Enzymes Practice Questions

Q: Which of the following enzymes directly catalyzes the decomposition of hydrogen peroxide to water and oxygen without requiring an additional reducing substrate?

Catalase. ***Catalase*** - **Catalase** directly catalyzes the breakdown of **hydrogen peroxide (H2O2)** into **water (H2O)** and **oxygen (O2)** without requiring any additional reducing substrate. - The reaction is: **2 H2O2 → 2 H2O + O2** - This unique mechanism makes catalase extremely efficient at detoxifying high concentrations of H2O2, particularly in peroxisomes and red blood cells. - Catalase has one of the highest turnover rates of all enzymes (millions of molecules per second). *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts the **superoxide radical (O2•−)** into oxygen and **hydrogen peroxide**. - The reaction is: **2 O2•− + 2 H+ → H2O2 + O2** - While it detoxifies superoxide, it produces hydrogen peroxide, which then requires other enzymes like catalase for further detoxification. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** reduces **hydrogen peroxide** to water using **reduced glutathione (GSH)** as a reducing substrate. - The reaction is: **H2O2 + 2 GSH → 2 H2O + GSSG** - Unlike catalase, GPx requires glutathione as a co-substrate and works in concert with glutathione reductase to regenerate GSH. *Peroxiredoxin* - **Peroxiredoxins** reduce **hydrogen peroxide** to water using **thioredoxin or glutaredoxin** as reducing substrates. - The reaction requires: **H2O2 + thioredoxin(red) → 2 H2O + thioredoxin(ox)** - They are abundant antioxidant enzymes but depend on the thioredoxin/thioredoxin reductase system, unlike catalase which acts independently.

Q: What is the significance of the 'Flipping effect' in clinical biochemistry?

LDH 1 > LDH 2 in myocardial infarction. ***LDH 1 > LDH 2 in myocardial infarction*** - The "flipping effect" refers to the specific finding where **lactate dehydrogenase isoenzyme 1 (LDH 1)** levels become higher than **LDH 2** levels. - This inversion is a classic, though now less commonly used, indicator of **acute myocardial infarction (MI)** due to the release of cardiac LDH. *LDH 2 > LDH 1 in liver disease* - In healthy individuals, **LDH 2** is typically higher than **LDH 1**. - While liver disease can cause elevated total LDH, it primarily involves **LDH 5**, and does not typically present with the "flipping effect." *LDH 2 > LDH 3 in hemolysis* - **Hemolysis** primarily elevates **LDH 1** and **LDH 2**, as these isoenzymes are abundant in red blood cells. - The characteristic "flipping effect" specifically describes LDH 1 exceeding LDH 2, not LDH 2 exceeding LDH 3. *LDH 3 > LDH 2 in other conditions* - **LDH 3** is typically found in organs like the lung, kidney, pancreas, and spleen. - While its elevation can indicate pathology in these areas, it does not constitute the "flipping effect," which is defined by the specific ratio of **LDH 1** to **LDH 2**.

Q: Which enzyme is primarily responsible for the transfer of hydrogen ions in oxidation-reduction reactions?

Dehydrogenase. ***Dehydrogenase*** - **Dehydrogenases** are a class of enzymes that facilitate the transfer of **hydrogen ions (protons)** and electrons from one molecule to another. - They are crucial in **oxidation-reduction (redox) reactions** by removing hydrogen from a substrate, often transferring it to coenzymes like **NAD+** or **FAD**. *Hydratase* - **Hydratases** are enzymes that catalyze the **addition** or **removal of water** to and from a substrate. - These enzymes are involved in **hydration** or **dehydration reactions**, not directly in the transfer of hydrogen ions in redox reactions. *Oxidase* - **Oxidases** are enzymes that catalyze **oxidation-reduction reactions** specifically involving **molecular oxygen (O2)** as an electron acceptor. - While they are involved in redox, their primary role is not the direct transfer of hydrogen ions but rather the **reduction of oxygen**. *Peroxidase* - **Peroxidases** are enzymes that catalyze the breakdown of **hydrogen peroxide**, often using it to oxidize another substrate. - They are important in **detoxification** and **antioxidant defense**, but they do not primarily transfer hydrogen ions in typical redox reactions of metabolism.

Q: Which of the following is a constitutive enzyme?

