Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 501: 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 502: Which enzyme is primarily responsible for the metabolism of alcohol?

A. Alcohol dehydrogenase (Correct Answer)
B. MEOS
C. Catalase
D. Aldehyde dehydrogenase

Explanation: ***Alcohol dehydrogenase*** - **Alcohol dehydrogenase (ADH)** is the primary enzyme in the **cytosol** of hepatocytes responsible for the initial breakdown of ethanol to **acetaldehyde**. - This is the main pathway for alcohol metabolism, particularly at **low to moderate alcohol concentrations**, accounting for approximately **80-90%** of alcohol metabolism. *MEOS* - The **Microsomal Ethanol Oxidizing System (MEOS)**, primarily involving **CYP2E1**, becomes significant at **higher alcohol concentrations** or in chronic alcohol users. - While it contributes to alcohol metabolism, it is not the *primary* enzyme at typical consumption levels; ADH handles the bulk. *Catalase* - **Catalase** can metabolize alcohol, but its contribution is quantitatively **minor** compared to ADH and MEOS, accounting for less than **10%** of alcohol metabolism. - Catalase's primary role is to break down **hydrogen peroxide** into water and oxygen in peroxisomes. *Aldehyde dehydrogenase* - **Aldehyde dehydrogenase (ALDH)** is responsible for the **second step** of alcohol metabolism, converting acetaldehyde (produced by ADH) to acetate. - While crucial for alcohol metabolism, it acts on the **product** of ADH activity, not on ethanol itself, making ADH the primary enzyme for alcohol metabolism.

Question 503: What is the classification of urease?

A. Oxidoreductase
B. Lyase
C. Ligase
D. Hydrolase (Correct Answer)

Explanation: ***Hydrolase*** - **Urease** catalyzes the hydrolysis of urea into **ammonia** and **carbon dioxide**. - **Hydrolases** are enzymes that catalyze the cleavage of a chemical bond by adding water. *Oxidoreductase* - **Oxidoreductases** catalyze **oxidation-reduction reactions** by transferring electrons. - Urease does not facilitate electron transfer; rather, it performs a **hydrolytic** breakdown. *Lyase* - **Lyases** catalyze the cleavage of various chemical bonds by means other than hydrolysis or oxidation, often forming a new double bond or ring structure. - Urease uses water to break a bond and does not form double bonds. *Ligase* - **Ligases** catalyze the **joining of two large molecules** by forming a new chemical bond, typically with ATP hydrolysis. - Urease breaks down a molecule, it does not join molecules.

Question 504: 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.

Question 505: Which of the following is a ribozyme?

A. Peptidyl Transferase (Correct Answer)
B. Ribonuclease
C. Transpeptidase
D. Poly A polymerase

Explanation: ***Peptidyl Transferase*** - This enzyme, a component of the **large ribosomal subunit**, is responsible for forming **peptide bonds** between amino acids during protein synthesis. - It is unique because its catalytic activity is performed by **ribosomal RNA (rRNA)**, making it a ribozyme rather than a protein enzyme. *Ribonuclease* - Ribonucleases are a class of enzymes that **catalyze the degradation of RNA** into smaller components. - They are typically **protein-based enzymes** and do not exhibit catalytic activity stemming from RNA itself. *Transpeptidase* - Transpeptidases are protein enzymes primarily involved in **bacterial cell wall synthesis**, catalyzing the cross-linking of peptidoglycan chains. - They are the target of **beta-lactam antibiotics** like penicillin, which inhibit their protein-based enzymatic activity. *Poly A polymerase* - This enzyme adds a **polyadenosine (Poly A) tail** to the 3' end of messenger RNA (mRNA) precursors. - Poly A polymerase is a **protein enzyme** and its activity is not derived from an RNA molecule.

Question 506: Alpha-1 antitrypsin primarily inhibits which enzyme?

A. Trypsin
B. Neutrophil elastase (Correct Answer)
C. Chymotrypsin
D. Trypsinogen

Explanation: ***Neutrophil elastase*** - **Alpha-1 antitrypsin (A1AT)** is a serpin that primarily inactivates **neutrophil elastase**, which is released by neutrophils during inflammation. - Deficiency in A1AT leads to unopposed **elastase activity** in the lungs, causing tissue destruction and conditions like **emphysema**. *Trypsin* - **Trypsin** is a serine protease produced in the pancreas that aids in protein digestion in the small intestine. - While A1AT can inhibit trypsin to some extent, its primary and most clinically significant target is **neutrophil elastase**. *Chymotrypsin* - **Chymotrypsin** is another **serine protease** involved in protein digestion, also produced by the pancreas. - It is not the primary target of **alpha-1 antitrypsin**; its inactivation is less crucial in the context of A1AT's protective role in the lungs. *Trypsinogen* - **Trypsinogen** is the inactive precursor (zymogen) of trypsin, which is activated in the duodenum. - A1AT would not primarily inhibit trypsinogen, as it acts on active proteases like **trypsin** and **elastase**.

