Enzymes Practice Questions

Q: What is true about ribozymes?

Peptidyl transferase activity. **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.

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

The enzyme exhibits cooperative binding.. ***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.

Q: Which enzyme is activated by covalent phosphorylation?

Glycogen phosphorylase. **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).

Q: Fumarase is classified as which type of enzyme?

Lyase. **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).

Q: Dehydrogenation of succinic acid to fumaric acid requires which of the following hydrogen carriers?

Flavoprotein. **Explanation:** The conversion of succinate to fumarate is a key step in the **Tricarboxylic Acid (TCA) Cycle**, catalyzed by the enzyme **Succinate Dehydrogenase (SDH)**. **Why Flavoprotein is correct:** Succinate dehydrogenase is unique because it is the only enzyme in the TCA cycle that is membrane-bound (located in the inner mitochondrial membrane as **Complex II** of the Electron Transport Chain). The reaction involves the removal of two hydrogen atoms from succinate. The free energy change ($\Delta G$) of this specific reaction is insufficient to reduce $NAD^+$. Therefore, it requires a stronger oxidizing agent, **FAD (Flavin Adenine Dinucleotide)**. FAD is covalently bound to the enzyme, making SDH a **flavoprotein**. The FAD accepts electrons to become $FADH_2$, which then transfers them directly into the respiratory chain via Coenzyme Q. **Why other options are incorrect:** * **NAD+:** Used by other TCA cycle dehydrogenases (Isocitrate, $\alpha$-ketoglutarate, and Malate dehydrogenases) where the energy change is higher. * **NADP+:** Primarily functions as a reducing agent in anabolic pathways (like fatty acid synthesis) and the Hexose Monophosphate (HMP) shunt, not as an electron acceptor in the TCA cycle. * **Glutathione:** Acts as a major intracellular antioxidant and a cofactor for enzymes like glutathione peroxidase to neutralize free radicals; it does not participate in the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Competitive Inhibition:** This reaction is classically inhibited by **Malonate**, which is a structural analog of succinate. * **Dual Role:** Succinate Dehydrogenase is the only enzyme that participates in both the TCA cycle and the Electron Transport Chain (Complex II). * **Marker Enzyme:** It is often used as a marker enzyme for the inner mitochondrial membrane.

Q: Which of the following is NOT a true statement about glucokinase?

It is inhibited by Glucose-6-phosphate.. **Explanation** The key to this question lies in distinguishing between the two major hexokinase isoenzymes: **Hexokinase (Types I-III)** and **Glucokinase (Hexokinase Type IV)**. **Why Option D is the correct answer:** Unlike Hexokinase, **Glucokinase is NOT inhibited by its product, Glucose-6-phosphate (G6P).** Instead, it is regulated by the **Glucokinase Regulatory Protein (GKRP)** in the liver. This lack of feedback inhibition allows the liver to continue phosphorylating glucose even when G6P levels are high, facilitating the storage of excess glucose as glycogen after a meal. **Analysis of Incorrect Options:** * **Option A:** Glucokinase has a **high Km** (approx. 10 mmol/L), which is higher than normal fasting blood glucose levels. This means it has a low affinity for glucose and only becomes highly active when blood glucose levels rise significantly (post-prandial). * **Option B:** Glucokinase is primarily located in the **liver parenchymal cells** and the **beta cells of the pancreas**. * **Option C:** In the liver and pancreas, glucose entry is mediated by **GLUT2**, a high-capacity, high-Km transporter. This ensures that the intracellular glucose concentration equilibrates rapidly with the blood glucose concentration, allowing glucokinase to act as a "glucose sensor." **High-Yield Clinical Pearls for NEET-PG:** * **Glucose Sensor:** Glucokinase acts as the glucose sensor for insulin release in pancreatic beta cells. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) Type 2. * **Sigmoidal Kinetics:** Unlike the hyperbolic curve of Hexokinase, Glucokinase exhibits sigmoidal (positive cooperativity) kinetics. * **Induction:** Glucokinase is induced by **Insulin**, whereas Hexokinase is constitutive.

