Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 81: The activity of pyruvate carboxylase is dependent upon the positive allosteric effector?

A. Succinate
B. AMP
C. Isocitrate
D. Acetyl CoA (Correct Answer)

Explanation: **Explanation:** **Pyruvate Carboxylase** is the first regulatory enzyme of **gluconeogenesis**. It catalyzes the conversion of pyruvate to oxaloacetate (OAA) within the mitochondria. This reaction requires ATP, Biotin (as a CO2 carrier), and Manganese. **1. Why Acetyl CoA is correct:** Acetyl CoA acts as an **obligatory activator** (positive allosteric effector) for pyruvate carboxylase. When Acetyl CoA levels rise in the mitochondria (due to increased fatty acid oxidation), it signals that the TCA cycle is saturated or that energy levels are high. Acetyl CoA binds to the enzyme, inducing a conformational change that allows it to convert pyruvate into oxaloacetate. This OAA can then enter the gluconeogenic pathway to produce glucose or replenish the TCA cycle (anaplerosis). **2. Why other options are incorrect:** * **Succinate & Isocitrate:** These are intermediates of the TCA cycle. While they regulate enzymes like Isocitrate Dehydrogenase, they do not directly regulate the initial step of gluconeogenesis. * **AMP:** This is a signal of low cellular energy. It typically inhibits gluconeogenic enzymes (like Fructose-1,6-bisphosphatase) and activates glycolytic enzymes (like PFK-1). It does not activate Pyruvate Carboxylase. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **ABC Enzyme:** Pyruvate Carboxylase is an "ABC" enzyme—it requires **A**TP, **B**iotin, and **C**O2. * **Biotin Deficiency:** Can lead to lactic acidosis because pyruvate cannot be converted to OAA and is instead shunted to lactate. * **Localization:** This reaction occurs exclusively in the **mitochondria**. * **Reciprocal Regulation:** High Acetyl CoA inhibits the **Pyruvate Dehydrogenase (PDH) complex** while simultaneously activating Pyruvate Carboxylase, effectively diverting pyruvate from oxidation to gluconeogenesis.

Question 82: Which enzyme is deficient in alkaptonuria?

A. Homogentisic acid oxidase (Correct Answer)
B. Tyrosinase I
C. Tyrosinase II
D. Acid maltase

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. The correct answer is **Homogentisic acid oxidase** (Option A). This enzyme is responsible for converting homogentisic acid into maleylacetoacetic acid. When deficient, homogentisic acid accumulates in the blood and is excreted in the urine. Upon exposure to air, it oxidizes to form a brownish-black pigment (alkapton), which is a classic diagnostic sign. **Analysis of Incorrect Options:** * **Tyrosinase I & II (Options B & C):** Deficiency of Tyrosinase leads to **Oculocutaneous Albinism**, characterized by a lack of melanin in the skin, hair, and eyes. It is not related to the accumulation of homogentisic acid. * **Acid Maltase (Option D):** Also known as $\alpha$-1,4-glucosidase, its deficiency causes **Pompe Disease** (Glycogen Storage Disease Type II), which primarily affects cardiac and skeletal muscles. **Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (bluish-black pigmentation of connective tissues like the sclera and ear cartilage), and 3. Arthritis (usually involving large joints and the spine). * **Diagnosis:** Ferric chloride test (turns urine deep blue/green) and Silver nitrate test. * **Management:** Low protein diet (restricted Phenylalanine and Tyrosine) and **Nitisinone**, which inhibits 4-hydroxyphenylpyruvate dioxygenase to reduce homogentisic acid production.

Question 83: Which of the following statements regarding the Michaelis constant (Km) is TRUE?

A. Km is dependent on enzyme concentration.
B. The Km value is inversely proportional to the affinity of the enzyme for its substrate. (Correct Answer)
C. The Km value denotes the product concentration at half maximal velocity.
D. The Km value can be the same for two different enzymes.

Explanation: ### Explanation **1. Why Option B is Correct:** The Michaelis constant (**Km**) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity ($V_{max}$). It serves as a measure of the **affinity** between an enzyme and its substrate. * **Inverse Relationship:** A **low Km** indicates high affinity (the enzyme reaches half-maximal velocity at low substrate concentrations). Conversely, a **high Km** indicates low affinity (more substrate is needed to saturate the enzyme). This is a fundamental concept in enzyme kinetics. **2. Why the Other Options are Incorrect:** * **Option A:** Km is an **intrinsic property** of an enzyme-substrate pair. It is independent of enzyme concentration. While $V_{max}$ increases with more enzyme, the Km remains constant. * **Option C:** Km denotes the **substrate concentration** ($[S]$), not the product concentration, at half-maximal velocity ($1/2 V_{max}$). * **Option D:** While theoretically possible by coincidence, Km is generally unique to a specific enzyme-substrate pair under defined conditions (pH, temperature). In the context of competitive inhibition, the *apparent* Km changes, but the true Km is a characteristic constant. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hexokinase vs. Glucokinase:** This is the most common clinical application. **Hexokinase** has a **low Km** (high affinity) for glucose, allowing it to function even during fasting. **Glucokinase** (in the liver/pancreas) has a **high Km** (low affinity), functioning only when glucose levels are high (post-prandial). * **Lineweaver-Burk Plot:** On a double-reciprocal plot, the **x-intercept is $-1/Km$**. * **Competitive Inhibition:** Km **increases** (affinity decreases), but $V_{max}$ remains unchanged. * **Non-competitive Inhibition:** Km **remains unchanged**, but $V_{max}$ decreases.

