Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 71: Cytochromes are

A. Pyridine nucleotides
B. Metal containing flavoproteins
C. Peroxidases
D. Iron-porphyrin proteins (Correct Answer)

Explanation: **Explanation:** **Why the Correct Answer is Right:** Cytochromes are a group of colored proteins (pigments) that function as electron carriers in the Electron Transport Chain (ETC) and oxidative metabolism. Structurally, they are **hemeproteins**, meaning they consist of a protein part conjugated to an **iron-porphyrin ring (Heme)**. The iron atom within the porphyrin ring undergoes reversible valency changes between the ferrous ($Fe^{2+}$) and ferric ($Fe^{3+}$) states, allowing them to transfer electrons. **Analysis of Incorrect Options:** * **A. Pyridine nucleotides:** These refer to $NAD^+$ and $NADP^+$, which are coenzymes derived from Niacin (Vitamin $B_3$). They do not contain porphyrin rings. * **B. Metal containing flavoproteins:** These are enzymes containing Riboflavin (Vitamin $B_2$) derivatives like FMN or FAD (e.g., Succinate dehydrogenase). While they may contain iron-sulfur centers, they are not porphyrin-based. * **C. Peroxidases:** While some peroxidases (like Catalase) are hemeproteins, "Cytochromes" as a class are specifically defined by their role in electron transfer rather than being classified as peroxidases. **High-Yield Clinical Pearls for NEET-PG:** * **Cytochrome $a_3$:** Contains **Copper** ($Cu$) in addition to iron; it is the only cytochrome that can react directly with molecular oxygen (part of Complex IV). * **Inhibitors:** Cyanide, Carbon Monoxide (CO), and Azide inhibit Cytochrome oxidase (Complex IV), halting the ETC. * **Cytochrome P450:** Located in the endoplasmic reticulum (microsomes), it is crucial for Phase I drug metabolism and steroidogenesis. * **Cytochrome c:** A peripheral membrane protein that, when released into the cytosol, acts as a key trigger for **Apoptosis** (Intrinsic pathway).

Question 72: Pepsin is a type of enzyme classified as:

A. Oxidoreductase
B. Ligase
C. Transferase
D. Hydrolase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Hydrolase (Option D)**. Enzymes are classified into six major classes by the IUBMB (International Union of Biochemistry and Molecular Biology) based on the type of reaction they catalyze. **Pepsin** is a proteolytic enzyme (protease) found in the stomach that breaks down proteins into smaller peptides. It functions by catalyzing the **hydrolytic cleavage** of peptide bonds—a process where a water molecule is added to break a chemical bond. All digestive enzymes (e.g., trypsin, amylase, lipase) are classified as Hydrolases (Class 3). **Why other options are incorrect:** * **Oxidoreductases (Class 1):** Catalyze oxidation-reduction reactions (e.g., LDH, Cytochrome oxidase). Pepsin does not involve electron or hydrogen transfer. * **Transferases (Class 2):** Catalyze the transfer of functional groups (like methyl or phosphate groups) from one substrate to another (e.g., Hexokinase, ALT/AST). * **Ligases (Class 6):** Catalyze the joining of two molecules coupled with the hydrolysis of ATP (e.g., DNA Ligase, Pyruvate carboxylase). Pepsin breaks molecules apart rather than joining them. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Secretion:** Pepsin is secreted as an inactive precursor, **pepsinogen**, by the **Chief cells** (Peptic cells) of the stomach. * **Activation:** It is activated by the acidic pH (HCl) secreted by **Parietal cells**. Once some pepsin is formed, it activates more pepsinogen (Auto-catalysis). * **Specificity:** Pepsin is an endopeptidase that preferentially cleaves peptide bonds involving aromatic amino acids (Phenylalanine, Tyrosine, Tryptophan). * **Optimal pH:** It functions best at a highly acidic pH (~1.5 to 2.5).

Question 73: Carboxylases require which of the following vitamins?