Q: Which of the following enzymes catalyzes the formation of AMP from two molecules of ADP?

Adenylate kinase. ### Explanation **Correct Option: A. Adenylate kinase** Adenylate kinase (also known as **myokinase**) is a phosphotransferase enzyme that maintains adenine nucleotide equilibrium within the cell. It catalyzes the reversible reaction: **2 ADP ⇌ ATP + AMP** This reaction is crucial for cellular energy homeostasis. When a cell rapidly consumes ATP (e.g., during vigorous muscle contraction), ADP levels rise. Adenylate kinase "recycles" two ADP molecules to generate one ATP for immediate energy and one AMP, which acts as a potent metabolic signal to activate pathways like glycolysis and fatty acid oxidation (via AMPK). **Why the other options are incorrect:** * **B. Adenylyl cyclase:** This enzyme converts **ATP into cyclic AMP (cAMP)** and pyrophosphate. It is a key component of G-protein signaling pathways, not nucleotide interconversion. * **C. Adenosine kinase:** This enzyme catalyzes the phosphorylation of **adenosine to AMP** using one molecule of ATP. It does not utilize ADP as a substrate. * **D. Adenosine deaminase (ADA):** This is a hydrolase involved in purine catabolism that converts **adenosine to inosine**. Deficiency of ADA leads to Severe Combined Immunodeficiency (SCID). **High-Yield Clinical Pearls for NEET-PG:** * **AMP as a Metabolic Sensor:** An increase in the [AMP]/[ATP] ratio (driven by adenylate kinase) activates **AMP-activated protein kinase (AMPK)**, the "master metabolic switch" that shifts the cell from anabolic to catabolic states. * **Myokinase in Muscle:** In skeletal muscle, this enzyme allows for the rapid regeneration of ATP during the initial seconds of exercise. * **Directionality:** The reaction is freely reversible; however, in a metabolically active cell, it typically proceeds toward AMP formation to signal energy depletion.

Q: Trypsin is a:

Serine protease. **Explanation:** **1. Why Serine Protease is Correct:** Trypsin is a proteolytic enzyme (protease) secreted by the pancreas as the inactive zymogen, trypsinogen. It belongs to the **Serine Protease** family because it contains a highly conserved **"Catalytic Triad"** consisting of three amino acids: **Serine (Ser 195), Histidine (His 57), and Aspartate (Asp 102)**. The serine residue acts as a nucleophile that attacks the peptide bond of the substrate. Trypsin specifically cleaves peptide bonds on the carboxyl side of basic amino acids, namely **Lysine and Arginine**. **2. Why the other options are incorrect:** * **Lecithinase:** Also known as Phospholipase C, this enzyme breaks down lecithin (phosphatidylcholine). It is a classic virulence factor for *Clostridium perfringens* (alpha-toxin), not a protease. * **Phospholipase:** These are enzymes that hydrolyze phospholipids into fatty acids and other lipophilic substances. While the pancreas secretes Phospholipase A2, it is distinct from trypsin. * **Elastase:** While Elastase is *also* a serine protease, it is a **distinct enzyme** from trypsin. Elastase specifically targets elastin and is characterized by its ability to cleave bonds next to small neutral amino acids like Alanine, Valine, or Serine. **3. NEET-PG High-Yield Pearls:** * **The Master Activator:** Trypsin is known as the "master activator" because once activated by **Enteropeptidase (Enterokinase)** in the duodenum, it autocatalytically activates more trypsinogen and other zymogens (Chymotrypsinogen, Procarboxypeptidase, and Proelastase). * **Inhibitor:** Pancreatic secretory trypsin inhibitor (PSTI/SPINK1) prevents premature activation of trypsin within the pancreas to prevent **Acute Pancreatitis**. * **Diagnostic Marker:** Serum Immunoreactive Trypsin (IRT) is used as a screening test for **Cystic Fibrosis** in newborns.

Q: Isocitrate dehydrogenase is linked to which cofactor?

