Enzymes Practice Questions

Q: The conversion of CO2 and H2O into carbonic acid during the formation of aqueous humour is catalysed by which one of the following enzymes

Carbonic anhydrase. ***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.

Q: Km of an enzyme is

The substrate concentration at half maximum velocity. ***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**.

Q: Which of the following indicates the 'flipping effect'?

LDH1 > LDH2. ***LDH1 > LDH2*** - The "flipping effect" specifically refers to an **increase in LDH1 activity** such that it surpasses LDH2 activity. - This pattern is a classic indicator of **myocardial injury**, particularly a **myocardial infarction (heart attack)**. *LDH2 > LDH1* - This is the **normal pattern** of LDH isoenzyme distribution in healthy individuals. - In healthy serum, **LDH2 is typically higher than LDH1**. *LDH2 > LDH3* - While LDH2 is normally higher than LDH3, this comparison is **not characteristic** of the "flipping effect." - The "flipping effect" specifically involves the relationship between **LDH1 and LDH2**. *LDH2 > LDH4* - This relationship is generally true in both normal and abnormal conditions, as **LDH2 is typically abundant** in serum. - It does not represent the specific **diagnostic pattern** known as the "flipping effect."

Q: Which of the following is true about non-competitive inhibition?

Km remains same, Vmax decreases. ***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.

Q: 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?

No change in Vmax. ***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].

Q: Evaluate the biochemical effects of competitive inhibition of succinate dehydrogenase by malonate.

Accumulation of succinate. ***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.

Q: 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?

Increase KM, no change in Vmax. ***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.

Q: Which enzyme is typically elevated 4 to 6 hours after a myocardial infarction and returns to normal within 2 to 3 days?

CPK-MB. ***CPK-MB*** - **Creatine phosphokinase-MB (CPK-MB)** is a cardiac-specific enzyme that rises 4 to 6 hours after an MI, peaks at 18-24 hours, and typically returns to normal in **2 to 3 days** (48-72 hours). - Its presence confirms myocardial damage and helps in diagnosing **acute myocardial infarction**. *Troponin T* - **Troponin T** is a highly sensitive and specific cardiac marker, but it remains elevated for a longer period (up to **7-14 days**) after an MI, unlike the 2-3 day return to normal specified. - While it rises within 3-6 hours, its prolonged elevation makes it unsuitable for the given timeframe. *LDH* - **Lactate dehydrogenase (LDH)** is a non-specific marker that rises later (24-48 hours after MI) and remains elevated for a much longer duration (up to **10-14 days**), making it not fit the specified time course. - Its elevation can also be due to other conditions, such as **hemolytic anemia** or **liver disease**, reducing its diagnostic specificity for MI. *Myoglobin* - **Myoglobin** is one of the first markers to rise (within 1-4 hours) after an MI, but it is **not cardio-specific** and also returns to normal very quickly (within **12-24 hours**). - Its rapid return to normal is shorter than the 2-3 day timeframe specified in the question, and its lack of specificity means it can be elevated in muscle injury from other causes.

Q: How does the Lineweaver-Burk plot change with a non-competitive inhibitor?

Increased y-intercept, same x-intercept. ***Increased y-intercept, same x-intercept*** - A **non-competitive inhibitor** binds to an allosteric site on the enzyme, reducing its catalytic efficiency (Vmax). - On a Lineweaver-Burk plot, a decrease in **Vmax** results in an **increased y-intercept (1/Vmax)**, while the **x-intercept (-1/Km)** remains unchanged because the inhibitor does not affect substrate binding affinity. *Increased x-intercept, same y-intercept* - This pattern is characteristic of a **competitive inhibitor**, which increases the apparent **Km** (shifting the x-intercept toward zero). - The **Vmax** remains unchanged, so the y-intercept does not change. *Both intercepts increase* - This does not accurately describe any standard inhibition pattern. - In **uncompetitive inhibition**, both Vmax and Km decrease proportionally, creating parallel lines (both intercepts increase, but the slope remains constant). - In **mixed inhibition**, both Km and Vmax are affected, but the pattern varies depending on whether the inhibitor binds preferentially to the enzyme or enzyme-substrate complex. *No change* - No change would indicate that the substance is **not an inhibitor** or has no effect on enzyme kinetics. - Inhibitors, by definition, alter the enzyme's activity and therefore impact the Lineweaver-Burk plot.

