Post-Translational Modifications Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Post-Translational Modifications. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Post-Translational Modifications Indian Medical PG Question 1: Which of the following statements regarding collagen synthesis is incorrect?
- A. Hydroxylation of lysine occurs in ER
- B. Synthesized in ribosomes as preprocollagen
- C. Triple helix assembly occurs in ER
- D. Hydroxylation of proline occurs in Golgi apparatus (Correct Answer)
Post-Translational Modifications Explanation: ***Hydroxylation of proline occurs in Golgi apparatus***
- This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus.
- This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**.
*Synthesized in ribosomes as preprocollagen*
- This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide.
- The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**.
*Hydroxylation of lysine occurs in ER*
- This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine.
- This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule.
*Triple helix assembly occurs in ER*
- This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**.
- This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.
Post-Translational Modifications Indian Medical PG Question 2: Ubiquitin is involved in what process?
- A. Protein folding
- B. Protein degradation (Correct Answer)
- C. Synthesis of nucleic acid
- D. Glycosylation of proteins
Post-Translational Modifications Explanation: ***Protein degradation***
- **Ubiquitin** is a small regulatory protein that attaches to other proteins as a signal, primarily for their **degradation** by the **proteasome**.
- This process, known as **ubiquitination**, marks misfolded, damaged, or no longer needed proteins for targeted destruction.
*Protein folding*
- This process is primarily mediated by **chaperone proteins**, which assist in the correct three-dimensional structuring of polypeptides.
- While ubiquitin can sometimes influence protein folding indirectly by marking misfolded proteins for degradation, its direct role is not in the folding itself.
*Synthesis of nucleic acid*
- The synthesis of **nucleic acids** (DNA and RNA) is carried out by **DNA polymerases** and **RNA polymerases**, respectively.
- Ubiquitin is not involved in the enzymatic processes of replication or transcription.
*Glycosylation of proteins*
- **Glycosylation** is the enzymatic addition of carbohydrate moieties to proteins, typically occurring in the **endoplasmic reticulum** and **Golgi apparatus**.
- This process is crucial for protein function, trafficking, and cell-cell recognition, but ubiquitin has no direct role in it.
Post-Translational Modifications Indian Medical PG Question 3: Carboxylation of clotting factors by vitamin K is required for them to be biologically active. Which amino acid is carboxylated?
- A. Histidine
- B. Serine
- C. Glutamate (Correct Answer)
- D. Aspartate
Post-Translational Modifications Explanation: ***Glutamate***
- **Vitamin K** acts as a cofactor for the enzyme **gamma-glutamyl carboxylase**, which carboxylates specific **glutamate residues** in clotting factors (II, VII, IX, and X).
- This carboxylation forms **gamma-carboxyglutamate (Gla)** residues, which allows the clotting factors to bind **calcium ions** and thereby to phospholipid surfaces, enabling the coagulation cascade.
*Histidine*
- **Histidine** is an amino acid that plays a role in enzyme active sites and metal ion chelation, but it is not the target for **vitamin K-dependent carboxylation**.
- Its imidazole ring can donate and accept protons, giving it a role in **pH buffering** and catalytic mechanisms.
*Serine*
- **Serine** is an amino acid that undergoes post-translational modifications such as **phosphorylation** (by kinases) and **O-glycosylation**.
- However, it is not the amino acid specifically targeted by **vitamin K-dependent carboxylation** for the activation of clotting factors.
*Aspartate*
- **Aspartate** is an acidic amino acid that can bind **metal ions** and participates in various metabolic pathways and enzyme active sites.
- While structurally similar to glutamate, it is not the amino acid specifically targeted by **vitamin K-dependent carboxylation** for the activation of clotting factors.
Post-Translational Modifications Indian Medical PG Question 4: What is the major site of protein glycosylation?
- A. Ribosome and Golgi body
- B. ER and Ribosome
- C. Ribosome and Cytoplasm
- D. ER and Golgi body (Correct Answer)
Post-Translational Modifications Explanation: ***ER and Golgi body***
- The **endoplasmic reticulum (ER)** is the primary site for **N-linked glycosylation**, where carbohydrates are added to the asparagine residues of nascent proteins.
- The **Golgi apparatus** is crucial for further modification and processing of these N-linked glycans, as well as the site for **O-linked glycosylation**, where sugars are added to serine or threonine residues.
*Ribosome and Golgi body*
- **Ribosomes** are responsible for **protein synthesis (translation)** but do not directly perform glycosylation, which is a post-translational modification.
- While the **Golgi body** is a site of glycosylation, the ribosome's inclusion makes this option incorrect as the ribosome's role precedes glycosylation.
*ER and Ribosome*
- The **ER** is a major site of protein glycosylation, especially N-linked glycosylation.
- However, **ribosomes** are involved in protein synthesis and lack the enzymatic machinery for adding sugar moieties to proteins.
*Ribosome and Cytoplasm*
- **Ribosomes** synthesize proteins, but glycosylation does not occur there.
- The **cytoplasm** is the site for many metabolic pathways, but major protein glycosylation events mostly occur within the ER and Golgi.
