Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 201: Enzymes act by reducing which of the following?

A. Activation energy (Correct Answer)
B. Binding energy
C. Heat energy
D. Covalent energy

Explanation: ### Explanation **1. Why Activation Energy is Correct:** In any biochemical reaction, substrates must reach a high-energy, unstable state known as the **Transition State** before they can be converted into products. The energy required to reach this state is called the **Activation Energy ($E_a$)**. Enzymes function as biological catalysts by stabilizing the transition state and providing an alternative reaction pathway. This significantly **lowers the activation energy barrier**, allowing more substrate molecules to have sufficient energy to react at body temperature, thereby increasing the reaction rate. **2. Why Other Options are Incorrect:** * **Binding Energy:** This is the energy released when the enzyme interacts with its substrate through non-covalent bonds. Enzymes actually **maximize** binding energy to stabilize the transition state; they do not reduce it. * **Heat Energy:** Enzymes do not reduce heat energy; in fact, they allow reactions to occur efficiently at a constant physiological temperature (37°C) without requiring an increase in thermal energy. * **Covalent Energy:** This refers to the energy within chemical bonds. While enzymes may form transient covalent bonds (covalent catalysis), they do not "reduce" covalent energy as a mechanism of action. **3. NEET-PG High-Yield Pearls:** * **Thermodynamics:** Enzymes change the **rate** of the reaction but **do not** alter the equilibrium constant ($K_{eq}$) or the standard free energy change ($\Delta G$). * **Transition State:** Enzymes have the highest affinity for the **transition state** of the substrate, not the substrate itself (Linus Pauling’s principle). * **Michaelis-Menten Kinetics:** A lower $K_m$ indicates a higher affinity of the enzyme for its substrate. * **Clinical Correlation:** Many drugs act as enzyme inhibitors (e.g., Statins inhibit HMG-CoA reductase) by interfering with the enzyme's ability to lower activation energy.

Question 202: Sulfonamides inhibit bacterial synthesis of folic acid by:

A. Uncompetitive inhibition
B. Allosteric inhibition
C. Competitive inhibition (Correct Answer)
D. Non-competitive inhibition

Explanation: ### Explanation **1. Why Competitive Inhibition is Correct:** Sulfonamides (Sulfa drugs) are structural analogs of **Para-Aminobenzoic Acid (PABA)**. In bacteria, the enzyme **Dihydropteroate Synthase** normally uses PABA as a substrate to synthesize folic acid. Because sulfonamides closely resemble PABA in structure, they compete for the same active site on the enzyme. By binding to the active site, sulfonamides prevent PABA from binding, thereby halting folic acid synthesis. Since humans obtain folic acid from their diet and do not synthesize it intracellularly, this mechanism selectively targets bacteria. **2. Why Other Options are Incorrect:** * **Uncompetitive Inhibition:** The inhibitor binds only to the enzyme-substrate (ES) complex, not the free enzyme. Sulfonamides bind to the free enzyme. * **Allosteric Inhibition:** The inhibitor binds to a site other than the active site (allosteric site), causing a conformational change. Sulfonamides compete directly for the active site. * **Non-competitive Inhibition:** The inhibitor binds to a site distinct from the active site, and its effect cannot be overcome by increasing substrate concentration. Competitive inhibition (like sulfonamides) *can* be reversed by increasing the concentration of PABA. **3. High-Yield Clinical Pearls for NEET-PG:** * **Kₘ and Vₘₐₓ:** In competitive inhibition, **Kₘ increases** (affinity decreases) while **Vₘₐₓ remains unchanged**. * **Sequential Blockade:** Trimethoprim is often combined with Sulfamethoxazole (Cotrimoxazole) to inhibit the next step in the pathway (Dihydrofolate Reductase), providing a synergistic effect. * **Other Competitive Inhibitors:** * **Statins** (compete with HMG-CoA) * **Methotrexate** (competes with Dihydrofolate) * **Methanol poisoning treatment:** Ethanol/Fomepizole (compete for Alcohol Dehydrogenase).