Q: In the context of enzyme inhibition, which approach is generally considered more effective for targeting highly conserved metabolic enzymes?

Targeting allosteric sites for better specificity. ***Targeting allosteric sites for better specificity*** - **Allosteric sites** are distinct from the active site and are generally less conserved between different species or even isoforms of the same enzyme, providing better opportunities for **selective inhibition** of specific enzymes over others. - Modulators binding to allosteric sites induce **conformational changes** that can either activate or inhibit enzyme activity, allowing for a more nuanced and **tunable control** over metabolic pathways. *Targeting active sites directly* - Inhibiting the **active site** of highly conserved metabolic enzymes can lead to significant **off-target effects** due to the structural similarity of active sites across various enzymes. - This lack of **specificity** can result in toxicity and undesirable side effects, making it less ideal for differentiating between host and pathogen enzymes or between different metabolic contexts. *Both methods are equally effective in all cases* - The effectiveness of targeting approaches is highly **context-dependent**, influenced by factors such as the specific enzyme, the desired therapeutic outcome, and the potential for off-target effects. - Generalizing that both methods are equally effective overlooks the inherent differences in **selectivity and mechanism of action** offered by active versus allosteric site targeting. *Effectiveness depends on the specific enzyme and context* - While it's true that effectiveness depends on specific enzyme and context, the question asks which approach is *generally* more effective for *highly conserved* metabolic enzymes. - For such enzymes, the **inherent specificity advantage** of allosteric targeting makes it a generally preferred approach over active site targeting.

Question 1

The conversion of CO2 and H2O into carbonic acid during the formation of aqueous humour is catalysed by which one of the following enzymes

Accepted Answer

Carbonic anhydrase

Answer

Carbonic deoxygenase

Answer

Carboxylase

Answer

Carbamylase

Question 2

Km of an enzyme is

Accepted Answer

The substrate concentration at half maximum velocity

Answer

Numerically identical for all isozymes that catalyze a given reaction

Answer

The normal physiological substrate concentration

Answer

Dissociation constant

Question 3

Which of the following indicates the 'flipping effect'?

Accepted Answer

LDH1 > LDH2

Answer

LDH2 > LDH1

Answer

LDH2 > LDH3

Answer

LDH2 > LDH4

Question 4

Which of the following is true about non-competitive inhibition?

Accepted Answer

Km remains same, Vmax decreases

Answer

Km increases, Vmax remains same

Answer

Km decreases, Vmax increases

Answer

Km increases, Vmax increases

Question 5

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?

Accepted Answer

No change in Vmax

Answer

Increase in both Km and Vmax

Answer

Decrease in Km

Answer

Decrease in Vmax

Question 6

Evaluate the biochemical effects of competitive inhibition of succinate dehydrogenase by malonate.

Accepted Answer

Accumulation of succinate

Answer

Decreased fumarate levels

Answer

No change in fumarate levels

Answer

Decreased ATP production

Question 7

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?

Accepted Answer

Increase KM, no change in Vmax

Answer

Decrease both Vmax and KM

Answer

No change in KM, decrease Vmax

Answer

Decrease Vmax, increase KM

Question 8

Which enzyme is typically elevated 4 to 6 hours after a myocardial infarction and returns to normal within 2 to 3 days?

Accepted Answer

CPK-MB

Answer

Troponin T

Answer

LDH

Answer

Myoglobin

Question 9

How does the Lineweaver-Burk plot change with a non-competitive inhibitor?

Accepted Answer

Increased y-intercept, same x-intercept

Answer

Increased x-intercept, same y-intercept

Answer

Both intercepts increase

Answer

No change

Question 10

In the context of enzyme inhibition, which approach is generally considered more effective for targeting highly conserved metabolic enzymes?

Accepted Answer

Targeting allosteric sites for better specificity

Answer

Targeting active sites directly

Answer

Both methods are equally effective in all cases

Answer

Effectiveness depends on the specific enzyme and context

Enzymes — MCQs

Enzymes — MCQs

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