Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 491: Which of the following is a constitutive enzyme?

A. Hexokinase (Correct Answer)
B. Glucokinase
C. Cyclooxygenase-2
D. β-galactosidase

Explanation: ***Hexokinase*** - **Hexokinase** is a **constitutive enzyme**, meaning it is consistently expressed at relatively constant levels in most tissues regardless of substrate availability. - It catalyzes the phosphorylation of glucose to glucose-6-phosphate, a crucial initial step in **glycolysis**, and is essential for basal cellular energy metabolism. *Glucokinase* - **Glucokinase** is an **inducible enzyme** primarily found in the liver and pancreatic beta cells, and its activity is significantly regulated by glucose levels. - Its expression increases in response to high glucose concentrations, promoting glucose storage and insulin secretion, unlike constitutive enzymes. *β-galactosidase* - **β-galactosidase** is a classic example of an **inducible enzyme**, whose synthesis is activated in the presence of lactose (its substrate) as part of the *lac operon* in bacteria. - It is typically present in very low amounts in the absence of lactose and is not constitutively expressed. *Cyclooxygenase-2* - **Cyclooxygenase-2 (COX-2)** is an **inducible enzyme** whose expression is significantly upregulated in response to inflammatory stimuli, cytokines, and growth factors. - It plays a major role in inflammation and pain, while **COX-1** is the constitutive isoform expressed under normal physiological conditions.

Question 492: What is the primary mechanism of action of 5-alpha reductase?

A. Reduction of C4-C5 double bond (Correct Answer)
B. Breakage of C-N bond
C. Breakage of amide bond
D. Breakage of N-N bond

Explanation: ***Reduction of C4-C5 double bond*** - 5-alpha reductase catalyzes the **reduction of testosterone** to dihydrotestosterone (DHT) by adding hydrogen atoms across the **C4-C5 double bond** in the steroid A-ring. - This reaction involves the **saturation of this specific double bond** through a reduction reaction, which is critical for its biological action. - The enzyme uses **NADPH as a cofactor** to donate hydrogen atoms, converting the double bond to a single bond. *Breakage of C-N bond* - The breaking of a **carbon-nitrogen bond** is not the mechanism of 5-alpha reductase. - This type of bond cleavage is characteristic of other enzymatic reactions, such as those involving **peptide hydrolysis** or certain types of **deamination**. *Breakage of amide bond* - An **amide bond** (-CO-NH-) is typically cleaved by enzymes like **amidases** or **proteases**. - This type of bond is not present in the substrate (testosterone) and is therefore not targeted by 5-alpha reductase. *Breakage of N-N bond* - The breaking of an **nitrogen-nitrogen bond** is a rare enzymatic process and is not involved in the metabolism of steroid hormones. - This type of reaction is seen in certain specialized enzymes involved in **nitrogen fixation** or **redox reactions** of nitrogen-containing compounds.

Question 493: Which enzyme is primarily responsible for the transfer of hydrogen ions in oxidation-reduction reactions?

A. Hydratase
B. Peroxidase
C. Oxidase
D. Dehydrogenase (Correct Answer)

Explanation: ***Dehydrogenase*** - **Dehydrogenases** are a class of enzymes that facilitate the transfer of **hydrogen ions (protons)** and electrons from one molecule to another. - They are crucial in **oxidation-reduction (redox) reactions** by removing hydrogen from a substrate, often transferring it to coenzymes like **NAD+** or **FAD**. *Hydratase* - **Hydratases** are enzymes that catalyze the **addition** or **removal of water** to and from a substrate. - These enzymes are involved in **hydration** or **dehydration reactions**, not directly in the transfer of hydrogen ions in redox reactions. *Oxidase* - **Oxidases** are enzymes that catalyze **oxidation-reduction reactions** specifically involving **molecular oxygen (O2)** as an electron acceptor. - While they are involved in redox, their primary role is not the direct transfer of hydrogen ions but rather the **reduction of oxygen**. *Peroxidase* - **Peroxidases** are enzymes that catalyze the breakdown of **hydrogen peroxide**, often using it to oxidize another substrate. - They are important in **detoxification** and **antioxidant defense**, but they do not primarily transfer hydrogen ions in typical redox reactions of metabolism.

Question 494: What is the primary function of the enzyme tyrosinase?

