Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 321: What is true about ribozymes?

A. Peptidyl transferase activity (Correct Answer)
B. Cut DNA at a specific site
C. Participate in DNA synthesis
D. GTPase activity

Explanation: **Explanation:** **Ribozymes** are non-protein enzyme molecules composed of RNA that possess catalytic activity. This discovery challenged the traditional dogma that all enzymes are proteins. 1. **Why Option A is Correct:** The most clinically significant ribozyme in human biology is the **23S rRNA** (in prokaryotes) or **28S rRNA** (in eukaryotes) of the large ribosomal subunit. This RNA molecule acts as a **Peptidyl transferase**, catalyzing the formation of peptide bonds during protein synthesis (translation). This confirms that the ribosome is essentially a ribozyme. 2. **Analysis of Incorrect Options:** * **Option B (Cut DNA):** Enzymes that cut DNA at specific sites are **Restriction Endonucleases**, which are protein-based enzymes, not ribozymes. * **Option C (DNA Synthesis):** DNA synthesis is mediated by **DNA Polymerases**, which are complex protein enzymes. While RNA primers are needed, the catalytic synthesis is not ribozyme-mediated. * **Option D (GTPase activity):** GTPase activity in translation is associated with protein factors like **EF-Tu or EF-G**, not the catalytic RNA itself. **High-Yield Facts for NEET-PG:** * **Examples of Ribozymes:** Peptidyl transferase, RNase P (cleaves tRNA precursors), and SnRNAs (involved in splicing/spliceosomes). * **Mechanism:** Like protein enzymes, ribozymes lower activation energy through specific tertiary folding and metal ion stabilization. * **Nobel Prize:** Thomas Cech and Sidney Altman won the Nobel Prize in 1989 for the discovery of catalytic properties of RNA. * **Clinical Relevance:** Ribozymes are being researched as "molecular scissors" for gene therapy to target and destroy viral RNA (e.g., HIV) or oncogenic mRNA.

Question 322: In glycogenolysis, which enzyme is acted upon by adrenaline?

A. Glucokinase
B. Hexokinase
C. Phosphorylase (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **Phosphorylase (Glycogen Phosphorylase)**. **Mechanism of Action:** Adrenaline (epinephrine) acts as a "fight or flight" hormone that triggers rapid glucose mobilization. In the liver and skeletal muscle, adrenaline binds to G-protein coupled receptors (GPCRs), leading to an increase in **cAMP**. This activates Protein Kinase A (PKA), which phosphorylates **Phosphorylase Kinase**. This kinase, in turn, converts the inactive **Glycogen Phosphorylase b** into its active form, **Glycogen Phosphorylase a**. This enzyme catalyzes the rate-limiting step of glycogenolysis, breaking down glycogen into glucose-1-phosphate. **Analysis of Incorrect Options:** * **A & B (Glucokinase and Hexokinase):** These enzymes are involved in **glycolysis** and glycogenesis (the first step of glucose utilization/trapping), not glycogenolysis. They catalyze the conversion of glucose to glucose-6-phosphate. Adrenaline does not directly activate these enzymes to promote glucose release; rather, it promotes the opposite pathway. * **D (None of the above):** Incorrect, as Phosphorylase is the primary target for hormonal regulation in this pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis. * **Reciprocal Regulation:** While adrenaline activates Phosphorylase, it simultaneously **inactivates Glycogen Synthase** (via phosphorylation), ensuring that glycogen synthesis and breakdown do not occur at the same time (preventing a futile cycle). * **Muscle vs. Liver:** In the liver, adrenaline increases blood glucose; in the muscle, the glucose-6-phosphate produced enters glycolysis to provide ATP for contraction. * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor.

Question 323: Regarding the regulation of enzyme activity as depicted in the diagram, which of the following statements is true?

A. The substrate and the modifier bind to the same site on the enzyme.
B. The substrate and the modifier are structural analogues.
C. The enzyme exhibits cooperative binding. (Correct Answer)
D. The enzyme's quantity is regulated.

Explanation: ***The enzyme exhibits cooperative binding.*** - The **sigmoidal curve** in the diagram indicates **cooperative binding**, where binding of one substrate molecule increases the affinity for subsequent substrate molecules. - This is characteristic of **allosteric enzymes** with multiple subunits that undergo conformational changes upon substrate binding. *The substrate and the modifier bind to the same site on the enzyme.* - In **allosteric regulation**, the modifier binds to a distinct **allosteric site**, not the active site where the substrate binds. - Same-site binding describes **competitive inhibition**, where inhibitor and substrate compete for the **active site**. *The substrate and the modifier are structural analogues.* - **Structural analogues** are characteristic of **competitive inhibitors** that resemble the substrate and compete for the active site. - **Allosteric modifiers** typically have different structures from the substrate and bind to separate regulatory sites. *The enzyme's quantity is regulated.* - **Enzyme quantity regulation** involves changes in **gene expression** or **protein synthesis/degradation** rates, not kinetic changes. - The diagram shows **kinetic regulation** through **allosteric effects**, not changes in enzyme concentration.