Question 507: Which one of the following shows allosteric inhibition of glycolysis?

A. Amino acid alanine
B. 2,3 BPG
C. Citrate (Correct Answer)
D. Malonic acid

Explanation: ***Citrate*** - **Citrate** is a classic **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, the key regulatory enzyme in glycolysis - It binds to an allosteric site (distinct from the active site), reducing PFK-1's affinity for **fructose-6-phosphate** - This is a **negative feedback mechanism** - when citrate accumulates (indicating sufficient ATP production via the citric acid cycle), glycolysis slows down *Malonic acid* - **Malonic acid** is a **competitive inhibitor** (NOT allosteric) of succinate dehydrogenase in the citric acid cycle - It structurally resembles succinate and competes for the active site directly *2,3-BPG* - **2,3-Bisphosphoglycerate (2,3-BPG)** is an **allosteric effector** of hemoglobin (decreases oxygen affinity), not an enzyme inhibitor in glycolysis - It binds to hemoglobin, not to glycolytic enzymes *Amino acid alanine* - **Alanine** is an allosteric inhibitor of **pyruvate kinase** (not a glycolytic regulator in this context) - While it does show allosteric inhibition, it acts on gluconeogenesis regulation in the liver, not as a direct glycolytic inhibitor

Question 508: The predominant isozyme of LDH in lung is:

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

Explanation: ***LD-3*** - **LD-3** is the predominant **LDH isozyme** found in the **lungs**, spleen, pancreas, and lymph nodes. - Its elevation often suggests conditions affecting these organs, such as pulmonary embolism or pancreatitis. *LD-1* - **LD-1** is primarily associated with the **heart** and **red blood cells**. - Elevated levels are typically seen in conditions like myocardial infarction and hemolytic anemia. *LD-2* - **LD-2** is also found in the **heart** and **red blood cells**, though typically in lower concentrations than LD-1 in the heart. - It is often elevated after an MI, but typically LD-1 is elevated higher than LD-2 after an MI. *LD-5* - **LD-5** is predominantly found in the **liver** and **skeletal muscle**. - Its increase is indicative of liver damage or muscle injury, such as hepatitis or muscular dystrophy.

Question 509: Citrate synthase is inhibited by -

A. Insulin
B. Glucagon
C. ADP
D. ATP (Correct Answer)

Explanation: ***ATP*** - **Citrate synthase**, a key enzyme in the Krebs cycle, is inhibited by **high levels of ATP**, indicating a high energy state in the cell. - This allosteric inhibition helps regulate the metabolic flux through the cycle, slowing it down when energy is abundant. *ADP* - **ADP** typically signifies a low energy state and would generally act as an **activator** rather than an inhibitor for metabolic pathways that produce ATP. - In this context, ADP would promote the activity of enzymes involved in energy generation, including those in the Krebs cycle. *Insulin* - **Insulin** is a hormone that promotes fuel storage and utilization, generally **activating** metabolic pathways rather than directly inhibiting enzymes like citrate synthase. - Its primary role is to regulate blood glucose levels and promote glucose uptake and utilization. *Glucagon* - **Glucagon** is a hormone that mobilizes fuel from storage and is typically associated with **catabolic processes**, often increasing metabolic activity in response to low blood glucose. - It does not directly inhibit citrate synthase; its main actions are on glucoregulation.

Question 510: Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?

A. Glucokinase is induced by insulin. (Correct Answer)
B. Hexokinase is specific for glucose.
C. Glucokinase is inhibited by glucose-6-phosphate.
D. Hexokinase has a high Km for glucose.

Explanation: ***Glucokinase is induced by insulin.*** - **Insulin** promotes glucose uptake and utilization in the liver and pancreatic beta cells, where glucokinase is primarily expressed. - Induction of **glucokinase** by insulin ensures that glucose is efficiently phosphorylated and trapped within hepatocytes when blood glucose levels are high. - This is a key mechanism for postprandial glucose homeostasis. *Incorrect: Hexokinase is specific for glucose.* - **Hexokinase** is NOT specific for glucose; it can phosphorylate various hexoses including **fructose**, **mannose**, and **galactose**. - Its broad substrate specificity distinguishes it from glucokinase, which has greater specificity for glucose. *Incorrect: Glucokinase is inhibited by glucose-6-phosphate.* - Unlike **hexokinase**, which is subject to product inhibition by glucose-6-phosphate, **glucokinase is NOT inhibited** by its product. - This lack of feedback inhibition allows glucokinase to continue phosphorylating glucose even when glucose-6-phosphate levels are elevated, which is appropriate for its role as a glucose sensor in liver and pancreatic beta cells. *Incorrect: Hexokinase has a high Km for glucose.* - **Hexokinase** has a **low Km** (~0.1 mM) for glucose, meaning it has high affinity and is saturated at normal blood glucose levels. - In contrast, **glucokinase** has a high Km (~10 mM), allowing it to respond proportionally to changes in blood glucose concentration.

Practice by Chapter

Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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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 Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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