Hexokinase. ***Hexokinase*** - **Hexokinase** is a **constitutive enzyme**, meaning it is consistently expressed at relatively constant levels in most tissues regardless of substrate availability. - It catalyzes the phosphorylation of glucose to glucose-6-phosphate, a crucial initial step in **glycolysis**, and is essential for basal cellular energy metabolism. *Glucokinase* - **Glucokinase** is an **inducible enzyme** primarily found in the liver and pancreatic beta cells, and its activity is significantly regulated by glucose levels. - Its expression increases in response to high glucose concentrations, promoting glucose storage and insulin secretion, unlike constitutive enzymes. *β-galactosidase* - **β-galactosidase** is a classic example of an **inducible enzyme**, whose synthesis is activated in the presence of lactose (its substrate) as part of the *lac operon* in bacteria. - It is typically present in very low amounts in the absence of lactose and is not constitutively expressed. *Cyclooxygenase-2* - **Cyclooxygenase-2 (COX-2)** is an **inducible enzyme** whose expression is significantly upregulated in response to inflammatory stimuli, cytokines, and growth factors. - It plays a major role in inflammation and pain, while **COX-1** is the constitutive isoform expressed under normal physiological conditions.

Q: Which of the following statements about glucokinase is true?

It has a high Km for glucose.. ***It has a high Km for glucose.*** - A **high Km** indicates that **glucokinase** has a **low affinity for glucose**, allowing it to become active only when glucose concentrations are high, such as after a meal. - This characteristic is crucial for its role in the **liver** and **pancreatic β-cells** as a glucose sensor, facilitating glucose uptake and metabolism only when glucose is abundant. *It is present in all tissues.* - **Glucokinase** is primarily found in the **liver** and **pancreatic β-cells** where it plays a critical role in glucose sensing and metabolism. - In most other tissues, **hexokinase** is the primary enzyme responsible for phosphorylating glucose. *It is an enzyme that is always active.* - The activity of **glucokinase** is regulated by **glucose concentration** and **hormonal signals (e.g., insulin)**, meaning it is not always active. - Its activity significantly increases post-prandially in response to elevated blood glucose levels. *It is inhibited by glucose 6-phosphate.* - Unlike **hexokinase**, which is strongly inhibited by its product **glucose 6-phosphate**, **glucokinase** is not inhibited by glucose 6-phosphate. - This allows the liver to continue taking up and phosphorylating glucose even when intracellular glucose 6-phosphate levels are high, which is important for replenishing glycogen stores.

Q: Which of the following is the UNIQUE characteristic feature of glutamate dehydrogenase that distinguishes it from other dehydrogenases?

Can use both NAD+ and NADP+ as coenzymes. ***Can use both NAD+ and NADP+ as coenzymes*** - **Glutamate dehydrogenase (GDH)** is unique among dehydrogenases in its ability to utilize both oxidized forms of nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+) as coenzymes for its reversible reaction. - This **dual coenzyme specificity** is the distinguishing characteristic that sets GDH apart from most other dehydrogenases, which typically use only one type of cofactor. - This feature allows GDH to participate in metabolic pathways that require either cofactor, contributing to its critical role in linking **amino acid and carbohydrate metabolism**. *Liver mitochondrial enzyme, catalyzing reversible oxidative deamination* - While this is a true statement about GDH, many other enzymes are also **mitochondrial** and found in **liver**. - Reversible oxidative deamination is a function but not a **unique distinguishing feature** compared to other dehydrogenases. *Catalyzes the conversion of glutamate to alpha-ketoglutarate and ammonia* - This statement describes the **forward reaction** (oxidative deamination), which is accurate. - However, this describes the **function** of the enzyme rather than a unique characteristic that distinguishes it from other dehydrogenases. - Many enzymes catalyze similar deamination reactions. *Activated by ADP and inhibited by GTP* - This statement correctly describes the **allosteric regulation** of GDH. - However, allosteric regulation by energy status indicators (ADP/ATP, GDP/GTP) is common among metabolic enzymes and is not a **unique distinguishing feature** of GDH specifically among dehydrogenases.

Q: Which of the following is a serine protease?

