Enzymes Practice Questions

Q: Aldolase is classified as which type of enzyme?

Lyase. **Explanation:** **1. Why Lyase is the Correct Answer:** Enzymes are classified into six major classes by the IUBMB (International Union of Biochemistry and Molecular Biology). **Lyases (Class 4)** are enzymes that catalyze the cleavage of C-C, C-O, C-N, and other 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. **Aldolase** (specifically Aldolase A in glycolysis) catalyzes the reversible cleavage of **Fructose 1,6-bisphosphate** (a 6-carbon sugar) into two 3-carbon molecules: **Dihydroxyacetone phosphate (DHAP)** and **Glyceraldehyde 3-phosphate (GAP)**. Since this involves breaking a C-C bond without the use of water (hydrolysis) or redox changes, it is a classic example of a Lyase. **2. Why Other Options are Incorrect:** * **Transferases (Class 2):** These transfer functional groups (e.g., methyl, phosphate) from one substrate to another. Example: Hexokinase. * **Isomerases (Class 5):** These catalyze structural rearrangements within a single molecule. Example: Phosphofructoisomerase. * **Reductases (Class 1 - Oxidoreductases):** These catalyze oxidation-reduction reactions involving the transfer of electrons/hydrogen. Example: Lactate dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoenzymes:** Aldolase A (Muscle/RBCs), Aldolase B (Liver/Kidney), Aldolase C (Brain). * **Clinical Correlation:** **Aldolase B deficiency** leads to **Hereditary Fructose Intolerance (HFI)**. In HFI, Fructose-1-phosphate accumulates, depleting intracellular phosphate and causing severe hypoglycemia and liver damage. * **Diagnostic Marker:** Serum Aldolase A levels are elevated in skeletal muscle disorders like Duchenne Muscular Dystrophy (DMD) and inflammatory myopathies.

Q: H2O2 is broken down or formed by which of the following enzymes?

Oxidase. **Explanation:** Enzymes involved in oxidation-reduction reactions are classified as **Oxidoreductases**. The question focuses on the specific handling of Hydrogen Peroxide ($H_2O_2$). **1. Why Oxidase is Correct:** Oxidases catalyze the removal of hydrogen from a substrate using **oxygen as a hydrogen acceptor**. In many of these reactions, the oxygen is reduced specifically to **Hydrogen Peroxide ($H_2O_2$)** rather than water. A classic example is *Xanthine Oxidase*. Additionally, the enzyme **Peroxidase** (a sub-type of oxidase) specifically breaks down $H_2O_2$ into water. Therefore, oxidases are the primary group associated with the formation or degradation of $H_2O_2$. **2. Why Other Options are Incorrect:** * **Oxygenases:** These enzymes do not produce $H_2O_2$. Instead, they catalyze the **incorporation of oxygen atoms** directly into the substrate molecule. They are divided into Dioxygenases (incorporate both atoms of $O_2$) and Monooxygenases (incorporate one atom as -OH and reduce the other to $H_2O$). * **Hydrolases:** These enzymes catalyze the cleavage of bonds (C-O, C-N, C-C) by the **addition of water**. They are not involved in redox reactions or $H_2O_2$ metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Catalase:** A hemoprotein containing four heme groups. it is one of the fastest enzymes known and specifically protects the body from oxidative damage by breaking down $H_2O_2$ into $H_2O$ and $O_2$. * **Superoxide Dismutase (SOD):** Converts the superoxide free radical ($O_2^-$) into $H_2O_2$ and $O_2$. * **Glutathione Peroxidase:** A **Selenium-dependent** enzyme that helps neutralize $H_2O_2$ in RBCs, protecting them from hemolysis. * **Peroxisomes:** The cellular organelles where most $H_2O_2$-producing oxidases are localized.

