Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 141: The predominant isozyme of lactate dehydrogenase (LDH) in cardiac muscle is:

A. LD-1 (Correct Answer)
B. LD-2
C. LD-3
D. LD-5

Explanation: **Explanation:** Lactate dehydrogenase (LDH) is a tetrameric enzyme composed of two types of subunits: **H (Heart)** and **M (Muscle)**. These subunits combine in five different ways to form isozymes (LD1 to LD5), which are tissue-specific. **Why LD-1 is correct:** LD-1 is composed of four 'H' subunits (**H4**). It is the predominant isozyme found in **cardiac muscle** and **erythrocytes**. Because the heart relies on aerobic metabolism, LD-1 is specialized to favor the conversion of lactate to pyruvate for energy production. **Analysis of incorrect options:** * **LD-2 (H3M1):** Found primarily in the **Reticuloendothelial system** and serum. In a healthy individual, LD-2 is the most abundant isozyme in the blood. * **LD-3 (H2M2):** Predominantly found in the **Lungs** and spleen. * **LD-5 (M4):** Predominantly found in the **Liver** and **Skeletal muscle**. It favors the conversion of pyruvate to lactate under anaerobic conditions. **High-Yield Clinical Pearls for NEET-PG:** 1. **LDH Flip:** Normally, serum LD-2 > LD-1. However, in **Myocardial Infarction (MI)**, LD-1 levels rise significantly, leading to a "flipped pattern" where **LD-1 > LD-2**. 2. **Diagnostic Window:** LDH levels begin to rise 12–24 hours after an MI, peak at 48 hours, and remain elevated for 7–10 days (useful for late diagnosis). 3. **Hemolysis:** Since LD-1 is high in RBCs, any hemolytic condition will cause a significant rise in LD-1. 4. **LD-4:** Found in the kidneys and pancreas.

Question 142: Which of the following enzymes contains copper?

A. Cytochrome oxidase (Correct Answer)
B. Catalase
C. Lactate dehydrogenase (LDH)
D. None of the above

Explanation: **Explanation:** The correct answer is **Cytochrome oxidase** (also known as Complex IV of the Electron Transport Chain). **1. Why Cytochrome Oxidase is correct:** Cytochrome oxidase is a large transmembrane protein complex that contains **two copper centers (CuA and CuB)** and two heme groups ($a$ and $a_3$). These copper ions are essential for the transfer of electrons from cytochrome $c$ to molecular oxygen, reducing it to water. Copper facilitates the redox reactions necessary for ATP production in the mitochondria. **2. Why the other options are incorrect:** * **Catalase:** This is a **heme-containing enzyme** (Iron/Fe). It protects cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide ($H_2O_2$) into water and oxygen. * **Lactate Dehydrogenase (LDH):** This is a glycolytic enzyme that requires **no metal cofactor** (it uses the coenzyme $NAD^+$). However, many other dehydrogenases are zinc-dependent; LDH is not one of them. **3. High-Yield Clinical Pearls for NEET-PG:** * **Copper-containing enzymes (Mnemonics: "C-C-C-S-T-L"):** **C**ytochrome oxidase, **C**eruloplasmin (Ferroxidase), **C**atalase (Note: Catalase is Fe, but **Superoxide Dismutase** is Cu-Zn), **S**uperoxide Dismutase (cytosolic), **T**yrosinase (deficiency leads to Albinism), and **L**ysyl oxidase (essential for collagen cross-linking; deficient in Menkes disease). * **Zinc-containing enzymes:** Carbonic anhydrase, Alcohol dehydrogenase, Carboxypeptidase, and DNA/RNA polymerases. * **Molybdenum-containing enzymes:** Xanthine oxidase and Sulfite oxidase. * **Selenium-containing enzyme:** Glutathione peroxidase.

Question 143: All known effects of cyclic AMP in eukaryotic cells result from which of the following?

