Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 511: What is the mechanism of conversion of trypsinogen to trypsin?

A. Hydrolysis
B. Phosphorylation
C. Removal of part of protein (Correct Answer)
D. Removal of Carboxyl group

Explanation: ***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.

Question 512: Enzyme activity is expressed as?

A. Millimoles/lit
B. Milli gm/lit
C. Mg/dl
D. Micromoles/min (Correct Answer)

Explanation: ***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.

Question 513: Which of the following is an example of an exopeptidase?

A. Trypsin
B. Chymotrypsin
C. Elastase
D. Carboxypeptidases (Correct Answer)

Explanation: ***Carboxypeptidases*** - **Carboxypeptidases** are enzymes that cleave the **C-terminal** (carboxyl end) amino acid from a polypeptide chain, making them a type of exopeptidase. - They are crucial in protein digestion, releasing individual amino acids from the end of protein chains. *Trypsin* - **Trypsin** is an **endopeptidase** that cleaves peptide bonds within protein chains, specifically at the carboxyl side of **lysine** or **arginine** residues. - It does not cleave amino acids from the ends of polypeptide chains. *Chymotrypsin* - **Chymotrypsin** is an **endopeptidase** that cleaves peptide bonds within a polypeptide chain, primarily at the carboxyl side of **tyrosine**, **tryptophan**, or **phenylalanine**. - Its action is internal to the protein sequence, not at the termini. *Elastase* - **Elastase** is also an **endopeptidase** that cleaves peptide bonds internally, specifically targeting small, uncharged amino acid residues like **alanine**, **glycine**, and **valine**. - Its primary role is to break down elastin, an elastic protein in connective tissues, but it does so by internal cleavage.

Question 514: How many isoenzymes does lactate dehydrogenase (LDH) have?

A. 5, based on H and M polypeptide subunits (Correct Answer)
B. 7, based on H and M polypeptide subunits
C. 9, based on H and M polypeptide subunits
D. 3, based on H and M polypeptide subunits

Explanation: **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 515: Chymotrypsinogen is activated into chymotrypsin by:

A. Trypsin (Correct Answer)
B. Pepsin
C. Renin
D. HCl

Explanation: ***Activation of Chymotrypsinogen by Trypsin*** - **Trypsin** is the primary enzyme responsible for the activation of **chymotrypsinogen** into its active form, **chymotrypsin**, by cleaving a specific peptide bond. - This activation is part of a cascade of proteolytic enzyme activations in the **pancreatic juice**, crucial for protein digestion in the small intestine. *Pepsin* - **Pepsin** is a protease active in the **stomach**, requiring an acidic environment for its activity, and is involved in the initial breakdown of proteins. - It does not play a role in the activation of pancreatic zymogens like chymotrypsinogen; its primary function is protein digestion in the gastric lumen. *Renin* - **Renin** is an enzyme primarily involved in the **renin-angiotensin-aldosterone system** (RAAS), which regulates blood pressure and fluid balance. - Its action involves cleaving **angiotensinogen** to form angiotensin I, and it has no role in the activation of digestive enzymes like chymotrypsinogen. *HCl* - **Hydrochloric acid (HCl)** is produced in the stomach and is essential for providing the acidic environment required for **pepsin's activity** and for denaturing proteins. - While HCl is crucial for digestion, it does not directly activate chymotrypsinogen; this activation is an enzymatic process carried out by another protease.

Question 516: Kcat/Km is a measure of which of the following?

A. Speed of enzymatic reaction
B. Concentration of substrate
C. Enzyme turnover
D. Enzyme efficiency (Correct Answer)

Explanation: **Correct: Enzyme efficiency** - The ratio **kcat/Km** is the definitive measure of an enzyme's **catalytic efficiency** or **specificity constant** - It reflects how effectively an enzyme converts substrate to product at low substrate concentrations - A higher **kcat/Km** value indicates greater efficiency, combining high catalytic rate (kcat) with strong substrate affinity (low Km) - This is the most important parameter for comparing different enzymes or different substrates for the same enzyme *Incorrect: Speed of enzymatic reaction* - **kcat** (turnover number) alone measures the maximum speed when enzyme is saturated with substrate - **kcat/Km** is a more comprehensive measure that includes substrate binding affinity, not just reaction speed - Speed also depends on enzyme and substrate concentrations, which kcat/Km doesn't directly represent *Incorrect: Concentration of substrate* - **Km** (Michaelis constant) represents the substrate concentration at which reaction velocity is half of Vmax - **kcat/Km** is a ratio that describes enzyme performance across substrate concentrations, not the concentration itself - It's particularly useful for predicting enzyme behavior at physiological (low) substrate concentrations *Incorrect: Enzyme turnover* - **kcat** specifically measures enzyme turnover: the number of substrate molecules converted per enzyme molecule per unit time at saturation - **kcat/Km** incorporates both kcat and Km, providing overall efficiency rather than just turnover rate - Turnover is only one component of the efficiency measure

Question 517: What is the mechanism by which mercury causes damage?