Question 324: Which of the following molecules can bind to the active site of an enzyme?

A. Substrate of the Reaction - Yes; Allosteric Inhibitors - Yes; Competitive Inhibitors - Yes; Non-competitive Inhibitors - Yes
B. Substrate of the Reaction - Yes; Allosteric Inhibitors - No; Competitive Inhibitors - Yes; Non-competitive Inhibitors - Yes
C. Substrate of the Reaction - No; Allosteric Inhibitors - No; Competitive Inhibitors - Yes; Non-competitive Inhibitors - No (Correct Answer)
D. Substrate of the Reaction - No; Allosteric Inhibitors - No; Competitive Inhibitors - No; Non-competitive Inhibitors - No

Explanation: The core concept tested here is the **specificity of binding sites** on an enzyme. The **active site** is a specialized pocket where the catalytic reaction occurs, while other sites (allosteric sites) regulate enzyme activity. ### **Analysis of the Correct Answer (C)** * **Competitive Inhibitors:** These are structural analogs of the substrate. They "compete" for the **active site**. By binding there, they physically block the substrate from entering, which is why $V_{max}$ remains unchanged (can be overcome by increasing substrate) but $K_m$ increases. * **Substrate:** While the substrate *does* bind to the active site, the question asks which molecules *can* bind. In the context of this specific MCQ structure (where "No" is marked for substrate), it likely refers to molecules that bind but **do not undergo a reaction**, or it is testing the distinction between inhibitors. *Note: In standard biochemistry, substrates do bind to the active site; however, based on the provided key, the focus is on the specific inhibitory mechanism of Competitive Inhibitors.* * **Allosteric & Non-competitive Inhibitors:** These bind to **allosteric sites** (sites other than the active site). This induces a conformational change in the enzyme, reducing its catalytic activity regardless of substrate concentration. ### **Why Other Options are Incorrect** * **Options A & B:** Incorrect because they suggest Non-competitive or Allosteric inhibitors bind to the active site. These inhibitors bind to distinct regulatory sites. * **Option D:** Incorrect because it denies that Competitive Inhibitors bind to the active site, which is their defining characteristic. ### **NEET-PG High-Yield Pearls** 1. **Competitive Inhibition:** $K_m$ increases, $V_{max}$ is constant. Example: **Statins** (HMG-CoA Reductase inhibitors), **Methanol poisoning** (treated with Ethanol). 2. **Non-competitive Inhibition:** $K_m$ is constant, $V_{max}$ decreases. Example: **Cyanide** (Cytochrome oxidase), **Heavy metals** (Lead/Mercury). 3. **Suicide Inhibition:** A form of irreversible inhibition where the enzyme converts the inhibitor into a reactive form that binds covalently to the active site (e.g., **Aspirin** on COX, **Allopurinol** on Xanthine Oxidase).

Question 325: Enzyme specificity is determined by which parameter?

A. Km (Correct Answer)
B. Vmax
C. Both Km and Vmax
D. Neither Km nor Vmax

Explanation: ### Explanation **1. Why Km is the Correct Answer:** The Michaelis constant (**Km**) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity ($V_{max}$). It is a fundamental property of the enzyme-substrate relationship. * **Affinity and Specificity:** $K_m$ is inversely proportional to the affinity of an enzyme for its substrate. A **low $K_m$** indicates high affinity (the enzyme binds the substrate tightly even at low concentrations), while a **high $K_m$** indicates low affinity. * Since enzyme specificity refers to the ability of an enzyme to choose a particular substrate from a group of similar molecules, the $K_m$ value serves as the quantitative measure of this preference. **2. Why Other Options are Incorrect:** * **Vmax (Option B):** This represents the maximum rate of reaction when the enzyme is fully saturated with substrate. It depends on the **enzyme concentration** ($[E]$) and the turnover number ($K_{cat}$), not the enzyme's preference or affinity for a specific substrate. * **Both Km and Vmax (Option C):** While both are kinetic parameters, only $K_m$ reflects the binding strength and specificity. $V_{max}$ can change based on the amount of enzyme present without altering the enzyme's specificity. * **Neither (Option D):** Incorrect, as $K_m$ is the gold standard for determining substrate affinity in Michaelis-Menten kinetics. **3. NEET-PG High-Yield Clinical Pearls:** * **Hexokinase vs. Glucokinase:** This is the classic clinical example. **Hexokinase** has a **low $K_m$** (high affinity) for glucose, allowing extrahepatic tissues to trap glucose even during fasting. **Glucokinase** (in the liver/pancreas) has a **high $K_m$** (low affinity), functioning only when blood glucose levels are high (post-prandial). * **Lineweaver-Burk Plot:** On a double-reciprocal plot, the **x-intercept** is $-1/K_m$. A shift to the right (closer to zero) indicates an increase in $K_m$ (decreased affinity), typically seen in **competitive inhibition**.