Q: The enzyme carbamoyl phosphate synthetase requires:

Mg++. **Explanation:** **Carbamoyl Phosphate Synthetase (CPS)** is a critical enzyme in nitrogen metabolism. There are two isoforms: **CPS-I** (found in mitochondria for the Urea Cycle) and **CPS-II** (found in the cytosol for Pyrimidine Synthesis). Both isoforms catalyze reactions that are highly energy-dependent, requiring the hydrolysis of **ATP**. **Why Mg++ is the correct answer:** Most enzymes that utilize ATP as a substrate require **Magnesium (Mg++)** as a mandatory cofactor. Mg++ binds to the phosphate groups of ATP, neutralizing their negative charge and facilitating the nucleophilic attack. Specifically, for CPS-I, Mg++ is essential for the activation of bicarbonate and the subsequent phosphorylation steps. Without Mg++, the enzyme cannot stabilize the high-energy intermediates required to form carbamoyl phosphate. **Why other options are incorrect:** * **Ca++ (Calcium):** While calcium is a vital secondary messenger and a cofactor for enzymes like α-amylase and certain clotting factors, it does not typically act as a cofactor for ATP-dependent ligases like CPS. * **K+ (Potassium):** Potassium is the major intracellular cation and is required for the activity of specific enzymes like **Pyruvate Kinase**, but it does not play a direct role in the CPS reaction mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **CPS-I vs. CPS-II:** Remember that CPS-I requires **N-Acetylglutamate (NAG)** as an absolute allosteric activator. CPS-II does not require NAG. * **Location:** CPS-I is in the **Mitochondria** (Mnemonic: **M**itochondria for **M**etabolism of Ammonia/Urea); CPS-II is in the **Cytosol** (Mnemonic: **C**ytosol for **C**yrimidine synthesis). * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the Urea Cycle. Deficiency leads to **Type I Hyperammonemia**, characterized by neurological symptoms and low citrulline levels.

Q: Which of the following enzymes is FAD-linked?

Succinate dehydrogenase. **Explanation:** **Succinate Dehydrogenase (SDH)** is the correct answer because it is a unique enzyme of the TCA cycle that is directly embedded in the inner mitochondrial membrane (acting as **Complex II** of the Electron Transport Chain). Unlike most other dehydrogenases that use NAD+, SDH requires **FAD (Flavin Adenine Dinucleotide)** as its prosthetic group. This is because the free energy change associated with the oxidation of succinate to fumarate is insufficient to reduce NAD+ but is adequate to reduce FAD to FADH₂. **Analysis of Incorrect Options:** * **Isocitrate Dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle and primarily uses **NAD+** (mitochondrial) or NADP+ (cytosolic) as a coenzyme. * **Pyruvate Dehydrogenase (PDH):** While the PDH complex *contains* FAD (as part of the E3 subunit), it is a multi-enzyme complex where the final electron acceptor that leaves the complex is **NAD+** (forming NADH). In the context of "FAD-linked" in competitive exams, SDH is the classic textbook example of a direct FAD-dependent reaction. * **Enoyl Reductase:** This enzyme is part of the Fatty Acid Synthase complex and utilizes **NADPH** as a reducing agent for fatty acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Role:** SDH is the only enzyme that participates in both the **TCA Cycle** and the **Electron Transport Chain**. * **Inhibitor:** **Malonate** is a classic competitive inhibitor of Succinate Dehydrogenase due to its structural similarity to succinate. * **Riboflavin Link:** Since FAD is derived from **Vitamin B2 (Riboflavin)**, deficiencies in this vitamin directly impair SDH activity. * **Marker Enzyme:** SDH is often used as a marker enzyme for the inner mitochondrial membrane.

Q: Which enzyme splits hydrogen peroxide into water and oxygen?

