Enzymes Practice Questions

Q: Glutathione peroxidase, which protects erythrocytes against hemolysis, contains which of the following elements?

Selenium. **Explanation:** **Glutathione Peroxidase (GPx)** is a critical antioxidant enzyme that protects cells, particularly erythrocytes, from oxidative damage. It catalyzes the reduction of hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides into water and alcohols, respectively, using reduced glutathione (GSH) as a donor. 1. **Why Selenium is Correct:** Selenium is an essential trace element that is incorporated into the active site of Glutathione Peroxidase as the 21st amino acid, **Selenocysteine**. This "selenoprotein" structure is vital for the enzyme's catalytic activity. Without selenium, GPx cannot neutralize peroxides, leading to oxidative stress and subsequent hemolysis of red blood cells. 2. **Why Other Options are Incorrect:** * **Chromium (A):** Primarily functions as a component of the Glucose Tolerance Factor (GTF), enhancing the action of insulin. * **Manganese (B):** Acts as a cofactor for enzymes like Pyruvate Carboxylase and **Manganese Superoxide Dismutase (Mn-SOD)**, which is found in the mitochondria. * **Zinc (C):** A cofactor for over 300 enzymes, including Carbonic Anhydrase, Alcohol Dehydrogenase, and **Cytoplasmic SOD (Cu-Zn SOD)**. **High-Yield Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy caused by Selenium deficiency. * **Kashin-Beck Disease:** An osteoarthropathy associated with Selenium deficiency. * **Glutathione Reductase:** Unlike the peroxidase, this enzyme requires **Vitamin B2 (Riboflavin)** as FAD to regenerate reduced glutathione. * **G6PD Deficiency:** Leads to hemolysis because the cell cannot produce NADPH, which is required by Glutathione Reductase to keep Glutathione in its protective reduced state.

Q: Which of the following enzymes does not require copper for its action?

Carbonic anhydrase. **Explanation:** The correct answer is **Carbonic anhydrase** because it is a **zinc-containing metalloenzyme**, not a copper-dependent one. ### 1. Why Carbonic Anhydrase is Correct Carbonic anhydrase (found in RBCs and renal tubules) requires **Zinc (Zn²⁺)** as a cofactor to catalyze the reversible hydration of carbon dioxide ($CO_2 + H_2O \rightleftharpoons H_2CO_3$). It is one of the fastest known enzymes and is essential for acid-base balance and $CO_2$ transport. ### 2. Analysis of Incorrect Options (Copper-containing Enzymes) * **Tyrosinase:** A copper-containing enzyme essential for melanin synthesis. It converts Tyrosine to DOPA and then to Dopaquinone. Deficiency leads to **Albinism**. * **Superoxide dismutase (SOD):** The cytoplasmic form (Cu-Zn SOD) requires both **Copper** and Zinc to scavenge free radicals. (Note: The mitochondrial form requires Manganese). * **Ceruloplasmin:** This is a ferroxidase enzyme that carries 95% of plasma copper. It is vital for iron metabolism (converting $Fe^{2+}$ to $Fe^{3+}$) and is deficient in **Wilson’s Disease**. ### 3. High-Yield Clinical Pearls for NEET-PG * **Other Copper Enzymes:** Cytochrome c oxidase (Complex IV of ETC), Lysyl oxidase (collagen cross-linking), and Dopamine $\beta$-hydroxylase. * **Zinc Enzymes:** Alcohol dehydrogenase, Carboxypeptidase, DNA/RNA polymerase, and Alkaline phosphatase (ALP). * **Menkes Kinky Hair Syndrome:** Due to impaired copper absorption/transport (ATP7A mutation), leading to deficiency of copper-dependent enzymes (e.g., Lysyl oxidase deficiency causes brittle hair and connective tissue issues).

Q: Fluorouracil undergoes in vivo chemical changes resulting in a covalent complex bound to both thymidylate synthase and methylene-tetrahydrofolate. What type of inhibition is responsible for the subsequent blockage of deoxythymidilate formation and cell division?