A. Activation of the catalytic unit of adenylate cyclase
B. Activation of synthetase
C. Activation of protein kinase (Correct Answer)
D. Phosphorylation of G protein

Explanation: **Explanation:** The correct answer is **C. Activation of protein kinase.** In eukaryotic cells, cyclic AMP (cAMP) acts as a secondary messenger. Its primary and most well-documented mechanism of action is the activation of **Protein Kinase A (PKA)**. **Mechanism:** PKA is a heterotetramer consisting of two regulatory (R) subunits and two catalytic (C) subunits. In its inactive state, the R subunits inhibit the C subunits. When cAMP levels rise, four molecules of cAMP bind to the regulatory subunits, causing a conformational change that releases the active catalytic subunits. These active subunits then phosphorylate specific serine and threonine residues on target proteins, altering their biological activity (e.g., activating glycogen phosphorylase kinase). **Why other options are incorrect:** * **Option A:** Adenylate cyclase is the enzyme that *produces* cAMP from ATP; it is activated by G-proteins (Gs), not by cAMP itself (this would be a circular reaction). * **Option B:** Synthetases (like Glycogen synthase) are generally **inactivated** by the phosphorylation cascade initiated by cAMP, not directly activated by it. * **Option D:** Phosphorylation of G-proteins is a regulatory mechanism for signal termination or desensitization, but it is not the primary pathway through which cAMP exerts its physiological effects. **High-Yield Clinical Pearls for NEET-PG:** * **Signal Termination:** cAMP is degraded into 5'-AMP by the enzyme **Phosphodiesterase (PDE)**. Drugs like Theophylline and Caffeine inhibit PDE, thereby prolonging cAMP action. * **Transcription Factor:** PKA can translocate to the nucleus and phosphorylate **CREB** (cAMP Response Element Binding protein), which regulates gene expression. * **Exceptions:** While PKA is the major effector, cAMP can also directly gate certain ion channels (e.g., in olfactory neurons) and activate **Epac** (Exchange protein directly activated by cAMP).

Question 144: All are true about the Northern blotting technique EXCEPT:

A. It involves electrophoresis.
B. It requires hybridization probes.
C. It is used to detect RNA molecules. (Correct Answer)
D. It requires restriction endonucleases.

Explanation: The question asks for the **EXCEPT** statement regarding Northern blotting. Note that the provided key lists Option C as the "correct" answer (the false statement), but in standard molecular biology, Northern blotting **is** used to detect RNA. However, in the context of competitive exams like NEET-PG, this question typically highlights that **Option D** is the false statement. ### Explanation **Why Option D is the correct answer (The False Statement):** Northern blotting is used to analyze **RNA**. Unlike DNA, RNA molecules are already short enough to be separated by size via electrophoresis and do not possess the double-stranded stability required for consistent recognition by restriction enzymes. Therefore, **Restriction Endonucleases are NOT required** for Northern blotting; they are a hallmark of Southern blotting (DNA analysis). **Analysis of Other Options:** * **Option A (Electrophoresis):** True. RNA fragments are separated based on size using gel electrophoresis (usually formaldehyde-agarose gel to prevent RNA secondary structure formation). * **Option B (Hybridization Probes):** True. After transferring RNA to a membrane (nitrocellulose or nylon), a labeled complementary DNA or RNA probe is used to identify specific sequences. * **Option C (Detects RNA):** True. Northern blotting is the gold standard for measuring gene expression by detecting specific mRNA levels. ### High-Yield Clinical Pearls for NEET-PG: To remember the blotting techniques, use the **SNOW DROP** mnemonic: * **S**outhern = **D**NA (Requires Restriction Endonucleases) * **N**orthern = **R**NA (No Restriction Endonucleases) * **O** = **O** (Nothing) * **W**estern = **P**roteins (Uses Antibodies) * **Southwestern Blotting:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Eastern Blotting:** Used to detect post-translational modifications of proteins.