Question 84: Damage to RBC resulting in their lysis increases which isoenzyme in blood?

A. LDH-1 (Correct Answer)
B. LDH-3
C. LDH-4
D. LDH-5

Explanation: **Explanation:** **Lactate Dehydrogenase (LDH)** is a tetrameric enzyme composed of two subunits: H (Heart) and M (Muscle). It exists in five isoenzyme forms (LDH-1 to LDH-5), which are distributed tissue-specifically. **1. Why LDH-1 is the correct answer:** LDH-1 (H4) is predominantly found in **Red Blood Cells (RBCs)** and **Cardiac Muscle**. When RBCs undergo lysis (hemolysis), they release large quantities of LDH-1 into the bloodstream. Therefore, an increase in LDH-1 is a hallmark biochemical marker for hemolytic anemia and myocardial infarction. **2. Analysis of Incorrect Options:** * **LDH-3 (H2M2):** Primarily found in the **lungs** and spleen. It increases in conditions like pulmonary embolism or pneumonia. * **LDH-4 (H1M3):** Found in the **kidneys**, placenta, and pancreas. * **LDH-5 (M4):** Predominantly found in **Skeletal Muscle** and the **Liver**. It is a sensitive marker for hepatitis, liver congestion, or skeletal muscle injury (e.g., rhabdomyolysis). **3. High-Yield Clinical Pearls for NEET-PG:** * **LDH Flip:** In normal serum, LDH-2 is higher than LDH-1 (LDH-2 > LDH-1). In cases of **Myocardial Infarction** or **Hemolysis**, LDH-1 levels rise significantly, surpassing LDH-2. This is known as the "Flipped LDH Pattern." * **Total LDH:** While non-specific, total LDH is used as a marker of high cell turnover (e.g., malignancies like Lymphoma or Seminoma). * **Megaloblastic Anemia:** This condition often shows the highest elevations of LDH-1 due to ineffective erythropoiesis (intramedullary hemolysis).

Question 85: In the study of enzymes, a sigmoidal plot of substrate concentration versus reaction velocity may indicate which of the following?

A. Michaelis-Menten kinetics
B. Competitive inhibition
C. Noncompetitive inhibition
D. Cooperative bindings (Correct Answer)

Explanation: **Explanation:** **1. Why "Cooperative Bindings" is correct:** In biochemistry, a **sigmoidal (S-shaped) curve** is the hallmark of **allosteric enzymes**. These enzymes do not follow simple Michaelis-Menten kinetics. Instead, they exhibit **cooperative binding**, where the binding of a substrate molecule to one active site increases the affinity of other active sites for the substrate (positive cooperativity). This results in a slow initial velocity that increases rapidly once the first few substrate molecules bind, creating the characteristic "S" shape. A classic non-enzymatic example of this is the oxygen-dissociation curve of **Hemoglobin**. **2. Why the other options are incorrect:** * **A. Michaelis-Menten kinetics:** These enzymes produce a **hyperbolic curve**, not a sigmoidal one. They typically consist of a single polypeptide chain with one active site (e.g., Myoglobin). * **B & C. Competitive and Noncompetitive inhibition:** Both of these follow Michaelis-Menten kinetics. While they alter the $V_{max}$ or $K_m$, the fundamental shape of the velocity-substrate plot remains **hyperbolic**. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzymes:** Most rate-limiting steps in metabolic pathways (e.g., **PFK-1** in glycolysis) are allosteric and show sigmoidal kinetics. * **Hill Coefficient ($n$):** This measures the degree of cooperativity. If $n > 1$, there is positive cooperativity (sigmoidal curve); if $n = 1$, it is non-cooperative (hyperbolic curve). * **T and R states:** Allosteric enzymes transition from a **T (Tense/Low affinity)** state to an **R (Relaxed/High affinity)** state upon substrate binding. * **Allosteric Effectors:** These shift the sigmoidal curve: **Activators** shift it to the **left** (decreasing $K_{0.5}$), while **inhibitors** shift it to the **right** (increasing $K_{0.5}$).

