Enzymes Practice Questions

Q: Fluoride, used in the collection of blood samples, inhibits which enzyme?

Enolase. ***Enolase*** - Fluoride acts as a potent inhibitor of **enolase** by forming a complex with magnesium and phosphate at the enzyme's active site. - By inhibiting the conversion of **2-phosphoglycerate** to **phosphoenolpyruvate**, fluoride effectively halts **glycolysis**, preserving blood glucose levels for laboratory measurement. *Glucokinase* - This enzyme catalyzes the phosphorylation of glucose in the **liver** and **pancreas** and is not inhibited by fluoride ions. - Its activity is regulated by **insulin** and the glucokinase regulatory protein (GKRP), rather than external anticoagulants used in blood vials. *Glucose-6-phosphatase* - This enzyme is responsible for modernizing blood sugar by converting **glucose-6-phosphate** into free glucose during **gluconeogenesis** and glycogenolysis. - It is primarily located in the **endoplasmic reticulum** of liver and kidney cells and is not the target of glycolytic inhibitors in blood tubes. *Hexokinase* - Hexokinase is the first regulatory enzyme of glycolysis that traps glucose in cells as **glucose-6-phosphate**. - Although it is a key glycolytic enzyme, fluoride specifically targets the downstream step catalyzed by **enolase** to achieve effective glucose preservation.

Q: All are cofactors for Dehydrogenase except:

SAM. ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.

Q: Zinc is cofactor of which enzyme?

Carbonic anhydrase. ***Carbonic anhydrase*** - **Zinc** is an essential cofactor for **carbonic anhydrase**, crucial for its enzymatic activity in catalyzing the reversible hydration of carbon dioxide. - This enzyme plays a vital role in processes like **pH regulation**, **carbon dioxide transport**, and **bicarbonate production** in various tissues. *Carboxylase* - Carboxylases typically require **biotin** as a cofactor for their activity, which involves the addition of a carboxyl group to a substrate. - Examples include **pyruvate carboxylase** and **acetyl-CoA carboxylase**, which are fundamental in metabolic pathways. *Kinase* - Kinases are enzymes that catalyze the transfer of a **phosphate group** from a high-energy donor molecule (like ATP) to a substrate. - Their activity often depends on cofactors like **magnesium (Mg2+)** or **manganese (Mn2+)**, not zinc. *Lysyl oxidase* - **Lysyl oxidase** is an enzyme that requires **copper** as a cofactor for its activity. - It plays a critical role in the **cross-linking of collagen and elastin**, essential for the integrity of connective tissues.

Q: Km increases, but Vmax remains same. This is which type of inhibition?

Competitive. ***Competitive*** - In **competitive inhibition**, the inhibitor **reversibly binds** to the **active site** of the enzyme, competing with the substrate. - This competition means that a higher substrate concentration is required to achieve half-maximal velocity, thus **increasing the Km**, while the maximum velocity (**Vmax**) remains unchanged if sufficient substrate is present. *Uncompetitive* - **Uncompetitive inhibition** involves the inhibitor binding only to the **enzyme-substrate complex**. - This type of inhibition typically leads to a **decrease in both Km and Vmax**. *Non-competitive* - In **non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site) on either the free enzyme or the enzyme-substrate complex. - This binding usually **decreases the Vmax** (due to reduced enzyme efficiency) but does not affect the Km (as substrate binding is not directly hindered). *Irreversible* - **Irreversible inhibition** involves the formation of a strong, often covalent, bond between the inhibitor and the enzyme, permanently inactivating it. - This type of inhibition effectively **reduces the concentration of active enzyme**, leading to a **decrease in Vmax** (as fewer enzyme molecules are available to catalyze the reaction) with varying effects on Km depending on the mechanism.

Q: Enzymes, which play an important role in calcification, are:

Alkaline phosphatase and pyrophosphatase. ***Alkaline phosphatase and pyrophosphatase*** - **Alkaline phosphatase** plays a crucial role in bone mineralization by hydrolyzing pyrophosphate, thereby increasing the local concentration of **inorganic phosphate** necessary for calcium phosphate crystal formation. - **Pyrophosphatase** (often referring to the activity of alkaline phosphatase itself or other pyrophosphate-hydrolyzing enzymes) is critical because **pyrophosphate** is a potent inhibitor of calcification; its hydrolysis removes this inhibitor, allowing mineralization to proceed. *Pyrophosphatase and carbonic anhydrase* - While **pyrophosphatase** is involved, **carbonic anhydrase** primarily catalyzes the rapid interconversion of carbon dioxide and water to bicarbonate and protons. - Carbonic anhydrase is important for **pH regulation** and **bicarbonate transport**, but its direct role in calcification is not as central as alkaline phosphatase. *Enolase and Carbonic anhydrase* - **Enolase** is a key enzyme in glycolysis, involved in energy metabolism, and has no direct role in calcification. - As mentioned, **carbonic anhydrase** is involved in pH regulation and bicarbonate metabolism, not directly in the enzymatic cascade of calcification. *Alkaline phosphatase and catalase* - **Alkaline phosphatase** is indeed critical for calcification. - **Catalase** is an enzyme that catalyzes the decomposition of hydrogen peroxide to water and oxygen, protecting cells from **oxidative damage**, and has no role in calcification.

