Enzymes Practice Questions

Q: Regan isoenzyme is increased in which of the following conditions?

Seminoma. **Explanation:** **Regan isoenzyme** is a heat-stable alkaline phosphatase (ALP) isoenzyme that is biochemically identical to the placental ALP (PALP) but is produced ectopically by certain tumors. 1. **Why Seminoma is correct:** Regan isoenzyme is a classic **oncofetal protein**. It is most characteristically associated with **Seminoma** (germ cell tumor of the testis) and dysgerminoma of the ovary. It serves as a useful tumor marker for monitoring treatment response and recurrence in these patients. 2. **Why the other options are incorrect:** * **Paget’s disease & Osteoporosis:** These conditions involve increased bone turnover. The ALP elevated here is the **Bone isoenzyme** (heat-labile), not the placental-like Regan isoenzyme. * **Cholestasis:** This leads to an elevation of the **Liver isoenzyme** of ALP due to increased synthesis and "leaking" from the canalicular membrane into the blood. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Stability Rule:** "Regan is Resistant." Unlike the bone isoenzyme (which is heat-labile), the Regan isoenzyme is highly heat-stable (resists denaturation at 65°C). * **Nagao Isoenzyme:** A variant of the Regan isoenzyme, also found in germ cell tumors and metastatic carcinomas, but inhibited by L-leucine. * **ALP Isoenzyme Sources (Mnemonic: BLP-R):** * **B**one (Heat-labile) * **L**iver (Most common) * **P**lacental (Normal in 3rd trimester) * **R**egan (Pathological/Ectopic)

Q: Enzymes that catalyze synthetic reactions where two molecules are joined together and ATP is used belong to which class?

Ligase. **Explanation:** The International Union of Biochemistry (IUB) classifies enzymes into seven major classes based on the type of reaction they catalyze. **1. Why Ligase is Correct:** **Ligases (Class 6)** are enzymes that catalyze the joining (ligation) of two large molecules. This process involves the formation of new chemical bonds (C-O, C-S, C-N, or C-C). Crucially, these reactions are endergonic and require energy, which is provided by the **hydrolysis of ATP** or other high-energy nucleoside triphosphates. A classic example is *DNA Ligase*, which joins DNA strands, or *Pyruvate Carboxylase*. **2. Why Other Options are Incorrect:** * **Hydrolases (Class 3):** These catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They do not join molecules using ATP. * **Lyases (Class 4):** These catalyze the cleavage of bonds by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds. They do not require ATP for synthetic joining. * **Transferases (Class 2):** These catalyze the transfer of a specific functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Classes:** **O T H L I L H** (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases, and the newly added Translocases). * **Synthetase vs. Synthase:** In medical biochemistry, the term **Synthetase** is synonymous with **Ligase** (requires ATP), whereas **Synthase** belongs to other classes (like Lyases) and does **not** require ATP directly. * **Key Example:** *Pyruvate Carboxylase* (a ligase) is the rate-limiting enzyme for gluconeogenesis, requiring Biotin and ATP.

Q: Alkaline phosphatase contains which of the following trace metals?

Zinc. **Explanation:** **Alkaline Phosphatase (ALP)** is a metalloenzyme that requires **Zinc (Zn²⁺)** for its catalytic activity and structural integrity. Specifically, each monomer of the enzyme contains two zinc ions and one magnesium ion. Zinc acts as a cofactor that stabilizes the enzyme’s active site, allowing it to hydrolyze phosphate esters at an alkaline pH. **Analysis of Options:** * **Zinc (Correct):** Zinc is the primary metal constituent of ALP. It is also a cofactor for other high-yield enzymes like Carbonic Anhydrase, Alcohol Dehydrogenase, and DNA/RNA Polymerase. * **Cobalt:** While cobalt is a central component of **Vitamin B12 (Cobalamin)**, it is not found in ALP. * **Iron:** Iron is found in **Heme-containing enzymes** (Cytochromes, Catalase, Peroxidase) and non-heme proteins like Aconitase. * **Copper:** Copper is a cofactor for enzymes involved in redox reactions, such as **Cytochrome c oxidase**, Superoxide Dismutase (cytosolic), and Lysyl Oxidase. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Significance:** ALP levels are significantly elevated in **Obstructive Jaundice** (cholestasis) and **Bone diseases** with increased osteoblastic activity (e.g., Paget’s disease, Rickets). * **Heat Stability Test:** Used to differentiate ALP isoenzymes. The **Placental** isoenzyme (Regan) is the most heat-stable, while the **Bone** isoenzyme is the most heat-labile (*Mnemonic: "Bone is Burned"*). * **Zinc Deficiency:** Can lead to decreased ALP activity, along with clinical features like acrodermatitis enteropathica, poor wound healing, and hypogeusia (loss of taste).

