Enzymes Practice Questions

Q: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?

NAD+. ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.

Q: Selenocysteine is associated with ?

Deiodinase. ***Deiodinase*** - Selenocysteine is a critical component of **iodothyronine deiodinases**, a family of enzymes that regulate **thyroid hormone metabolism**. - These enzymes catalyze the removal of iodine from thyroid hormones, converting **thyroxine (T4)** into the more active **triiodothyronine (T3)** or inactive forms. *Carbonic anhydrase* - This enzyme contains **zinc** as its essential metal cofactor and is involved in the interconversion of **carbon dioxide** and **bicarbonate**. - Its primary role is in pH regulation and CO2 transport, without any direct association with selenocysteine. *Catalase* - Catalase is an enzyme primarily found in **peroxisomes** and contains **iron-porphyrin** groups as its prosthetic group. - Its function is to convert **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. *Transferase* - Transferases are a broad class of enzymes that catalyze the transfer of **functional groups** (e.g., methyl, glucose) from one molecule to another. - While essential for many metabolic processes, there is no inherent association of the general class of transferases with selenocysteine.

Q: Type of inhibition of aconitase by trans-aconitate is?

Competitive. ***Competitive*** - **Competitive inhibition** occurs when the inhibitor (trans-aconitate) structurally resembles the enzyme's natural substrate (cis-aconitate) and binds to the **active site**, preventing the substrate from binding. - This type of inhibition can be overcome by increasing the concentration of the **substrate**. *Non-competitive* - **Non-competitive inhibitors** bind to a site on the enzyme other than the active site, causing a conformational change that reduces the enzyme's efficiency, regardless of substrate concentration. - Trans-aconitate's structural similarity to aconitate's substrate points away from a non-competitive mechanism. *Allosteric* - **Allosteric inhibition** involves an inhibitor binding to a regulatory site (allosteric site) on the enzyme, which is distinct from the active site, to alter enzyme activity. - While allosteric regulation is a type of non-competitive inhibition, trans-aconitate's direct structural resemblance to the substrate makes competitive inhibition the more specific and accurate description. *None of the options* - This option is incorrect because **competitive inhibition** accurately describes the mechanism by which trans-aconitate inhibits aconitase, given its structural similarity to the natural substrate. - The other options are less fitting due to the specific characteristics of trans-aconitate's action.

Q: Aldehyde dehydrogenase requires NAD as ?

Coenzyme. ***Coenzyme*** - **NAD** (nicotinamide adenine dinucleotide) acts as a **coenzyme** for aldehyde dehydrogenase, serving as the **most specific and accurate classification** for its role. - As a coenzyme, **NAD** is an **organic, non-protein molecule** that binds reversibly to the enzyme and acts as a **transient carrier of electrons** (hydride ions, H⁻) during aldehyde oxidation. - **NAD⁺** accepts electrons from the aldehyde substrate, becoming reduced to **NADH**, which then dissociates and transfers electrons elsewhere in metabolism. - This is the **preferred answer** because coenzyme precisely describes NAD's organic nature, vitamin origin (niacin/B3), and its role as a mobile electron carrier. *Cofactor* - While technically **NAD is a type of cofactor** (cofactors include coenzymes, prosthetic groups, and metal ions), this term is **too general** for this context. - In biochemistry nomenclature, when both a general and specific term apply, the **more specific term (coenzyme) is preferred** to demonstrate precise understanding. - Choosing "cofactor" would be like calling a "cardiologist" a "doctor" - true but less specific. *Apoenzyme* - An **apoenzyme** is the **protein component** of an enzyme without its cofactor - it refers to the enzyme itself, not to NAD. - In this case, **aldehyde dehydrogenase** (the protein) is the apoenzyme, and **NAD** is the coenzyme that binds to it. - Together they form the active **holoenzyme** (apoenzyme + coenzyme = holoenzyme). *None of the options* - Incorrect because **coenzyme** is the accurate and specific term for NAD's role in aldehyde dehydrogenase function.

Q: How do enzymes function in biochemical reactions?

Decrease in activation energy. ***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.

Q: What is the specific activity of an enzyme?

Enzyme units per mg of protein. ***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.

Q: Enzyme causing covalent bond cleavage without hydrolysis ?

Lyase. ***Lyase*** - **Lyases** are enzymes that catalyze the cleavage of **covalent bonds** (C-C, C-O, C-N, and others) by means other than hydrolysis or oxidation, often creating a new double bond or a ring structure. - They remove groups from substrates to form double bonds, or conversely, add groups to double bonds. - **Examples:** Aldolase (cleaves C-C bonds in glycolysis), carbonic anhydrase (reversible cleavage of C-O bond), fumarase (C-C bond cleavage in TCA cycle). *Ligase* - **Ligases** are enzymes that join two large molecules by forming a new chemical bond, usually accompanied by the **hydrolysis of ATP**. - They are involved in synthesis reactions, not the cleavage of bonds. *Hydrolase* - **Hydrolases** specifically catalyze the hydrolysis of a chemical bond, involving the **addition of water** across the bond. - They break down large molecules into smaller ones using water - this is the key difference from lyases. *Transferase* - **Transferases** catalyze the transfer of a **functional group** from one molecule (the donor) to another (the acceptor). - They do not cause covalent bond cleavage without hydrolysis but rather move existing groups between molecules.