A. Causes toxicity through various mechanisms
B. Binds to -SH groups of enzymes (Correct Answer)
C. Inhibits electron transport chain
D. Inhibits protein synthesis

Explanation: ***Binds to -SH groups of enzymes*** - Mercury, particularly its inorganic and organic forms, has a high affinity for **sulfhydryl (-SH) groups** found in **cysteine residues** of proteins and enzymes. - This binding disrupts the **tertiary structure** and **catalytic activity** of vital enzymes, leading to widespread cellular dysfunction and toxicity. *Causes toxicity through various mechanisms (not specific to -SH binding)* - While mercury can indeed cause toxicity through various mechanisms, the **most prominent and fundamental mechanism** underpins many of these downstream effects. - This option is too general and does not pinpoint the primary molecular interaction responsible for mercury's widespread cellular damage. *Indirectly inhibits the electron transport chain (ETC) by enzyme disruption* - This statement is partially true in that mercury's enzyme disruption can affect the ETC, but it's an **indirect consequence** rather than the primary mechanism itself. - The direct mechanism involves the initial binding to -SH groups, which then leads to the dysfunction of enzymes, including those involved in the ETC. *Indirectly inhibits protein synthesis by disrupting enzyme function* - Similar to ETC inhibition, mercury's disruption of enzyme function can ultimately impair protein synthesis, but this is an **effect down the causal chain**. - The initial and direct molecular interaction is the binding to sulfhydryl groups of key enzymes involved in various cellular processes, including protein synthesis.

Question 518: Trypsinogen is converted to trypsin by?

A. Combination of 2 molecules of trypsinogen
B. Phosphorylation
C. Addition of alkyl group
D. Removal of specific amino acids from trypsinogen (Correct Answer)

Explanation: ***Removal of specific amino acids from trypsinogen*** - Trypsinogen is an **inactive zymogen** that is activated by the enzymatic cleavage of a **short N-terminal peptide**. - This cleavage event, primarily catalyzed by **enteropeptidase** (or trypsin itself), transforms trypsinogen into active **trypsin**, a process known as **proteolytic activation**. *Combination of 2 molecules of trypsinogen* - The activation of trypsinogen to trypsin is a **unimolecular conformational change** followed by proteolytic cleavage, not a combination reaction between two zymogen molecules. - While trypsin can activate other trypsinogen molecules, the initial activation does not involve the physical combination of two zymogen molecules. *Phosphorylation* - **Phosphorylation** is a common regulatory mechanism in proteins but is not the primary method for activating inactive trypsinogen. - Trypsinogen activation relies on a **proteolytic cleavage event**, rather than the addition of a phosphate group. *Addition of alkyl group* - The addition of an **alkyl group** is not a known mechanism for the physiological activation of trypsinogen. - Enzymatic activation typically involves **hydrolysis of peptide bonds** or other specific post-translational modifications.

Question 519: Which kinetic parameter is primarily associated with enzyme specificity?

A. Both
B. Km
C. Vmax
D. None of the options (Correct Answer)

Explanation: ***None of the options*** - **Enzyme specificity** is primarily determined by the unique three-dimensional **active site structure** of the enzyme, which allows it to bind only to specific substrates through complementary shape and chemical interactions. - This structural complementarity involves steric fit and specific non-covalent interactions (hydrogen bonds, van der Waals forces, electrostatic interactions) between the enzyme and its substrate. - **Neither Km nor Vmax are determinants of enzyme specificity**—they are kinetic parameters that describe enzyme behavior, not structural selectivity. *Km (Michaelis constant)* - Represents the substrate concentration at which the reaction rate is half of Vmax. - Indicates the **affinity** of an enzyme for its substrate (lower Km = higher affinity). - While enzymes may show different Km values for different substrates, **Km reflects binding affinity, not the structural basis of specificity**. *Vmax (Maximum velocity)* - The maximum rate of reaction when the enzyme is saturated with substrate. - Reflects **catalytic efficiency** and the amount of active enzyme present. - Does not relate to the enzyme's ability to discriminate between different substrate molecules. *Both* - Incorrect because neither Km nor Vmax determines which substrates an enzyme can recognize and bind. - Enzyme specificity is a **structural property** of the active site, while Km and Vmax are **kinetic properties** that describe reaction rates.

Question 520: According to IUB system, hydrolases belong to which class?

A. EC-1
B. EC-2
C. EC-3 (Correct Answer)
D. EC-4

Explanation: ***EC-3*** - **Hydrolases** catalyze the **hydrolysis** of chemical bonds, which involves the addition of water to break the bond. - This class includes enzymes like **esterases**, **peptidases**, and **glycosidases**, all of which use water to cleave molecules. *EC-1* - **EC-1** refers to **oxidoreductases**, which catalyze **oxidation-reduction reactions**. - These enzymes are involved in the transfer of electrons or hydrogen atoms, not the hydrolysis of bonds. *EC-2* - **EC-2** represents **transferases**, enzymes that catalyze the **transfer of a functional group** from one molecule to another. - Examples include **kinases** and **transaminases**, which are distinct from hydrolytic enzymes. *EC-4* - **EC-4** encompasses **lyases**, which catalyze the **cleavage of various bonds** by means other than hydrolysis or oxidation, often forming double bonds. - This class includes enzymes like **decarboxylases** and **aldolases**, which are not primarily involved in breaking bonds with water.

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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