Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 51: Which of the following is a NAD-dependent enzyme?

A. Glucose 6 Phosphate Dehydrogenase
B. Malate dehydrogenase
C. Fatty acyl CoA dehydrogenase
D. Succinate dehydrogenase (Correct Answer)

Explanation: ### Explanation The question asks to identify the NAD-dependent enzyme. However, based on biochemical principles, there is a discrepancy in the provided key: **Succinate Dehydrogenase is actually FAD-dependent**, while **Malate Dehydrogenase is NAD-dependent**. Let’s clarify the coenzyme requirements for each: #### 1. Why Malate Dehydrogenase is the standard NAD-dependent enzyme: In the Citric Acid Cycle (TCA), **Malate Dehydrogenase** catalyzes the conversion of Malate to Oxaloacetate. This reaction reduces **$\text{NAD}^+$ to $\text{NADH} + \text{H}^+$**. Most dehydrogenases in the TCA cycle (Isocitrate DH, $\alpha$-Ketoglutarate DH, and Malate DH) utilize NAD as the electron acceptor. #### 2. Analysis of the Options: * **Succinate Dehydrogenase (Option D):** This enzyme converts Succinate to Fumarate. It is unique because it is embedded in the inner mitochondrial membrane (Complex II of ETC) and uses **FAD** as a prosthetic group, not NAD. * **Glucose-6-Phosphate Dehydrogenase (Option A):** This is the rate-limiting enzyme of the Pentose Phosphate Pathway (HMP Shunt). It specifically uses **NADP+** to produce NADPH for reductive biosynthesis. * **Fatty Acyl CoA Dehydrogenase (Option C):** Involved in the first step of $\beta$-oxidation, this enzyme utilizes **FAD** to create a double bond and produce $\text{FADH}_2$. #### Clinical Pearls for NEET-PG: * **Mnemonic for TCA Coenzymes:** "3-1-1" — 3 NADH produced (Isocitrate, $\alpha$-KG, Malate), 1 $\text{FADH}_2$ (Succinate), and 1 GTP. * **Succinate Dehydrogenase:** It is the only enzyme of the TCA cycle that is **membrane-bound** (Inner Mitochondrial Membrane) and also functions as **Complex II** of the Electron Transport Chain. It is inhibited by **Malonate** (competitive inhibition). * **NADP+ vs NAD:** Remember that NADP+ is generally used in **anabolic** pathways (synthesis), while NAD+ is used in **catabolic** pathways (breakdown).

Question 52: Which of the following enzymes does not belong to the isomerase class?

A. Mutase
B. Racemase
C. Phosphohexoisomerase
D. Transaminase (Correct Answer)

Explanation: **Explanation:** Enzymes are classified into six major functional classes based on the International Union of Biochemistry (IUB) system. **Isomerases (Class 5)** catalyze structural or geometric changes within a single molecule, converting one isomer into another (e.g., D-form to L-form or aldose to ketose). **Why Transaminase is the correct answer:** **Transaminases** (also known as Aminotransferases) belong to the **Transferase (Class 2)** category. They catalyze the transfer of an amino group (–NH₂) from an amino acid to a keto acid (typically α-ketoglutarate). This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory co-factor. Since it involves the transfer of a functional group between two different molecules rather than an internal rearrangement, it is not an isomerase. **Analysis of Incorrect Options:** * **Mutase:** These are isomerases that catalyze the internal transfer of a functional group (like a phosphate) from one position to another within the same molecule (e.g., Phosphoglycerate mutase). * **Racemase:** These catalyze the interconversion of stereoisomers, specifically between D and L enantiomers (e.g., Alanine racemase). * **Phosphohexoisomerase:** This enzyme catalyzes the conversion of Glucose-6-phosphate (an aldose) to Fructose-6-phosphate (a ketose) in glycolysis, a classic example of an isomerase reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** (**O**xidoreductase, **T**ransferase, **H**ydrolase, **L**yase, **I**somerase, **L**igase). * **Transaminase Markers:** ALT (SGPT) and AST (SGOT) are critical clinical markers for liver injury. * **Isomerase in Glycolysis:** Triose phosphate isomerase is considered a "catalytically perfect" enzyme.