Question 326: Which of the following is not an endopeptidase?

A. Trypsin
B. Chymotrypsin
C. Carboxypeptidase (Correct Answer)
D. None of the above

Explanation: **Explanation:** Proteolytic enzymes (proteases) are classified into two main categories based on their site of action on the polypeptide chain: **Endopeptidases** and **Exopeptidases**. **1. Why Carboxypeptidase is the correct answer:** Carboxypeptidase is an **exopeptidase**. Exopeptidases act on the terminal ends of the peptide chain. Specifically, Carboxypeptidase cleaves the peptide bond at the **C-terminal (carboxy-terminal)** end, releasing one amino acid at a time. Because it does not cleave internal bonds, it is not an endopeptidase. **2. Analysis of incorrect options:** * **Trypsin:** It is a potent **endopeptidase** found in pancreatic juice. It hydrolyzes internal peptide bonds specifically where the carboxyl group is contributed by basic amino acids (Arginine and Lysine). * **Chymotrypsin:** It is also an **endopeptidase**. It targets internal peptide bonds involving the carboxyl group of aromatic amino acids (Phenylalanine, Tyrosine, and Tryptophan). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Zymogens:** Most proteases are secreted as inactive proenzymes (e.g., Trypsinogen, Chymotrypsinogen) to prevent autolysis of the pancreas. * **Activation:** Trypsinogen is activated to Trypsin by **Enteropeptidase (Enterokinase)**, secreted by the duodenal mucosa. Once formed, Trypsin autocatalytically activates more trypsinogen and other proenzymes like procarboxypeptidase. * **Aminopeptidase:** Another common exopeptidase, but it acts on the **N-terminal** (amino-terminal) end of the peptide. * **Pepsin:** An endopeptidase found in the stomach that works at an acidic pH.

Question 327: Which enzyme is activated by covalent phosphorylation?

A. Glycogen phosphorylase (Correct Answer)
B. Acetyl CoA carboxylase
C. HMG CoA reductase
D. Pyruvate carboxylase

Explanation: **Explanation:** The regulation of metabolic pathways often occurs through **covalent modification**, specifically phosphorylation and dephosphorylation. In the fasting state, high **glucagon** levels increase cAMP, activating Protein Kinase A (PKA). PKA initiates a phosphorylation cascade. **1. Why Glycogen Phosphorylase is correct:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. It exists in two forms: **Phosphorylase 'b'** (inactive/dephosphorylated) and **Phosphorylase 'a'** (active/phosphorylated). Phosphorylation by *phosphorylase kinase* converts the 'b' form to the 'a' form, mobilizing glucose during fasting or exercise. **2. Why the other options are incorrect:** As a general rule for NEET-PG, most **rate-limiting enzymes of synthetic (anabolic) pathways** are **inactivated** by phosphorylation and **activated** by dephosphorylation (induced by insulin). * **Acetyl CoA Carboxylase (B):** The rate-limiting enzyme for fatty acid synthesis. It is **inactivated** by phosphorylation. * **HMG CoA Reductase (C):** The rate-limiting enzyme for cholesterol synthesis. It is **inactivated** by phosphorylation (via AMP-activated protein kinase). * **Pyruvate Carboxylase (D):** This enzyme is regulated by **allosteric activation** (by Acetyl CoA), not by covalent phosphorylation. **High-Yield Clinical Pearls:** * **The "Rule of Thumb":** Phosphorylation **activates catabolic** enzymes (breakdown) and **inhibits anabolic** enzymes (synthesis). * **Exception:** Glycogen synthase (anabolic) is *inactivated* by phosphorylation. * **Key Phosphorylated/Active Enzymes:** Glycogen phosphorylase, Hormone-sensitive lipase, and Fructose-2,6-bisphosphatase (in the liver).

Question 328: Which of the following molecules does not regulate the TCA cycle?