Question 203: In myocardial infarction (MI), what happens to the ratio of Lactate Dehydrogenase (LDH) isoenzymes 1 and 2?

A. LDH-1 is greater than LDH-2 (Correct Answer)
B. LDH-2 is greater than LDH-1
C. LDH-1 is equal to LDH-2
D. The ratio remains unchanged

Explanation: **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme consisting of two subunits: H (Heart) and M (Muscle). It has five isoenzymes (LDH 1-5). **1. Why Option A is Correct:** In a healthy physiological state, **LDH-2 (H3M1)** is the predominant isoenzyme in the serum, meaning the ratio of LDH-1 to LDH-2 is less than 1. However, **LDH-1 (H4)** is found in high concentrations within cardiac myocytes. Following a Myocardial Infarction (MI), the damaged cardiac cells release large amounts of LDH-1 into the bloodstream. This causes the serum level of LDH-1 to rise above LDH-2, a phenomenon known as the **"LDH Flipped Ratio"** (LDH-1 > LDH-2). **2. Why Other Options are Incorrect:** * **Option B:** This represents the normal physiological state in healthy individuals. * **Option C:** While the levels may briefly cross during the rise of LDH-1, it is not the diagnostic hallmark of MI. * **Option D:** The ratio must change because LDH-1 is specific to the myocardium, whereas LDH-2 is more generalized (predominantly in the reticuloendothelial system). **3. Clinical Pearls for NEET-PG:** * **Timing:** LDH levels begin to rise 12–24 hours after MI, peak at 48–72 hours, and remain elevated for 10–14 days. * **Clinical Utility:** Due to its late peak and prolonged elevation, LDH is a useful marker for **late diagnosis of MI** (when Troponins and CK-MB have already returned to baseline). * **Other Sources of LDH-1:** Apart from MI, a flipped ratio can also be seen in **Hemolytic Anemia**, as LDH-1 is also abundant in Red Blood Cells.

Question 204: Allosteric enzymes show the following characteristics, except?

A. Co-operative binding of substrate
B. Obey Sigmoid saturation kinetics
C. Effector site overlap with the active site (Correct Answer)
D. Active site and effector site need not be located on the same subunit

Explanation: ### Explanation **Why Option C is the Correct Answer:** Allosteric enzymes are characterized by the presence of **distinct, spatially separate sites**. The **active site** (catalytic site) is where the substrate binds, while the **allosteric site** (effector/regulatory site) is where modulators bind. By definition, these sites **do not overlap**. When an effector binds to the allosteric site, it induces a conformational change in the enzyme that alters the affinity of the active site for the substrate. If the sites overlapped, it would be a case of competitive inhibition rather than allosterism. **Analysis of Other Options:** * **Option A (Co-operative binding):** Most allosteric enzymes are oligomeric (multiple subunits). The binding of a substrate molecule to one subunit increases the affinity of other subunits for the substrate, a phenomenon known as **positive cooperativity**. * **Option B (Sigmoid kinetics):** Unlike simple enzymes that follow Michaelis-Menten (hyperbolic) kinetics, allosteric enzymes exhibit a **Sigmoid (S-shaped) curve** when plotting velocity against substrate concentration, reflecting their cooperative nature. * **Option C (Subunit location):** In many allosteric enzymes (like Aspartate Transcarbamoylase), the catalytic and regulatory sites are located on **different polypeptide chains** (subunits), though they can occasionally be on the same chain in different domains. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzymes:** Most rate-limiting steps in metabolic pathways (e.g., **PFK-1** in glycolysis) are regulated by allosteric enzymes. * **K-series vs. V-series:** Allosteric effectors that change the $K_{0.5}$ (affinity) are K-series enzymes; those changing $V_{max}$ are V-series. * **Aspartate Transcarbamoylase (ATCase):** The classic model for allosteric regulation; inhibited by CTP and activated by ATP. * **2,3-BPG:** Acts as an allosteric effector for Hemoglobin, shifting the dissociation curve to the right (promoting oxygen release).

