Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 381: According to the IUB classification of enzymes, what does the fourth digit in an enzyme's code refer to?

A. The main class
B. The subclass
C. The sub-subclass
D. The individual enzyme (Correct Answer)

Explanation: The International Union of Biochemistry (IUB) developed a systematic numerical nomenclature known as the **Enzyme Commission (EC) number**. This system classifies enzymes based on the specific chemical reaction they catalyze, using a four-digit code (e.g., EC 2.7.1.1 for Hexokinase). ### **Breakdown of the EC Number:** * **1st Digit (Main Class):** Represents the general type of reaction (e.g., 1: Oxidoreductases, 2: Transferases). There are currently 7 major classes (Mnemonic: **OTH LIL** – Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases, and the recently added Translocases). * **2nd Digit (Subclass):** Refers to the specific group or type of bond acted upon (e.g., for transferases, it identifies the group being transferred). * **3rd Digit (Sub-subclass):** Further narrows down the reaction, often identifying the specific co-enzyme or acceptor molecule involved. * **4th Digit (Individual Enzyme):** This is the **serial number** or specific identifier for the individual enzyme within its sub-subclass. It distinguishes the enzyme from others that perform similar but distinct reactions. ### **Why Other Options are Incorrect:** * **Option A:** The first digit represents the main class. * **Option B:** The second digit represents the subclass. * **Option C:** The third digit represents the sub-subclass. ### **High-Yield Clinical Pearls for NEET-PG:** * **Class 7 (Translocases):** This is the newest addition to the IUB classification, encompassing enzymes that catalyze the movement of ions or molecules across membranes (e.g., ATPases). * **Isoenzymes:** These are enzymes that catalyze the same reaction (same EC number) but differ in physical/chemical properties and amino acid sequence (e.g., LDH1 vs. LDH5). * **Pro-enzymes (Zymogens):** Inactive precursors (e.g., Pepsinogen) that require cleavage to become active.

Question 382: Fumarase is an example of which class of enzymes?

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

Explanation: **Explanation:** **1. Why Lyase is the correct answer:** Enzymes are classified by the IUBMB into seven classes based on the reactions they catalyze. **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** (fumarate hydratase) catalyzes the reversible addition of a water molecule across the double bond of fumarate to form L-malate in the TCA cycle. Since it adds water to a double bond without breaking a bond via hydrolysis, it is classified as a Lyase (specifically a hydro-lyase). **2. Why other options are incorrect:** * **Hydrolases (Class 3):** These enzymes catalyze the cleavage of bonds (like ester, peptide, or glycosidic bonds) by the **addition of water**. While Fumarase involves water, it does not "split" a molecule into two smaller components via water; it adds water to a double bond. * **Ligases (Class 6):** These enzymes catalyze the joining of two large molecules, coupled with the **hydrolysis of ATP** (e.g., Pyruvate carboxylase). Fumarase does not require ATP for its reaction. **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. * **Oncogene Link:** Mutations in the fumarate hydratase (FH) gene are associated with **Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC)**, as fumarate acts as an "oncometabolite" when it accumulates. * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** **T** (Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase, Translocase).

Question 383: Which of the following is NOT a rate-limiting enzyme?