Question 86: Which of the following statements is true about glutamate dehydrogenase?

A. It is present in the inner mitochondrial membrane.
B. It is elevated in acute viral hepatitis.
C. It favours the formation of glutamate.
D. It is used as a marker to assess drug safety. (Correct Answer)

Explanation: **Explanation:** **Glutamate Dehydrogenase (GDH)** is a mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to $\alpha$-ketoglutarate and ammonia. **Why Option D is Correct:** GDH is primarily located in the **mitochondrial matrix** of hepatocytes, specifically in the **perivenous (Zone 3)** region of the liver lobule. Because it is sequestered within the mitochondria, its release into the serum indicates severe hepatocellular necrosis or mitochondrial damage. In pharmaceutical research and clinical trials, GDH is used as a **highly specific biomarker for Drug-Induced Liver Injury (DILI)**. It is more specific for mitochondrial damage than ALT, making it a critical marker for assessing drug safety. **Analysis of Incorrect Options:** * **Option A:** GDH is located in the **mitochondrial matrix**, not the inner mitochondrial membrane. * **Option B:** In acute viral hepatitis, **ALT and AST** are the primary markers elevated due to cytoplasmic leakage. GDH is a marker of **centrilobular necrosis** (often seen in toxic injury or ischemia) rather than typical viral hepatitis. * **Option C:** Under physiological conditions, the reaction strongly **favors the oxidative deamination** (formation of $\alpha$-ketoglutarate and ammonia) to facilitate the entry of nitrogen into the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Allosteric Regulation:** GDH is uniquely regulated; it is **inhibited by GTP/ATP** (high energy) and **activated by ADP/GDP** (low energy). * **Coenzyme Versatility:** It is one of the few enzymes that can use either **NAD+** (for catabolism) or **NADP+** (for anabolism). * **Clinical Marker:** A high **GDH/ALT ratio** is often suggestive of alcoholic liver disease or ischemic hepatitis.

Question 87: Which molecule activates glutamate dehydrogenase in mitochondria?

A. ATP
B. GTP
C. NADH
D. ADP (Correct Answer)

Explanation: **Explanation:** **Glutamate Dehydrogenase (GDH)** is a unique mitochondrial enzyme that catalyzes the reversible oxidative deamination of Glutamate to $\alpha$-ketoglutarate and ammonia. It serves as a critical bridge between amino acid metabolism and the TCA cycle. **Why ADP is the correct answer:** GDH is an allosterically regulated enzyme that responds to the energy status of the cell. When cellular energy levels are low, **ADP** (and GDP) concentrations rise. ADP acts as a potent **allosteric activator** of GDH, promoting the breakdown of glutamate into $\alpha$-ketoglutarate. This $\alpha$-ketoglutarate then enters the TCA cycle to facilitate ATP production. **Why the other options are incorrect:** * **ATP and GTP (Options A & B):** These are indicators of high cellular energy. They act as **allosteric inhibitors** of GDH. When energy is abundant, the cell does not need to divert amino acids into the TCA cycle for fuel. * **NADH (Option C):** As a product of the GDH reaction and a signal of high energy status, NADH also acts as an inhibitor of the enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Coenzyme Specificity:** GDH is one of the few enzymes that can use either **NAD+** (oxidative deamination) or **NADP+** (reductive amination) as a coenzyme. * **Hyperinsulinism-Hyperammonemia Syndrome:** Mutations that lead to the loss of allosteric inhibition of GDH (making it permanently active) result in this syndrome. Overactive GDH increases $\alpha$-ketoglutarate, stimulating the TCA cycle and triggering excessive insulin release (hypoglycemia), while simultaneously increasing ammonia production (hyperammonemia). * **Directionality:** In the liver, the reaction primarily proceeds toward the formation of ammonia for the urea cycle.

Question 88: Which one of the following statements regarding enzyme kinetics is true?

A. A high Km indicates high enzyme-substrate affinity.
B. A low Km indicates high enzyme-substrate affinity. (Correct Answer)
C. Non-competitive inhibition decreases Km.
D. Non-competitive inhibition increases Km.

