Ribozymes and Catalytic RNA — MCQs
During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?
Which of the following is a function of ribozymes?
Which type of RNA is primarily involved in gene silencing?
Which RNA is used in RNA splicing?
Which condition is associated with defects in pre-mRNA splicing and SMN protein dysfunction?
Kcat/Km is a measure of which of the following?
Carboxypeptidase contains which mineral?
Which of the following genetic disorders is treated with enzyme replacement therapy?
Which statement is false about allosteric regulation?
Non-competitive inhibition is:
Ribozymes and Catalytic RNA Indian Medical PG Practice Questions and MCQs
Question 1: During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?
- A. eIF2
- B. eIF4A
- C. eIF4G
- D. eIF4E (Correct Answer)
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.
Question 2: Which of the following is a function of ribozymes?
- A. Peptidyl transferase activity (Correct Answer)
- B. Cut DNA at specific site
- C. GTPase activity
- D. Participate in DNA synthesis
Explanation: ***Peptidyl transferase activity*** - The **ribosome's large subunit**, which contains **ribosomal RNA (rRNA)**, catalyzes the formation of peptide bonds during protein synthesis. - This **rRNA enzyme**, known as a **ribozyme**, exhibits **peptidyl transferase activity**. *Cut DNA at specific site* - This function is primarily carried out by **restriction enzymes**, which are **proteins**, not ribozymes. - **Ribozymes** are **RNA molecules** with catalytic activity and do not typically cleave DNA. *Participate in DNA synthesis* - **DNA synthesis** is mediated by **DNA polymerases** and other **protein enzymes**, not ribozymes. - Ribozymes' primary roles involve **RNA processing** and **peptide bond formation**. *GTPase activity* - **GTPase activity** is characteristic of **G-proteins**, which are **protein enzymes** involved in signal transduction and cell regulation. - While some ribosomal activities are **GTP-dependent**, the **GTPase itself is a protein**, not the ribozyme component.
Question 3: Which type of RNA is primarily involved in gene silencing?
- A. rRNA
- B. tRNA
- C. miRNA (Correct Answer)
- D. mRNA
Explanation: ***miRNA*** - **miRNA** (microRNA) is a small non-coding RNA molecule that plays a crucial role in **post-transcriptional regulation of gene expression**. - It functions by binding to complementary messenger RNA (mRNA) molecules, leading to **mRNA degradation** or **inhibition of translation**, thereby silencing genes. - miRNA is the primary RNA type involved in **gene silencing** through the RNA interference (RNAi) pathway. *rRNA* - **rRNA** (ribosomal RNA) is a primary component of **ribosomes**, the cellular machinery responsible for protein synthesis. - Its main function is to **catalyze peptide bond formation** and provide structural integrity to the ribosome, not gene silencing. *tRNA* - **tRNA** (transfer RNA) is responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - It acts as an adapter molecule, translating the **genetic code** in mRNA into an amino acid sequence. *mRNA* - **mRNA** (messenger RNA) carries genetic information from **DNA to ribosomes** for protein synthesis. - While mRNA can be targeted by gene silencing mechanisms (like miRNA), it is not the RNA type that performs the silencing function itself.
Question 4: Which RNA is used in RNA splicing?
- A. mRNA
- B. tRNA
- C. rRNA
- D. Small nuclear RNA (snRNA) (Correct Answer)
Explanation: ***Small nuclear RNA (snRNA)*** - **snRNAs** are key components of **spliceosomes**, the molecular machines that catalyze the removal of introns from pre-mRNA. - They bind to specific sequences within the pre-mRNA and facilitate the splicing reactions. *mRNA* - **mRNA (messenger RNA)** carries the genetic code from DNA to the ribosomes for **protein synthesis**. - While it is the molecule that gets spliced, it does not directly participate in the splicing machinery itself. *rRNA* - **rRNA (ribosomal RNA)** is a structural and catalytic component of **ribosomes**, where protein synthesis occurs. - It plays no direct role in the process of RNA splicing. *tRNA* - **tRNA (transfer RNA)** molecules are responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - They are involved in translation, not in the processing of RNA by splicing.
Question 5: Which condition is associated with defects in pre-mRNA splicing and SMN protein dysfunction?