Irreversible inhibition. **Explanation:** **Mechanism of Action (Why C is correct):** 5-Fluorouracil (5-FU) is a pyrimidine analog that acts as a "suicide inhibitor." Once inside the cell, it is converted to **5-FdUMP** (5-fluorodeoxyuridine monophosphate). This metabolite acts as a false substrate and forms a stable, **covalent ternary complex** with the enzyme **Thymidylate Synthase (TS)** and the cofactor **5,10-methylene tetrahydrofolate**. Because the bond is covalent and the enzyme is permanently inactivated, this is classified as **Irreversible Inhibition**. This prevents the conversion of dUMP to dTMP, leading to a "thymineless death" of the cell. **Analysis of Incorrect Options:** * **A. Allosteric inhibition:** This involves binding to a site other than the active site to induce a conformational change. 5-FU binds directly to the active site. * **B. Competitive inhibition:** While 5-FdUMP competes for the active site initially, the formation of a permanent covalent bond makes the inhibition irreversible (non-competitive in kinetics over time), as it cannot be reversed by increasing substrate concentration. * **D. Noncovalent inhibition:** The hallmark of 5-FU’s action is the formation of a **covalent** bond; noncovalent interactions are reversible and weaker. **NEET-PG High-Yield Pearls:** * **Rescue Agent:** Leucovorin (folinic acid) is administered with 5-FU not to reduce toxicity, but to **potentiate** its efficacy by stabilizing the ternary complex. * **Rate-limiting Enzyme:** Dihydropyrimidine dehydrogenase (DPD) degrades 5-FU. Patients with **DPD deficiency** are at high risk of severe 5-FU toxicity (pancytopenia, mucosal ulceration). * **Specific Toxicity:** Hand-foot syndrome (palmar-plantar erythrodysesthesia) is a characteristic side effect.

Q: Which enzyme protects the brain from free radical injury?

Superoxide dismutase. ### Explanation **Correct Option: C. Superoxide dismutase (SOD)** Superoxide dismutase is a critical antioxidant enzyme that protects cells from oxidative stress. It catalyzes the dismutation of the highly reactive **superoxide radical ($O_2^-$)** into oxygen and hydrogen peroxide ($H_2O_2$). In the brain, which is highly susceptible to oxidative damage due to its high oxygen consumption and lipid content, SOD acts as the first line of defense against reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A. Myeloperoxidase:** Found primarily in neutrophils, this enzyme produces hypochlorous acid (HOCl) from $H_2O_2$ and chloride ions. It is involved in the respiratory burst to kill bacteria, actually *generating* free radicals rather than scavenging them. * **B. Monoamine oxidase (MAO):** This enzyme breaks down neurotransmitters like dopamine and serotonin. A byproduct of this reaction is $H_2O_2$, which can contribute to oxidative stress if not neutralized; thus, MAO is a source of potential injury, not a protector. * **D. Hydroxylase:** These enzymes (e.g., Phenylalanine hydroxylase) add hydroxyl groups to substrates. They are metabolic enzymes and do not possess specific antioxidant properties. **High-Yield Clinical Pearls for NEET-PG:** * **SOD Isoforms:** SOD1 (Cytosolic, contains Cu-Zn), SOD2 (Mitochondrial, contains Mn), and SOD3 (Extracellular, contains Cu-Zn). * **Clinical Link:** Mutations in the **SOD1 gene** are associated with **Amyotrophic Lateral Sclerosis (ALS)**, highlighting the enzyme's vital role in protecting motor neurons. * **Antioxidant Trio:** Remember the sequence: **SOD** converts $O_2^-$ to $H_2O_2$, then **Catalase** or **Glutathione Peroxidase** converts $H_2O_2$ to water.

Q: Glutathione peroxidase contains which trace element?

Selenium (Se). **Explanation:** **Glutathione Peroxidase (GPx)** is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. The correct answer is **Selenium (Se)** because this enzyme contains the unique amino acid **Selenocysteine** at its active site. Selenocysteine is often referred to as the "21st amino acid," where selenium replaces the sulfur atom found in cysteine, allowing the enzyme to efficiently catalyze redox reactions. **Analysis of Incorrect Options:** * **Copper (Cu):** While copper is a vital cofactor, it is associated with enzymes like **Superoxide Dismutase (Cytosolic SOD)**, Cytochrome c Oxidase, and Tyrosinase, not GPx. * **Iron (Fe):** Iron is the cofactor for **Catalase** (which also breaks down $H_2O_2$) and various cytochromes. It is not the functional trace element in GPx. * **Mercury (Hg):** Mercury is a heavy metal toxin. It has no physiological role as a cofactor and actually inhibits various enzymes by binding to sulfhydryl groups. **High-Yield Clinical Pearls for NEET-PG:** * **Selenium Deficiency:** Leads to **Keshan Disease** (an endemic cardiomyopathy) and **Kashin-Beck Disease** (an osteoarthropathy). * **Glutathione System:** GPx requires **Reduced Glutathione (GSH)** as a hydrogen donor. The resulting oxidized glutathione (GSSG) is recycled back to GSH by **Glutathione Reductase**, which requires **NADPH** (derived from the HMP Shunt). * **Other Selenoenzymes:** Include **Thioredoxin reductase** and **Deiodinase** (involved in converting $T_4$ to $T_3$).