Question 145: Extracellular binding domain for the ligand is present in all types of receptors except?

A. G-protein coupled receptors
B. Enzyme linked receptors
C. Transcription factors (Correct Answer)
D. None of the above

Explanation: **Explanation:** The location of a receptor is primarily determined by the chemical nature of its ligand. Ligands that are large or hydrophilic cannot cross the lipid bilayer and must bind to **cell surface receptors**, whereas small, lipophilic ligands cross the membrane to bind to **intracellular receptors**. **Why Transcription Factors are the Correct Answer:** Transcription factors (such as **Steroid hormone receptors**, Thyroid hormone receptors, and Vitamin D receptors) are **intracellular receptors**. They are located either in the cytoplasm or directly within the nucleus. Because their ligands (e.g., estrogen, cortisol, thyroxine) are lipid-soluble, they diffuse through the plasma membrane. Therefore, these receptors lack an extracellular binding domain; instead, they possess a ligand-binding domain, a DNA-binding domain (often with zinc fingers), and a transcription-activating domain. **Analysis of Incorrect Options:** * **G-protein coupled receptors (GPCRs):** These are transmembrane receptors with 7-alpha helical passes. They have an **extracellular N-terminus** for ligand binding and an intracellular C-terminus that interacts with G-proteins. * **Enzyme-linked receptors:** Examples include Receptor Tyrosine Kinases (e.g., Insulin receptor). These are single-pass transmembrane proteins with a distinct **extracellular ligand-binding domain** and an intracellular catalytic (enzymatic) domain. **High-Yield Clinical Pearls for NEET-PG:** * **Zinc Fingers:** The DNA-binding domain of steroid receptors (transcription factors) typically contains zinc finger motifs. * **Speed of Action:** GPCRs and Ion-channel receptors act within seconds, while Transcription Factors (Intracellular receptors) take hours to days as they require new protein synthesis. * **Exceptions:** Though most transcription factors are nuclear, the **Glucocorticoid receptor** is primarily cytoplasmic and translocates to the nucleus only after ligand binding.

Question 146: All of the following enzymes are regulated by calcium/calmodulin, EXCEPT?

A. Hexokinase (Correct Answer)
B. Nitric oxide synthase
C. Pyruvate dehydrogenase
D. Phosphatidyl Inositol 3 kinase

Explanation: **Explanation:** The regulation of enzymes by the **Calcium-Calmodulin (Ca²⁺-CaM) complex** is a vital mechanism for integrating intracellular signaling with metabolic activity. Calmodulin acts as a calcium sensor; upon binding Ca²⁺, it undergoes a conformational change that allows it to activate specific target enzymes. **1. Why Hexokinase is the correct answer:** **Hexokinase** is the first enzyme of glycolysis. It is primarily regulated by **allosteric inhibition by its product, Glucose-6-Phosphate**, and is not dependent on the Ca²⁺-CaM complex for its activity. In the liver, its isoenzyme (Glucokinase) is regulated by the Glucokinase Regulatory Protein (GKRP), but neither is Ca²⁺-CaM dependent. **2. Analysis of Incorrect Options:** * **Nitric Oxide Synthase (NOS):** Both endothelial (eNOS) and neuronal (nNOS) isoforms are strictly dependent on the Ca²⁺-CaM complex for activation to produce Nitric Oxide (NO). * **Pyruvate Dehydrogenase (PDH):** The PDH complex is activated by **PDH Phosphatase**, which is stimulated by **Calcium**. This ensures that during muscle contraction (where Ca²⁺ rises), ATP production via the TCA cycle is increased. * **Phosphatidylinositol 3-kinase (PI3K):** Certain isoforms of PI3K are regulated by Ca²⁺-CaM, facilitating signaling pathways involved in cell growth and glucose transport. **High-Yield Clinical Pearls for NEET-PG:** * **Other Ca²⁺-CaM dependent enzymes:** Phosphorylase kinase (key in glycogenolysis), Adenylate cyclase (brain isoform), and Myosin Light Chain Kinase (MLCK). * **Troponin C** is structurally related to Calmodulin and serves as the calcium sensor in skeletal and cardiac muscle. * **Calmodulin** has four binding sites for Calcium (EF-hand motifs).