- A. Sickle cell disease
- B. Huntington's disease
- C. Spinal muscular atrophy (Correct Answer)
- D. α-Thalassemia
Explanation: ***Spinal muscular atrophy*** - **Spinal muscular atrophy (SMA)** is primarily caused by mutations in the **SMN1 gene**, leading to insufficient production of the **survival motor neuron (SMN) protein**. - Without adequate SMN protein, defects occur in the **pre-mRNA splicing** of motor neuron genes, leading to the degeneration of **alpha motor neurons** in the spinal cord. *Sickle cell disease* - **Sickle cell disease** is an inherited **hemoglobinopathy** caused by a point mutation in the beta-globin gene, leading to the production of abnormal **hemoglobin S**. - This condition does not involve defects in pre-mRNA splicing or SMN protein dysfunction, but rather the **polymerization of hemoglobin S** under low oxygen conditions. *Huntington's disease* - **Huntington's disease** (formerly called Huntington chorea) is a neurodegenerative disorder caused by an **expanded CAG trinucleotide repeat** in the huntingtin gene. - Huntington's disease involves protein misfolding and aggregation, but not primary defects in pre-mRNA splicing or SMN protein dysfunction. *α-Thalassemia* - **α-Thalassemia** is a group of inherited blood disorders characterized by reduced or absent production of **alpha-globin chains**, typically due to **gene deletions** on chromosome 16. - This condition affects the assembly of hemoglobin and does not involve pre-mRNA splicing defects or SMN protein dysfunction.
Question 6: Kcat/Km is a measure of which of the following?
- A. Speed of enzymatic reaction
- B. Concentration of substrate
- C. Enzyme turnover
- D. Enzyme efficiency (Correct Answer)
Explanation: **Correct: Enzyme efficiency** - The ratio **kcat/Km** is the definitive measure of an enzyme's **catalytic efficiency** or **specificity constant** - It reflects how effectively an enzyme converts substrate to product at low substrate concentrations - A higher **kcat/Km** value indicates greater efficiency, combining high catalytic rate (kcat) with strong substrate affinity (low Km) - This is the most important parameter for comparing different enzymes or different substrates for the same enzyme *Incorrect: Speed of enzymatic reaction* - **kcat** (turnover number) alone measures the maximum speed when enzyme is saturated with substrate - **kcat/Km** is a more comprehensive measure that includes substrate binding affinity, not just reaction speed - Speed also depends on enzyme and substrate concentrations, which kcat/Km doesn't directly represent *Incorrect: Concentration of substrate* - **Km** (Michaelis constant) represents the substrate concentration at which reaction velocity is half of Vmax - **kcat/Km** is a ratio that describes enzyme performance across substrate concentrations, not the concentration itself - It's particularly useful for predicting enzyme behavior at physiological (low) substrate concentrations *Incorrect: Enzyme turnover* - **kcat** specifically measures enzyme turnover: the number of substrate molecules converted per enzyme molecule per unit time at saturation - **kcat/Km** incorporates both kcat and Km, providing overall efficiency rather than just turnover rate - Turnover is only one component of the efficiency measure
Question 7: Carboxypeptidase contains which mineral?
- A. Copper
- B. Zinc (Correct Answer)
- C. Iron
- D. None of the options
Explanation: ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.
Question 8: Which of the following genetic disorders is treated with enzyme replacement therapy?
- A. Gaucher's disease (Correct Answer)
- B. Krabbe's disease
- C. Metachromatic leukodystrophy
- D. Tay-Sachs disease
Explanation: **Explanation:** **Gaucher’s Disease (Option A)** is the correct answer because it was the first lysosomal storage disorder (LSD) for which **Enzyme Replacement Therapy (ERT)** was developed. It is caused by a deficiency of the enzyme **Glucocerebrosidase** (Acid $\beta$-glucosidase), leading to the accumulation of glucosylceramide in macrophages (Gaucher cells). Recombinant enzymes like **Imiglucerase** are administered intravenously to clear these deposits, particularly improving hepatosplenomegaly and hematological parameters in Type 1 Gaucher’s. **Why the other options are incorrect:** * **Krabbe’s disease (Option B):** Caused by **Galactocerebrosidase** deficiency. ERT is not the standard of care because the enzyme cannot cross the blood-brain barrier (BBB) to treat the severe central nervous system (CNS) demyelination. Hematopoietic stem cell transplantation (HSCT) is the preferred intervention. * **Metachromatic leukodystrophy (Option C):** Caused by **Arylsulfatase A** deficiency. Similar to Krabbe’s, the primary pathology is in the CNS, making standard ERT ineffective. Gene therapy and HSCT are the focus of current management. * **Tay-Sachs disease (Option D):** Caused by **Hexosaminidase A** deficiency. It involves rapid neurodegeneration. ERT cannot reach the brain tissues effectively, and currently, treatment remains supportive. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" appearance of the cytoplasm. * **ERT Success:** ERT is highly effective for LSDs with significant **systemic/visceral** involvement (e.g., Gaucher Type 1, Fabry, Pompe, and MPS I/Hurler) but is generally ineffective for purely **neurodegenerative** conditions due to the BBB. * **Alternative Treatment:** Substrate Reduction Therapy (SRT) using **Miglustat** is also used in Gaucher’s to decrease the synthesis of the accumulating substrate.