Q: One of the following groups of enzymes does not exhibit stereospecificity?

Isomerases. ### Explanation **Why Isomerases are the Correct Answer:** Stereospecificity refers to the ability of an enzyme to act exclusively on one specific stereoisomer (e.g., L-amino acids or D-sugars). **Isomerases** are unique because their primary function is to catalyze the **interconversion** of optical or geometric isomers. For an enzyme to convert a D-isomer into an L-isomer (or vice versa), it must interact with both configurations. Therefore, by definition, isomerases lack the absolute stereospecificity seen in other enzyme classes, as they must accommodate and transform different stereochemical arrangements of the same molecule. **Analysis of Incorrect Options:** * **Oxidoreductases:** These are highly stereospecific. For example, Lactate Dehydrogenase (LDH) acts specifically on L-lactate, not D-lactate. Alcohol dehydrogenase also distinguishes between the pro-R and pro-S hydrogens of ethanol. * **Lyases:** These enzymes catalyze the addition or removal of groups to form double bonds. They exhibit high stereospecificity; for instance, Fumarase adds water to Fumarate (trans) but cannot act on Maleate (cis). * **Transferases:** These enzymes transfer functional groups (like phosphate or methyl groups) with high spatial precision. Hexokinase, for example, is specific for D-glucose and will not phosphorylate L-glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Racemases and Epimerases:** These are sub-classes of Isomerases. A classic example is **Mutarotase**, which converts α-D-glucose to β-D-glucose. * **Absolute Specificity:** Most enzymes exhibit "Optical Specificity," meaning they only work on one isomer (e.g., human enzymes almost exclusively utilize **L-amino acids** and **D-sugars**). * **Exception to the Rule:** While most enzymes are proteins, **Ribozymes** (RNA-based enzymes) also exhibit high specificity.

Q: In the Citric acid cycle, which enzyme is inhibited by arsenite?

alpha-ketoglutarate Dehydrogenase. **Explanation:** The correct answer is **B. $\alpha$-ketoglutarate Dehydrogenase.** **Mechanism of Inhibition:** Arsenite (the trivalent form of arsenic) has a high affinity for **sulfhydryl (-SH) groups**. Specifically, it binds to the **lipoic acid** (lipoamide) cofactor. The $\alpha$-ketoglutarate dehydrogenase complex requires five cofactors: Thiamine pyrophosphate (TPP), Lipoic acid, CoA, FAD, and NAD+. By binding to the thiol groups of lipoic acid, arsenite forms a stable chelate, rendering the enzyme inactive. This halts the Citric Acid Cycle, leading to a decrease in ATP production and an accumulation of upstream metabolites. **Analysis of Incorrect Options:** * **A. Isocitrate Dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle, regulated primarily by the NADH/NAD+ ratio and ADP/ATP levels, not by arsenite. * **C. Succinate Dehydrogenase:** This enzyme is inhibited by **Malonate** (a classic example of competitive inhibition) because malonate is a structural analog of succinate. * **D. Aconitase:** This enzyme is inhibited by **Fluoroacetate** (via "suicide inhibition" after conversion to fluorocitrate). **High-Yield Clinical Pearls for NEET-PG:** * **Pyruvate Dehydrogenase (PDH):** Like $\alpha$-ketoglutarate dehydrogenase, PDH also uses lipoic acid and is similarly inhibited by arsenite. This leads to lactic acidosis. * **Clinical Presentation:** Arsenic poisoning manifests with a "garlic breath" odor, skin hyperpigmentation (raindrop appearance), and Mees' lines on nails. * **Treatment:** Dimercaprol (BAL) is used as an antidote because it provides competing sulfhydryl groups to displace the arsenic.

Q: What cofactor does the enzyme transketolase require?