A. Vitamin B12
B. Folic acid
C. Niacin
D. Biotin (Correct Answer)

Explanation: **Explanation:** **Biotin (Vitamin B7)** is the essential coenzyme for all **carboxylase enzymes** that catalyze the addition of a carboxyl group ($CO_2$) to a substrate. It acts as a carrier of activated carbon dioxide. The mechanism involves the covalent attachment of biotin to a lysine residue of the enzyme, forming **biocytin**. **Why Biotin is Correct:** Biotin is required for four key enzymes in human metabolism: 1. **Pyruvate Carboxylase:** Converts pyruvate to oxaloacetate (Gluconeogenesis). 2. **Acetyl-CoA Carboxylase:** Converts Acetyl-CoA to Malonyl-CoA (Fatty acid synthesis). 3. **Propionyl-CoA Carboxylase:** Converts Propionyl-CoA to Methylmalonyl-CoA (Odd-chain fatty acid oxidation). 4. **Methylcrotonyl-CoA Carboxylase:** Involved in leucine catabolism. **Why Other Options are Incorrect:** * **Vitamin B12 (Cobalamin):** Acts as a coenzyme for Methylmalonyl-CoA mutase and Methionine synthase. It is involved in isomerizations and methyl transfers, not carboxylations. * **Folic Acid (B9):** Functions as a carrier of **one-carbon units** (formyl, methyl, etc.) at various oxidation levels, primarily for nucleic acid synthesis. * **Niacin (B3):** Forms NAD+ and NADP+, which are essential for **redox reactions** (electron transfer). **High-Yield Clinical Pearls for NEET-PG:** * **Avidin:** A protein in raw egg whites that binds biotin with high affinity, preventing its absorption and leading to deficiency. * **ABC Enzymes:** Remember that Carboxylases require **A**TP, **B**iotin, and **C**O2. * **Holocarboxylase Synthetase:** The enzyme that attaches biotin to carboxylases; its deficiency leads to Multiple Carboxylase Deficiency.

Question 74: A 44-year-old man sustains a myocardial infarction and is admitted to the hospital. His serum chemistries reveal a two-fold elevation of his LDL cholesterol. He is prescribed lovastatin. Lovastatin acts by inhibiting which of the following enzymes?

A. Acetyl-CoA carboxylase
B. Carbamoyl phosphate synthetase I
C. Hydroxymethylglutaryl-CoA reductase (Correct Answer)
D. Pyruvate dehydrogenase

Explanation: **Explanation:** The patient has hypercholesterolemia (elevated LDL) following a myocardial infarction. **Lovastatin** belongs to the "statin" class of drugs, which are competitive inhibitors of **HMG-CoA Reductase (3-hydroxy-3-methylglutaryl-CoA reductase)**. This enzyme catalyzes the rate-limiting step of de novo cholesterol synthesis: the conversion of HMG-CoA to mevalonate. By inhibiting this enzyme, statins decrease intracellular cholesterol levels, leading to the up-regulation of LDL receptors on hepatocytes, which subsequently increases the clearance of LDL from the plasma. **Analysis of Incorrect Options:** * **A. Acetyl-CoA carboxylase:** This is the rate-limiting enzyme for **fatty acid synthesis** (converting Acetyl-CoA to Malonyl-CoA). It is regulated by insulin and glucagon, not statins. * **B. Carbamoyl phosphate synthetase I (CPS-I):** This is the rate-limiting enzyme of the **Urea Cycle**, located in the mitochondria. It is activated by N-acetylglutamate. * **D. Pyruvate dehydrogenase (PDH):** This multienzyme complex converts pyruvate to Acetyl-CoA, linking glycolysis to the TCA cycle. Deficiency leads to lactic acidosis. **NEET-PG High-Yield Pearls:** * **Mechanism:** Statins are **structural analogs** of HMG-CoA and act via **competitive inhibition**. * **Pleiotropic Effects:** Beyond lowering LDL, statins stabilize atherosclerotic plaques and have anti-inflammatory properties. * **Side Effects:** Most common high-yield side effects are **myopathy/rhabdomyolysis** (monitored via Creatine Kinase) and **hepatotoxicity** (monitored via LFTs). * **Timing:** Cholesterol synthesis is maximal at night; hence, short-acting statins (like Lovastatin) are traditionally administered at bedtime.