A. ALA synthase
B. Phosphofructokinase
C. Acetyl CoA carboxylase
D. Malonate dehydrogenase (Correct Answer)

Explanation: **Explanation:** In metabolic pathways, a **rate-limiting enzyme** is typically the slowest step (bottleneck), often regulated by allosteric effectors or hormones to control the flux of the entire pathway. **Why Malonate Dehydrogenase is the correct answer:** There is no enzyme called "Malonate dehydrogenase" in human metabolism. **Malonate** is actually a classic **competitive inhibitor** of the enzyme *Succinate dehydrogenase* in the TCA cycle. It mimics the structure of succinate but cannot be dehydrogenated. Therefore, it is a pharmacological/biochemical tool rather than a functional rate-limiting enzyme. **Analysis of Incorrect Options:** * **ALA Synthase (Aminolevulinic acid synthase):** This is the key rate-limiting and committed step of **Heme synthesis**. It requires Pyridoxal Phosphate (B6) as a cofactor and is feedback-inhibited by Heme. * **Phosphofructokinase-1 (PFK-1):** This is the most important rate-limiting enzyme of **Glycolysis**. It is allosterically activated by Fructose-2,6-bisphosphate and AMP, and inhibited by ATP and Citrate. * **Acetyl CoA Carboxylase (ACC):** This is the rate-limiting step for **Fatty Acid Synthesis**. It converts Acetyl CoA to Malonyl CoA and is activated by Citrate and inhibited by Palmitoyl-CoA. **Clinical Pearls for NEET-PG:** * **HMG-CoA Reductase:** Rate-limiting for Cholesterol synthesis (Target of Statins). * **Carbamoyl Phosphate Synthetase I (CPS-I):** Rate-limiting for the Urea Cycle (Activated by N-acetylglutamate). * **Fructose-1,6-Bisphosphatase:** Rate-limiting for Gluconeogenesis. * **Glycogen Synthase:** Rate-limiting for Glycogenesis.

Question 384: Which of the following acts as a coenzyme and not as a co-factor?

A. Ascorbic acid (Correct Answer)
B. Biotin
C. Thiamine
D. Folic acid

Explanation: **Explanation:** The distinction between a **coenzyme** and a **cofactor** lies in their chemical nature and function. While "cofactor" is often used as a broad term for any non-protein molecule required for enzyme activity, in a strict biochemical sense, **coenzymes** are complex organic molecules (often derived from B-vitamins) that act as transient carriers of specific functional groups. **Why Ascorbic Acid (Vitamin C) is the correct answer:** Ascorbic acid is unique because it functions primarily as a **co-antioxidant** and a **reducing agent** rather than a traditional coenzyme. In reactions like the hydroxylation of proline and lysine (essential for collagen synthesis), it maintains the iron atom of the enzyme (prolyl hydroxylase) in its reduced ferrous ($Fe^{2+}$) state. It does not carry a specific functional group to the substrate, which is the hallmark of a coenzyme. Therefore, in many classification systems, it is considered a cofactor/reducing agent rather than a classic coenzyme. **Analysis of Incorrect Options:** * **Biotin (B7):** A classic coenzyme for **carboxylation** reactions (e.g., Pyruvate carboxylase). It carries $CO_2$. * **Thiamine (B1):** As Thiamine Pyrophosphate (TPP), it is a coenzyme for **oxidative decarboxylation** (e.g., Pyruvate dehydrogenase) and transketolase reactions. * **Folic Acid (B9):** As Tetrahydrofolate (THF), it is the essential coenzyme for **one-carbon metabolism** (transferring methyl, formyl, etc., groups). **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Ascorbic acid leads to defective collagen cross-linking due to failure of proline hydroxylation, presenting with bleeding gums and petechiae. * **Prosthetic Group:** If a coenzyme is covalently or very tightly bound to the enzyme (like Biotin or FAD), it is specifically called a prosthetic group. * **Metal Ions:** Inorganic elements like $Zn^{2+}$ (Carbonic anhydrase) or $Mg^{2+}$ (Hexokinase) are strictly referred to as metal ion cofactors.

Question 385: Which one of the following enzymes is not a protein, but an RNA molecule?