TPP (Thiamine pyrophosphate). **Explanation:** **Transketolase** is a key enzyme in the **Pentose Phosphate Pathway (Hexose Monophosphate Shunt)**. It catalyzes the transfer of a two-carbon unit (ketol group) from a ketose to an aldose. This reaction is strictly dependent on **Thiamine Pyrophosphate (TPP)**, the active form of Vitamin B1, which acts as a prosthetic group to stabilize the carbanion intermediate during the carbon transfer. **Analysis of Options:** * **TPP (Correct):** Besides transketolase, TPP is a vital cofactor for three other major enzyme complexes: Pyruvate Dehydrogenase (PDH), $\alpha$-ketoglutarate dehydrogenase, and Branched-chain $\alpha$-ketoacid dehydrogenase. * **FAD & FMN:** These are derivatives of Vitamin B2 (Riboflavin). They act as redox cofactors in the Electron Transport Chain and for enzymes like Succinate Dehydrogenase. * **PLP:** This is the active form of Vitamin B6. It is primarily involved in **transamination**, decarboxylation, and deamination reactions of amino acids. **Clinical Pearls for NEET-PG:** 1. **Diagnostic Marker:** Measuring **Erythrocyte Transketolase Activity (ETKA)** is the most reliable laboratory method to diagnose **Thiamine deficiency**. An increase in enzyme activity upon adding TPP in vitro (TPP effect >15-25%) confirms the deficiency. 2. **Wernicke-Korsakoff Syndrome:** This condition is caused by thiamine deficiency (often in chronic alcoholics). It is characterized by the triad of ataxia, ophthalmoplegia, and confusion. 3. **Metabolic Link:** Transketolase provides a reversible link between the HMP shunt and Glycolysis (producing Glyceraldehyde-3-P and Fructose-6-P).

Question 1

Glutathione peroxidase, which protects erythrocytes against hemolysis, contains which of the following elements?

Accepted Answer

Selenium

Answer

Chromium

Answer

Manganese

Answer

Zinc

Question 2

Which of the following enzymes does not require copper for its action?

Accepted Answer

Carbonic anhydrase

Answer

Tyrosinase

Answer

Superoxide dismutase

Answer

Ceruloplasmin

Question 3

The Michaelis Menten hypothesis states that:

Accepted Answer

The formation of an enzyme-substrate complex is essential in enzymatic reactions.

Answer

The rate of an enzymatic reaction is independent of substrate concentration.

Answer

The rate of a non-enzymatic reaction is proportional to the substrate concentration.

Answer

Km represents the enzyme-substrate complex dissociation constant.

Question 4

Fluorouracil undergoes in vivo chemical changes resulting in a covalent complex bound to both thymidylate synthase and methylene-tetrahydrofolate. What type of inhibition is responsible for the subsequent blockage of deoxythymidilate formation and cell division?

Accepted Answer

Irreversible inhibition

Answer

Allosteric inhibition

Answer

Competitive inhibition

Answer

Noncovalent inhibition

Question 5

Which enzyme protects the brain from free radical injury?

Accepted Answer

Superoxide dismutase

Answer

Myeloperoxidase

Answer

Monoamine oxidase (MAO)

Answer

Hydroxylase

Question 6

Glutathione peroxidase contains which trace element?

Accepted Answer

Selenium (Se)

Answer

Copper (Cu)

Answer

Iron (Fe)

Answer

Mercury (Hg)

Question 7

All of the following enzymes are active within a cell, except?

Accepted Answer

Hexokinase

Answer

Trypsin

Answer

Fumarase

Answer

Alcohol dehydrogenase

Question 8

One of the following groups of enzymes does not exhibit stereospecificity?

Accepted Answer

Isomerases

Answer

Oxidoreductases

Answer

Lyases

Answer

Transferases

Question 9

In the Citric acid cycle, which enzyme is inhibited by arsenite?

Accepted Answer

alpha-ketoglutarate Dehydrogenase

Answer

Isocitrate Dehydrogenase

Answer

Succinate Dehydrogenase

Answer

Aconitase

Question 10

What cofactor does the enzyme transketolase require?

Accepted Answer

TPP (Thiamine pyrophosphate)

Answer

FAD (Flavin adenine dinucleotide)

Answer

PLP (Pyridoxal phosphate)

Answer

FMN (Flavin mononucleotide)

Enzymes — MCQs

Enzymes — MCQs

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