Enzymes — MCQs
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The enzyme primarily responsible for the conversion of T4 to T3 in the periphery is
The conversion of CO2 and H2O into carbonic acid during the formation of aqueous humour is catalysed by which one of the following enzymes
Km of an enzyme is
Enzyme that can be traced in semen sample of 8-10 weeks is:
Which of the following is true about non-competitive inhibition?
Enzyme activated by decrease in Insulin: glucagon ratio:
Which enzyme catalyzes the conversion of succinyl-CoA to succinate?
Evaluate the biochemical effects of competitive inhibition of succinate dehydrogenase by malonate.
A biochemistry lab is investigating an enzyme with a KM of 0.5 mM and a Vmax of 100 µmol/min. How will a competitive inhibitor with a KI of 0.1 mM affect the enzyme's activity if the substrate concentration is 1 mM?
Which enzyme is most commonly deficient in patients with Alzheimer's disease?
Enzymes Indian Medical PG Practice Questions and MCQs
Question 471: The enzyme primarily responsible for the conversion of T4 to T3 in the periphery is
- A. D3 thyroid deiodinase
- B. Thyroid peroxidase
- C. D1 thyroid deiodinase (Correct Answer)
- D. D2 thyroid deiodinase
Explanation: ***D1 thyroid deiodinase (Correct Answer)*** - **Type 1 deiodinase (D1)** is predominantly found in the **liver, kidney, and thyroid**, playing a significant role in the conversion of T4 to T3, particularly in conditions of iodine sufficiency. - This enzyme removes the 5′ iodine from T4, converting it into the more potent **T3**, which is essential for systemic thyroid hormone action. - **D1 is responsible for approximately 80% of circulating T3**, making it the primary enzyme for peripheral conversion. *D3 thyroid deiodinase (Incorrect)* - **Type 3 deiodinase (D3)** primarily **inactivates** thyroid hormones by converting T4 to **reverse T3 (rT3)** and T3 to **T2**, thereby reducing their biological activity. - D3 is highly expressed in the **placenta** and brain, protecting developing tissues from excessive thyroid hormone exposure. *Thyroid peroxidase (Incorrect)* - **Thyroid peroxidase (TPO)** is an enzyme involved in the **synthesis of thyroid hormones** within the thyroid gland, not peripheral conversion. - TPO catalyzes the **iodination of tyrosine residues** on thyroglobulin and the **coupling of iodotyrosines** to form T4 and T3. *D2 thyroid deiodinase (Incorrect)* - **Type 2 deiodinase (D2)** is found in tissues like the **brain, pituitary, and skeletal muscle**, where it converts T4 to T3 for **local tissue availability**. - While D2 is important for local T3 production and contributes ~20% to circulating T3, **D1 is primarily responsible for the circulating levels of T3** that act on peripheral tissues.
Question 472: The conversion of CO2 and H2O into carbonic acid during the formation of aqueous humour is catalysed by which one of the following enzymes
- A. Carbonic deoxygenase
- B. Carbonic anhydrase (Correct Answer)
- C. Carboxylase
- D. Carbamylase
Explanation: ***Carbonic anhydrase*** - **Carbonic anhydrase** is the enzyme responsible for catalysing the rapid interconversion of carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) into carbonic acid ($\text{H}_2\text{CO}_3$). - In the eye, this enzyme is crucial in the **ciliary epithelium** for the production of **aqueous humour**, where bicarbonate ions are actively secreted, drawing water into the posterior chamber. *Carbonic deoxygenase* - This term is **not a recognized enzyme** in standard biochemical pathways. - Enzymes involved in oxygenation typically add oxygen, while **deoxygenases** would remove oxygen. *Carboxylase* - **Carboxylase** enzymes catalyze the addition of a **carboxyl group** ($\text{COOH}$) to a substrate. - This reaction typically involves **bicarbonate** as the source of the carboxyl group and requires **ATP**, which is different from the hydration of $\text{CO}_2$ to form carbonic acid. *Carbamylase* - This term is not a widely recognized enzyme, but a similar enzyme, **carbamoyl phosphate synthetase**, catalyzes the formation of **carbamoyl phosphate**. - This enzyme is involved in the **urea cycle** and pyrimidine synthesis, not the direct conversion of $\text{CO}_2$ and $\text{H}_2\text{O}$ to carbonic acid.