Question 147: A drug that competes for an active binding site is called:

A. Competitive inhibitor (Correct Answer)
B. Non-competitive inhibitor
C. Covalent inhibitor
D. None of these

Explanation: ### Explanation **1. Why Option A is Correct:** In **competitive inhibition**, the inhibitor molecule structurally resembles the substrate. Because of this similarity, it "competes" with the substrate for the same **active site** on the enzyme. * **Mechanism:** When the inhibitor binds to the active site, it prevents the substrate from binding. * **Kinetics:** This can be overcome by increasing the substrate concentration ($[S]$); therefore, the **$V_{max}$ remains unchanged**, but the **$K_m$ increases** (affinity appears to decrease). **2. Why Other Options are Incorrect:** * **B. Non-competitive inhibitor:** These inhibitors bind to an **allosteric site** (a site other than the active site). They do not compete for the active binding site; instead, they change the enzyme's conformation, reducing its catalytic activity. Here, $V_{max}$ decreases, but $K_m$ remains unchanged. * **C. Covalent inhibitor:** These form strong, irreversible covalent bonds with the enzyme (often at the active site), permanently inactivating it. While they may target the active site, the term "competes" specifically refers to the reversible process defined in Option A. **3. NEET-PG High-Yield Clinical Pearls:** * **Classic Examples of Competitive Inhibitors:** * **Statins** (e.g., Atorvastatin) compete with HMG-CoA for HMG-CoA reductase. * **Methanol poisoning treatment:** Ethanol or Fomepizole competes for Alcohol Dehydrogenase. * **Methotrexate** competes with Dihydrofolate for Dihydrofolate Reductase. * **Sulfonamides** compete with PABA for Dihydropteroate synthase. * **Lineweaver-Burk Plot:** In competitive inhibition, the lines intersect on the **Y-axis** ($1/V_{max}$ is the same).

Question 148: The name Cytochrome P450 is derived from which characteristic?

A. Molecular weight of 450 Daltons
B. Production by 450 genes
C. Absorption of light at 450 nanometers (Correct Answer)
D. Presence of 450 isoforms

Explanation: **Explanation:** The name **Cytochrome P450 (CYP450)** is derived from its unique spectral property. These are hemeproteins that, when in a reduced state (ferrous form, $Fe^{2+}$) and bound to **carbon monoxide (CO)**, exhibit a characteristic absorption peak (Soret peak) at a wavelength of **450 nanometers**. The "P" stands for "pigment," and "450" refers to this specific absorption maximum. **Analysis of Options:** * **Option A (Molecular weight):** Incorrect. The molecular weight of CYP450 enzymes is typically between 45,000 to 55,000 Daltons (45–55 kDa), not 450. * **Option B (450 genes):** Incorrect. While the CYP superfamily is large, humans have approximately 57 functional genes and 58 pseudogenes, not 450. * **Option D (450 isoforms):** Incorrect. There are many isoforms (classified into families and subfamilies like CYP3A4, CYP2D6), but the number 450 does not represent the count of these isoforms. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Primarily found in the smooth endoplasmic reticulum (microsomes) of hepatocytes. * **Function:** They are **Monooxygenases** (Mixed Function Oxidases) involved in Phase I drug metabolism (hydroxylation). * **Key Isoform:** **CYP3A4** is the most abundant isoform in the liver and is responsible for metabolizing nearly 50% of commonly used drugs. * **Inducers vs. Inhibitors:** Knowledge of CYP inducers (e.g., Rifampicin, Phenytoin) and inhibitors (e.g., Ketoconazole, Grapefruit juice) is crucial for understanding drug-drug interactions. * **Requirement:** They require **NADPH** and **NADPH-cytochrome P450 reductase** for their catalytic cycle.