Q: Carbonic anhydrase is an enzyme that occurs in plants, bacteria, and animals and is involved in the formation of which chemical?

Carbonic acid from carbon dioxide and water. **Explanation:** **1. Why the Correct Answer is Right:** Carbonic anhydrase (CA) is a zinc-containing metalloenzyme that catalyzes the reversible hydration of carbon dioxide ($CO_2$). The primary reaction involves the combination of $CO_2$ and water ($H_2O$) to form **carbonic acid ($H_2CO_3$)**. This reaction is essential for the transport of $CO_2$ from tissues to the lungs and for maintaining the acid-base balance in the body. While $H_2CO_3$ spontaneously dissociates into bicarbonate ($HCO_3^-$) and hydrogen ions ($H^+$), the specific chemical formed directly via the enzymatic action of CA is carbonic acid. **2. Why Incorrect Options are Wrong:** * **Option A:** The formation of $CO_2$ from elemental carbon and oxygen is a combustion process, not a biological enzymatic reaction. * **Option C:** The dissociation of carbonic acid into bicarbonate and $H^+$ is a spontaneous ionic dissociation that occurs rapidly without the need for an enzyme, although CA facilitates the overall equilibrium. * **Option D:** Hydrochloric acid ($HCl$) is secreted by parietal cells in the stomach. While CA is involved in providing the $H^+$ ions for this process, it does not directly synthesize $HCl$. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Zinc Cofactor:** Carbonic anhydrase is the classic example of an enzyme requiring **Zinc ($Zn^{2+}$)** for its catalytic activity. * **Isoforms:** CA II is the most active isoform found in RBCs; CA IV is found in the renal brush border. * **Clinical Application:** Carbonic anhydrase inhibitors like **Acetazolamide** are used to treat glaucoma (by reducing aqueous humor production), altitude sickness, and as a weak diuretic (acting on the proximal convoluted tubule). * **Bohr Effect:** CA plays a crucial role in the Bohr effect by facilitating the production of $H^+$ ions, which decrease hemoglobin's affinity for oxygen in peripheral tissues.

Q: Malonate inhibits which enzyme of glycolysis?

Succinate dehydrogenase. **Explanation:** The question contains a common conceptual trap: **Malonate does not inhibit an enzyme of glycolysis; it inhibits an enzyme of the Citric Acid Cycle (TCA Cycle).** Among the options provided, Succinate Dehydrogenase is the correct target. **1. Why Succinate Dehydrogenase is Correct:** Malonate is a classic example of a **competitive inhibitor**. It is a structural analogue of **succinate** (the substrate for succinate dehydrogenase). Because of its structural similarity, malonate competes for the active site of the enzyme **Succinate Dehydrogenase (Complex II)**, thereby blocking the conversion of succinate to fumarate. This inhibition halts the TCA cycle and the Electron Transport Chain. **2. Why Other Options are Incorrect:** * **Aconitase:** Inhibited by **Fluoroacetate** (which is converted to fluorocitrate, a "suicide inhibitor"). * **Alpha-ketoglutarate dehydrogenase:** Inhibited by **Arsenite** and high levels of Ammonia. It requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, NAD). * **Isocitrate dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle, primarily regulated by the **ATP/ADP ratio** and NADH levels, not by malonate. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Location:** Succinate Dehydrogenase is the **only** enzyme of the TCA cycle located in the **inner mitochondrial membrane** (others are in the matrix). It also functions as **Complex II** of the Electron Transport Chain. * **Competitive Inhibition:** In competitive inhibition, **$V_{max}$ remains unchanged**, but **$K_m$ increases**. This can be overcome by increasing the concentration of the substrate (succinate). * **Glycolysis vs. TCA:** Always read the question carefully. While the question asks about "glycolysis," the options provided are all TCA cycle enzymes. In such cases, select the biochemically accurate inhibitor-enzyme pair.

Q: What is the predominant isozyme of lactate dehydrogenase (LDH) in the heart?