Question 53: Which enzyme is regulated by phosphorylation?

A. Glucokinase
B. Glycogen synthetase (Correct Answer)
C. Pyruvate dehydrogenase
D. Isocitrate

Explanation: **Explanation:** **1. Why Glycogen Synthetase is Correct:** Glycogen synthetase is the rate-limiting enzyme of glycogenesis. It is regulated by **covalent modification** through phosphorylation/dephosphorylation. A key concept in metabolism is that most regulatory enzymes of glycogen metabolism and glycolysis are **inactive when phosphorylated** (with the notable exception of Glycogen Phosphorylase). * **Glycogen Synthetase 'a'** (Dephosphorylated) = Active. * **Glycogen Synthetase 'b'** (Phosphorylated) = Inactive. Glucagon and Epinephrine trigger phosphorylation via Protein Kinase A to inhibit glycogen synthesis during fasting or stress. **2. Analysis of Incorrect Options:** * **A. Glucokinase:** Regulated primarily by **compartmentalization** (binding to Glucokinase Regulatory Protein - GKRP) and induced by insulin at the transcriptional level, but not by direct phosphorylation. * **C. Pyruvate Dehydrogenase (PDH):** While PDH is indeed regulated by phosphorylation (inactive when phosphorylated), in the context of standard medical examinations, **Glycogen Synthetase** is the classic textbook example of reciprocal hormonal regulation via the cAMP cascade. *Note: If this were a multiple-choice question where PDH was also considered correct, Glycogen Synthetase remains the more "high-yield" primary answer for glycogen metabolism questions.* * **D. Isocitrate:** This is a substrate/intermediate in the TCA cycle, not an enzyme. The enzyme is Isocitrate Dehydrogenase, which is regulated by allosteric effectors (ADP, NADH), not phosphorylation. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "Phosphorylation = Off" for most rate-limiting enzymes in carbohydrate metabolism (e.g., Glycogen synthase, PFK-2, Pyruvate Kinase), except for **Glycogen Phosphorylase**, which turns **ON** when phosphorylated. * **Insulin** acts via phosphatases to dephosphorylate and activate Glycogen Synthetase. * **Glucagon** acts via kinases to phosphorylate and inhibit it.

Question 54: Dehydrogenases use all of the following as coenzymes, except?

A. FMN
B. FAD
C. NADOP+
D. Ferriprotoporphyrin (Correct Answer)

Explanation: ### Explanation **Core Concept:** Dehydrogenases are a subclass of **oxidoreductases** that catalyze the removal of hydrogen atoms from a substrate, transferring them to a specific electron carrier (coenzyme). These enzymes are highly dependent on nicotinamide and flavin nucleotides to function as hydrogen acceptors. **Why Ferriprotoporphyrin is the Correct Answer:** Ferriprotoporphyrin (also known as **Heme**) is a prosthetic group characterized by a porphyrin ring complexed with iron ($Fe^{3+}$). While it is essential for oxygen transport (hemoglobin) and electron transfer in the respiratory chain (**Cytochromes**), it does **not** act as a coenzyme for dehydrogenases. Dehydrogenases specifically transfer protons ($H^+$) and electrons, whereas heme-containing proteins like Cytochrome C oxidase or Catalase are involved in direct electron transfer or peroxide breakdown. **Analysis of Incorrect Options:** * **NAD+ / NADP+:** These are nicotinamide derivatives (Vitamin B3). NAD+ is a universal coenzyme for dehydrogenases in glycolysis and the TCA cycle (e.g., Lactate dehydrogenase). NADP+ is typically used in reductive biosynthesis (e.g., G6PD). * **FMN / FAD:** These are riboflavin derivatives (Vitamin B2). They act as prosthetic groups for flavoproteins. FAD is the coenzyme for Succinate dehydrogenase, while FMN is used by NADH dehydrogenase (Complex I). **NEET-PG Clinical Pearls:** * **Succinate Dehydrogenase:** A high-yield fact is that it is the only enzyme of the TCA cycle located in the **inner mitochondrial membrane** (also part of Complex II). * **Niacin Deficiency (Pellagra):** Leads to a deficiency in NAD+/NADP+, impairing dehydrogenase activity across multiple metabolic pathways. * **Riboflavin Deficiency:** Affects FMN/FAD-dependent enzymes, often clinically presenting as cheilosis and glossitis.