Catalase. **Explanation:** **1. Why Catalase is the correct answer:** Catalase is a hemeprotein found primarily in **peroxisomes**. Its primary function is to protect cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and molecular oxygen ($O_2$). The reaction is: $2H_2O_2 \rightarrow 2H_2O + O_2$. It has one of the highest turnover numbers among all enzymes, processing millions of molecules per second. **2. Analysis of Incorrect Options:** * **Cytochromes (A):** These are heme-containing proteins involved in the Electron Transport Chain (ETC). They function as electron carriers (undergoing $Fe^{2+}/Fe^{3+}$ redox cycles) rather than decomposing peroxides. * **Cytochrome P450 (B):** This superfamily of enzymes is primarily involved in the hydroxylation of drugs and steroids (Phase I metabolism) in the endoplasmic reticulum of the liver. * **Superoxide dismutase (C):** This enzyme acts on the **superoxide radical** ($O_2^{\bullet-}$), converting it into hydrogen peroxide ($H_2O_2$) and oxygen. It precedes the action of catalase in the antioxidant defense pathway. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Glutathione Peroxidase:** Another key enzyme that neutralizes $H_2O_2$ in the cytosol; it requires **Selenium** as a cofactor. * **Catalase Test:** Used in Microbiology to differentiate *Staphylococci* (Catalase positive) from *Streptococci* (Catalase negative). * **Acatalasia:** A rare genetic deficiency of catalase leading to oral ulcerations and gangrene. * **Peroxisomes:** Known as "microbodies," they contain catalase and are essential for the $\beta$-oxidation of Very Long Chain Fatty Acids (VLCFA).

Q: Which of the following conditions is associated with an increase in serum gamma-glutamyl transpeptidase (GGT)?

Alcoholism. **Explanation:** **Gamma-glutamyl transpeptidase (GGT)** is a membrane-bound enzyme primarily found in the liver, biliary tract, and kidneys. It is a highly sensitive marker for hepatobiliary diseases. **Why Alcoholism is the Correct Answer:** GGT is the most sensitive indicator of **chronic alcohol consumption**. Alcohol induces the microsomal expression of GGT in hepatocytes, leading to elevated serum levels even in the absence of overt liver disease. It is frequently used in clinical practice to monitor abstinence in recovering alcoholics. Additionally, GGT is elevated in **obstructive jaundice** and cholestasis, often paralleling Alkaline Phosphatase (ALP) levels. **Why the Other Options are Incorrect:** * **A & C (Muscle Injury & Muscular Dystrophy):** GGT is **not** found in skeletal muscle. Therefore, it remains normal in myopathies. This makes GGT a crucial tool to differentiate whether an elevated ALP is originating from the bone or the liver. * **D (Myocardial Infarction):** While GGT is present in the heart in trace amounts, it is not a marker for MI. Enzymes like Troponins, CK-MB, and LDH are the preferred markers for cardiac injury. **High-Yield Clinical Pearls for NEET-PG:** * **GGT vs. ALP:** If both GGT and ALP are elevated, the source of ALP is likely the **liver/biliary tract**. If ALP is high but GGT is normal, the source is likely **bone** (e.g., Paget’s disease, rickets). * **Enzyme Induction:** Besides alcohol, drugs like **Phenytoin** and **Phenobarbital** can also induce GGT. * **Sensitivity:** GGT is the first enzyme to rise in biliary obstruction.

Q: Which of the following enzymes is active in its phosphorylated form?

HMG-CoA reductase kinase. **Explanation:** The metabolic regulation of enzymes via covalent modification (phosphorylation/dephosphorylation) is a high-yield concept. In general, **catabolic enzymes** (breakdown) are active when phosphorylated, while **anabolic enzymes** (synthesis) are active when dephosphorylated. **Why Option C is Correct:** **HMG-CoA reductase kinase** (also known as AMP-activated protein kinase or AMPK) is part of a phosphorylation cascade. Its role is to inhibit cholesterol synthesis when cellular energy is low. To perform its function of phosphorylating (and thus inactivating) HMG-CoA reductase, the kinase itself must be **activated by phosphorylation**. This is an exception to the general rule that "kinases" are the actors; here, the regulator itself is regulated. **Analysis of Incorrect Options:** * **A. Pyruvate Dehydrogenase (PDH):** This enzyme links glycolysis to the TCA cycle. It is **inactivated** by phosphorylation (via PDH kinase) and activated by dephosphorylation (via PDH phosphatase). * **B. HMG-CoA Reductase:** This is the rate-limiting enzyme for cholesterol synthesis. It follows the "anabolic rule": it is **active in the dephosphorylated state** and inactive when phosphorylated. * **D. Pyruvate Kinase:** A key glycolytic enzyme. In the liver, it is **inactivated** by phosphorylation (stimulated by glucagon) to prevent glycolysis during fasting. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "P" for Phosphorylated = "P" for Pumped up (Catabolic) / "D" for Dephosphorylated = "D" for Done building (Anabolic). * **Key Exception:** Glycogen Phosphorylase is **active** when phosphorylated (catabolic), whereas Glycogen Synthase is **inactive** when phosphorylated (anabolic). * **HMG-CoA Reductase** is the target of **Statins**, which are competitive inhibitors used in dyslipidemia.

