation of collagen?

Accepted Answer

Ascorbic acid

Answer

Proline

Answer

Copper and zinc

Answer

Phenylalanine

Question 5

Which of the following cell pathways occurs exclusively in cell cytoplasm?

Accepted Answer

Glycolysis

Answer

TCA cycle

Answer

Urea cycle

Answer

Beta oxidation

Question 6

Nitric oxide is synthesized from which of the following amino acids?

Accepted Answer

Arginine

Answer

Tryptophan

Answer

Lysine

Answer

Tyrosine

Question 7

Identify the type of inhibition shown in the graph.

Accepted Answer

Noncompetitive inhibition

Answer

Competitive inhibition

Answer

Allosteric Inhibition

Answer

Uncompetitive inhibition

Question 8

Which source and cell produce testosterone?

Accepted Answer

Cholesterol and Leydig cells

Answer

Dihydrotestosterone and Leydig cells

Answer

FSH and Leydig cells

Answer

Cholesterol and Sertoli cells

Question 9

A 68-year-old female patient has had a diabetic cataract for 3 months. Accumulation of which of the following substances is responsible for this?

Accepted Answer

Sorbitol + Fructose

Answer

Fructose

Answer

Lactose + Glucose

Answer

Glucose

Question 10

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

FMGE 2023 — Biochemistry