Question 55: Zinc is a component of which of the following enzymes, except?

A. Hexokinase (Correct Answer)
B. Carbonic anhydrase
C. Insulin
D. Carboxypeptidase

Explanation: **Explanation:** The correct answer is **Hexokinase**. In biochemistry, enzymes often require metal ions for catalytic activity. These are categorized as metalloenzymes (tightly bound) or metal-activated enzymes (loosely bound). **1. Why Hexokinase is the correct answer:** Hexokinase, the first enzyme of glycolysis, requires **Magnesium ($Mg^{2+}$)** as a cofactor, not Zinc. Magnesium binds to ATP to form a Mg-ATP complex, which is the actual substrate for the reaction. This is a high-yield distinction: most kinases involved in phosphate transfer require $Mg^{2+}$ or $Mn^{2+}$. **2. Analysis of other options:** * **Carbonic Anhydrase:** This is the classic example of a Zinc-containing metalloenzyme. Zinc is essential for the hydration of $CO_2$ to bicarbonate. * **Insulin:** While not an enzyme (it is a hormone), Zinc is crucial for its structural integrity. In the pancreas, insulin is stored as a **zinc-insulin hexamer**. * **Carboxypeptidase:** This proteolytic digestive enzyme requires Zinc for its catalytic mechanism to cleave peptide bonds at the carboxyl terminal. **Clinical Pearls for NEET-PG:** * **Zinc-containing enzymes (Mnemonic: "C-ALCO"):** **C**arbonic anhydrase, **A**lcohol dehydrogenase, **L**actate dehydrogenase, **C**arboxypeptidase, and Superoxide dismutase (cytosolic). * **DNA/RNA Polymerases:** These are also Zinc-dependent. * **Clinical Correlation:** Zinc deficiency leads to **Acrodermatitis enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. * **Key Distinction:** If a question asks for the most common metal cofactor in the body, it is often $Mg^{2+}$ (for kinases) or $Zn^{2+}$ (for a wide variety of enzyme classes).

Question 56: Allopurinol acts by inhibiting which enzyme?

A. Uric acid carboxylase
B. Hypoxanthine oxidase
C. Uric acid synthase
D. Xanthine oxidase (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Xanthine Oxidase** Allopurinol is a structural analog of hypoxanthine and acts as a potent **competitive inhibitor** of the enzyme **Xanthine Oxidase (XO)**. In the purine degradation pathway, XO is responsible for converting hypoxanthine to xanthine and subsequently xanthine to uric acid. Interestingly, XO converts allopurinol into **Alloxanthine (Oxypurinol)**. Alloxanthine then binds tightly to the molybdenum-iron center of the enzyme, acting as a **suicide inhibitor** (irreversible inhibition). By blocking this pathway, allopurinol reduces the production of uric acid, thereby lowering serum urate levels and preventing the deposition of urate crystals in joints and tissues. **Analysis of Incorrect Options:** * **A & C (Uric acid carboxylase/synthase):** These are not recognized enzymes in the human purine metabolic pathway. Uric acid is the end product of purine catabolism in humans, not a substrate for carboxylation. * **B (Hypoxanthine oxidase):** While the enzyme does oxidize hypoxanthine, its official and medically recognized name is Xanthine Oxidase. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Allopurinol is the first-line agent for the chronic management of **Gout** (intercritical and chronic tophaceous gout). * **Tumor Lysis Syndrome:** It is used to prevent hyperuricemia in patients undergoing chemotherapy. * **Drug Interaction:** Since **6-Mercaptopurine** and **Azathioprine** are metabolized by Xanthine Oxidase, co-administration with Allopurinol leads to their toxicity. Doses of these drugs must be reduced by 75%. * **Hypersensitivity:** Watch for **Stevens-Johnson Syndrome (SJS)**, especially in patients with the HLA-B*5801 allele.