A. Synthesis of melanin (Correct Answer)
B. Synthesis of norepinephrine
C. Synthesis of dopamine
D. Synthesis of thyroxine

Explanation: ***Synthesis of melanin*** - **Tyrosinase** is a copper-containing enzyme that catalyzes the hydroxylation of **tyrosine** to DOPA and the oxidation of DOPA to dopaquinone. - These steps are crucial for the biosynthesis of **melanin**, the primary pigment responsible for skin, hair, and eye color. *Synthesis of norepinephrine* - **Norepinephrine** synthesis involves a series of enzymatic steps starting from **tyrosine**, but tyrosinase is not directly involved in its formation. - The conversion of **dopamine** to norepinephrine is catalyzed by **dopamine β-hydroxylase**. *Synthesis of dopamine* - **Dopamine** is synthesized from **L-DOPA** by the enzyme **DOPA decarboxylase**. - While DOPA is an intermediate in melanin synthesis, **tyrosinase** is not the primary enzyme for dopamine production, although it can produce DOPA from tyrosine. *Synthesis of thyroxine* - **Thyroxine (T4)** is a thyroid hormone synthesized from **tyrosine residues** on **thyroglobulin** by the enzyme **thyroid peroxidase**. - This process is distinct from tyrosinase's role in melanin synthesis.

Question 495: Which of the following statements about glucokinase is true?

A. It is present in all tissues.
B. It is an enzyme that is always active.
C. It has a high Km for glucose. (Correct Answer)
D. It is inhibited by glucose 6-phosphate.

Explanation: ***It has a high Km for glucose.*** - A **high Km** indicates that **glucokinase** has a **low affinity for glucose**, allowing it to become active only when glucose concentrations are high, such as after a meal. - This characteristic is crucial for its role in the **liver** and **pancreatic β-cells** as a glucose sensor, facilitating glucose uptake and metabolism only when glucose is abundant. *It is present in all tissues.* - **Glucokinase** is primarily found in the **liver** and **pancreatic β-cells** where it plays a critical role in glucose sensing and metabolism. - In most other tissues, **hexokinase** is the primary enzyme responsible for phosphorylating glucose. *It is an enzyme that is always active.* - The activity of **glucokinase** is regulated by **glucose concentration** and **hormonal signals (e.g., insulin)**, meaning it is not always active. - Its activity significantly increases post-prandially in response to elevated blood glucose levels. *It is inhibited by glucose 6-phosphate.* - Unlike **hexokinase**, which is strongly inhibited by its product **glucose 6-phosphate**, **glucokinase** is not inhibited by glucose 6-phosphate. - This allows the liver to continue taking up and phosphorylating glucose even when intracellular glucose 6-phosphate levels are high, which is important for replenishing glycogen stores.

Question 496: Which enzyme is considered a marker for the endoplasmic reticulum?

A. Catalase
B. LDH
C. Glucose-6-phosphatase (Correct Answer)
D. Acid phosphatase

Explanation: ***Glucose-6-phosphatase*** - This enzyme is uniquely localized to the **endoplasmic reticulum (ER) membrane**, playing a crucial role in the final step of gluconeogenesis and glycogenolysis. - Its presence and activity are used as a **biochemical marker** to identify and characterize ER fractions in cell biology studies. *Catalase* - **Catalase** is predominantly found within **peroxisomes**, where it catalyzes the decomposition of hydrogen peroxide into water and oxygen. - While peroxisomes can bud off from the ER, catalase itself is not considered a direct marker for the ER membrane. *LDH* - **Lactate dehydrogenase (LDH)** is a ubiquitous **cytoplasmic enzyme** involved in glycolysis, converting pyruvate to lactate. - It is a marker for general cellular damage when found in extracellular fluids, but not specifically for the endoplasmic reticulum. *Acid phosphatase* - **Acid phosphatase** is primarily localized within **lysosomes**, where it plays a role in the hydrolysis of phosphate esters in an acidic environment. - Therefore, it serves as a marker for lysosomes, not the endoplasmic reticulum.

Question 497: Which of the following enzymes is not a free radical scavenger?

A. Glutathione peroxidase
B. Superoxide dismutase
C. Catalase
D. Xanthine oxidase (Correct Answer)

Explanation: ***Xanthine oxidase*** - **Xanthine oxidase** is involved in the production of **superoxide radicals** during the metabolism of purines, particularly the conversion of **hypoxanthine to xanthine** and xanthine to uric acid. - It is known to contribute to **oxidative stress** by generating **reactive oxygen species**, rather than scavenging them. *Glutathione peroxidase* - This enzyme **reduces hydrogen peroxide** to water and organic hydroperoxides to their corresponding alcohols, using **glutathione** as a reducing agent. - It plays a crucial role in protecting cells from **oxidative damage** by neutralizing harmful peroxides. *Superoxide dismutase* - **Superoxide dismutase (SOD)** catalyzes the dismutation of the **superoxide radical** into molecular oxygen and hydrogen peroxide. - This enzyme is a primary defense against the **toxic effects of superoxide** in various organisms. *Catalase* - **Catalase** functions to convert **hydrogen peroxide** into water and oxygen. - It is an important enzyme in **peroxisomes** and protects the cell from damage by **reactive oxygen species**.

