Enzyme Inhibition: Competitive and Non-competitive — MCQs
Acetazolamide is given to a patient with angle closure glaucoma. It is a reversible competitive inhibitor of carbonic anhydrase enzyme. Which of the following should be the effect of this drug?
Which of the following is an example of allosteric inhibition?
Which one of the following shows allosteric inhibition of glycolysis?
Km increases, but Vmax remains same. This is which type of inhibition?
Type of inhibition of aconitase by trans-aconitate is?
Which enzyme is competitively inhibited by malonate in the Krebs cycle?
Fluoride, used in the collection of blood samples, inhibits which enzyme?
Atorvastatin is used as an anti-dyslipidemic drug. These drugs inhibit their target enzyme by:-
Which of the following is true about non-competitive inhibition?
What is the mechanism by which mercury causes damage?
Enzyme Inhibition: Competitive and Non-competitive Indian Medical PG Practice Questions and MCQs
Question 1: Acetazolamide is given to a patient with angle closure glaucoma. It is a reversible competitive inhibitor of carbonic anhydrase enzyme. Which of the following should be the effect of this drug?
- A. No change in Vmax (Correct Answer)
- B. Increase in both Km and Vmax
- C. Decrease in Km
- D. Decrease in Vmax
Explanation: ***No change in Vmax*** - **Competitive inhibitors** bind reversibly to the active site of the enzyme, competing with the substrate for binding [1]. - At sufficiently high substrate concentrations, the substrate can outcompete the inhibitor, allowing the enzyme to reach its **maximum velocity (Vmax)**. - Therefore, Vmax remains unchanged in competitive inhibition, though more substrate is needed to achieve it [2]. *Decrease in Vmax* - A decrease in Vmax is characteristic of **non-competitive inhibitors**, which bind to a site other than the active site and reduce the enzyme's catalytic efficiency. - In competitive inhibition, Vmax is not decreased because high substrate concentrations can overcome the inhibition [2]. *Increase in both Km and Vmax* - While competitive inhibition does **increase Km** (apparent Km increases because more substrate is needed to reach half-maximal velocity), **Vmax remains unchanged**, not increased. - An increase in Vmax would indicate enhanced enzyme activity, which does not occur with inhibitors. *Decrease in Km* - A **decrease in Km** indicates higher enzyme affinity for substrate, meaning less substrate is needed to reach half-maximal velocity. - Competitive inhibition actually **increases Km** (decreases apparent affinity) because the inhibitor competes with substrate for the active site [1]. *Clinical Application* - Acetazolamide is used preoperatively in acute angle-closure glaucoma to lower intraocular pressure [3].
Question 2: Which of the following is an example of allosteric inhibition?
- A. Decreased synthesis of glucokinase by glucagon
- B. Inactivation of glycogen synthase by phosphorylation
- C. Inhibition of PFK-1 by citrate (Correct Answer)
- D. None of the options
Explanation: ***Inhibition of PFK-1 by citrate*** - **Citrate** acts as an **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, a key enzyme in glycolysis. - Citrate binds to a site distinct from the active site, inducing a conformational change that reduces PFK-1's affinity for **fructose-6-phosphate**, thus slowing glycolysis. *Inactivation of glycogen synthase by phosphorylation* - This is an example of **covalent modification** (phosphorylation), not allosteric regulation. - Phosphorylation alters the enzyme's activity by adding a phosphate group, changing its structure and function. *Decreased synthesis of glucokinase by glucagon* - This describes **transcriptional regulation** or **gene expression control**, where glucagon affects the amount of enzyme produced. - It is not an example of allosteric regulation, which involves direct binding of a molecule to an enzyme to alter its activity. *None of the options* - This option is incorrect because the inhibition of PFK-1 by citrate is a classic example of allosteric inhibition.
Question 3: 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 4: Km increases, but Vmax remains same. This is which type of inhibition?