Q: Arsenic inhibits all except :

Enolase. ***Enolase*** - Enolase is **NOT directly inhibited** by arsenic's primary toxic mechanism. - Arsenic (arsenite) primarily exerts its toxic effects by binding to **sulfhydryl (-SH) groups** of enzymes and cofactors. - Enolase, a glycolytic enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate, does not depend on sulfhydryl-containing cofactors and is therefore **not a direct target** of arsenic toxicity. - While arsenate can interfere with glycolysis at the **glyceraldehyde-3-phosphate dehydrogenase** step (forming unstable 1-arseno-3-phosphoglycerate), enolase itself remains functionally intact. *Lipoic acid* - Arsenic (arsenite) directly binds to the **sulfhydryl groups** of lipoic acid, inactivating this essential cofactor. - This binding disrupts the function of all lipoic acid-dependent enzyme complexes. *α-KG dehydrogenase* - The **α-ketoglutarate dehydrogenase complex** requires **lipoic acid** as a critical cofactor. - Arsenic's binding to lipoic acid's sulfhydryl groups **directly inhibits** this TCA cycle enzyme, impairing cellular respiration and ATP production. *PDH* - The **pyruvate dehydrogenase (PDH) complex** requires **lipoic acid** as an essential cofactor. - Arsenic directly inhibits PDH by binding to lipoic acid's sulfhydryl groups, blocking the conversion of pyruvate to acetyl-CoA and disrupting energy metabolism.

Q: SGPT is found in:

Cytoplasm of hepatocytes. ***Cytoplasm of hepatocytes*** - **SGPT** (serum glutamic-pyruvic transaminase), also known as **ALT** (alanine aminotransferase), is predominantly found in the **cytoplasm of hepatocytes**. - Its release into the bloodstream indicates **hepatocellular injury**, as the damaged cell membranes allow cytoplasmic enzymes to leak out. *Nucleus of hepatocytes* - The nucleus of hepatocytes primarily contains the cell's **genetic material** (DNA) and machinery for replication and transcription. - It does not contain significant amounts of **SGPT**. *Mitochondria of hepatocytes* - Mitochondria are responsible for **energy production** and contain enzymes like **SGOT** (AST), but not SGPT in significant quantities. - While mitochondrial damage can release other enzymes, **SGPT** is a cytoplasmic enzyme. *All of the options* - This statement is incorrect because **SGPT** is primarily located in the **cytoplasm** and not significantly distributed across all mentioned cellular compartments within hepatocytes. - Its cellular localization is specific and helps in differentiating various forms of liver injury.

Q: Which of the following enzymes can be found in CSF?

LDH & CK. ***LDH & CK*** - Both **lactate dehydrogenase (LDH)** and **creatine kinase (CK)** enzymes can be found in the cerebrospinal fluid (CSF). - Elevated levels of these enzymes in CSF can indicate **damage to brain tissue**, such as from ischemic stroke, trauma, or meningitis. - **CK-BB isoform** is the brain-specific form found in CSF. *ALT & LDH* - While **LDH** is found in CSF, **alanine aminotransferase (ALT)** is primarily a liver enzyme and is not typically found in significant concentrations in healthy CSF. - Presence of ALT in CSF would be highly unusual and not indicative of common neurological pathology. *ALT & CK* - **CK** is found in CSF, but **ALT** is an enzyme predominantly associated with **liver function** and is not a common or expected finding in CSF analysis. - Its presence in CSF would suggest contamination or severe, diffuse organ damage. *All of the options* - This option is incorrect because **ALT** is not typically found in CSF, making the combinations including ALT incorrect. - Only LDH and CK are consistently detected in CSF among the enzymes listed.

Q: The enzyme primarily responsible for the conversion of T4 to T3 in the periphery is

D1 thyroid deiodinase. ***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.

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.

Question 1

Fluoride, used in the collection of blood samples, inhibits which enzyme?

Accepted Answer

Enolase

Answer

Glucokinase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Question 2

All are cofactors for Dehydrogenase except:

Accepted Answer

SAM

Answer

NADP

Answer

NAD

Answer

FAD

Question 3

Zinc is cofactor of which enzyme?

Accepted Answer

Carbonic anhydrase

Answer

Carboxylase

Answer

Kinase

Answer

Lysyl oxidase

Question 4

Km increases, but Vmax remains same. This is which type of inhibition?

Accepted Answer

Competitive

Answer

Uncompetitive

Answer

Non-competitive

Answer

Irreversible

Question 5

Enzymes, which play an important role in calcification, are:

Accepted Answer

Alkaline phosphatase and pyrophosphatase

Answer

Pyrophosphatase and Carbonic anhydrase

Answer

Enolase and Carbonic anhydrase

Answer

Alkaline phosphatase and Catalase

Question 6

Arsenic inhibits all except :

Accepted Answer

Enolase

Answer

Lipoic acid

Answer

α-KG dehydrogenase

Answer

PDH

Question 7

SGPT is found in:

Accepted Answer

Cytoplasm of hepatocytes

Answer

Nucleus of hepatocytes

Answer

Mitochondria of hepatocytes

Answer

All of the options

Question 8

Which of the following enzymes can be found in CSF?

Accepted Answer

LDH & CK

Answer

ALT & LDH

Answer

ALT & CK

Answer

All of the options

Question 9

The enzyme primarily responsible for the conversion of T4 to T3 in the periphery is

Accepted Answer

D1 thyroid deiodinase

Answer

D3 thyroid deiodinase

Answer

Thyroid peroxidase

Answer

D2 thyroid deiodinase

Question 10

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

Enzymes — MCQs

Enzymes — MCQs

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