Question 75: Which of the following best describes the role of NAD+ in the reaction catalyzed by aldehyde dehydrogenase?

A. Cofactor
B. Apoenzyme
C. Coenzyme (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **Coenzyme**. **1. Why Coenzyme is correct:** Enzymes often require non-protein components called **cofactors** to function. Cofactors are broadly divided into inorganic ions (e.g., $Mg^{2+}$, $Zn^{2+}$) and organic molecules. Organic cofactors are specifically termed **coenzymes**. NAD+ (Nicotinamide Adenine Dinucleotide) is an organic molecule derived from Vitamin B3 (Niacin). In the reaction catalyzed by **Aldehyde Dehydrogenase**, NAD+ acts as an electron acceptor (oxidizing agent), facilitating the conversion of acetaldehyde to acetate. Since it is an organic carrier molecule that is loosely bound to the enzyme, it is classified as a coenzyme. **2. Why other options are incorrect:** * **Cofactor:** While "cofactor" is a broad umbrella term that includes coenzymes, "Coenzyme" is the more specific and accurate description for an organic molecule like NAD+. In medical exams, always choose the most specific term. * **Apoenzyme:** This refers to the **protein portion** of the enzyme that is catalytically inactive without its required cofactor. NAD+ is the non-protein component, not the protein part. **High-Yield Clinical Pearls for NEET-PG:** * **Holoenzyme:** The complete, catalytically active unit (Apoenzyme + Cofactor). * **Prosthetic Group:** A coenzyme that is **tightly or covalently** bound to the enzyme (e.g., FAD, Heme, Biotin). NAD+ is NOT a prosthetic group because it binds loosely and dissociates. * **Clinical Correlation:** Aldehyde Dehydrogenase (ALDH) is inhibited by **Disulfiram**, leading to the accumulation of acetaldehyde, which causes the "Disulfiram-like reaction" (flushing, tachycardia, nausea). * **Vitamin Precursor:** Deficiency of Niacin (the precursor to NAD+) leads to **Pellagra** (3Ds: Dermatitis, Diarrhea, Dementia).

Question 76: Which enzyme is deficient in diabetes mellitus?

A. Glucokinase (Correct Answer)
B. Hexokinase
C. Phosphorylase
D. Pyrophosphate dehydrogenase

Explanation: **Explanation:** In Diabetes Mellitus (DM), the deficiency or relative inactivity of **Glucokinase** is a key metabolic feature. Glucokinase (Hexokinase IV) is primarily found in the liver and pancreatic beta cells. Its expression is **insulin-dependent**; insulin induces the synthesis of this enzyme. In DM, low insulin levels or insulin resistance leads to decreased glucokinase activity, resulting in reduced glucose uptake by the liver and impaired glucose sensing by the pancreas. **Analysis of Options:** * **Glucokinase (Correct):** Unlike other hexokinases, it has a high $K_m$ (low affinity) and high $V_{max}$. It acts as a "glucose sensor." Its deficiency leads to hyperglycemia because the liver cannot efficiently trap glucose as Glucose-6-Phosphate during the fed state. * **Hexokinase:** Found in most extrahepatic tissues, it is **not** induced by insulin. It has a low $K_m$ (high affinity), allowing tissues to take up glucose even at low blood concentrations. It remains functional in DM. * **Phosphorylase:** This enzyme is involved in glycogenolysis (breaking down glycogen). In DM, glucagon action is unopposed, often leading to *increased* activity of glycogen phosphorylase, contributing to hyperglycemia. * **Pyrophosphate dehydrogenase (PDH):** While PDH activity may be secondary decreased due to high Acetyl-CoA levels in DM, it is not the primary enzyme "deficient" in the context of glucose sensing and initial phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **MODY Type 2:** Mutations in the Glucokinase gene cause Maturity-Onset Diabetes of the Young Type 2. * **Glucokinase vs. Hexokinase:** Glucokinase is NOT inhibited by its product (Glucose-6-P), whereas Hexokinase is. * **Location:** Glucokinase is also known as Hexokinase IV. Remember: "Liver and Pancreas = Glucokinase."