Question 473: Km of an enzyme is
- A. Numerically identical for all isozymes that catalyze a given reaction
- B. The substrate concentration at half maximum velocity (Correct Answer)
- C. The normal physiological substrate concentration
- D. Dissociation constant
Explanation: ***The substrate concentration at half maximum velocity*** - **Km** (Michaelis constant) represents the **substrate concentration** at which the reaction velocity is **half of the maximum velocity (Vmax)**. - A **low Km** indicates a **high affinity** of the enzyme for its substrate, meaning it achieves half-maximal velocity at a lower substrate concentration. *Numerically identical for all isozymes that catalyses a given reaction* - **Isozymes** are different forms of an enzyme that catalyze the same reaction but may have **different Km values**, reflecting varying affinities for their substrate and different regulatory properties. - For example, **hexokinase** and **glucokinase** are isozymes with different Kms for glucose, reflecting their different physiological roles. *The normal physiological substrate concentration* - While Km is often in the range of **physiological substrate concentrations**, it is not defined as the normal physiological concentration itself. - It is a **kinetic parameter** determined experimentally and describes the enzyme's affinity, not the in vivo substrate availability. *Dissociation constant* - **Km** is technically an **apparent dissociation constant** but is not strictly equivalent to the true dissociation constant (Kd) of the enzyme-substrate complex. - It approximates Kd only under specific conditions where the **rate of ES breakdown to product is much slower than the dissociation of ES back to E + S**.
Question 474: Enzyme that can be traced in semen sample of 8-10 weeks is:
- A. CPK enzyme
- B. LDH
- C. ALP test
- D. Acid phosphatase test (Correct Answer)
Explanation: ***Acid phosphatase test*** - The **acid phosphatase (AP) test** is a crucial forensic test for identifying seminal fluid, even in aged or degraded samples. - While detectable for months, it remains a reliable indicator in semen samples for at least **8-10 weeks** due to its relative stability. *CPK enzyme* - **Creatine phosphokinase (CPK)** is primarily associated with muscle and brain tissue damage, not a specific marker for semen. - It is not routinely traced in semen samples for forensic analysis due to its low specificity. *LDH* - **Lactate dehydrogenase (LDH)** is an enzyme found in various tissues throughout the body, reflecting general cellular damage or metabolism. - It lacks the specificity to be a reliable forensic marker for the presence of semen. *ALP test* - **Alkaline phosphatase (ALP)** is commonly used in clinical settings to assess liver and bone health. - It is not a principal enzyme marker used for the forensic identification of seminal fluid due to its widespread distribution in the body.
Question 475: 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 476: Enzyme activated by decrease in Insulin: glucagon ratio:
- A. PFK
- B. Glucose 6 phosphatase (Correct Answer)
- C. Glucokinase
- D. Hexokinase
Explanation: ***Glucose 6 phosphatase*** - A decreased **insulin:glucagon ratio** signifies a catabolic state, promoting glucose release into the blood. - **Glucose-6-phosphatase** is the key enzyme in **gluconeogenesis** and **glycogenolysis** in the liver, dephosphorylating **glucose-6-phosphate** to **free glucose**, which can then be exported from the liver. *PFK* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, which is inhibited in a state of low insulin:glucagon ratio. - Its activity would decrease, not increase, to reduce glucose utilization. *Glucokinase* - **Glucokinase** phosphorylates glucose to **glucose-6-phosphate** in the liver, trapping it for metabolism; its activity is increased by high insulin levels. - In a low insulin:glucagon ratio, its activity would be reduced to conserve glucose. *Hexokinase* - **Hexokinase** phosphorylates glucose in most peripheral tissues but has a lower Km for glucose than glucokinase, becoming saturated even at low glucose concentrations. - Its activity is not primarily regulated by the insulin:glucagon ratio; it is generally involved in glucose uptake for cellular energy needs.
Question 477: Which enzyme catalyzes the conversion of succinyl-CoA to succinate?
- A. Malate dehydrogenase
- B. Succinyl-CoA synthetase (Correct Answer)
- C. Isocitrate dehydrogenase
- D. Succinate dehydrogenase
Explanation: ***Succinyl-CoA synthetase*** - This enzyme (also known as succinate thiokinase) directly catalyzes the reversible conversion of **succinyl-CoA to succinate**, coupled with the phosphorylation of GDP to GTP (or ADP to ATP). - This reaction is a key step in the **Krebs cycle** (or citric acid cycle), generating a **GTP molecule** which can be converted to ATP. *Malate dehydrogenase* - This enzyme catalyzes the reversible oxidation of **malate to oxaloacetate**, utilizing NAD+ as a coenzyme in the Krebs cycle. - It does not act on succinyl-CoA or succinate. *Isocitrate dehydrogenase* - This enzyme catalyzes the oxidative decarboxylation of **isocitrate to α-ketoglutarate**, producing NADH and CO2 in the Krebs cycle. - It is an important regulatory step in the pathway but is not involved in succinyl-CoA metabolism. *Succinate dehydrogenase* - This enzyme catalyzes the oxidation of **succinate to fumarate**, which is a subsequent step in the Krebs cycle. - It does not interconvert succinyl-CoA and succinate; instead, it acts on succinate after it has been formed.
Question 478: Evaluate the biochemical effects of competitive inhibition of succinate dehydrogenase by malonate.