NAD. **Explanation:** **Isocitrate Dehydrogenase (ICDH)** is a critical rate-limiting enzyme of the **TCA cycle (Krebs cycle)**. It catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate. 1. **Why NAD is correct:** In the mitochondrial matrix, the primary isoform of ICDH involved in the TCA cycle utilizes **NAD+** as an electron acceptor, reducing it to **NADH**. This NADH then enters the Electron Transport Chain (Complex I) to generate ATP. 2. **Why other options are incorrect:** * **FAD/FMN:** These are flavin nucleotides. FAD is specifically used by **Succinate Dehydrogenase** (Complex II) in the TCA cycle. ICDH does not utilize flavin cofactors. * **NADP:** While an NADP-dependent isoform of ICDH exists (found in the cytosol and mitochondria), it is primarily involved in providing NADPH for reductive biosynthesis and antioxidant defense, not the primary energy-yielding steps of the TCA cycle. In the context of standard metabolic questions, the NAD-linked mitochondrial enzyme is the intended answer. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** Isocitrate dehydrogenase is the most important rate-limiting enzyme of the TCA cycle. * **Regulation:** It is allosterically **activated by ADP and Ca²⁺** and **inhibited by ATP and NADH**. * **First Decarboxylation:** This reaction marks the first of two CO₂ molecules released in the TCA cycle. * **IDH Mutations:** Mutations in IDH1 and IDH2 are significant markers in neuro-oncology (e.g., Gliomas) and AML, leading to the production of the oncometabolite 2-hydroxyglutarate.

Question 1

What is true about ribozymes?

Accepted Answer

Peptidyl transferase activity

Answer

Cut DNA at a specific site

Answer

Participate in DNA synthesis

Answer

GTPase activity

Question 2

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

Accepted Answer

The enzyme exhibits cooperative binding.

Answer

The substrate and the modifier bind to the same site on the enzyme.

Answer

The substrate and the modifier are structural analogues.

Answer

The enzyme's quantity is regulated.

Question 3

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

Accepted Answer

Substrate of the Reaction - No; Allosteric Inhibitors - No; Competitive Inhibitors - Yes; Non-competitive Inhibitors - No

Answer

Substrate of the Reaction - Yes; Allosteric Inhibitors - Yes; Competitive Inhibitors - Yes; Non-competitive Inhibitors - Yes

Answer

Substrate of the Reaction - Yes; Allosteric Inhibitors - No; Competitive Inhibitors - Yes; Non-competitive Inhibitors - Yes

Answer

Substrate of the Reaction - No; Allosteric Inhibitors - No; Competitive Inhibitors - No; Non-competitive Inhibitors - No

Question 4

Which enzyme is activated by covalent phosphorylation?

Accepted Answer

Glycogen phosphorylase

Answer

Acetyl CoA carboxylase

Answer

HMG CoA reductase

Answer

Pyruvate carboxylase

Question 5

Fumarase is classified as which type of enzyme?

Accepted Answer

Lyase

Answer

Ligase

Answer

Hydrolase

Answer

None of the above

Question 6

Dehydrogenation of succinic acid to fumaric acid requires which of the following hydrogen carriers?

Accepted Answer

Flavoprotein

Answer

NAD+

Answer

NADP+

Answer

Glutathione

Question 7

Which of the following is NOT a true statement about glucokinase?

Accepted Answer

It is inhibited by Glucose-6-phosphate.

Answer

Its Km value is higher than normal blood glucose levels.

Answer

It is found in the liver.

Answer

Glucose enters cells via GLUT2 transporters when glucokinase is involved.

Question 8

Which of the following enzymes catalyzes the formation of AMP from two molecules of ADP?

Accepted Answer

Adenylate kinase

Answer

Adenylyl cyclase

Answer

Adenosine kinase

Answer

Adenosine deaminase

Question 9

Trypsin is a:

Accepted Answer

Serine protease

Answer

Lecithinase

Answer

Phospholipase

Answer

Elastase

Question 10

Isocitrate dehydrogenase is linked to which cofactor?

Accepted Answer

NAD

Answer

FAD

Answer

NADP

Answer

FMN

Enzymes — MCQs

Enzymes — MCQs

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