Explanation: **Explanation:** **1. Understanding Km and Affinity (The Correct Answer)** The Michaelis constant (**Km**) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity ($V_{max}$). It is an inverse measure of the enzyme's affinity for its substrate. * **Low Km:** The enzyme reaches half-saturation at a low substrate concentration, indicating **high affinity**. * **High Km:** The enzyme requires a high substrate concentration to reach half-saturation, indicating **low affinity**. **2. Analysis of Incorrect Options** * **Option A:** Incorrect. As explained above, a high Km signifies that the enzyme binds weakly to the substrate (low affinity). * **Options C & D:** In **Non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site). This reduces the overall concentration of functional enzymes, thereby **decreasing $V_{max}$**. However, because the inhibitor does not compete for the active site, the binding of the substrate to the enzyme is unaffected; therefore, **Km remains unchanged**. **3. High-Yield Clinical Pearls for NEET-PG** * **Competitive Inhibition:** The inhibitor resembles the substrate. **Km increases** (affinity decreases), but **$V_{max}$ remains unchanged** (can be overcome by adding more substrate). *Example: Statins inhibiting HMG-CoA reductase.* * **Non-competitive Inhibition:** **$V_{max}$ decreases**, but **Km remains unchanged**. *Example: Cyanide poisoning of Cytochrome oxidase.* * **Lineweaver-Burk Plot:** * X-intercept = $-1/Km$ * Y-intercept = $1/V_{max}$ * **Hexokinase vs. Glucokinase:** Hexokinase has a **low Km** (high affinity) for glucose, allowing it to function even at low blood sugar levels, whereas Glucokinase has a **high Km** (low affinity), functioning primarily after meals (high blood sugar).

Question 89: Copper is involved in collagen synthesis by which enzyme?

A. Lysyl oxidase (Correct Answer)
B. Lysyl hydroxylase
C. Cytochrome oxidase
D. Tyrosinase

Explanation: **Explanation:** **1. Why Lysyl Oxidase is correct:** Collagen synthesis involves several post-translational modifications. **Lysyl oxidase** is a copper-dependent extracellular enzyme responsible for the final step: the formation of **cross-links** between collagen fibrils. It oxidatively deaminates specific lysine and hydroxylysine residues into reactive aldehydes (allysine). These aldehydes then undergo spontaneous condensation to form covalent cross-links, which provide collagen with its structural integrity and high tensile strength. **2. Why other options are incorrect:** * **Lysyl hydroxylase:** This enzyme is responsible for the hydroxylation of lysine residues *inside* the cell. Crucially, it requires **Vitamin C (Ascorbic acid)** and Iron (Fe²⁺) as cofactors, not copper. Deficiency leads to Scurvy. * **Cytochrome oxidase:** While this is a copper-containing enzyme (Complex IV of the electron transport chain), it is involved in cellular respiration and ATP production, not collagen synthesis. * **Tyrosinase:** This is also a copper-containing enzyme, but its role is in the rate-limiting step of **melanin synthesis**. Deficiency results in Albinism. **3. Clinical Pearls & High-Yield Facts:** * **Menkes Disease:** A defect in copper absorption (ATP7A gene) leads to copper deficiency. This results in reduced lysyl oxidase activity, causing "kinky hair," connective tissue defects, and aortic aneurysms. * **Lathyrism:** Consumption of sweet peas (*Lathyrus odoratus*) containing β-aminopropionitrile inhibits lysyl oxidase, leading to skeletal and vascular deformities. * **Cofactor Summary:** Remember **"C for Cross-linking and Copper"** (Lysyl oxidase) vs. **"H for Hydroxylation and Hydroxylase"** (Vitamin C).

Question 90: What is a characteristic feature of alkaline phosphatase deficiency?

A. Absence of cementum. (Correct Answer)
B. Resorption of bone.
C. Ankylosis of teeth.
D. All of the above.

Explanation: **Explanation:** Alkaline Phosphatase (ALP) is a crucial enzyme for mineralization. In the genetic disorder **Hypophosphatasia**, there is a deficiency of the tissue-nonspecific alkaline phosphatase (TNSALP) isoenzyme. **1. Why "Absence of cementum" is correct:** ALP is essential for the formation of hydroxyapatite crystals. In the oral cavity, ALP activity is mandatory for the development of **acellular cementum** (the layer covering the tooth root). A deficiency leads to either complete absence or severe hypoplasia of cementum. Without cementum, the periodontal ligaments cannot attach the teeth to the alveolar bone, leading to the hallmark clinical sign: **premature spontaneous exfoliation of deciduous teeth** (especially incisors) without any signs of inflammation. **2. Why other options are incorrect:** * **Resorption of bone:** ALP deficiency causes a failure of mineralization (osteomalacia/rickets), not active resorption. Bone resorption is typically mediated by osteoclasts and acid phosphatase, which are not primarily affected here. * **Ankylosis of teeth:** Ankylosis is the pathological fusion of the tooth root to the bone. In ALP deficiency, the opposite occurs—teeth are loose and fall out because they fail to anchor to the bone due to the lack of cementum. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Low serum ALP levels and **elevated urinary phosphoethanolamine** are diagnostic. * **Radiology:** "Beaten copper" appearance of the skull and "bowing" of long bones. * **Key Symptom:** Premature loss of primary teeth (before age 4) is often the first presenting sign of the childhood form.

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Isoenzymes and Clinical Significance

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Enzymes — MCQs

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