- A. Decreased fumarate levels
- B. No change in fumarate levels
- C. Accumulation of succinate (Correct Answer)
- D. Decreased ATP production
Explanation: ***Accumulation of succinate*** - **Malonate** is a **structural analog** of **succinate** and acts as a **competitive inhibitor** of **succinate dehydrogenase**. - When **succinate dehydrogenase** is inhibited, its normal substrate, **succinate**, cannot be converted to **fumarate**, leading to its **accumulation**. - This is the **primary and most direct biochemical effect** of competitive inhibition - substrate accumulation upstream of the blocked enzyme. *Decreased fumarate levels* - While **fumarate levels would indeed decrease** due to reduced conversion from succinate, this is a **secondary downstream effect**. - In competitive inhibition questions, the **accumulation of substrate** (succinate) is considered the more direct and significant biochemical effect compared to depletion of product (fumarate). - The question asks to "evaluate biochemical effects" - substrate accumulation is the hallmark finding of enzyme inhibition. *No change in fumarate levels* - This is incorrect because the inhibition of **succinate dehydrogenase** directly prevents the conversion of **succinate** to **fumarate**. - Therefore, **fumarate production** will be reduced, leading to decreased fumarate levels over time. *Decreased ATP production* - While inhibition of the **TCA cycle** at **succinate dehydrogenase** could eventually decrease **ATP production** over time, this is a **tertiary downstream consequence**. - The immediate and most direct biochemical effect is the **accumulation of succinate** at the site of enzyme inhibition. - The impact on **ATP production** is an indirect systemic effect, not the primary biochemical hallmark of competitive inhibition.
Question 479: A biochemistry lab is investigating an enzyme with a KM of 0.5 mM and a Vmax of 100 µmol/min. How will a competitive inhibitor with a KI of 0.1 mM affect the enzyme's activity if the substrate concentration is 1 mM?
- A. Decrease both Vmax and KM
- B. Increase KM, no change in Vmax (Correct Answer)
- C. No change in KM, decrease Vmax
- D. Decrease Vmax, increase KM
Explanation: ***Increase KM, no change in Vmax*** - A **competitive inhibitor** binds reversibly to the enzyme's active site, competing with the substrate. This effectively **increases the apparent KM** because a higher substrate concentration is needed to achieve half Vmax due to increased competition. - While it increases the apparent KM, a competitive inhibitor can be overcome by sufficiently high substrate concentrations, meaning the **Vmax** (the maximum reaction rate) **remains unchanged**. *Decrease Vmax, increase KM* - This pattern of inhibition is characteristic of **mixed non-competitive inhibition**, where the inhibitor binds to both the enzyme and the enzyme-substrate complex. - In such cases, the inhibitor reduces the catalytic efficiency, leading to a decrease in Vmax, and can also affect substrate binding, altering KM. *Decrease both Vmax and KM* - This scenario describes **uncompetitive inhibition**, where the inhibitor binds only to the enzyme-substrate complex. - This binding reduces the effective concentration of enzyme-substrate complex, leading to a decrease in both the apparent Vmax and the apparent KM. *No change in KM, decrease Vmax* - This is typical of **pure non-competitive inhibition**, where the inhibitor binds to a site other than the active site on both the free enzyme and the enzyme-substrate complex. - The inhibitor does not interfere with substrate binding, so KM remains unchanged, but it reduces the enzyme's catalytic efficiency, thus decreasing Vmax.
Question 480: Which enzyme is most commonly deficient in patients with Alzheimer's disease?
- A. Acetylcholinesterase
- B. Monoamine oxidase
- C. Dopamine β-hydroxylase
- D. Choline acetyltransferase (Correct Answer)
Explanation: ***Choline acetyltransferase*** - **Choline acetyltransferase (ChAT)** is the enzyme responsible for synthesizing **acetylcholine**, a neurotransmitter crucial for memory and learning. - In Alzheimer's disease, there is a characteristic **degeneration of cholinergic neurons** and a significant reduction in ChAT activity, leading to **acetylcholine deficiency**. *Acetylcholinesterase* - **Acetylcholinesterase (AChE)** is the enzyme that breaks down acetylcholine in the synaptic cleft. - While it's involved in cholinergic signaling, its deficiency is not the primary pathology in Alzheimer's; rather, **inhibitors of AChE** are used therapeutically to increase acetylcholine levels. *Monoamine oxidase* - **Monoamine oxidase (MAO)** is involved in the metabolism of monoamine neurotransmitters like dopamine, norepinephrine, and serotonin. - Its dysfunction is more commonly associated with conditions like **Parkinson's disease** and depression, not the primary deficit in Alzheimer's. *Dopamine β-hydroxylase* - **Dopamine β-hydroxylase (DBH)** catalyzes the conversion of dopamine to norepinephrine. - Dysregulation of norepinephrine pathways can occur in Alzheimer's, but a primary deficiency of DBH is **not a hallmark** of the disease.
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
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