Question 77: The adenylate cyclase system is mediated by which of the following molecules?

A. cAMP (Correct Answer)
B. Phosphodiesterase
C. GTP regulating proteins
D. Nuclear receptors

Explanation: **Explanation:** The **Adenylate Cyclase (AC) system** is a major signal transduction pathway used by many hormones (e.g., Glucagon, ACTH, PTH). When a ligand binds to a G-protein coupled receptor (GPCR), it activates Adenylate Cyclase, which catalyzes the conversion of ATP into **cyclic AMP (cAMP)**. 1. **Why cAMP is correct:** cAMP acts as the **second messenger** in this system. It diffuses through the cell to activate **Protein Kinase A (PKA)**, which then phosphorylates specific enzymes to alter cellular activity. Therefore, the physiological effects of the AC system are directly mediated by cAMP. 2. **Why other options are incorrect:** * **Phosphodiesterase (PDE):** This enzyme terminates the signal by degrading cAMP into 5'-AMP. It regulates the system but does not mediate the forward signaling. * **GTP regulating proteins (G-proteins):** While G-proteins (Gs/Gi) are essential for *activating* or *inhibiting* the AC enzyme, they are upstream components of the membrane complex, not the intracellular mediator itself. * **Nuclear receptors:** These are used by lipophilic hormones (like steroids or thyroid hormones) that act directly on DNA. They do not utilize the AC/cAMP second messenger system. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae toxin** works by ADP-ribosylation of the **Gs protein**, keeping Adenylate Cyclase constitutively active, leading to high cAMP and secretory diarrhea. * **Pertussis toxin** inhibits the **Gi protein**, preventing the inhibition of Adenylate Cyclase, also resulting in increased cAMP levels. * **Caffeine and Theophylline** inhibit Phosphodiesterase, thereby prolonging the action of cAMP.

Question 78: Hexokinase is classified as which type of enzyme?

A. Ligase
B. Transferase (Correct Answer)
C. Oxidoreductase
D. Reductase

Explanation: **Explanation:** Hexokinase is the first enzyme of the glycolysis pathway. It catalyzes the conversion of Glucose to Glucose-6-Phosphate by transferring a phosphate group from ATP to the C6 hydroxyl group of glucose. **1. Why Transferase is correct:** According to the IUBMB classification, **Transferases (Class 2)** are enzymes that catalyze the transfer of a functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. Since Hexokinase transfers a phosphoryl group from ATP to a hexose sugar, it belongs to the sub-class of **Kinases**, which are all categorized under Transferases. **2. Why other options are incorrect:** * **Ligases (Class 6):** These enzymes join two molecules together, usually coupled with the hydrolysis of ATP (e.g., DNA Ligase, Pyruvate Carboxylase). Hexokinase does not "join" two large molecules; it modifies one. * **Oxidoreductases (Class 1):** These catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen (e.g., Lactate Dehydrogenase). Hexokinase does not involve a change in oxidation state. * **Reductase:** This is a sub-type of Oxidoreductase. It specifically reduces a substrate (e.g., HMG-CoA Reductase). **Clinical Pearls for NEET-PG:** * **Hexokinase vs. Glucokinase:** Hexokinase is found in most extrahepatic tissues, has a **low Km** (high affinity for glucose), and is inhibited by its product (Glucose-6-P). Glucokinase (Hexokinase IV) is found in the liver and pancreatic beta cells, has a **high Km**, and is NOT inhibited by Glucose-6-P. * **Irreversible Step:** The reaction catalyzed by Hexokinase is one of the three irreversible, rate-limiting steps of glycolysis. * **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 79: What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

