Enzymes Practice Questions

Q: Which of the following enzymes is not a free radical scavenger?

Xanthine oxidase. ***Xanthine oxidase*** - **Xanthine oxidase** is involved in the production of **superoxide radicals** during the metabolism of purines, particularly the conversion of **hypoxanthine to xanthine** and xanthine to uric acid. - It is known to contribute to **oxidative stress** by generating **reactive oxygen species**, rather than scavenging them. *Glutathione peroxidase* - This enzyme **reduces hydrogen peroxide** to water and organic hydroperoxides to their corresponding alcohols, using **glutathione** as a reducing agent. - It plays a crucial role in protecting cells from **oxidative damage** by neutralizing harmful peroxides. *Superoxide dismutase* - **Superoxide dismutase (SOD)** catalyzes the dismutation of the **superoxide radical** into molecular oxygen and hydrogen peroxide. - This enzyme is a primary defense against the **toxic effects of superoxide** in various organisms. *Catalase* - **Catalase** functions to convert **hydrogen peroxide** into water and oxygen. - It is an important enzyme in **peroxisomes** and protects the cell from damage by **reactive oxygen species**.

Q: What is the predominant isoform of lactate dehydrogenase (LDH) found in skeletal muscles?

LDH-5. ***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.

Q: Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?

Malonate. ***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.

Q: Which of the following is not a cofactor required by pyruvate dehydrogenase?

Biotin. ***NAD*** - **NAD+ (Nicotinamide adenine dinucleotide)** is required by the dihydrolipoyl dehydrogenase (E3) component of the pyruvate dehydrogenase complex to accept electrons and form NADH. - This cofactor is crucial for the regeneration of the oxidized lipoamide, allowing the complex to continue its catalytic cycle. *FAD* - **FAD (Flavin adenine dinucleotide)** is also a cofactor for the dihydrolipoyl dehydrogenase (E3) enzyme, accepting electrons from reduced lipoamide before transferring them to NAD+. - It is tightly bound to the E3 enzyme and undergoes reversible oxidation-reduction during the reaction. *Biotin* - **Biotin** is primarily a cofactor for **carboxylase enzymes**, such as pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate. - It is **not involved** in the pyruvate dehydrogenase complex reaction, which is an oxidative decarboxylation, not a carboxylation. *CoA* - **Coenzyme A (CoA)** is essential for the pyruvate dehydrogenase complex, as it accepts the acetyl group from pyruvate to form **acetyl-CoA**. - Acetyl-CoA is the product of the reaction and serves as the entry molecule into the **Krebs cycle**.

Q: Which of the following statements about acid phosphatase is correct?

Erythrocyte isoform is inhibited by cupric ions. ***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.

Q: What is the classification of urease?

Hydrolase. ***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.

Q: Which enzyme is primarily responsible for the metabolism of alcohol?

Alcohol dehydrogenase. ***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.

Q: Transferases are classified as which of the following?

EC-2 (Transferases). ***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.

Q: Which of the following statements is true about noncompetitive enzyme inhibitors?

Km remains same, Vmax decreases. ***Km remains same, Vmax decreases*** - **Noncompetitive inhibitors** bind to an allosteric site on the enzyme, altering its conformation and **reducing its catalytic efficiency** without affecting substrate binding affinity. - This results in a decreased **Vmax** (maximum reaction rate) because the enzyme's ability to process substrate is impaired, while the **Km** (substrate concentration at half Vmax) remains unchanged as the substrate can still bind efficiently to the active site. *Noncompetitive inhibitors increase Km and decrease Vmax* - This statement is incorrect because noncompetitive inhibitors primarily affect the enzyme's **catalytic activity** (Vmax) rather than its affinity for the substrate (Km). - An increase in **Km** would imply a decrease in the enzyme's affinity for the substrate, which is characteristic of **competitive inhibition**, not noncompetitive inhibition. *Km decreases, Vmax increases* - This statement describes an effect not typically seen with noncompetitive inhibition; a decreased **Km** would imply increased substrate affinity, and an increased **Vmax** would suggest enhanced catalytic efficiency. - Noncompetitive inhibitors are known to **reduce** the maximum reaction rate, not increase it. *Km increases, Vmax increases* - This scenario, where both **Km** and **Vmax** increase, is not characteristic of any standard type of enzyme inhibition. - An increase in **Vmax** implies an enhancement in enzyme activity, which is contrary to the action of an inhibitor.

Q: Alpha-1 antitrypsin primarily inhibits which enzyme?

Neutrophil elastase. ***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 1

Which of the following enzymes is not a free radical scavenger?

Accepted Answer

Xanthine oxidase

Answer

Glutathione peroxidase

Answer

Superoxide dismutase

Answer

Catalase

Question 2

What is the predominant isoform of lactate dehydrogenase (LDH) found in skeletal muscles?

Accepted Answer

LDH-5

Answer

LDH-2

Answer

LDH-3

Answer

LDH-1

Question 3

Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?

Accepted Answer

Malonate

Answer

Fluoroacetate

Answer

Arsenite

Answer

Fluoride

Question 4

Which of the following is not a cofactor required by pyruvate dehydrogenase?

Accepted Answer

Biotin

Answer

CoA

Answer

NAD

Answer

FAD

Question 5

Which of the following statements about acid phosphatase is correct?

Accepted Answer

Erythrocyte isoform is inhibited by cupric ions

Answer

Acts at pH 8-9

Answer

Prostate isoform is tartrate resistant

Answer

None of the options

Question 6

What is the classification of urease?

Accepted Answer

Hydrolase

Answer

Oxidoreductase

Answer

Lyase

Answer

Ligase

Question 7

Which enzyme is primarily responsible for the metabolism of alcohol?

Accepted Answer

Alcohol dehydrogenase

Answer

MEOS

Answer

Catalase

Answer

Aldehyde dehydrogenase

Question 8

Transferases are classified as which of the following?

Accepted Answer

EC-2 (Transferases)

Answer

EC-1 (Oxidoreductases)

Answer

EC-3 (Hydrolases)

Answer

EC-4 (Lyases)

Question 9

Which of the following statements is true about noncompetitive enzyme inhibitors?

Accepted Answer

Km remains same, Vmax decreases

Answer

Noncompetitive inhibitors increase Km and decrease Vmax

Answer

Km decreases, Vmax increases

Answer

Km increases, Vmax increases

Question 10

Alpha-1 antitrypsin primarily inhibits which enzyme?

Accepted Answer

Neutrophil elastase

Answer

Trypsin

Answer

Chymotrypsin

Answer

Trypsinogen

Enzymes — MCQs

Enzymes — MCQs

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