- A. Uncompetitive
- B. Non-competitive
- C. Competitive (Correct Answer)
- D. Irreversible
Explanation: ***Competitive*** - In **competitive inhibition**, the inhibitor **reversibly binds** to the **active site** of the enzyme, competing with the substrate. - This competition means that a higher substrate concentration is required to achieve half-maximal velocity, thus **increasing the Km**, while the maximum velocity (**Vmax**) remains unchanged if sufficient substrate is present. *Uncompetitive* - **Uncompetitive inhibition** involves the inhibitor binding only to the **enzyme-substrate complex**. - This type of inhibition typically leads to a **decrease in both Km and Vmax**. *Non-competitive* - In **non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site) on either the free enzyme or the enzyme-substrate complex. - This binding usually **decreases the Vmax** (due to reduced enzyme efficiency) but does not affect the Km (as substrate binding is not directly hindered). *Irreversible* - **Irreversible inhibition** involves the formation of a strong, often covalent, bond between the inhibitor and the enzyme, permanently inactivating it. - This type of inhibition effectively **reduces the concentration of active enzyme**, leading to a **decrease in Vmax** (as fewer enzyme molecules are available to catalyze the reaction) with varying effects on Km depending on the mechanism.
Question 5: Type of inhibition of aconitase by trans-aconitate is?
- A. Competitive (Correct Answer)
- B. Non-competitive
- C. Allosteric
- D. None of the options
Explanation: ***Competitive*** - **Competitive inhibition** occurs when the inhibitor (trans-aconitate) structurally resembles the enzyme's natural substrate (cis-aconitate) and binds to the **active site**, preventing the substrate from binding. - This type of inhibition can be overcome by increasing the concentration of the **substrate**. *Non-competitive* - **Non-competitive inhibitors** bind to a site on the enzyme other than the active site, causing a conformational change that reduces the enzyme's efficiency, regardless of substrate concentration. - Trans-aconitate's structural similarity to aconitate's substrate points away from a non-competitive mechanism. *Allosteric* - **Allosteric inhibition** involves an inhibitor binding to a regulatory site (allosteric site) on the enzyme, which is distinct from the active site, to alter enzyme activity. - While allosteric regulation is a type of non-competitive inhibition, trans-aconitate's direct structural resemblance to the substrate makes competitive inhibition the more specific and accurate description. *None of the options* - This option is incorrect because **competitive inhibition** accurately describes the mechanism by which trans-aconitate inhibits aconitase, given its structural similarity to the natural substrate. - The other options are less fitting due to the specific characteristics of trans-aconitate's action.
Question 6: Which enzyme is competitively inhibited by malonate in the Krebs cycle?
- A. Fumarate dehydrogenase
- B. Succinate thiokinase
- C. Aconitase
- D. Succinate dehydrogenase (Correct Answer)
Explanation: ***Succinate dehydrogenase*** - **Malonate** is structurally similar to **succinate**, allowing it to bind to the active site of **succinate dehydrogenase**, thus competitively inhibiting the enzyme. - This enzyme is crucial for the conversion of **succinate to fumarate** in the **Krebs cycle** (or tricarboxylic acid cycle). *Fumarate dehydrogenase* - This enzyme is not a standard component of the Krebs cycle; instead, a **fumarase** enzyme catalyzes the hydration of **fumarate to malate**. - Malonate does not directly inhibit **fumarase**. *Succinate thiokinase* - This enzyme, also known as **succinyl-CoA synthetase**, catalyzes the conversion of **succinyl-CoA to succinate**. - It is not competitively inhibited by malonate during this step. *Aconitase* - **Aconitase** is an enzyme that catalyzes the isomerization of **citrate to isocitrate** via _cis_-aconitate in the Krebs cycle. - Its activity is not affected by malonate; it is instead inhibited by **fluoroacetate**, which is metabolized to **fluorocitrate**.
Question 7: Fluoride, used in the collection of blood samples, inhibits which enzyme?
- A. Enolase (Correct Answer)
- B. Glucokinase
- C. Glucose-6-phosphatase
- D. Hexokinase
Explanation: ***Enolase*** - Fluoride is a potent inhibitor of **enolase**, an enzyme in the **glycolytic pathway**. - Inhibition of enolase prevents the conversion of **2-phosphoglycerate** to **phosphoenolpyruvate**, thereby halting glycolysis in collected blood samples. *Glucokinase* - Glucokinase is an enzyme primarily found in the **liver** and **pancreatic beta cells** that phosphorylates glucose. - Fluoride does not directly inhibit glucokinase; its primary site of action for preventing glycolysis in blood samples is enolase. *Glucose-6-phosphatase* - This enzyme is crucial for **glucose production** in the liver and kidneys, facilitating the dephosphorylation of **glucose-6-phosphate** to glucose. - Fluoride does not specifically target glucose-6-phosphatase as its mechanism for preventing glycolysis. *Hexokinase* - Hexokinase catalyzes the first step of glycolysis, phosphorylating **glucose to glucose-6-phosphate**. - While essential for glycolysis, hexokinase is not the primary target of fluoride's inhibitory action in blood collection, which specifically aims to stop the entire pathway further downstream at enolase.