Question 57: Which coenzyme is involved in decarboxylation reactions?

A. Niacin
B. Biotin
C. Pyridoxine (Correct Answer)
D. Riboflavin

Explanation: **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. In its active form, **Pyridoxal Phosphate (PLP)**, it serves as an essential coenzyme for enzymes involved in amino acid metabolism. PLP acts by forming a Schiff base with the amino group of the substrate, stabilizing the transition state to facilitate the removal of the carboxyl group as $CO_2$. **Why Pyridoxine is correct:** PLP is the universal coenzyme for **decarboxylation** of amino acids. Key examples include: * Histidine → Histamine (via Histidine decarboxylase) * Glutamate → GABA (via Glutamate decarboxylase) * 5-HTP → Serotonin * DOPA → Dopamine **Analysis of Incorrect Options:** * **Niacin (B3):** Functions as NAD/NADP, primarily involved in **redox (oxidation-reduction)** reactions. * **Biotin (B7):** Acts as a coenzyme for **carboxylation** reactions (adding $CO_2$), such as Pyruvate carboxylase and Acetyl-CoA carboxylase. (Mnemonic: "ABC" – ATP, Biotin, $CO_2$). * **Riboflavin (B2):** Functions as FAD/FMN, also involved in **redox** reactions (e.g., Succinate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** 1. **PLP Functions:** Besides decarboxylation, PLP is mandatory for **Transamination** (ALT/AST), **Deamination**, and **Heme synthesis** (ALA synthase). 2. **Isoniazid (INH) Interaction:** This anti-tubercular drug inhibits pyridoxine kinase, leading to B6 deficiency and peripheral neuropathy. Always supplement B6 with INH. 3. **Cystathionine Synthase:** PLP is a cofactor here; its deficiency can lead to **Homocystinuria**. 4. **Transsulfuration Pathway:** PLP is required to convert Homocysteine to Cysteine.

Question 58: All of the following enzymes are involved in oxidation-reduction reactions, except?

A. Dehydrogenases
B. Hydrolases (Correct Answer)
C. Oxygenases
D. Peroxidases

Explanation: **Explanation:** The classification of enzymes is a high-yield topic for NEET-PG. Enzymes are categorized into six major classes based on the type of reaction they catalyze (EC classification). **1. Why Hydrolases is the Correct Answer:** **Hydrolases (Class 3)** catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They do not involve the transfer of electrons or changes in oxidation states. Common examples include digestive enzymes like pepsin, trypsin, and alkaline phosphatase. Since they do not facilitate redox reactions, they are the exception in this list. **2. Analysis of Incorrect Options (Oxidoreductases):** The other three options belong to **Class 1: Oxidoreductases**, which catalyze the transfer of electrons ($H^+$ or $e^-$) from a reductant to an oxidant. * **Dehydrogenases:** These transfer hydrogen from a substrate to an electron acceptor like $NAD^+$ or $FAD$ (e.g., Lactate Dehydrogenase). * **Oxygenases:** These catalyze the direct incorporation of oxygen into a substrate (e.g., Cytochrome P450). * **Peroxidases:** These use hydrogen peroxide ($H_2O_2$) as an electron acceptor to oxidize substrates (e.g., Glutathione peroxidase). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**I**L** (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Lyases vs. Hydrolases:** Lyases (Class 4) also break bonds but do so without water or oxidation, often forming double bonds (e.g., Carbonic anhydrase). * **Ligases (Class 6):** These join two molecules together and **require ATP** (e.g., Pyruvate carboxylase).

