Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 521: 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 522: Carboxypeptidase contains which mineral?

A. Copper
B. Zinc (Correct Answer)
C. Iron
D. None of the options

Explanation: ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.

Question 523: 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 524: 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 525: Which of the following enzymes is classified as a serine protease?

A. Pepsin
B. Trypsin (Correct Answer)
C. Carboxypeptidase
D. None of the options

Explanation: ***Trypsin*** - **Trypsin** is a digestive enzyme belonging to the **serine protease** family, characterized by a crucial **serine residue** in its active site. - It plays a vital role in protein digestion in the small intestine, cleaving peptide bonds on the carboxyl side of **lysine** or **arginine** residues. *Pepsin* - **Pepsin** is an aspartic protease, meaning it utilizes an **aspartate residue** in its active site for catalysis. - It primarily functions in the stomach, digesting proteins into smaller peptides in an **acidic environment**. *Carboxypeptidase* - **Carboxypeptidase** is a **metalloexopeptidase** that contains a zinc ion in its active site. - It removes amino acids one by one from the **carboxyl-terminal** end of polypeptide chains. *None of the options* - This option is incorrect because **trypsin** is indeed a well-known example of a serine protease.

Question 526: What is the specific activity of an enzyme?

A. Enzyme units per mg of protein (Correct Answer)
B. Concentration of substrate transformed per minute
C. Enzyme units per mg of substrate
D. Limit of enzyme per gram of substrate

Explanation: ***Enzyme units per mg of protein*** - **Specific activity** is defined as the number of **enzyme units** (representing catalytic activity) per milligram of total protein in the sample. - It is a measure of **purity**, indicating the amount of active enzyme relative to other proteins in a preparation. - Formula: Specific activity = Units of enzyme activity / mg of total protein - Used to track enzyme purification progress during isolation procedures. *Concentration of substrate transformed per minute* - This describes the **reaction velocity** or rate of catalysis, but not the specific activity of the enzyme. - While related to enzyme activity, it does not normalize the activity to the amount of **total protein**. - This would be expressed as reaction rate or velocity (V), not specific activity. *Enzyme units per mg of substrate* - This is an incorrect formulation that confuses substrate with protein. - **Specific activity** is normalized to the amount of **protein** in the enzyme preparation, not the substrate. - This option represents a common misconception in enzyme kinetics terminology. *Limit of enzyme per gram of substrate* - This phrase does not correspond to any standard biochemical measure of enzyme activity or concentration. - It does not provide information about the **catalytic efficiency** or **purity** of the enzyme preparation. - The term "limit" is not used in the context of specific activity measurements.

Question 527: How do enzymes function in biochemical reactions?

A. Increase in activation energy
B. Decrease in activation energy (Correct Answer)
C. Shift equilibrium constant
D. Provide energy to the reaction

Explanation: ***Decrease in activation energy*** - Enzymes act as **biological catalysts** by providing an alternative reaction pathway with a lower **transition state energy**. - This reduction in the **activation energy** allows a higher proportion of reactant molecules to overcome the energy barrier and react, thereby increasing the reaction rate. *Increase in activation energy* - This statement is incorrect as increasing activation energy would slow down the reaction rate, which is contrary to the function of enzymes. - Enzymes are designed to accelerate reactions, not inhibit them, by making them energetically more favorable to proceed. *Shift equilibrium constant* - Enzymes catalyze both the forward and reverse reactions equally, meaning they accelerate the rate at which equilibrium is reached but **do not alter the equilibrium constant (Keq)** of a reaction. - The equilibrium constant is determined by the difference in free energy between reactants and products, which enzymes do not change. *Provide energy to the reaction* - This statement is incorrect because enzymes do **not provide energy** to reactions; they only lower the activation energy barrier. - Enzymes facilitate reactions by stabilizing the transition state, not by adding energy to the system, which would violate thermodynamic principles.

Question 528: Apoenzyme is ?