Post-Translational Modifications Indian Medical PG Question 5: Which of the following is not a characteristic feature of Alzheimer's disease?
- A. Neurofibrillary tangles
- B. Senile (neuritic) plaques
- C. Amyloid Angiopathy
- D. Lewy bodies (Correct Answer)
Post-Translational Modifications Explanation: ***Correct Answer: Lewy bodies***
- **Lewy bodies** are abnormal aggregates of protein, primarily **alpha-synuclein**, found in the brains of individuals with **Parkinson's disease** and **Lewy body dementia**, but **not typically in Alzheimer's disease** [1].
- While there can be overlap in pathologies, the presence of Lewy bodies as a primary feature suggests a different neurodegenerative process than Alzheimer's.
- This is the correct answer because it is **NOT a characteristic feature** of Alzheimer's disease.
*Incorrect: Neurofibrillary tangles*
- **Neurofibrillary tangles** are intracellular aggregates of hyperphosphorylated **tau protein** and are a **hallmark pathological feature** of Alzheimer's disease.
- These tangles disrupt neuronal function and axonal transport, leading to neuronal death.
- This IS a characteristic feature of Alzheimer's disease.
*Incorrect: Senile (neuritic) plaques*
- **Senile plaques**, also known as **neuritic plaques**, are extracellular deposits of **beta-amyloid protein** that are a **characteristic pathological feature** of Alzheimer's disease.
- These plaques accumulate outside neurons, leading to inflammation and neuronal dysfunction.
- This IS a characteristic feature of Alzheimer's disease.
*Incorrect: Amyloid Angiopathy*
- **Cerebral amyloid angiopathy** (CAA) is the deposition of **amyloid-beta protein** in the walls of small and medium-sized blood vessels in the central nervous system, particularly the cerebral cortex and leptomeninges.
- It is present in **over 90% of Alzheimer's disease cases** and is a significant contributor to cognitive decline and hemorrhagic events in these patients.
- This IS a characteristic feature of Alzheimer's disease.
**References:**
[1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1296-1298.
Post-Translational Modifications Indian Medical PG Question 6: Which of the following is considered a poor prognostic marker in multiple myeloma (MM)?
- A. Serum Creatinine
- B. Hypercalcemia
- C. B2 microglobulins (Correct Answer)
- D. Telomerase
Post-Translational Modifications Explanation: ***B2 microglobulins***
- Elevated levels of **B2 microglobulin** are a significant indicator of increased tumor burden and are used in the **International Staging System (ISS)** for multiple myeloma, correlating with shorter survival [1].
- This protein is present on the surface of most nucleated cells and its accumulation reflects **renal impairment** or increased cell turnover.
*Serum Creatinine*
- While elevated **serum creatinine** can indicate **renal insufficiency**, a common complication in MM, it is not a direct measure of tumor burden or aggression in the same way as B2 microglobulin [1].
- **Renal failure** can be a poor prognostic factor in MM, but creatinine itself is a marker of organ damage rather than disease progression or malignancy.
*Hypercalcemia*
- **Hypercalcemia** is a common complication in MM due to increased **bone resorption**, but it is generally manageable and not considered a primary prognostic marker for disease aggressiveness itself.
- While severe hypercalcemia can contribute to overall morbidity, its presence doesn't directly stage the disease or predict survival as significantly as B2 microglobulin.
*Telomerase*
- **Telomerase** is an enzyme involved in maintaining telomere length and is often overexpressed in various cancers, including MM, allowing cells to proliferate indefinitely.
- While it plays a role in the **pathogenesis** of MM, its utility as a prognostic marker in routine clinical practice is not as established or as widely used as B2 microglobulin.
Post-Translational Modifications Indian Medical PG Question 7: Statement 1 - A 59-year-old patient presents with flaccid bullae. Histopathology shows a suprabasal acantholytic split.
Statement 2 - The row of tombstones appearance is diagnostic of Pemphigus vulgaris.