LD-1. **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 tissue-specific isozymes. **1. Why LD-1 is correct:** LD-1 consists of four H subunits (**H₄**). It is the predominant isozyme found in the **heart** and **erythrocytes** (RBCs). Because the heart relies on aerobic metabolism, LD-1 is specialized to favor the conversion of lactate to pyruvate for energy production. **2. Analysis of Incorrect Options:** * **LD-2 (H₃M₁):** Predominantly found 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 lymphoid tissue. * **LD-5 (M₄):** Predominantly found in the **Liver** and **Skeletal Muscle**. It favors the conversion of pyruvate to lactate under anaerobic conditions. **3. Clinical Pearls for NEET-PG:** * **LDH Flip:** Normally, serum LD-2 > LD-1. However, in **Myocardial Infarction (MI)** or **Hemolytic Anemia**, LD-1 levels rise above LD-2. This reversal is known as the "Flipped LDH" pattern. * **Diagnostic Timing:** LDH levels begin to rise 12–24 hours after an MI, peak at 48–72 hours, and remain elevated for 7–10 days. This makes it a useful marker for **late diagnosis of MI** (though Troponins are now the gold standard). * **LD-4 (HM₃):** Primarily found in the kidneys and pancreas.

Q: An 18-year-old is brought to the emergency room following an automobile accident. Serum enzyme studies demonstrate a thousand-fold elevation of creatine kinase (CK). Which of the following tissues is most likely to be the source of the enzyme?

Skeletal muscle. **Explanation:** The correct answer is **Skeletal muscle**. **1. Why Skeletal Muscle is Correct:** Creatine Kinase (CK) is an enzyme primarily found in tissues with high energy demands. In the context of an automobile accident, the most likely cause of a massive (thousand-fold) elevation in serum CK is **Rhabdomyolysis**—the rapid breakdown of skeletal muscle due to mechanical trauma or crush injury. Skeletal muscle contains the highest total concentration of CK in the body (specifically the **CK-MM** isoenzyme). While other tissues contain CK, only the massive bulk of skeletal muscle can release enough enzyme to reach such extreme levels (e.g., >100,000 U/L). **2. Why Other Options are Incorrect:** * **Bone:** Bone tissue does not contain significant amounts of Creatine Kinase. Elevations in bone-related pathologies usually involve Alkaline Phosphatase (ALP). * **Brain:** The brain contains the **CK-BB** isoenzyme. However, the blood-brain barrier typically prevents significant amounts of CK-BB from entering the systemic circulation, and the total mass of the brain is insufficient to cause a thousand-fold serum elevation. * **Heart:** The myocardium contains **CK-MB**. While Myocardial Infarction (MI) causes an increase in CK, the elevation is typically 10–20 times the normal limit. A thousand-fold increase is clinically inconsistent with a cardiac event and points toward systemic muscle trauma. **3. High-Yield NEET-PG Pearls:** * **Isoenzymes:** CK-MM (Skeletal Muscle), CK-MB (Cardiac), CK-BB (Brain). * **CK-MB Index:** If CK-MB is >5% of total CK, suspect MI; if <3%, suspect skeletal muscle injury. * **Clinical Complication:** In rhabdomyolysis, massive CK elevation is often accompanied by **myoglobinuria**, which can lead to Acute Tubular Necrosis (ATN) and renal failure. * **Marker of Choice:** While CK-MB was traditionally used for MI, **Cardiac Troponins (I and T)** are now the gold standard due to higher specificity.

Q: What is the classification of tyrosinase?

Oxidase. **Explanation:** **Tyrosinase** is a copper-containing enzyme that belongs to the class of **Oxidoreductases** (EC 1). Specifically, it acts as an **oxidase** because it uses molecular oxygen ($O_2$) as an electron acceptor to catalyze the rate-limiting steps in melanin synthesis: the hydroxylation of Tyrosine to L-DOPA and the subsequent oxidation of L-DOPA to Dopaquinone. **Why the other options are incorrect:** * **Transferases:** These enzymes catalyze the transfer of a functional group (e.g., methyl, phosphate, or amino groups) from one substrate to another. Tyrosinase does not transfer groups; it facilitates redox reactions. * **Lyases:** These enzymes catalyze the cleavage of C-C, C-O, or C-N bonds by means other than hydrolysis or oxidation, often forming double bonds. * **Isomerases:** These enzymes catalyze structural rearrangements within a single molecule (e.g., converting glucose to fructose). **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Albinism:** A congenital deficiency or absence of the enzyme **Tyrosinase** leads to Oculocutaneous Albinism (Type 1), characterized by a lack of melanin in the skin, hair, and eyes. 2. **Cofactor:** Tyrosinase requires **Copper ($Cu^{2+}$)** for its activity. This is why copper deficiency can sometimes lead to hypopigmentation. 3. **Melanoma Marker:** Tyrosinase is often used as a specific immunohistochemical marker for identifying malignant melanoma cells. 4. **Pathway:** Tyrosine $\xrightarrow{\text{Tyrosinase}}$ DOPA $\xrightarrow{\text{Tyrosinase}}$ Dopaquinone $\rightarrow$ Melanin.