A. Peptidyl transferase (Correct Answer)
B. RNA polymerase
C. Restriction endonuclease
D. Reverse transcriptase

Explanation: ### Explanation The correct answer is **Peptidyl transferase**. **1. Why Peptidyl Transferase is Correct:** Traditionally, all enzymes were thought to be proteins. However, **ribozymes** are RNA molecules that possess catalytic activity. Peptidyl transferase is a classic example of a ribozyme. It is located in the large ribosomal subunit (28S rRNA in eukaryotes and 23S rRNA in prokaryotes). During translation, it catalyzes the formation of peptide bonds between amino acids. Because the catalytic activity resides in the RNA component rather than a protein, it is classified as a non-protein enzyme. **2. Why the Other Options are Incorrect:** * **RNA Polymerase (B):** This is a complex protein enzyme responsible for synthesizing RNA from a DNA template during transcription. * **Restriction Endonuclease (C):** These are bacterial protein enzymes (often called "molecular scissors") used in recombinant DNA technology to cut DNA at specific palindromic sequences. * **Reverse Transcriptase (D):** This is an RNA-dependent DNA polymerase protein, famously found in retroviruses like HIV, which synthesizes DNA from an RNA template. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other Ribozymes:** Apart from Peptidyl transferase, other notable ribozymes include **RNase P** (involved in tRNA processing) and **SnRNAs** (involved in splicing). * **Antibiotic Link:** Many antibiotics target the 50S subunit (e.g., Chloramphenicol), effectively inhibiting the peptidyl transferase activity of the ribozyme. * **Abzymes:** These are catalytic antibodies (proteins) that mimic enzymes; do not confuse them with ribozymes (RNA).

Question 386: Pentostatin acts by inhibiting which enzyme?

A. RNA dependent DNA polymerase
B. Aldolase
C. Adenosine deaminase (Correct Answer)
D. Adenylyl cyclase

Explanation: **Explanation:** **Pentostatin** (also known as 2'-deoxycoformycin) is a potent transition-state analog that irreversibly inhibits **Adenosine Deaminase (ADA)**. 1. **Mechanism of Action:** ADA is a critical enzyme in the purine salvage pathway that converts adenosine to inosine and deoxyadenosine to deoxyinosine. By inhibiting ADA, Pentostatin leads to an intracellular accumulation of **deoxyadenosine triphosphate (dATP)**. High levels of dATP are toxic to lymphocytes as they inhibit ribonucleotide reductase, thereby halting DNA synthesis and inducing apoptosis. 2. **Clinical Application:** Because it specifically targets lymphocytes, Pentostatin is primarily used as a chemotherapeutic agent in the treatment of **Hairy Cell Leukemia** and certain T-cell lymphomas. **Analysis of Incorrect Options:** * **A. RNA-dependent DNA polymerase:** Also known as Reverse Transcriptase; this is the target of NRTIs (like Zidovudine) used in HIV treatment. * **B. Aldolase:** An enzyme in glycolysis (Aldolase A) and fructose metabolism (Aldolase B). Deficiency of Aldolase B leads to Hereditary Fructose Intolerance. * **D. Adenylyl cyclase:** This enzyme converts ATP to cAMP. It is regulated by G-proteins and is the target of various bacterial toxins (e.g., Cholera toxin, Pertussis toxin) rather than Pentostatin. **High-Yield NEET-PG Pearls:** * **ADA Deficiency:** A genetic deficiency of Adenosine Deaminase is the second most common cause of **Autosomal Recessive SCID** (Severe Combined Immunodeficiency). * **Drug of Choice:** While Pentostatin is effective, **Cladribine** (a purine analog) is currently considered the first-line treatment for Hairy Cell Leukemia. * **Transition State Analog:** Pentostatin is a classic example of a drug that mimics the transition state of a substrate to achieve high-affinity enzyme inhibition.

Question 387: Which among the following controls is an allosteric inhibitor of the TCA cycle?