Question 149: The 'Lock and Key' model of enzyme action was proposed by whom?

A. Emil Fischer (Correct Answer)
B. Daniel Koshland
C. Leonor Michaelis
D. Maud Menten

Explanation: **Explanation:** The **Lock and Key model**, proposed by **Emil Fischer in 1894**, is a fundamental concept in enzymology. It posits that the enzyme's active site (the "lock") has a rigid, pre-defined geometric shape that is perfectly complementary to the specific substrate (the "key"). This model explains the high degree of **enzyme specificity**, as only a substrate with the exact matching shape can fit into the active site to initiate a reaction. **Analysis of Options:** * **Emil Fischer (Correct):** He introduced the rigid template concept, laying the groundwork for understanding how enzymes recognize specific molecules. * **Daniel Koshland:** He proposed the **Induced Fit Theory** (1958). Unlike Fischer’s rigid model, Koshland suggested that the active site is flexible and undergoes conformational changes to fit the substrate upon binding. This is currently the more widely accepted model. * **Michaelis and Menten:** Leonor Michaelis and Maud Menten are famous for the **Michaelis-Menten Equation**, which describes enzyme kinetics (the relationship between reaction velocity and substrate concentration), rather than the physical mechanism of binding. **High-Yield Clinical Pearls for NEET-PG:** * **Rigid vs. Flexible:** Remember: Fischer = Rigid (Lock & Key); Koshland = Flexible (Induced Fit). * **Specificity:** The Lock and Key model explains *absolute specificity* (e.g., Urease acting only on Urea). * **Transition State:** Modern enzymology suggests enzymes are actually most complementary to the **transition state** of the reaction, rather than the ground-state substrate, which helps lower the activation energy.

Question 150: Leucine aminopeptidase is elevated in obstruction of which of the following?

A. Ureter
B. Urethra
C. Common bile duct (Correct Answer)
D. Spermatic cord

Explanation: **Explanation:** **Leucine Aminopeptidase (LAP)** is a hydrolytic enzyme found in various tissues, with the highest concentrations located in the **liver, pancreas, and small intestine**. In the liver, it is specifically localized to the biliary canalicular membrane. **Why Common Bile Duct is correct:** LAP serves as a sensitive marker for **cholestasis** (interference with bile flow). When there is an obstruction in the **Common Bile Duct (CBD)**, such as by a gallstone or a tumor, the pressure within the biliary system increases. This leads to the regurgitation of LAP into the bloodstream. Its clinical significance is similar to **Alkaline Phosphatase (ALP)** and **Gamma-Glutamyl Transferase (GGT)**. It is particularly useful in differentiating the source of an elevated ALP; if both ALP and LAP are elevated, the pathology is likely hepatobiliary rather than bone-related. **Why incorrect options are wrong:** * **Ureter and Urethra:** Obstruction in the urinary tract (uropathy) leads to hydronephrosis or urinary retention. While enzymes like LDH or NAG might be studied in renal pathology, LAP is not a marker for urinary obstruction. * **Spermatic Cord:** Obstruction here (e.g., torsion or vasectomy) affects sperm transport or blood flow to the testes but has no correlation with hepatobiliary enzymes like LAP. **High-Yield Clinical Pearls for NEET-PG:** * **LAP vs. ALP:** LAP levels remain **normal in bone diseases** (e.g., Paget’s, rickets), making it a specific tool to confirm that an elevated ALP is of hepatic origin. * **Pregnancy:** LAP levels rise significantly during the third trimester of pregnancy (produced by the placenta), so it is less specific for liver disease in pregnant patients. * **Other Cholestatic Markers:** Always correlate LAP with **GGT** and **5'-Nucleotidase** for hepatobiliary questions.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

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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