A. Protein moiety (Correct Answer)
B. Organic cofactor
C. Inactive enzyme component
D. Non-protein component required for enzyme activity

Explanation: ***Protein moiety*** - An **apoenzyme** is the **protein component of an enzyme** that is catalytically inactive by itself. - It requires a **non-protein cofactor** (either an inorganic ion or an organic molecule) to become active. *Organic cofactor* - An **organic cofactor** is also known as a **coenzyme**, which binds to the apoenzyme to form a functional holoenzyme. - While essential for enzyme activity, the apoenzyme itself is the protein part, not the organic cofactor. *Inactive enzyme component* - While an apoenzyme is **inactive on its own**, this description is too broad and doesn't specify its chemical nature. - It is specifically the **protein component** that is inactive until bound to its cofactor. *Non-protein component required for enzyme activity* - This describes a **cofactor** (either inorganic or organic), not the apoenzyme itself. - The apoenzyme is the **protein portion**, which *requires* the non-protein component for activity.

Question 529: What is the cofactor required for the enzyme xanthine oxidase?

A. Selenium
B. Zinc
C. Molybdenum (Correct Answer)
D. Magnesium

Explanation: ***Molybdenum*** - **Xanthine oxidase** is a key enzyme in **purine metabolism**, responsible for the oxidation of **hypoxanthine to xanthine** and further to **uric acid**. - **Molybdenum** is an essential trace element that serves as a **cofactor** for several enzymes, including xanthine oxidase, where it helps facilitate electron transfer reactions. *Selenium* - **Selenium** is a cofactor for **glutathione peroxidase**, an enzyme involved in antioxidant defense. - It is not directly involved in the function of **xanthine oxidase**. *Zinc* - **Zinc** is a cofactor for a wide range of enzymes, including **carbonic anhydrase** and **alcohol dehydrogenase**. - It does not serve as a cofactor for **xanthine oxidase**. *Magnesium* - **Magnesium** is a critical cofactor for many enzymes, particularly those involved in **ATP hydrolysis and synthesis** and **DNA/RNA synthesis**. - It is not a cofactor for **xanthine oxidase**.

Question 530: Which of the following best describes the difference between glucokinase and hexokinase?

A. Glucokinase has higher Km for glucose compared to hexokinase. (Correct Answer)
B. Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.
C. Glucokinase has a low affinity for glucose.
D. Glucokinase activity increases with glucose concentration while hexokinase remains saturated.

Explanation: ***Glucokinase has higher Km for glucose compared to hexokinase*** - **Glucokinase** has a **Km of ~10 mM** for glucose, while **hexokinase** has a **Km of ~0.1 mM**, making glucokinase's Km approximately **100-fold higher** - This **high Km** is the fundamental biochemical parameter that defines glucokinase's unique role as a **glucose sensor** in liver and pancreatic β-cells - The high Km means glucokinase activity is **proportional to blood glucose concentration** in the physiological range (5-15 mM), allowing it to regulate glucose metabolism in response to feeding - This is the **most precise biochemical descriptor** of the difference, from which other functional characteristics derive *Glucokinase has a low affinity for glucose* - While this statement is **correct** (high Km = low affinity), it is a **qualitative description** of what Km quantifies - Option stating "higher Km" is more specific and biochemically precise than simply stating "low affinity" *Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase* - This is a **correct and important regulatory difference** - **Hexokinase** is allosterically inhibited by its product **glucose-6-phosphate**, providing feedback regulation to prevent excessive glucose phosphorylation when cellular needs are met - **Glucokinase** lacks this product inhibition, allowing the liver to continue glucose uptake and storage even when G6P levels are high after meals - However, this describes a regulatory difference rather than the fundamental kinetic parameter *Glucokinase activity increases with glucose concentration while hexokinase remains saturated* - This statement is **correct** and describes the **functional consequence** of the different Km values - **Hexokinase** with its low Km (~0.1 mM) is saturated at normal blood glucose levels (5 mM), operating at Vmax - **Glucokinase** with its high Km (~10 mM) shows increasing activity as glucose rises from 5 to 15 mM postprandially - This is a physiological consequence rather than the fundamental biochemical parameter

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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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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