A. Pyruvate dehydrogenase
B. Ketoglutarate dehydrogenase
C. Isocitrate dehydrogenase
D. Malate dehydrogenase (Correct Answer)

Explanation: **Explanation** The Citric Acid Cycle (TCA) is regulated primarily by the energy status of the cell, signaled by ratios of ATP/ADP and NADH/NAD⁺. **Why Malate Dehydrogenase (MDH) is the correct answer:** Malate dehydrogenase catalyzes the final step of the TCA cycle, converting Malate to Oxaloacetate. This reaction is highly endergonic ($\Delta G^\circ$ is positive), meaning it is naturally unfavorable. It is strictly controlled by the **NADH/NAD⁺ ratio**. When NADH levels are high (signaling high energy), **NADH acts as a potent allosteric inhibitor** of MDH. This feedback inhibition prevents the unnecessary accumulation of oxaloacetate and slows the cycle when energy supplies are sufficient. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** While inhibited by NADH and Acetyl-CoA, PDH is technically a **link reaction** enzyme and not a component of the TCA cycle itself. * **Isocitrate Dehydrogenase (ICD):** This is the **rate-limiting enzyme** of the TCA cycle. It is allosterically *activated* by ADP and inhibited by ATP and NADH. While it is a control point, MDH is the specific answer often tested in the context of product-based allosteric inhibition in this question format. * **$\alpha$-Ketoglutarate Dehydrogenase:** This enzyme is inhibited by its products, Succinyl-CoA and NADH, but it is not the primary answer when MDH is provided as a specific inhibitory control point. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** Isocitrate Dehydrogenase. * **Only membrane-bound enzyme:** Succinate Dehydrogenase (also part of Complex II of ETC). * **Substrate-level phosphorylation:** Occurs at the Succinate Thiokinase (Succinyl-CoA Synthetase) step, producing GTP. * **Fluoroacetate:** A potent inhibitor of the TCA cycle (inhibits Aconitase).

Question 388: Plasma cholinesterase is reduced in all of the following conditions except?

A. Pregnancy
B. Liver disease
C. Malnutrition
D. Hemolysis (Correct Answer)

Explanation: ### Explanation **Plasma Cholinesterase (Pseudocholinesterase)** is a glycoprotein enzyme synthesized by the **liver** and secreted into the blood. It is clinically significant because it hydrolyzes drugs like succinylcholine and mivacurium. **Why Hemolysis is the correct answer:** Hemolysis involves the destruction of Red Blood Cells (RBCs). While RBCs contain **True Cholinesterase** (Acetylcholinesterase), they do not contain Plasma Cholinesterase. Therefore, the breakdown of RBCs does not decrease the levels of plasma cholinesterase; in fact, it has no significant effect on its concentration. **Why the other options are incorrect:** * **Liver Disease:** Since the liver is the primary site of synthesis for plasma cholinesterase, any hepatic dysfunction (cirrhosis, hepatitis) leads to decreased production and lower plasma levels. * **Pregnancy:** Plasma cholinesterase levels naturally decrease by about 20–30% during the first trimester and remain low until delivery, likely due to hemodilution and hormonal changes. * **Malnutrition:** As a protein synthesized by the liver, its levels drop in states of protein-energy malnutrition (like Kwashiorkor) due to a lack of substrate for protein synthesis. **Clinical Pearls for NEET-PG:** 1. **Succinylcholine Apnea:** Patients with inherited or acquired deficiency of plasma cholinesterase cannot metabolize succinylcholine, leading to prolonged neuromuscular blockade and respiratory paralysis. 2. **Organophosphate Poisoning:** Plasma cholinesterase is a sensitive marker for organophosphate poisoning (it decreases earlier than RBC cholinesterase), though RBC cholinesterase is more specific for monitoring chronic exposure. 3. **True vs. Pseudo:** Remember, **True** cholinesterase is found in the **N**erve endings, **G**ray matter, and **R**BCs (Mnemonic: **NGR**), while **Pseudo**cholinesterase is found in the **P**lasma, **L**iver, and **W**hite matter (Mnemonic: **PLW**).

Question 389: The Meister cycle uses which enzyme?

