Enzymes — MCQs
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What is the classification of urease?
Which of the following statements about acid phosphatase is correct?
Which enzyme is primarily responsible for the metabolism of alcohol?
Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?
Which of the following is a ribozyme?
Alpha-1 antitrypsin primarily inhibits which enzyme?
Which one of the following shows allosteric inhibition of glycolysis?
The predominant isozyme of LDH in lung is:
Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?
What is the mechanism of conversion of trypsinogen to trypsin?
Enzymes Indian Medical PG Practice Questions and MCQs
Question 501: 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 502: Which of the following statements about acid phosphatase is correct?
- A. Acts at pH 8-9
- B. Prostate isoform is tartrate resistant
- C. Erythrocyte isoform is inhibited by cupric ions (Correct Answer)
- D. None of the options
Explanation: ***Erythrocyte isoform is inhibited by cupric ions*** - The **erythrocyte isoform** of acid phosphatase, often referred to as **red cell acid phosphatase (ACP1)**, is known to be inhibited by **cupric ions (Cu2+)**. This characteristic is used in some forensic and biochemical applications. - This isoform is also important in forensic analysis for genetic typing from bloodstains, where its activity can be distinguished based on inhibitors and electrophoretic patterns. *Acts at pH 8-9* - Acid phosphatase, by definition, functions optimally in an **acidic environment**, typically with an optimal pH ranging from **4.5 to 5.5**. - An enzyme acting at pH 8-9 would be an **alkaline phosphatase**, not an acid phosphatase. *Prostate isoform is tartrate resistant* - The **prostate-specific acid phosphatase (PSAP)**, an isoform of acid phosphatase, is notably **inhibited by L-tartrate**. This property is used diagnostically to differentiate PSAP from other acid phosphatase isoforms. - Therefore, the statement that it is tartrate resistant is incorrect; it is actually **tartrate sensitive**. *None of the options* - This option is incorrect because the statement regarding the **erythrocyte isoform being inhibited by cupric ions** is factually accurate.
Question 503: 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 504: Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?
- A. Fluoroacetate
- B. Arsenite
- C. Malonate (Correct Answer)
- D. Fluoride
Explanation: ***Malonate*** - **Malonate** is a **competitive inhibitor** of **succinate dehydrogenase** because its structure is very similar to that of succinate, allowing it to bind to the enzyme's active site but preventing the catalytic reaction. - This enzyme, also known as Complex II, is vital for both the **Krebs cycle** and the **electron transport chain**, linking these two metabolic pathways. *Fluoroacetate* - **Fluoroacetate** is an inhibitor of **aconitase**, an enzyme in the Krebs cycle that converts citrate to isocitrate. - It is metabolically converted to **fluorocitrate**, which then acts as a potent inhibitor of aconitase. *Arsenite* - **Arsenite** inhibits enzymes that require **lipoic acid** as a coenzyme, such as the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**. - Its mechanism involves binding to the sulfhydryl groups of dihydrolipoyl transacetylase, preventing its function. *Fluoride* - **Fluoride** is known to inhibit **enolase**, an enzyme involved in **glycolysis**. - Its inhibitory action is typically enhanced in the presence of phosphate.
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: 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.
Question 510: What is the mechanism of conversion of trypsinogen to trypsin?
- A. Hydrolysis
- B. Phosphorylation
- C. Removal of part of protein (Correct Answer)
- D. Removal of Carboxyl group
Explanation: ***Removal of part of protein*** - The conversion of **trypsinogen to trypsin** is an example of **proteolytic activation**, where a specific part of the inactive precursor (zymogen) is cleaved off. - This cleavage occurs at the N-terminus of trypsinogen by **enteropeptidase (or enterokinase)** in the duodenum, exposing the active site and forming active trypsin. *Hydrolysis* - While the removal of a part of the protein involves **hydrolysis of peptide bonds**, this option is too general. - It does not specify the selective nature of the cleavage that leads to activation, nor the fact that it's a specific segment being removed. *Phosphorylation* - **Phosphorylation** is a common mechanism for regulating enzyme activity, but it involves the addition of a **phosphate group**, not the removal of a protein segment. - This process is typically mediated by kinases and does not activate trypsinogen. *Removal of Carboxyl group* - The activation of trypsinogen involves the removal of a small N-terminal peptide, not specifically the removal of a **carboxyl group** from the protein. - While enzymatic cleavage does involve breaking peptide bonds, stating "removal of carboxyl group" is imprecise and does not accurately describe the mechanism.
Practice by Chapter
Enzyme Classification and Nomenclature
Practice Questions
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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