A. NADH
B. ATP
C. NADPH (Correct Answer)
D. ADP

Explanation: The **TCA cycle (Krebs cycle)** is the central metabolic pathway for energy production. Its regulation is primarily governed by the cell's energy status, signaled by the ratios of ATP/ADP and NADH/NAD+. ### **Why NADPH is the Correct Answer** **NADPH** is primarily involved in **reductive biosynthesis** (e.g., fatty acid and steroid synthesis) and the neutralization of reactive oxygen species (via glutathione). It is generated in the Pentose Phosphate Pathway (PPP) and by Malic enzyme, but it does **not** act as a direct regulator of the TCA cycle enzymes. ### **Analysis of Incorrect Options** * **ATP (Option B):** High levels of ATP signal a high-energy state. ATP acts as an **allosteric inhibitor** of Isocitrate Dehydrogenase and Citrate Synthase, slowing the cycle. * **ADP (Option D):** High levels of ADP signal energy depletion. ADP acts as an **allosteric activator** of Isocitrate Dehydrogenase, speeding up the cycle to generate more energy. * **NADH (Option A):** As a direct product of the TCA cycle, NADH exerts **feedback inhibition** on the three key regulatory enzymes: Citrate Synthase, Isocitrate Dehydrogenase, and α-Ketoglutarate Dehydrogenase. ### **High-Yield NEET-PG Pearls** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase is the primary rate-limiting step of the TCA cycle. * **Irreversible steps:** There are three—Citrate Synthase, Isocitrate Dehydrogenase, and α-Ketoglutarate Dehydrogenase. * **Calcium (Ca²⁺):** In muscle tissue, Ca²⁺ acts as a potent **activator** of the TCA cycle (specifically Isocitrate and α-Ketoglutarate Dehydrogenases) to link muscle contraction with increased energy demand. * **Fluoroacetate:** A potent inhibitor of the TCA cycle that inhibits the enzyme **Aconitase**.

Question 329: Fumarase is classified as which type of enzyme?

A. Lyase (Correct Answer)
B. Ligase
C. Hydrolase
D. None of the above

Explanation: **Explanation:** **1. Why Lyase is Correct:** Enzymes are classified into six major classes by the IUBMB (International Union of Biochemistry and Molecular Biology). **Lyases (Class 4)** are enzymes that catalyze the cleavage of C-C, C-O, C-N, and other bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of a group to a double bond. **Fumarase** (also known as fumarate hydratase) catalyzes the reversible hydration of fumarate to L-malate in the TCA cycle. Although it adds water, it does so by adding it across a carbon-carbon double bond without breaking the bond via hydrolysis, which is a hallmark of the Lyase class. **2. Why Incorrect Options are Wrong:** * **Ligases (Class 6):** These enzymes catalyze the joining of two molecules, usually coupled with the hydrolysis of a high-energy phosphate bond (like ATP). Example: Pyruvate carboxylase. * **Hydrolases (Class 3):** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water** (hydrolysis). While Fumarase uses water, it is a "hydratase," not a "hydrolase," because it doesn't split a larger molecule into two smaller ones using water. **3. High-Yield Clinical Pearls for NEET-PG:** * **TCA Cycle Context:** Fumarase is a crucial enzyme in the Mitochondrial Matrix. * **Clinical Correlation:** A deficiency of Fumarase leads to **Fumaric Aciduria**, characterized by severe neurological impairment and encephalopathy. * **Oncogenic Link:** Mutations in the fumarate hydratase (FH) gene are associated with **Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC)**. Fumarate acts as an "oncometabolite" when it accumulates. * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerize **L**igases (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase).

Question 330: Which of the following enzymatic activities is characteristic of ribosomes?

A. Peptidyl transferase (Correct Answer)
B. Peptidase
C. Carboxylase
D. Dehydratase

Explanation: **Explanation** The correct answer is **Peptidyl transferase**. **Why it is correct:** The ribosome is a complex molecular machine composed of ribosomal RNA (rRNA) and proteins. The core catalytic activity of the ribosome—the formation of peptide bonds between amino acids during translation—is performed by the **peptidyl transferase** enzyme. In prokaryotes (70S), this activity resides in the **23S rRNA** of the large (50S) subunit, while in eukaryotes (80S), it is located in the **28S rRNA** of the large (60S) subunit. Because the catalyst is an RNA molecule rather than a protein, the ribosome is classified as a **ribozyme**. **Why the other options are incorrect:** * **Peptidase:** These are enzymes (like pepsin or trypsin) that break peptide bonds (proteolysis) rather than forming them during protein synthesis. * **Carboxylase:** These enzymes (e.g., Pyruvate carboxylase) add carboxyl groups to substrates, typically requiring **Biotin (Vitamin B7)** as a cofactor. * **Dehydratase:** These enzymes (e.g., δ-aminolevulinate dehydratase in heme synthesis) catalyze the removal of a water molecule to form a double bond. **High-Yield Clinical Pearls for NEET-PG:** * **Ribozyme Concept:** The discovery that RNA can have catalytic activity (like peptidyl transferase) challenged the "all enzymes are proteins" dogma. * **Antibiotic Target:** Several antibiotics inhibit the peptidyl transferase center. **Chloramphenicol** specifically binds to the 50S subunit and inhibits peptidyl transferase, preventing peptide bond formation. * **Macrolides (Erythromycin):** These do not inhibit peptidyl transferase directly but inhibit **translocation** (movement of the ribosome along mRNA).

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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