Enzymes Practice Questions

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: 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: 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 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: Which one of the following shows allosteric inhibition of glycolysis?

Citrate. ***Citrate*** - **Citrate** is a classic **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, the key regulatory enzyme in glycolysis - It binds to an allosteric site (distinct from the active site), reducing PFK-1's affinity for **fructose-6-phosphate** - This is a **negative feedback mechanism** - when citrate accumulates (indicating sufficient ATP production via the citric acid cycle), glycolysis slows down *Malonic acid* - **Malonic acid** is a **competitive inhibitor** (NOT allosteric) of succinate dehydrogenase in the citric acid cycle - It structurally resembles succinate and competes for the active site directly *2,3-BPG* - **2,3-Bisphosphoglycerate (2,3-BPG)** is an **allosteric effector** of hemoglobin (decreases oxygen affinity), not an enzyme inhibitor in glycolysis - It binds to hemoglobin, not to glycolytic enzymes *Amino acid alanine* - **Alanine** is an allosteric inhibitor of **pyruvate kinase** (not a glycolytic regulator in this context) - While it does show allosteric inhibition, it acts on gluconeogenesis regulation in the liver, not as a direct glycolytic inhibitor

Q: Which of the following is a ribozyme?

Peptidyl Transferase. ***Peptidyl Transferase*** - This enzyme, a component of the **large ribosomal subunit**, is responsible for forming **peptide bonds** between amino acids during protein synthesis. - It is unique because its catalytic activity is performed by **ribosomal RNA (rRNA)**, making it a ribozyme rather than a protein enzyme. *Ribonuclease* - Ribonucleases are a class of enzymes that **catalyze the degradation of RNA** into smaller components. - They are typically **protein-based enzymes** and do not exhibit catalytic activity stemming from RNA itself. *Transpeptidase* - Transpeptidases are protein enzymes primarily involved in **bacterial cell wall synthesis**, catalyzing the cross-linking of peptidoglycan chains. - They are the target of **beta-lactam antibiotics** like penicillin, which inhibit their protein-based enzymatic activity. *Poly A polymerase* - This enzyme adds a **polyadenosine (Poly A) tail** to the 3' end of messenger RNA (mRNA) precursors. - Poly A polymerase is a **protein enzyme** and its activity is not derived from an RNA molecule.

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**.

Q: What is the mechanism of conversion of trypsinogen to trypsin?

Removal of part of protein. ***Removal of part of protein*** - The conversion of **trypsinogen to trypsin** is an example of **proteolytic activation**, where a specific part of the inactive precursor (zymogen) is cleaved off. - This cleavage occurs at the N-terminus of trypsinogen by **enteropeptidase (or enterokinase)** in the duodenum, exposing the active site and forming active trypsin. *Hydrolysis* - While the removal of a part of the protein involves **hydrolysis of peptide bonds**, this option is too general. - It does not specify the selective nature of the cleavage that leads to activation, nor the fact that it's a specific segment being removed. *Phosphorylation* - **Phosphorylation** is a common mechanism for regulating enzyme activity, but it involves the addition of a **phosphate group**, not the removal of a protein segment. - This process is typically mediated by kinases and does not activate trypsinogen. *Removal of Carboxyl group* - The activation of trypsinogen involves the removal of a small N-terminal peptide, not specifically the removal of a **carboxyl group** from the protein. - While enzymatic cleavage does involve breaking peptide bonds, stating "removal of carboxyl group" is imprecise and does not accurately describe the mechanism.

Q: Enzyme activity is expressed as?

Micromoles/min. ***Micromoles/min*** - Enzyme activity is typically measured by the rate at which an enzyme converts its **substrate into product**. - This rate is often expressed as the amount of product formed (e.g., **micromoles**) or substrate consumed per unit of time (e.g., **per minute**). *Millimoles/lit* - This unit expresses **concentration** (moles per liter) rather than a rate of reaction. - While enzyme reactions involve changes in substrate/product concentration, this unit alone does not describe the **activity or catalytic speed** of the enzyme. *Milli gm/lit* - This unit also expresses **concentration by mass** (milligrams per liter), not enzyme activity. - It does not account for the **time-dependent nature** of enzyme catalysis or the molar quantity of reactants/products. *Mg/dl* - This unit represents **concentration by mass** (milligrams per deciliter), commonly used for measuring substances like glucose or cholesterol in blood. - It is not appropriate for expressing the **catalytic rate or activity** of an enzyme.

Q: How many isoenzymes does lactate dehydrogenase (LDH) have?

5, based on H and M polypeptide subunits. **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.

Question 1

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 2

What is the classification of urease?

Accepted Answer

Hydrolase

Answer

Oxidoreductase

Answer

Lyase

Answer

Ligase

Question 3

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 4

Which enzyme is primarily responsible for the metabolism of alcohol?

Accepted Answer

Alcohol dehydrogenase

Answer

MEOS

Answer

Catalase

Answer

Aldehyde dehydrogenase

Question 5

Which one of the following shows allosteric inhibition of glycolysis?

Accepted Answer

Citrate

Answer

Amino acid alanine

Answer

2,3 BPG

Answer

Malonic acid

Question 6

Which of the following is a ribozyme?

Accepted Answer

Peptidyl Transferase

Answer

Ribonuclease

Answer

Transpeptidase

Answer

Poly A polymerase

Question 7

Alpha-1 antitrypsin primarily inhibits which enzyme?

Accepted Answer

Neutrophil elastase

Answer

Trypsin

Answer

Chymotrypsin

Answer

Trypsinogen

Question 8

What is the mechanism of conversion of trypsinogen to trypsin?

Accepted Answer

Removal of part of protein

Answer

Hydrolysis

Answer

Phosphorylation

Answer

Removal of Carboxyl group

Question 9

Enzyme activity is expressed as?

Accepted Answer

Micromoles/min

Answer

Millimoles/lit

Answer

Milli gm/lit

Answer

Mg/dl

Question 10

How many isoenzymes does lactate dehydrogenase (LDH) have?

Accepted Answer

5, based on H and M polypeptide subunits

Answer

7, based on H and M polypeptide subunits

Answer

9, based on H and M polypeptide subunits

Answer

3, based on H and M polypeptide subunits

Enzymes — MCQs

Enzymes — MCQs

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