Question 9: Which statement is false about allosteric regulation?
- A. It is usually the mode of regulation for the first committed step in reaction pathways. (Correct Answer)
- B. Cellular response is faster with allosteric control than by controlling enzyme concentration in the cell.
- C. The regulation is important to the conservation of energy and materials in cells.
- D. Allosteric modulators bind non-covalently at sites other than the active site and induce conformational changes in the enzyme.
Explanation: ### Explanation **Why Option A is the Correct Answer (The False Statement):** In the context of this specific question, Option A is technically a **true** statement regarding biochemistry. However, in many NEET-PG style assessments, if this is marked as the "false" option, it is often due to a technicality in phrasing or a specific textbook context where allosteric regulation is contrasted with other forms of control. *Correction/Refinement:* Allosteric regulation **is** indeed the most common mode of regulation for the **first committed step** (rate-limiting step) of a metabolic pathway (e.g., PFK-1 in glycolysis). If the question identifies this as the "false" statement, it may be implying that not *all* committed steps are regulated *exclusively* by allosteric means (some use covalent modification or induction). **Analysis of Other Options:** * **Option B (True):** Allosteric control involves simple binding/unbinding of a ligand, causing an immediate conformational change. This is significantly faster than **enzyme induction/repression**, which requires transcription and translation (taking hours to days). * **Option C (True):** By inhibiting the first committed step via feedback inhibition, the cell prevents the unnecessary accumulation of intermediates and the wasteful expenditure of ATP and substrates. * **Option D (True):** By definition, allosteric ("other site") modulators bind **non-covalently** to a regulatory site. This induces a conformational change (T-state to R-state or vice versa) that alters the affinity of the active site for the substrate. **High-Yield Clinical Pearls for NEET-PG:** * **Kinetics:** Allosteric enzymes show a **Sigmoidal (S-shaped)** curve on a velocity-substrate plot, unlike the hyperbolic curve of Michaelis-Menten enzymes. * **Feedback Inhibition:** The end-product of a pathway often acts as a negative allosteric effector of the rate-limiting enzyme. * **Key Example:** **Phosphofructokinase-1 (PFK-1)** is the rate-limiting enzyme of glycolysis; it is allosterically inhibited by ATP and Citrate, and activated by AMP and Fructose 2,6-bisphosphate. * **Aspartate Transcarbamoylase (ATCase):** A classic example of allosteric regulation in pyrimidine synthesis, inhibited by CTP.
Question 10: Non-competitive inhibition is:
- A. Reversible
- B. Irreversible
- C. Any of the above (Correct Answer)
- D. None of the above
Explanation: ### Explanation In biochemistry, **Non-competitive inhibition** occurs when an inhibitor binds to a site other than the active site (the allosteric site). This binding induces a conformational change in the enzyme, reducing its catalytic activity regardless of whether the substrate is bound. **1. Why "Any of the above" is correct:** Non-competitive inhibition is traditionally categorized based on the nature of the bond formed between the inhibitor and the enzyme: * **Reversible Non-competitive Inhibition:** The inhibitor binds via weak, non-covalent interactions (e.g., hydrogen bonds). The inhibitor can dissociate, and the enzyme's function can be restored. * **Irreversible Non-competitive Inhibition:** The inhibitor binds via strong covalent bonds or destroys a functional group necessary for catalysis. This is often referred to as "irreversible inhibition" or "enzyme poisoning." Because the term "non-competitive" describes the **site and mechanism** of binding (not competing for the active site), it can technically be either reversible or irreversible. **2. Analysis of Incorrect Options:** * **Option A (Reversible):** While many classic examples (like Ferrochelatase inhibition by Lead) are reversible, this is too restrictive as it excludes irreversible inhibitors. * **Option B (Irreversible):** Similarly, many non-competitive inhibitors (like Cyanide) act irreversibly, but this option ignores the reversible class. **3. NEET-PG High-Yield Pearls:** * **Kinetics:** In non-competitive inhibition, **$V_{max}$ decreases** (the engine is broken), but **$K_m$ remains unchanged** (affinity for the substrate is the same). * **Classic Example:** Heavy metal poisoning (Lead, Mercury) and Cyanide (inhibiting Cytochrome Oxidase). * **Comparison:** Unlike Competitive inhibition, non-competitive inhibition **cannot** be overcome by increasing the substrate concentration. * **Graph:** On a Lineweaver-Burk plot, the lines intersect on the negative x-axis ($-1/K_m$).
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free