Question 498: Hepatomegaly with hypoglycemia occurs in deficiency of

A. Branching enzyme
B. Debranching enzyme
C. Hepatic phosphorylase
D. Glucose-6-phosphatase (Correct Answer)

Explanation: ***Acid maltase*** - Deficiency of acid maltase, also known as Pompe disease, leads to significant muscle and liver glycogen accumulation, which can cause an **AST/ALT ratio > 2** due to hepatocellular injury [1,2]. - It predominantly causes **myopathy** and hepatomegaly, making this enzyme deficiency clinically relevant in this context [1]. *Glucose-6-phosphotase* - This deficiency leads to **von Gierke disease**, characterized by severe hypoglycemia and increased lactate, but not specifically an AST/ALT ratio > 2 [1]. - It results in **glycogen accumulation in the liver**, affecting glucose metabolism but not primarily liver enzymes [1]. *Branching enzyme* - Branching enzyme deficiency results in **Andersen disease**, which is characterized by long unbranched glycogen chains, and predominantly causes **hepatic dysfunction** without a specific AST/ALT ratio > 2. - Clinical manifestations include **cirrhosis** and splenomegaly, but not elevation of liver transaminases in the described ratio. *Liver phosphorylase* - Liver phosphorylase deficiency results in **Cori disease**, which presents with hypoglycemic episodes and hepatomegaly, but not necessarily an AST/ALT ratio above 2 [1]. - The enzyme affects glycogen mobilization, and clinical features do not consistently include liver enzyme elevation as described. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 164-167. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 850-851.

Question 499: What is the predominant isoform of lactate dehydrogenase (LDH) found in skeletal muscles?

A. LDH-2
B. LDH-3
C. LDH-5 (Correct Answer)
D. LDH-1

Explanation: ***LDH-5*** - **LDH-5**, also known as **M4 (four M subunits)**, is the predominant isoform found in **skeletal muscles** and the **liver**. - Its high concentration in skeletal muscle reflects its role in converting **pyruvate to lactate** under anaerobic conditions, which is essential for muscle activity when oxygen is limited. *LDH-1* - **LDH-1 (H4)** is predominantly found in the **heart** and **red blood cells**. - It is efficient at converting **lactate back to pyruvate**, which is crucial for aerobic metabolism in organs like the heart. *LDH-2* - **LDH-2 (H3M1)** is found in various tissues but is particularly abundant in the **reticuloendothelial system** and **white blood cells**. - It represents an intermediate isoform with a balance of properties between LDH-1 and LDH-5. *LDH-3* - **LDH-3 (H2M2)** is a more widely distributed isoform, predominantly found in the **lungs**, **lymphatic tissue**, and **kidneys**. - It also has intermediate catalytic properties, reflecting the metabolic diversity of these tissues.

Question 500: Transferases are classified as which of the following?

A. EC-1 (Oxidoreductases)
B. EC-2 (Transferases) (Correct Answer)
C. EC-3 (Hydrolases)
D. EC-4 (Lyases)

Explanation: ***EC-2 (Transferases)*** - Transferases are enzymes that catalyze the **transfer of a functional group** (e.g., methyl, glycosyl, phosphate) from one molecule to another. - The Enzyme Commission (EC) number system classifies enzymes into **seven main classes**, with EC-2 specifically designating transferases. *EC-1 (Oxidoreductases)* - **Oxidoreductases** are enzymes that catalyze **oxidation-reduction reactions**, involving the transfer of electrons. - This class includes enzymes like **dehydrogenases** and **oxidases**, which are distinct from transferases as they do not transfer functional groups. *EC-3 (Hydrolases)* - **Hydrolases** are enzymes that catalyze the **hydrolysis of chemical bonds**, a process that involves the addition of water. - This group includes enzymes such as **esterases**, **peptidases**, and **glycosidases**, which break down molecules. *EC-4 (Lyases)* - **Lyases** are enzymes that catalyze the **breaking of various chemical bonds** by means other than hydrolysis or oxidation, often forming new double bonds or rings. - Examples include **decarboxylases** and **aldolases**, which remove groups without the involvement of water.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

Practice Questions

Enzyme Inhibition: Competitive and Non-competitive

Practice Questions

Allosteric Regulation

Practice Questions

Coenzymes and Cofactors

Practice Questions

Isoenzymes and Clinical Significance

Practice Questions

Enzyme Regulation: Covalent Modification

Practice Questions

Enzyme Regulation: Zymogen Activation

Practice Questions

Enzyme Induction and Repression

Practice Questions

Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

Practice Questions

Enzyme Therapy and Inhibitors as Drugs

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?