Q: In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:

Coenzyme-cosubstrate. **Explanation:** The correct answer is **B. Coenzyme-cosubstrate.** **1. Why it is correct:** Enzymes often require non-protein components called **cofactors** for activity. Cofactors are divided into inorganic ions and organic molecules called **coenzymes**. Coenzymes are further classified based on their binding affinity: * **Cosubstrates:** These are loosely and transiently bound to the enzyme. They bind, undergo a chemical change (like NAD+ being reduced to NADH), and then **dissociate** from the enzyme to be regenerated by a different reaction. * In this case, NAD+ acts as a cosubstrate because it shuttles electrons from the alcohol and leaves the alcohol dehydrogenase (ADH) complex as NADH. **2. Why other options are incorrect:** * **A. Apoenzyme:** This refers to the protein portion of the enzyme that is catalytically inactive without its cofactor. * **C. Coenzyme-prosthetic group:** A prosthetic group is a coenzyme that is **tightly or covalently bound** to the enzyme and does not dissociate during the reaction (e.g., FAD in Succinate Dehydrogenase or Heme in Cytochrome c). * **D. Cofactor:** While NAD+ is technically a cofactor, "Coenzyme-cosubstrate" is the more specific and accurate description required for NEET-PG level biochemistry. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alcohol Metabolism:** ADH is the rate-limiting enzyme in alcohol metabolism (Zero-order kinetics). It is inhibited by **Fomepizole** (used in methanol/ethylene glycol poisoning). * **Niacin (Vitamin B3):** NAD+ and NADP+ are derived from Niacin. Deficiency leads to **Pellagra** (4 Ds: Dermatitis, Diarrhea, Dementia, Death). * **Key Cosubstrates:** NAD+, NADP+, and Coenzyme A. * **Key Prosthetic Groups:** FAD, FMN, Biotin, and Pyridoxal Phosphate (PLP).

Q: Which of the following enzymes is activated in its dephosphorylated state?

HMG Co A reductase. ### Explanation In metabolic regulation, many key enzymes are controlled via **reversible covalent modification**, specifically phosphorylation and dephosphorylation. **1. Why HMG-CoA Reductase is Correct:** HMG-CoA reductase is the rate-limiting enzyme of cholesterol synthesis. It follows the general rule for **anabolic (synthetic) pathways**: they are typically **active in the dephosphorylated state**. * **Mechanism:** Insulin promotes the dephosphorylation of HMG-CoA reductase via protein phosphatase, thereby activating it. Conversely, Glucagon and AMPK trigger phosphorylation, which inactivates the enzyme to conserve energy. **2. Analysis of Incorrect Options:** * **B & C (Glycogen phosphorylase and Phosphorylase kinase):** These are key enzymes in **glycogenolysis** (a catabolic pathway). Catabolic enzymes are generally **active in their phosphorylated state**. This ensures that during fasting (high glucagon) or stress (high epinephrine), glycogen is broken down to release glucose. * **D (Citrate lyase):** ATP-citrate lyase, which provides acetyl-CoA for fatty acid synthesis, is actually **activated by phosphorylation** (specifically by Akt/PKB in response to insulin), making it an exception to the general rule that anabolic enzymes are dephosphorylated. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Thumb":** Most rate-limiting enzymes of **Anabolic** pathways (e.g., Glycogen synthase, HMG-CoA reductase, Acetyl-CoA carboxylase) are **Active when Dephosphorylated**. * Most **Catabolic** enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase) are **Active when Phosphorylated**. * **Statins:** These drugs are competitive inhibitors of HMG-CoA reductase, mimicking the structure of the HMG-CoA substrate. * **AMPK:** This "energy sensor" inhibits HMG-CoA reductase by phosphorylating it when cellular ATP levels are low.