Q: Which of the following is not a ligase?

Acetyl CoA Carboxylase. **Explanation:** Enzymes are classified into six major classes (IUBMB classification). **Ligases (Class 6)** are enzymes that catalyze the joining of two molecules, usually coupled with the hydrolysis of a high-energy phosphate bond (like ATP). **Why Acetyl CoA Carboxylase is the Correct Answer:** Most **Carboxylases** (including Acetyl CoA Carboxylase, Pyruvate Carboxylase, and Propionyl CoA Carboxylase) are classic examples of **Ligases**. They utilize ATP to join a CO₂ molecule to a substrate. Acetyl CoA Carboxylase is the rate-limiting enzyme in fatty acid synthesis, converting Acetyl CoA to Malonyl CoA using ATP and Biotin as a cofactor. **Analysis of Incorrect Options:** * **A. Aldolase:** This is a **Lyase (Class 4)**. It catalyzes the cleavage of Fructose 1,6-bisphosphate into DHAP and Glyceraldehyde 3-phosphate in glycolysis without using water or redox reactions. * **B. Enolase:** This is also a **Lyase**. It catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). * **C. Decarboxylase:** These are **Lyases**. Unlike carboxylases, they remove a CO₂ group without requiring ATP energy input (e.g., Histidine decarboxylase). **High-Yield NEET-PG Pearls:** * **Mnemonic for Enzyme Classes:** **O**ver **T**he **H**ill **L**yases **I**somerases **L**igases (**O**xidoreductases, **T**ransferases, **H**ydrolases, **L**yases, **I**somerases, **L**igases). * **Biotin (B7) Dependency:** All carboxylases (Ligases) require Biotin as a cofactor, except for Vitamin K-dependent γ-carboxylation of clotting factors. * **Synthetase vs. Synthase:** "Synthetase" implies a Ligase (requires ATP), whereas "Synthase" is usually a Lyase or Transferase (does not require ATP).

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

Succinate dehydrogenase. ### 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 1

Regan isoenzyme is increased in which of the following conditions?

Accepted Answer

Seminoma

Answer

Paget's disease

Answer

Osteoporosis

Answer

Cholestasis

Question 2

Enzymes that catalyze synthetic reactions where two molecules are joined together and ATP is used belong to which class?

Accepted Answer

Ligase

Answer

Hydrolase

Answer

Lyase

Answer

Transferase

Question 3

Alkaline phosphatase contains which of the following trace metals?

Accepted Answer

Zinc

Answer

Cobalt

Answer

Iron

Answer

Copper

Question 4

Carbonic anhydrase is an enzyme that occurs in plants, bacteria, and animals and is involved in the formation of which chemical?

Accepted Answer

Carbonic acid from carbon dioxide and water

Answer

Carbon dioxide from carbon and oxygen

Answer

Bicarbonate ion from carbonic acid

Answer

Hydrochloric acid

Question 5

Malonate inhibits which enzyme of glycolysis?

Accepted Answer

Succinate dehydrogenase

Answer

Aconitase

Answer

Alpha ketoglutarate dehydrogenase

Answer

Isocitrate dehydrogenase

Question 6

What is the predominant isozyme of lactate dehydrogenase (LDH) in the heart?

Accepted Answer

LD-1

Answer

LD-2

Answer

LD-3

Answer

LD-5

Question 7

An 18-year-old is brought to the emergency room following an automobile accident. Serum enzyme studies demonstrate a thousand-fold elevation of creatine kinase (CK). Which of the following tissues is most likely to be the source of the enzyme?

Accepted Answer

Skeletal muscle

Answer

Bone

Answer

Brain

Answer

Heart

Question 8

What is the classification of tyrosinase?

Accepted Answer

Oxidase

Answer

Transferase

Answer

Lyase

Answer

Isomerase

Question 9

Which of the following is not a ligase?

Accepted Answer

Acetyl CoA Carboxylase

Answer

Aldolase

Answer

Enolase

Answer

Decarboxylase

Question 10

Which of the following is a NAD-dependent enzyme?

Accepted Answer

Succinate dehydrogenase

Answer

Glucose 6 Phosphate Dehydrogenase

Answer

Malate dehydrogenase

Answer

Fatty acyl CoA dehydrogenase

Enzymes — MCQs

Enzymes — MCQs

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