Enzymes Practice Questions

Q: Which of the following is an example of allosteric inhibition?

Inhibition of PFK-1 by citrate. ***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.

Q: Which of the following is a structural component of matrix vesicles?

Alkaline phosphatase. ***Alkaline phosphatase*** - **Alkaline phosphatase** (ALP) is a **key membrane-bound enzyme** and structural component of matrix vesicles. - It plays a critical role in **modulating phosphate concentrations** by hydrolyzing pyrophosphate, thereby promoting mineralization. - ALP is considered the **most characteristic enzymatic marker** of matrix vesicles. *Calcium ATPase* - Calcium ATPase (PMCA) is indeed a **membrane protein present in matrix vesicles** and functions in calcium transport. - However, **alkaline phosphatase** is more widely recognized as the **primary structural and functional marker** of matrix vesicles in standard medical literature. - While both are membrane components, ALP is the classical answer for matrix vesicle identification. *Metallo proteinase* - **Metalloproteinases** are enzymes involved in **extracellular matrix remodeling** and degradation. - They are not structural components of matrix vesicles, although they may interact with the matrix during mineralization. *Calcium phosphate* - **Calcium phosphate** is the mineral that *forms within* and *is deposited by* matrix vesicles during biomineralization. - It is the **product** of matrix vesicle activity, not a structural component of the vesicle itself.

Q: Which enzyme is competitively inhibited by malonate in the Krebs cycle?

Succinate dehydrogenase. ***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**.

Q: Malonate is a competitive inhibitor of

Succinate dehydrogenase (Complex II). ***Succinate dehydrogenase (Complex II)*** - Malonate is a structural analog of **succinate**, the natural substrate of **succinate dehydrogenase**. - Its similar structure allows it to bind to the **active site** of the enzyme, competitively inhibiting its activity. *NADH dehydrogenase (Complex I)* - This complex is involved in the oxidation of **NADH**, not succinate, and thus malonate has no direct inhibitory effect. - Inhibitors of Complex I typically include substances like **rotenone** and **amytal**. *Cytochrome c oxidase (Complex IV)* - This enzyme catalyzes the reduction of oxygen to water and is inhibited by substances like **cyanide** and **carbon monoxide**. - Malonate's mechanism of action is unrelated to this final step of the electron transport chain. *Cytochrome bc1 complex (Complex III)* - This complex transfers electrons from ubiquinol to cytochrome c and is typically inhibited by agents like **antimycin A**. - Malonate does not structurally resemble the substrates or cofactors involved in Complex III activity.

Q: Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?

Creatine kinase (CK) and Lactate dehydrogenase (LDH). ***Creatine kinase (CK) and Lactate dehydrogenase (LDH)*** - **CK and LDH** are the most commonly measured enzyme markers in CSF for detecting **neuronal and glial cell damage**. - Elevated **LDH** in CSF indicates cellular injury and is seen in conditions like **meningitis, stroke, CNS malignancies, and traumatic brain injury**. - Elevated **CK** (particularly CK-BB isoform) in CSF indicates **brain tissue damage** and is elevated in conditions like **stroke, seizures, and CNS trauma**. - These enzymes are **not normally present in significant concentrations** in CSF but become elevated with cellular damage. *Gamma-glutamyl transferase (GGT) and Alkaline phosphatase (ALP)* - **GGT** is primarily a marker of **hepatobiliary disease** and is not routinely measured in CSF for neurological assessment. - **ALP** is found mainly in **liver, bone, and placenta**; it is not a standard CSF marker for neurological conditions. *Deaminase and Peroxidase* - **Adenosine deaminase (ADA)** can be elevated in CSF in **tuberculous meningitis**, but it is not a routine marker for general neurological damage. - **Peroxidase** is not a standard CSF enzyme marker for neurological disease assessment. *Alkaline phosphatase (ALP) and Creatine kinase (CK-MB)* - **CK-MB** is the cardiac-specific isoform used for **myocardial damage** detection, not for CNS assessment. - The brain-specific isoform is **CK-BB**, not CK-MB. - **ALP** is not a relevant CSF marker for neurological conditions.

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: 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.

Question 1

Which of the following is an example of allosteric inhibition?

Accepted Answer

Inhibition of PFK-1 by citrate

Answer

Decreased synthesis of glucokinase by glucagon

Answer

Inactivation of glycogen synthase by phosphorylation

Answer

None of the options

Question 2

Which of the following is a structural component of matrix vesicles?

Accepted Answer

Alkaline phosphatase

Answer

Metallo proteinase

Answer

Calcium ATPase

Answer

Calcium phosphate

Question 3

Which of the following statements about isozymes is true?

Accepted Answer

They catalyze the same reaction but may differ in structure.

Answer

They have the same quaternary structure.

Answer

They have the same enzyme classification but differ in number and name.

Answer

They are distributed uniformly across different organs.

Question 4

Which enzyme is competitively inhibited by malonate in the Krebs cycle?

Accepted Answer

Succinate dehydrogenase

Answer

Fumarate dehydrogenase

Answer

Succinate thiokinase

Answer

Aconitase

Question 5

Malonate is a competitive inhibitor of

Accepted Answer

Succinate dehydrogenase (Complex II)

Answer

NADH dehydrogenase (Complex I)

Answer

Cytochrome c oxidase (Complex IV)

Answer

Cytochrome bc1 complex (Complex III)

Question 6

Which of the following enzymes, when elevated in cerebrospinal fluid (CSF), are commonly used as markers of neurological damage or disease?

Accepted Answer

Creatine kinase (CK) and Lactate dehydrogenase (LDH)

Answer

Gamma-glutamyl transferase (GGT) and Alkaline phosphatase (ALP)

Answer

Deaminase and Peroxidase

Answer

Alkaline phosphatase (ALP) and Creatine kinase (CK-MB)

Question 7

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 8

Which isoenzyme of LDH is seen in the heart?

Accepted Answer

LDH 1

Answer

LDH 2

Answer

LDH 3

Answer

LDH 4

Question 9

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 10

Carbonic anhydrase requires which of the following?

Accepted Answer

Zinc

Answer

Copper

Answer

Iron

Answer

No cofactor required

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?