Question 205: Which trace element is present in carbonic anhydrase?

A. Zinc (Correct Answer)
B. Molybdenum
C. Copper
D. Magnesium

Explanation: **Explanation:** **Carbonic Anhydrase** is a classic example of a **metalloenzyme** where a metal ion is tightly bound and essential for catalytic activity. 1. **Why Zinc is correct:** Carbonic anhydrase contains a **Zinc ($Zn^{2+}$)** ion coordinated to three histidine residues at its active site. The Zinc ion facilitates the nucleophilic attack of water on carbon dioxide, catalyzing the reversible hydration of $CO_2$ to bicarbonate ($HCO_3^-$) and $H^+$. This reaction is vital for $CO_2$ transport in RBCs and acid-base regulation in the kidneys. 2. **Why other options are incorrect:** * **Molybdenum:** Found in enzymes like **Xanthine oxidase** (purine metabolism) and Sulfite oxidase. * **Copper:** Present in **Cytochrome c oxidase**, Superoxide dismutase (cytosolic), and Tyrosinase. * **Magnesium:** Acts as a cofactor for almost all enzymes utilizing ATP (e.g., **Hexokinase**, Phosphofructokinase) and DNA/RNA polymerases. **High-Yield Clinical Pearls for NEET-PG:** * **Carbonic Anhydrase Inhibitors:** Acetazolamide is a potent inhibitor used to treat glaucoma, altitude sickness, and idiopathic intracranial hypertension. * **Zinc-containing enzymes:** Other high-yield Zinc enzymes include **Alcohol dehydrogenase**, **Carboxypeptidase**, and **DNA polymerase**. * **Zinc Deficiency:** Characterized by growth retardation, impaired wound healing, and **Acrodermatitis enteropathica**. * **Speed:** Carbonic anhydrase is one of the fastest known enzymes, with a turnover number ($K_{cat}$) of $10^6$ reactions per second.

Question 206: Which enzyme is known as the 'suicidal enzyme'?

A. Lipoxygenase
B. Cyclooxygenase (Correct Answer)
C. Thromboxane synthase
D. 5'Nucleotidase

Explanation: **Explanation:** **Cyclooxygenase (COX)**, also known as Prostaglandin H2 synthase, is termed a **'suicidal enzyme'** because of its unique mechanism of **self-catalyzed inactivation**. During the conversion of arachidonic acid into Prostaglandin $H_2$, the enzyme generates highly reactive free radical intermediates. These radicals attack the protein structure of the enzyme itself, leading to its irreversible denaturation and loss of activity. Thus, each molecule of COX can only perform a limited number of catalytic cycles before "killing" itself. **Analysis of Options:** * **Lipoxygenase (Option A):** This enzyme converts arachidonic acid into leukotrienes. While it is part of the eicosanoid pathway, it does not undergo the same self-inactivation process as COX. * **Thromboxane synthase (Option C):** This enzyme acts downstream of COX to convert $PGH_2$ into Thromboxane $A_2$. It is not characterized by suicidal inhibition. * **5' Nucleotidase (Option D):** This is a marker enzyme for the plasma membrane and canalicular membrane of hepatocytes; it is used clinically to differentiate the source of elevated alkaline phosphatase (ALP). **High-Yield Clinical Pearls for NEET-PG:** * **Aspirin Connection:** Aspirin is a **suicide inhibitor** (irreversible inhibitor) of COX. It acetylates a specific serine residue in the active site. Since platelets lack a nucleus, they cannot synthesize new COX enzymes, leading to the lifelong (7–10 days) anti-platelet effect of aspirin. * **Isoforms:** COX-1 is constitutive (gastric protection, renal blood flow), while COX-2 is inducible (inflammation, pain). * **Suicide Inhibition vs. Suicidal Enzyme:** While the terms are related, "suicidal enzyme" specifically refers to the enzyme's inherent property of self-destruction during its normal catalytic cycle.

