Enzymes Practice Questions

Q: What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?

The action of carbonic anhydrase in erythrocytes. ***The action of carbonic anhydrase in erythrocytes*** - **Carbonic anhydrase** is an enzyme found in high concentrations within **red blood cells (erythrocytes)**, catalyzing the rapid interconversion of carbon dioxide and water to **carbonic acid**. - This enzyme is crucial for the efficient transport of carbon dioxide from the tissues to the lungs, as carbonic acid quickly dissociates into **bicarbonate ions**, which are easily transported in the plasma. *The high solubility of CO2 in water* - While **CO2** does have some solubility in water, this process is too slow on its own to account for the rapid and efficient transport of the large amounts of metabolic CO2 produced by the body. - The direct dissolution of CO2 in plasma accounts for only a small fraction of its total transport. *The role of hemoglobin in CO2 transport* - **Hemoglobin** does play a role in CO2 transport by forming **carbaminohemoglobin**, binding to the amino groups on the globin chains. - However, this mechanism represents only about 20-30% of CO2 transport and does not involve the conversion to **bicarbonate**. *The conversion of carbon dioxide to carbonic acid* - The conversion of CO2 to **carbonic acid (H2CO3)** is indeed an intermediate step in bicarbonate formation. - However, this reaction is very slow in the absence of an enzyme and does not address the primary catalyst responsible for this rapid conversion.

Q: Which isoenzyme of LDH is seen in the heart?

LDH 1. ***LDH 1*** - **LDH 1** is the predominant isoenzyme found in the **heart muscle**, particularly in the ventricles. Its elevation is a key indicator of cardiac tissue damage. - It is also found in **red blood cells** and the **kidneys**. *LDH 2* - **LDH 2** is also present in the **heart**, but it is typically the second most abundant isoenzyme after LDH 1 in cardiac tissue. - It is predominantly found in the **reticuloendothelial system**, including macrophages and lymphoid tissues. *LDH 3* - **LDH 3** is primarily associated with **pulmonary tissue**, the **spleen**, and **lymph nodes**. - Its elevation often suggests conditions affecting these organs, such as pulmonary embolism or pneumonia. *LDH 4* - **LDH 4** is found in the **kidney**, **placenta**, and **pancreas**, with some presence in skeletal muscle. - **LDH 5** (not LDH 4) is the primary isoenzyme in the **liver** and **skeletal muscle**.

Q: The rate-limiting enzyme in the synthesis of dopamine is which of the following?

Tyrosine hydroxylase. ***Tyrosine hydroxylase*** - **Tyrosine hydroxylase (TH)** catalyzes the conversion of **L-tyrosine to L-DOPA**, the first and **rate-limiting step** in the synthesis of catecholamines, including dopamine [1] - This enzyme's activity is crucial for regulating the overall **production rate** of dopamine and other catecholamines [1] - The rate-limiting nature means this is the slowest step that controls the overall pathway flux *Dopa decarboxylase* - **Dopa decarboxylase** converts **L-DOPA to dopamine**, which is a subsequent step in the synthesis pathway [1] - While essential for dopamine production, it is **not the rate-limiting enzyme** as its activity is generally higher than that of tyrosine hydroxylase [1] *Monoamine oxidase* - **Monoamine oxidase (MAO)** is an enzyme involved in the **breakdown and inactivation** of monoamine neurotransmitters, including dopamine, rather than its synthesis - It plays a role in regulating the **levels of dopamine** in the synapse but does not contribute to its production *Dopamine beta-hydroxylase* - **Dopamine beta-hydroxylase (DBH)** converts dopamine to **norepinephrine** (noradrenaline) - This enzyme is important for the synthesis of norepinephrine from dopamine but is not involved in the actual synthesis of dopamine itself

Q: Carbonic anhydrase requires which of the following?

Zinc. ***Zinc*** - **Zinc** is an essential **cofactor** for the enzyme carbonic anhydrase, playing a crucial role in its catalytic activity. - It directly participates in the enzyme's mechanism by coordinating a water molecule, facilitating the rapid interconversion of **carbon dioxide** and **bicarbonate**. *Copper* - **Copper** is a cofactor for several enzymes, such as **cytochrome c oxidase** and **superoxide dismutase**, but not for carbonic anhydrase. - Its presence is not required for the catalytic function of carbonic anhydrase. *Iron* - **Iron** is a vital component of many proteins, including hemoglobin and cytochromes, involved in **oxygen transport** and **electron transfer**. - However, **iron** does not serve as a cofactor for carbonic anhydrase. *No cofactor required* - This statement is incorrect because carbonic anhydrase is a **metalloenzyme** that absolutely requires a **metal ion cofactor** for its function. - Without a **cofactor**, specifically **zinc**, the enzyme would be catalytically inactive.

Q: Which of the following enzymes is NAD+ dependent?

Glycerol-3-phosphate dehydrogenase. ***Glycerol-3-phosphate dehydrogenase*** - This enzyme catalyzes the interconversion of **dihydroxyacetone phosphate (DHAP)** and **glycerol-3-phosphate**, using **NAD+** as a cofactor to oxidize glycerol-3-phosphate. - In the **glycerol-phosphate shuttle**, it enables the transfer of reducing equivalents from cytosolic NADH to the mitochondrial electron transport chain. *HMG CoA reductase* - This enzyme is a key regulatory step in **cholesterol biosynthesis** and utilizes **NADPH** as a reducing agent, not NAD+. - It catalyzes the reduction of HMG-CoA to **mevalonate**. *Acyl CoA dehydrogenase* - This enzyme is involved in the first step of **beta-oxidation of fatty acids** and uses **FAD** as a prosthetic group, not NAD+, which is reduced to FADH2. - It catalyzes the formation of a double bond between the alpha and beta carbons of the acyl-CoA. *Succinate dehydrogenase* - This enzyme is part of the **TCA cycle** (Complex II of the electron transport chain) and uses **FAD** as its electron acceptor, converting succinate to fumarate. - It is unique among TCA cycle enzymes as it is membrane-bound and directly links the cycle to the **electron transport chain**.