Question 59: An enzyme increases the rate of a chemical reaction through which one of the following effects?

A. Decreasing the free energy of the substrate
B. Increasing the free energy of the product
C. Decreasing the activation energy of the reaction (Correct Answer)
D. Shifting the equilibrium of the reaction toward the product

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** The fundamental mechanism by which enzymes function as biological catalysts is by **decreasing the activation energy ($E_a$)** of a reaction. Activation energy is the minimum energy barrier that substrates must overcome to reach the unstable **transition state** before turning into products. By stabilizing this transition state and providing an alternative reaction pathway, enzymes allow a larger fraction of substrate molecules to possess enough energy to react at body temperature, thereby increasing the reaction rate without being consumed. **2. Why Incorrect Options are Wrong:** * **Options A & B:** Enzymes do not alter the ground-state free energy ($G$) of the substrates or products. The overall free energy change ($\Delta G$) of the reaction remains constant. * **Option D:** Enzymes **do not shift the equilibrium** of a reaction. Equilibrium is determined solely by the thermodynamic properties of the reactants and products. An enzyme merely allows the reaction to reach that pre-defined equilibrium point faster by increasing the rate of both the forward and reverse reactions equally. **3. NEET-PG High-Yield Pearls:** * **Transition State:** Enzymes have the highest affinity for the transition state, not the substrate (Linus Pauling’s principle). * **Thermodynamics:** Enzymes change the **kinetics** (velocity) but never the **thermodynamics** ($\Delta G$, $K_{eq}$) of a reaction. * **Active Site:** The specific region where the substrate binds; it represents a small portion of the total enzyme volume and creates a unique microenvironment (often non-polar). * **Models:** The "Induced Fit Model" (Koshland) is more accurate than the "Lock and Key Model" (Fischer) as it accounts for the conformational flexibility of enzymes.

Question 60: What enzyme deficiency is tested in the Guthrie test?

A. Phenylalanine hydroxylase (Correct Answer)
B. Tyrosine transaminase
C. p-Hydroxyphenyl pyruvate dioxygenase
D. Homogentisate oxidase

Explanation: ### Explanation **Correct Answer: A. Phenylalanine hydroxylase** The **Guthrie Test** is a classic semi-quantitative bacterial inhibition assay used for neonatal screening of **Phenylketonuria (PKU)**. PKU is most commonly caused by a deficiency of the enzyme **Phenylalanine hydroxylase (PAH)**, which converts phenylalanine to tyrosine. * **Mechanism:** The test uses *Bacillus subtilis* spores incorporated into an agar medium containing **β-2-thienylalanine**, a metabolic inhibitor that prevents bacterial growth. If the infant's blood (collected via heel prick) contains high levels of phenylalanine (due to PAH deficiency), the phenylalanine overcomes the inhibition, allowing the bacteria to grow. The diameter of the growth zone is proportional to the concentration of phenylalanine in the blood. **Analysis of Incorrect Options:** * **B. Tyrosine transaminase:** Deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), characterized by palmoplantar keratosis and corneal ulcers. * **C. p-Hydroxyphenyl pyruvate dioxygenase:** Deficiency leads to **Tyrosinemia Type III**, a very rare condition involving neurological symptoms. * **D. Homogentisate oxidase:** Deficiency causes **Alkaptonuria**, characterized by ochronosis (darkening of tissues) and urine that turns black upon standing. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** The Guthrie test must be performed **after 48–72 hours of protein feeding** (breast milk/formula) to allow phenylalanine levels to rise; testing too early leads to false negatives. * **Hyperphenylalaninemia:** While 98% of PKU is due to PAH deficiency, 2% is due to a deficiency in **Dihydrobiopterin reductase** or BH4 synthesis (Malignant PKU). * **Dietary Management:** Treatment involves a diet low in phenylalanine and supplementation with **Tyrosine**, which becomes an **essential amino acid** in these patients.

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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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