A. Production of Lactate
B. Production of ATP
C. Replenishment of NAD+ (Correct Answer)
D. Replenishment of NADH

Explanation: **Explanation:** In anaerobic glycolysis, the primary objective is to maintain a continuous flow of glucose through the glycolytic pathway to generate ATP in the absence of oxygen. **1. Why Option C is Correct:** The enzyme **Glyceraldehyde-3-phosphate dehydrogenase (G3PDH)** requires **NAD+** as a co-factor to convert Glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. In aerobic conditions, NADH is oxidized back to NAD+ via the electron transport chain (ETC). However, in anaerobic conditions (like exercising muscle or in RBCs lacking mitochondria), the ETC cannot function. **Lactate Dehydrogenase (LDH)** solves this by reducing Pyruvate to Lactate, simultaneously oxidizing NADH back to **NAD+**. This replenishment of NAD+ ensures that glycolysis can continue. **2. Why other options are incorrect:** * **Option A:** While lactate is produced, it is a metabolic "end-product" or byproduct. The *purpose* of the reaction is not to make lactate, but to recycle the co-enzyme. * **Option B:** LDH does not directly produce ATP. ATP is produced by Phosphoglycerate kinase and Pyruvate kinase via substrate-level phosphorylation. * **Option D:** LDH *consumes* NADH; it does not replenish it. NADH is generated by G3PDH. **Clinical Pearls & High-Yield Facts:** * **RBCs:** Since RBCs lack mitochondria, they rely exclusively on LDH to regenerate NAD+ for energy. * **LDH Isoenzymes:** LDH is a tetramer. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is predominant in the liver and skeletal muscle. * **Myocardial Infarction:** Historically, an elevation in LDH-1 (causing an "LDH flip" where LDH-1 > LDH-2) was used as a diagnostic marker for MI. * **Warburg Effect:** Cancer cells often utilize anaerobic glycolysis (producing lactate) even in the presence of oxygen to support rapid growth.

Question 80: All of the following are biotin-independent carboxylation reactions, EXCEPT?

A. Addition of CO2 to form C6 in purine ring
B. Conversion of pyruvate to malate by malic enzyme
C. Acetyl-CoA carboxylase (Correct Answer)
D. Carbamoyl phosphate synthetase

Explanation: **Explanation:** The question asks to identify the reaction that **requires** biotin (the exception to the biotin-independent list). Biotin (Vitamin B7) acts as a coenzyme for most **carboxylation reactions** where CO2 is added to a substrate. These enzymes typically follow a "ABC" rule: they require **A**TP, **B**iotin, and **C**O2. **1. Why Acetyl-CoA Carboxylase (ACC) is the correct answer:** Acetyl-CoA carboxylase is a classic **biotin-dependent** enzyme. It catalyzes the rate-limiting step of fatty acid synthesis: the conversion of Acetyl-CoA to Malonyl-CoA. It requires biotin as a carrier to transfer the carboxyl group to the substrate. **2. Why the other options are incorrect (Biotin-Independent):** * **Addition of CO2 to form C6 in purine ring:** This step (catalyzed by AIR carboxylase) is unique because it does not require biotin or ATP; the CO2 is added directly to the imidazole ring. * **Malic Enzyme:** This enzyme converts pyruvate to malate (or vice versa) using NADPH. It is a decarboxylation/carboxylation reaction that does **not** utilize biotin. * **Carbamoyl Phosphate Synthetase (CPS I & II):** These enzymes incorporate CO2 (as bicarbonate) into carbamoyl phosphate for the Urea cycle and Pyrimidine synthesis. They require ATP but are notably **biotin-independent**. **High-Yield Clinical Pearls for NEET-PG:** * **The "Big Four" Biotin-Dependent Enzymes:** 1. **Pyruvate Carboxylase** (Gluconeogenesis) 2. **Acetyl-CoA Carboxylase** (Fatty acid synthesis) 3. **Propionyl-CoA Carboxylase** (Metabolism of VOMIT: Valine, Odd-chain FAs, Methionine, Isoleucine, Threonine) 4. **3-Methylcrotonyl-CoA Carboxylase** (Leucine catabolism) * **Avidin**, a protein in raw egg whites, binds biotin tightly and can lead to deficiency. * Biotin deficiency presents with dermatitis, alopecia, and enteritis.

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Enzymes — MCQs

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