Question 8: Atorvastatin is used as an anti-dyslipidemic drug. These drugs inhibit their target enzyme by:-
- A. Noncompetitive inhibition
- B. Competitive inhibition (Correct Answer)
- C. Irreversible inhibition
- D. Uncompetitive inhibition
Explanation: ***Competitive inhibition*** - Atorvastatin is a **statin**, which acts as a **competitive inhibitor** of **HMG-CoA reductase**, the rate-limiting enzyme in cholesterol synthesis. - It competes with the natural substrate, HMG-CoA, for binding to the **active site of the enzyme**, thereby reducing cholesterol production. *Uncompetitive* - **Uncompetitive inhibitors** bind only to the **enzyme-substrate complex**, not to the free enzyme. - This type of inhibition is characterized by a decrease in both **apparent Vmax** and **apparent Km**. *Noncompetitive inhibition* - **Noncompetitive inhibitors** bind to an allosteric site on the enzyme, distinct from the active site, and can bind to either the **free enzyme or the enzyme-substrate complex**. - This leads to a decrease in the **apparent Vmax** but does not affect Km. *Irreversible inhibition* - **Irreversible inhibitors** form a **strong covalent bond** with the enzyme, permanently inactivating it. - Statins do not form covalent bonds with HMG-CoA reductase; their inhibition is **reversible** upon drug discontinuation.
Question 9: Which of the following is true about non-competitive inhibition?
- A. Km increases, Vmax remains same
- B. Km decreases, Vmax increases
- C. Km increases, Vmax increases
- D. Km remains same, Vmax decreases (Correct Answer)
Explanation: ***Km remains same, Vmax decreases*** - In **non-competitive inhibition**, the inhibitor binds to an allosteric site on the enzyme, altering its conformation, thereby **reducing its catalytic efficiency**. - This binding does not affect the **enzyme's affinity for the substrate (Km remains the same)**, but it **reduces the maximum reaction rate (Vmax decreases)** because fewer enzyme molecules are able to perform catalysis per unit time. *Km increases, Vmax remains same* - This describes **competitive inhibition**, where the inhibitor competes with the substrate for the enzyme's active site. - While it **increases the apparent Km** (more substrate needed to reach half Vmax), **Vmax remains unchanged** as high substrate concentrations can overcome the inhibition. *Km decreases, Vmax increases* - This scenario would imply an activation rather than inhibition, where both enzyme affinity and catalytic efficiency are enhanced. - This is not characteristic of any standard **enzyme inhibition mechanism**. *Km increases, Vmax increases* - This combination is not observed in any typical **enzyme inhibition pattern**. - An increase in **Vmax** implies enhanced catalytic activity, while an increase in **Km** suggests reduced substrate affinity, which are contradictory effects for a single inhibitor.
Question 10: What is the mechanism by which mercury causes damage?
- A. Causes toxicity through various mechanisms
- B. Binds to -SH groups of enzymes (Correct Answer)
- C. Inhibits electron transport chain
- D. Inhibits protein synthesis
Explanation: ***Binds to -SH groups of enzymes*** - Mercury, particularly its inorganic and organic forms, has a high affinity for **sulfhydryl (-SH) groups** found in **cysteine residues** of proteins and enzymes. - This binding disrupts the **tertiary structure** and **catalytic activity** of vital enzymes, leading to widespread cellular dysfunction and toxicity. *Causes toxicity through various mechanisms (not specific to -SH binding)* - While mercury can indeed cause toxicity through various mechanisms, the **most prominent and fundamental mechanism** underpins many of these downstream effects. - This option is too general and does not pinpoint the primary molecular interaction responsible for mercury's widespread cellular damage. *Indirectly inhibits the electron transport chain (ETC) by enzyme disruption* - This statement is partially true in that mercury's enzyme disruption can affect the ETC, but it's an **indirect consequence** rather than the primary mechanism itself. - The direct mechanism involves the initial binding to -SH groups, which then leads to the dysfunction of enzymes, including those involved in the ETC. *Indirectly inhibits protein synthesis by disrupting enzyme function* - Similar to ETC inhibition, mercury's disruption of enzyme function can ultimately impair protein synthesis, but this is an **effect down the causal chain**. - The initial and direct molecular interaction is the binding to sulfhydryl groups of key enzymes involved in various cellular processes, including protein synthesis.
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