- A. Statements 1 & 2 are correct, 2 is not explaining 1 (Correct Answer)
- B. Statements 1 and 2 are correct and 2 is the correct explanation for 1
- C. Statements 1 and 2 are incorrect
- D. Statement 1 is incorrect
Post-Translational Modifications Explanation: ***Correct: Statements 1 & 2 are correct, 2 is not explaining 1***
**Analysis of Statement 1:**
- A 59-year-old patient with **flaccid bullae** and **suprabasal acantholytic split** on histopathology is the classic presentation of **Pemphigus vulgaris**
- The flaccid (easily ruptured) nature of bullae distinguishes it from tense bullae seen in bullous pemphigoid
- The suprabasal location of the split (just above the basal layer) with acantholysis (loss of cell-to-cell adhesion) is pathognomonic
- **Statement 1 is CORRECT** ✓
**Analysis of Statement 2:**
- The **"row of tombstones" or "tombstone appearance"** is indeed a diagnostic histopathological feature of Pemphigus vulgaris
- This appearance results from basal keratinocytes remaining attached to the basement membrane while suprabasal cells separate due to acantholysis
- The intact basal cells standing upright resemble a row of tombstones
- **Statement 2 is CORRECT** ✓
**Does Statement 2 explain Statement 1?**
- Statement 2 describes a **histopathological appearance** (tombstone pattern) that is a **consequence** of the suprabasal split
- However, it does NOT explain the **underlying cause** of the flaccid bullae or the suprabasal split
- The true explanation involves **IgG autoantibodies against desmoglein 3 (and desmoglein 1)**, which attack intercellular adhesion structures (desmosomes), causing **acantholysis**
- Therefore, **Statement 2 does NOT explain Statement 1** ✗
*Incorrect: Statement 2 is the correct explanation for Statement 1*
- While both statements describe features of Pemphigus vulgaris, the tombstone appearance is a descriptive finding, not an explanatory mechanism
*Incorrect: Statements 1 and 2 are incorrect*
- Both statements are medically accurate descriptions of Pemphigus vulgaris features
*Incorrect: Statement 1 is incorrect*
- Statement 1 correctly describes the cardinal clinical and histopathological features of Pemphigus vulgaris
Post-Translational Modifications Indian Medical PG Question 8: During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?
- A. eIF2
- B. eIF4A
- C. eIF4G
- D. eIF4E (Correct Answer)
Post-Translational Modifications Explanation: ***eIF4E***
- Insulin activates the **mTOR pathway**, which leads to activation of **Mnk1/2 kinases** that phosphorylate eIF4E at **Ser209**.
- This phosphorylation enhances eIF4E's **affinity for the 5' cap structure** and increases **cap-dependent translation initiation** efficiency.
*eIF4G*
- While eIF4G is essential for **eIF4F complex formation**, its phosphorylation is not the primary target enhanced by insulin signaling.
- Insulin's effect on eIF4G is mainly **indirect through 4E-BP1 phosphorylation**, which releases eIF4E to bind eIF4G.
*eIF2*
- **eIF2 phosphorylation** by kinases like **PERK, PKR, and GCN2** inhibits translation initiation during stress conditions.
- This is **opposite to insulin's anabolic effects**, as insulin signaling typically promotes conditions that reduce eIF2 phosphorylation.
*eIF4A*
- eIF4A functions as an **RNA helicase** in the eIF4F complex, unwinding mRNA secondary structures.
- While important for translation, **direct phosphorylation enhancement by insulin** is not a primary mechanism for eIF4A regulation.
Post-Translational Modifications Indian Medical PG Question 9: Protein segregation occurs in which organelle?
- A. Peroxisomes
- B. ER
- C. Mitochondria
- D. Golgi apparatus (Correct Answer)
Post-Translational Modifications Explanation: ***Golgi apparatus***
- The **Golgi apparatus** is a central organelle for **protein modification, sorting, and packaging** into vesicles for delivery to various cellular destinations.
- It acts as a "post office" of the cell, directing proteins to their correct locations through **segregation** into specific secretory or transport pathways.
*Peroxisomes*
- **Peroxisomes** are involved in **metabolic processes** such as fatty acid oxidation and detoxification.
- While they import some proteins, their primary role is not in the overall **segregation** and trafficking of proteins for diverse cellular destinations.
*ER*
- The **endoplasmic reticulum (ER)** is where proteins are synthesized (rough ER) and undergo initial folding and modification, including glycosylation.
- However, the ER's main function is protein synthesis and early modification, not the final **segregation** and sorting for transport to different cellular locations.
*Mitochondria*
- **Mitochondria** are primarily responsible for **ATP production** through cellular respiration and houses its own genome.
- While mitochondria import specific proteins necessary for their function, they are not involved in the general **segregation** of proteins destined for other organelles or secretion.
Post-Translational Modifications Indian Medical PG Question 10: Name the antigen marked as X determining blood group A.
- A. N-Acetyl-Glucosamine
- B. Dermatan sulphate
- C. Keratan sulfate
- D. N-Acetyl-Galactosamine (Correct Answer)
Post-Translational Modifications Explanation: ***N-Acetyl-Galactosamine***
- Blood group A antigens are formed by the addition of **N-acetylgalactosamine** to the H antigen precursor molecule on the surface of red blood cells.
- This sugar modification is catalyzed by the **A transferase enzyme**, which is specific for N-acetylgalactosamine.
*N-Acetyl-Glucosamine*
- While N-acetylglucosamine is a component of many glycans, it is not the terminal sugar that defines the **blood group A antigen**.
- **N-acetylglucosamine** is a key building block for the H antigen and other blood group precursors, but not the specific modifying sugar for A.
*Dermatan sulphate*
- **Dermatan sulfate** is a **glycosaminoglycan** primarily found in connective tissues, skin, and blood vessels.
- It plays a role in wound healing and coagulation, but is not involved in **ABO blood group determination**.
*Keratan sulfate*
- **Keratan sulfate** is another **glycosaminoglycan** found in cartilage, cornea, and bone.
- It contributes to tissue hydration and structural integrity, but it is not part of the **ABO blood group antigens**.
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