Question 207: When the pH of a solution of a weak acid, HA, is equal to the pKa, what is the ratio of the concentrations of the conjugate base [A-] to the weak acid [HA]?

A. 0
B. 1 (Correct Answer)
C. 2
D. 3

Explanation: ### Explanation The relationship between pH, pKa, and the dissociation of a weak acid is defined by the **Henderson-Hasselbalch equation**: $$pH = pKa + \log \frac{[A^-]}{[HA]}$$ **Why Option B is Correct:** When the pH of the solution is equal to the pKa ($pH = pKa$), the equation becomes: $$0 = \log \frac{[A^-]}{[HA]}$$ Since the logarithm of 1 is 0 ($\log 1 = 0$), the ratio of the conjugate base $[A^-]$ to the weak acid $[HA]$ must be **1**. This means the acid is exactly 50% dissociated and 50% undissociated. **Why Other Options are Incorrect:** * **Option A (0):** A ratio of 0 would imply there is no conjugate base present, which only occurs in extremely acidic conditions far below the pKa. * **Options C & D (2 & 3):** These ratios would occur if the pH were higher than the pKa (alkaline relative to the pKa). For example, if the ratio were 10, the pH would be exactly 1 unit higher than the pKa. **NEET-PG High-Yield Pearls:** 1. **Definition of pKa:** It is the pH at which a weak acid is half-ionized. 2. **Buffering Capacity:** A buffer is most effective at resisting pH changes when the **pH is equal to the pKa** (maximum buffering capacity). The effective buffer range is generally $pKa \pm 1$ pH unit. 3. **Clinical Relevance:** This principle determines the absorption of drugs. For instance, aspirin (a weak acid) is non-ionized (lipid-soluble) in the acidic gastric juice (pH < pKa), favoring its absorption across the stomach lining. 4. **Bicarbonate Buffer:** In humans, the $pKa$ of the carbonic acid/bicarbonate system is **6.1**. Since physiological pH is 7.4, the ratio of $[HCO_3^-]$ to $[H_2CO_3]$ in the blood is approximately **20:1**.

Question 208: A patient presents with recurrent kidney stones. Microscopic examination of the urine specimen is shown below. Which of the following is not seen in the urine of this patient?

A. Cysteine (Correct Answer)
B. Lysine
C. Arginine
D. Cystine

Explanation: ***Cysteine*** - **Cysteine** is the free, reduced monomer form that is **not found in urine** during cystinuria as it is readily reabsorbed by normal transporters. - The condition involves defective transport of **cystine** (the oxidized disulfide dimer) and dibasic amino acids, not the free cysteine monomer. *Lysine* - **Lysine** is one of the **dibasic amino acids** affected in cystinuria and is found in increased amounts in urine. - It is part of the **COAL mnemonic** (Cystine, Ornithine, Arginine, Lysine) representing amino acids with defective renal tubular transport. *Arginine* - **Arginine** is another **dibasic amino acid** that accumulates in urine due to defective renal tubular reabsorption. - Along with lysine, it contributes to the **alkaline pH** that promotes cystine stone formation in the urinary tract. *Cystine* - **Cystine** is the oxidized disulfide dimer that forms **hexagonal crystals** visible on urine microscopy in cystinuria patients. - It is poorly soluble in acidic urine and readily precipitates to form **recurrent kidney stones** characteristic of this genetic disorder.

Question 209: Arsenite inhibits which of the following enzymes?