A. Alanine aminotransferase (ALT)
B. Aspartate aminotransferase (AST)
C. Gamma-glutamyl transferase (GGT) (Correct Answer)
D. Alkaline Phosphatase

Explanation: **Explanation:** The **Meister Cycle** (also known as the **$\gamma$-glutamyl cycle**) is the primary metabolic pathway responsible for the transport of amino acids across cell membranes, particularly in the kidneys and intestines. **Why Gamma-glutamyl transferase (GGT) is correct:** GGT is the key membrane-bound enzyme of this cycle. It facilitates the transfer of the $\gamma$-glutamyl moiety from **Glutathione** (GSH) to an extracellular amino acid. This reaction forms a $\gamma$-glutamyl amino acid complex, which can then be transported into the cell. Inside the cell, the amino acid is released, and glutathione is resynthesized in a series of ATP-dependent steps. **Why the other options are incorrect:** * **ALT and AST (Options A & B):** These are transaminases involved in amino acid catabolism and gluconeogenesis (transferring amino groups to $\alpha$-ketoglutarate). They are markers of hepatocellular injury but play no role in the Meister cycle. * **Alkaline Phosphatase (Option D):** This enzyme is involved in removing phosphate groups from molecules and is a marker for cholestasis or bone turnover; it is unrelated to glutathione metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Requirement:** The Meister cycle requires **3 molecules of ATP** to transport a single amino acid, making it an energy-expensive process. * **GGT as a Marker:** In clinical practice, GGT is a highly sensitive marker for **cholestasis** and **alcohol consumption** (due to enzyme induction). * **5-Oxoprolinuria:** A deficiency in enzymes of the Meister cycle (like glutathione synthetase) leads to the accumulation of 5-oxoproline (pyroglutamic acid), causing metabolic acidosis.

Question 390: NAD+ acts as a coenzyme for which of the following enzymes?

A. Xanthine oxidase
B. L-amino acid oxidase
C. Succinate dehydrogenase
D. Malate dehydrogenase (Correct Answer)

Explanation: ### Explanation **1. Why Malate Dehydrogenase is Correct:** Malate dehydrogenase is a key enzyme in the **TCA cycle** and the **Malate-Aspartate shuttle**. It catalyzes the reversible oxidation of L-malate to oxaloacetate. This reaction involves the transfer of two electrons and a proton to **NAD+**, reducing it to NADH + H⁺. In biochemistry, most dehydrogenases involved in the oxidation of hydroxyl groups (like malate or lactate) to carbonyl groups utilize NAD+ as the electron acceptor. **2. Analysis of Incorrect Options:** * **Xanthine oxidase (Option A):** This enzyme, involved in purine catabolism, is a metalloflavoprotein. It utilizes **FAD** and Molybdenum as cofactors, not NAD+. * **L-amino acid oxidase (Option B):** This enzyme catalyzes the oxidative deamination of amino acids. It is a flavoprotein that uses **FMN** (Flavin Mononucleotide) as its coenzyme. * **Succinate dehydrogenase (Option C):** A unique enzyme that is part of both the TCA cycle and Complex II of the Electron Transport Chain. It utilizes **FAD** (covalently bound) because the free energy change of succinate oxidation is insufficient to reduce NAD+. **3. Clinical Pearls & High-Yield Facts:** * **Niacin (Vitamin B3):** NAD+ and NADP+ are derived from Niacin. Deficiency leads to **Pellagra** (3Ds: Dermatitis, Diarrhea, Dementia). * **NAD+ vs. NADPH:** Generally, **NAD+** is used in **catabolic** pathways (oxidative), while **NADPH** is used in **anabolic** pathways (reductive biosynthesis like fatty acid synthesis) and to maintain reduced glutathione. * **Mnemonic:** Most "Dehydrogenases" use NAD+, EXCEPT for **Succinate Dehydrogenase**, **Acyl-CoA Dehydrogenase**, and **Glycerol-3-Phosphate Dehydrogenase (mitochondrial)**, which use FAD.

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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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