Q: Glucose oxidase converts glucose to?

Gluconic acid. ***Gluconic acid*** - **Glucose oxidase** specifically catalyzes the oxidation of glucose, producing **gluconic acid** and hydrogen peroxide. - This reaction forms the basis for many common **glucose diagnostic tests**, such as those used in blood glucose monitors. *Glucuronic acid* - **Glucuronic acid** is formed from the oxidation of glucose at carbon 6, typically through the **uronic acid pathway**. - It is known for its role in **detoxification** and conjugation reactions in the liver, not as a direct product of glucose oxidase. *Galactonic acid* - **Galactonic acid** is an oxidized form of galactose, a different monosaccharide from glucose. - Its formation is not associated with the action of **glucose oxidase**, an enzyme specific to glucose. *Iduronic acid* - **Iduronic acid** is a C5 epimer of glucuronic acid and is a common component of various **glycosaminoglycans** like dermatan sulfate and heparan sulfate. - It is not produced by the action of **glucose oxidase** on glucose.

Q: Which isoform of LDH is raised in Anemia ?

LDH 2. ***Correct: LDH 2*** - **LDH 2 (H3M1)** is highly abundant in **red blood cells (RBCs)**, second only to LDH 1. - In **hemolytic anemia**, there is increased destruction of red blood cells, leading to the release of intracellular LDH isoforms into the bloodstream. - Both **LDH 1 and LDH 2 are significantly elevated** in hemolytic anemia, as RBCs contain predominantly these two isoforms. - The elevation of LDH 2 is particularly notable and diagnostically useful in anemia, especially hemolytic conditions. *Incorrect: LDH 1* - **LDH 1 (H4)** is the most abundant isoform in **heart muscle** and is also present in **red blood cells**. - While LDH 1 is indeed elevated in hemolytic anemia, this option is not the best answer in this context. - LDH 1 elevation is classically associated with **myocardial infarction**. *Incorrect: LDH 4* - **LDH 4 (HM3)** is present in various tissues, including **liver, skeletal muscle, and kidneys**, but in lower concentrations. - Not the primary isoform associated with anemia. *Incorrect: LDH 5* - **LDH 5 (M4)** is predominantly found in the **liver and skeletal muscles**. - Elevation of LDH 5 typically indicates **liver damage, muscle injury, or malignancies**, not primarily anemia.

Question 1

Which cofactor is primarily associated with the activity of glutamate dehydrogenase?

Accepted Answer

NAD+

Answer

FAD

Answer

FMN

Answer

FADH2

Question 2

Selenocysteine is associated with ?

Accepted Answer

Deiodinase

Answer

Carbonic anhydrase

Answer

Catalase

Answer

Transferase

Question 3

Type of inhibition of aconitase by trans-aconitate is?

Accepted Answer

Competitive

Answer

Non-competitive

Answer

Allosteric

Answer

None of the options

Question 4

Aldehyde dehydrogenase requires NAD as ?

Accepted Answer

Coenzyme

Answer

None of the options

Answer

Apoenzyme

Answer

Cofactor

Question 5

How do enzymes function in biochemical reactions?

Accepted Answer

Decrease in activation energy

Answer

Increase in activation energy

Answer

Shift equilibrium constant

Answer

Provide energy to the reaction

Question 6

What is the specific activity of an enzyme?

Accepted Answer

Enzyme units per mg of protein

Answer

Concentration of substrate transformed per minute

Answer

Enzyme units per mg of substrate

Answer

Limit of enzyme per gram of substrate

Question 7

Enzyme causing covalent bond cleavage without hydrolysis ?

Accepted Answer

Lyase

Answer

Ligase

Answer

Hydrolase

Answer

Transferase

Question 8

Glucose oxidase converts glucose to?

Accepted Answer

Gluconic acid

Answer

Glucuronic acid

Answer

Galactonic acid

Answer

Iduronic acid

Question 9

Which isoform of LDH is raised in Anemia ?

Accepted Answer

LDH 2

Answer

LDH 5

Answer

LDH 4

Answer

LDH 1

Question 10

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

Accepted Answer

Glucokinase has higher Km for glucose compared to hexokinase.

Answer

Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.

Answer

Glucokinase has a low affinity for glucose.

Answer

Glucokinase activity increases with glucose concentration while hexokinase remains saturated.

Enzymes — MCQs

Enzymes — MCQs

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