Q: Which of the following enzymes is not classified as an oxidoreductase?

Glucokinase. ***Glucokinase*** - **Glucokinase** is a **transferase** enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. - Its function is primarily in **glucose metabolism** and **insulin secretion**, not in oxidation or reduction reactions. *Catalase* - **Catalase** is an **oxidoreductase** that catalyzes the decomposition of **hydrogen peroxide** into water and oxygen. - This reaction involves the **oxidation and reduction** of substrates, fitting the definition of an oxidoreductase. *Alcohol dehydrogenase* - **Alcohol dehydrogenase** is an **oxidoreductase** that catalyzes the interconversion between alcohols and aldehydes or ketones with the concomitant reduction and oxidation of **NAD+** to **NADH**. - This enzyme is crucial in **detoxifying alcohol** by oxidizing it and is a classic example of an oxidoreductase. *Peroxidase* - **Peroxidase** is an **oxidoreductase** that catalyzes the oxidation of a substrate by **hydrogen peroxide**. - Peroxidases work by using hydrogen peroxide to accept electrons from another molecule, thereby **oxidizing** that molecule.

Q: Which of the following enzymes is not considered a rate-limiting enzyme in metabolic pathways?

Succinate dehydrogenase. ***Succinate dehydrogenase*** - **Succinate dehydrogenase** is an enzyme of the **Krebs cycle** and **electron transport chain** but is not considered a primary rate-limiting enzyme. - Its activity is generally high, and it operates close to its maximum velocity under most physiological conditions, thus not typically controlling the overall flux of its respective pathways in a rate-limiting manner. *HMG CoA reductase* - **HMG CoA reductase** is the **rate-limiting enzyme in cholesterol biosynthesis**. - It is a key target for **statins**, drugs that lower cholesterol by inhibiting this enzyme. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is the **rate-limiting enzyme in glycolysis**. - Its activity is tightly regulated by **allosteric modulators** like ATP, AMP, and citrate to control the flux of glucose metabolism. *Acetyl CoA carboxylase* - **Acetyl CoA carboxylase (ACC)** is the **rate-limiting enzyme in fatty acid synthesis**. - It catalyzes the irreversible carboxylation of acetyl-CoA to **malonyl-CoA**, a crucial committed step in the pathway.

Question 1

What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?

Accepted Answer

The action of carbonic anhydrase in erythrocytes

Answer

The high solubility of CO2 in water

Answer

The role of hemoglobin in CO2 transport

Answer

The conversion of carbon dioxide to carbonic acid

Question 2

Which of the following statements about the enzyme aldolase B is false?

Accepted Answer

The enzyme involved catalyzes the conversion of fructose to fructose-1 phosphate

Answer

The enzyme cleaves fructose-1-phosphate into dihydroxyacetone phosphate and glyceraldehyde

Answer

The enzyme is involved in fructose metabolism

Answer

The enzyme is found primarily in liver, kidney, and intestine

Question 3

Which isoenzyme of LDH is seen in the heart?

Accepted Answer

LDH 1

Answer

LDH 2

Answer

LDH 3

Answer

LDH 4

Question 4

If a chymotrypsin molecule undergoes a ser-195-ala mutation, then

Accepted Answer

Chymotrypsin will bind the substrate but will not cause cleavage

Answer

Chymotrypsin will not bind the substrate

Answer

Chymotrypsin will bind the substrate as well as cause cleavage

Answer

No effect will be observed

Question 5

The rate-limiting enzyme in the synthesis of dopamine is which of the following?

Accepted Answer

Tyrosine hydroxylase

Answer

Dopa decarboxylase

Answer

Dopamine beta-hydroxylase

Answer

Monoamine oxidase

Question 6

Carbonic anhydrase requires which of the following?

Accepted Answer

Zinc

Answer

Copper

Answer

Iron

Answer

No cofactor required

Question 7

Which of the following enzymes is NAD+ dependent?

Accepted Answer

Glycerol-3-phosphate dehydrogenase

Answer

HMG CoA reductase

Answer

Acyl CoA dehydrogenase

Answer

Succinate dehydrogenase

Question 8

Which of the following statements about cofactors and metalloenzymes is true?

Accepted Answer

Coenzymes are loosely bound cofactors that transiently bind to enzymes, while prosthetic groups are tightly bound cofactors

Answer

The most common cofactors are metal ions

Answer

Enzymes that require metal ion cofactors are termed as metalloenzymes

Answer

Cofactors bind in a transient, dissociable manner to enzymes

Question 9

Which of the following enzymes is not classified as an oxidoreductase?

Accepted Answer

Glucokinase

Answer

Alcohol dehydrogenase

Answer

Catalase

Answer

Peroxidase

Question 10

Which of the following enzymes is not considered a rate-limiting enzyme in metabolic pathways?

Accepted Answer

Succinate dehydrogenase

Answer

HMG CoA reductase

Answer

Phosphofructokinase

Answer

Acetyl CoA carboxylase

Enzymes — MCQs

Enzymes — MCQs

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