Q: All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?

Coenzyme A. The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that converts pyruvate to Acetyl-CoA. It requires five distinct cofactors. The distinction between "tightly bound" (prosthetic groups) and "mobile" (transient) cofactors is a high-yield NEET-PG concept. ### 1. Why Coenzyme A is the Correct Answer **Coenzyme A (CoA)** and **NAD+** are considered **mobile carriers** or "dissociable" cofactors. They are not permanently attached to the enzyme complex. Instead, they enter the reaction, pick up the products (the Acetyl group and electrons, respectively), and then diffuse away to participate in other metabolic pathways (like the TCA cycle or Electron Transport Chain). ### 2. Why the Other Options are Incorrect The remaining three cofactors are **prosthetic groups**, meaning they are covalently or very tightly bound to their respective sub-enzymes: * **Thiamine Pyrophosphate (TPP):** Tightly bound to **E1** (Pyruvate decarboxylase). It is essential for the decarboxylation step. * **Lipoic Acid (Lipoamide):** Tightly bound to **E2** (Dihydrolipoyl transacetylase) via a lysine residue. It swings between active sites to transfer the acetyl group. * **Flavin Adenine Dinucleotide (FAD):** Tightly bound to **E3** (Dihydrolipoyl dehydrogenase). It accepts electrons from lipoamide to become FADH₂. ### 3. Clinical Pearls for NEET-PG * **The "Tender Loving Care For No One" Mnemonic:** TPP, Lipoic Acid, CoA, FAD, NAD. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the **SH groups of Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * **Location:** The PDH complex is located in the **mitochondrial matrix**.

Question 1

Aldolase is classified as which type of enzyme?

Accepted Answer

Lyase

Answer

Transferase

Answer

Isomerase

Answer

Reductase

Question 2

H2O2 is broken down or formed by which of the following enzymes?

Accepted Answer

Oxidase

Answer

Oxygenase

Answer

Hydrolase

Answer

None of the above

Question 3

The enzyme carbamoyl phosphate synthetase requires:

Accepted Answer

Mg++

Answer

Ca++

Answer

K+

Answer

None of the above

Question 4

Which of the following enzymes is FAD-linked?

Accepted Answer

Succinate dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Enoyl reductase

Question 5

Which enzyme splits hydrogen peroxide into water and oxygen?

Accepted Answer

Catalase

Answer

Cytochromes

Answer

Cytochrome P450

Answer

Superoxide dismutase

Question 6

Which of the following conditions is associated with an increase in serum gamma-glutamyl transpeptidase (GGT)?

Accepted Answer

Alcoholism

Answer

Muscle injury

Answer

Muscular dystrophy

Answer

Myocardial infarction

Question 7

Which of the following enzymes is active in its phosphorylated form?

Accepted Answer

HMG-CoA reductase kinase

Answer

Pyruvate dehydrogenase

Answer

HMG-CoA reductase

Answer

Pyruvate kinase

Question 8

In the reaction catalyzed by alcohol dehydrogenase (ADH), an alcohol is oxidized to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. ADH requires NAD+ for catalytic activity. In this reaction, NAD+ is functioning as a:

Accepted Answer

Coenzyme-cosubstrate

Answer

Apoenzyme

Answer

Coenzyme-prosthetic group

Answer

Cofactor

Question 9

Which of the following enzymes is activated in its dephosphorylated state?

Accepted Answer

HMG Co A reductase

Answer

Glycogen phosphorylase

Answer

Glycogen phosphorylase kinase

Answer

Citrate lyase

Question 10

All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT?

Accepted Answer

Coenzyme A

Answer

Thiamine pyrophosphate

Answer

Lipoic acid

Answer

Flavin adenine dinucleotide

Enzymes — MCQs

Enzymes — MCQs

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