A. Enolase
B. Glucose-6-phosphate dehydrogenase (G-6-PD)
C. Alpha-ketoglutarate dehydrogenase (Correct Answer)
D. Hexokinase

Explanation: **Explanation:** **Correct Answer: C. Alpha-ketoglutarate dehydrogenase** Arsenite (the trivalent form of arsenic) acts as a potent metabolic poison by binding to **thiol (-SH) groups**. Specifically, it targets **lipoic acid**, an essential cofactor for multienzyme complexes. The **Alpha-ketoglutarate dehydrogenase** complex (as well as the Pyruvate Dehydrogenase complex) requires dihydrolipoyl transsuccinylase, which contains lipoic acid. Arsenite binds to the sulfhydryl groups of dihydrolipoate, forming a stable chelate that prevents the cofactor from regenerating. This halts the TCA cycle, leading to a decrease in ATP production and an accumulation of upstream metabolites like pyruvate and lactate. **Analysis of Incorrect Options:** * **A. Enolase:** This enzyme of glycolysis is inhibited by **Fluoride**. Fluoride removes magnesium (a necessary cofactor) by forming a magnesium-fluorophosphate complex. * **B. Glucose-6-phosphate dehydrogenase (G-6-PD):** This is the rate-limiting enzyme of the HMP shunt. It is primarily regulated by the NADPH/NADP+ ratio, not by arsenite. * **D. Hexokinase:** This enzyme is inhibited by its product, **Glucose-6-phosphate** (allosteric inhibition). **High-Yield Clinical Pearls for NEET-PG:** * **Arsenite vs. Arsenate:** While *Arsenite* inhibits lipoic acid-dependent enzymes, *Arsenate* (pentavalent) acts as a **phosphate analog**, uncoupling oxidative phosphorylation and glycolysis by substituting for Pi in the G3P dehydrogenase reaction. * **Antidote:** Dimercaprol (British Anti-Lewisite/BAL) is used in arsenic poisoning because it provides competing -SH groups to displace the arsenite. * **Clinical Sign:** Arsenic poisoning classically presents with "garlic breath," rice-water stools, and Mees' lines on nails.

Question 210: Copper is a constituent of which enzyme?

A. Lysyl oxidase (Correct Answer)
B. Glucose oxidase
C. Xanthine oxidase
D. Transketolase

Explanation: **Explanation:** **1. Why Lysyl Oxidase is the Correct Answer:** Lysyl oxidase is a copper-dependent enzyme essential for the cross-linking of collagen and elastin fibers. It oxidatively deaminates the side chains of lysine and hydroxylysine residues to form reactive aldehydes (allysine). these aldehydes then undergo spontaneous condensation to form stable covalent cross-links, providing structural integrity and tensile strength to the extracellular matrix. **2. Analysis of Incorrect Options:** * **Glucose oxidase:** This is a flavoprotein (containing **FAD**) used primarily in laboratory assays to measure blood glucose levels. * **Xanthine oxidase:** This enzyme, involved in purine catabolism (converting hypoxanthine to xanthine and xanthine to uric acid), requires **Molybdenum**, Iron, and FAD as cofactors. * **Transketolase:** A key enzyme in the Pentose Phosphate Pathway (HMP Shunt), it requires **Thiamine pyrophosphate (Vitamin B1)** and Magnesium ($Mg^{2+}$) as cofactors. **3. High-Yield Clinical Pearls for NEET-PG:** * **Menkes Kinky Hair Syndrome:** An X-linked recessive disorder caused by impaired copper absorption. The resulting deficiency in **Lysyl oxidase** activity leads to weak collagen, causing brittle hair, skeletal deformities, and arterial aneurysms. * **Other Copper-containing enzymes:** * **Cytochrome c oxidase** (Complex IV of the ETC) * **Superoxide dismutase (SOD)** (Cytosolic form; contains Cu and Zn) * **Tyrosinase** (Deficiency leads to Albinism) * **Ceruloplasmin** (Ferroxidase activity) * **Dopamine $\beta$-hydroxylase** (Converts dopamine to norepinephrine) * **Wilson’s Disease:** Characterized by excessive copper accumulation due to defective biliary excretion (ATP7B mutation).

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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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 Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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