Chymotrypsin. ***Chymotrypsin*** - **Chymotrypsin** is a digestive enzyme that belongs to the class of **serine proteases**, meaning it uses a serine residue in its active site to catalyze peptide bond hydrolysis. - It specifically cleaves peptide bonds adjacent to **aromatic amino acids** (e.g., phenylalanine, tryptophan, tyrosine). *Caspases* - **Caspases** are a family of cysteine proteases, meaning they use a **cysteine residue** in their active site for catalysis. - They play crucial roles in **apoptosis** (programmed cell death) and inflammation. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave the **C-terminal amino acid** from a polypeptide chain. - While they are proteases, they do not primarily rely on a serine residue in their active site in the same way as serine proteases; some are metalloproteases (e.g., carboxypeptidase A uses zinc). *Pepsin* - **Pepsin** is an aspartic protease, meaning it uses two **aspartic acid residues** in its active site to cleave peptide bonds. - It functions optimally in the **acidic environment of the stomach** and is responsible for initial protein digestion.

Q: What is the primary mechanism of action of 5-alpha reductase?

Reduction of C4-C5 double bond. ***Reduction of C4-C5 double bond*** - 5-alpha reductase catalyzes the **reduction of testosterone** to dihydrotestosterone (DHT) by adding hydrogen atoms across the **C4-C5 double bond** in the steroid A-ring. - This reaction involves the **saturation of this specific double bond** through a reduction reaction, which is critical for its biological action. - The enzyme uses **NADPH as a cofactor** to donate hydrogen atoms, converting the double bond to a single bond. *Breakage of C-N bond* - The breaking of a **carbon-nitrogen bond** is not the mechanism of 5-alpha reductase. - This type of bond cleavage is characteristic of other enzymatic reactions, such as those involving **peptide hydrolysis** or certain types of **deamination**. *Breakage of amide bond* - An **amide bond** (-CO-NH-) is typically cleaved by enzymes like **amidases** or **proteases**. - This type of bond is not present in the substrate (testosterone) and is therefore not targeted by 5-alpha reductase. *Breakage of N-N bond* - The breaking of an **nitrogen-nitrogen bond** is a rare enzymatic process and is not involved in the metabolism of steroid hormones. - This type of reaction is seen in certain specialized enzymes involved in **nitrogen fixation** or **redox reactions** of nitrogen-containing compounds.

Q: Which enzyme is considered a marker for the endoplasmic reticulum?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - This enzyme is uniquely localized to the **endoplasmic reticulum (ER) membrane**, playing a crucial role in the final step of gluconeogenesis and glycogenolysis. - Its presence and activity are used as a **biochemical marker** to identify and characterize ER fractions in cell biology studies. *Catalase* - **Catalase** is predominantly found within **peroxisomes**, where it catalyzes the decomposition of hydrogen peroxide into water and oxygen. - While peroxisomes can bud off from the ER, catalase itself is not considered a direct marker for the ER membrane. *LDH* - **Lactate dehydrogenase (LDH)** is a ubiquitous **cytoplasmic enzyme** involved in glycolysis, converting pyruvate to lactate. - It is a marker for general cellular damage when found in extracellular fluids, but not specifically for the endoplasmic reticulum. *Acid phosphatase* - **Acid phosphatase** is primarily localized within **lysosomes**, where it plays a role in the hydrolysis of phosphate esters in an acidic environment. - Therefore, it serves as a marker for lysosomes, not the endoplasmic reticulum.

Q: Which of the following enzymes is not a free radical scavenger?

Xanthine oxidase. ***Xanthine oxidase*** - **Xanthine oxidase** is involved in the production of **superoxide radicals** during the metabolism of purines, particularly the conversion of **hypoxanthine to xanthine** and xanthine to uric acid. - It is known to contribute to **oxidative stress** by generating **reactive oxygen species**, rather than scavenging them. *Glutathione peroxidase* - This enzyme **reduces hydrogen peroxide** to water and organic hydroperoxides to their corresponding alcohols, using **glutathione** as a reducing agent. - It plays a crucial role in protecting cells from **oxidative damage** by neutralizing harmful peroxides. *Superoxide dismutase* - **Superoxide dismutase (SOD)** catalyzes the dismutation of the **superoxide radical** into molecular oxygen and hydrogen peroxide. - This enzyme is a primary defense against the **toxic effects of superoxide** in various organisms. *Catalase* - **Catalase** functions to convert **hydrogen peroxide** into water and oxygen. - It is an important enzyme in **peroxisomes** and protects the cell from damage by **reactive oxygen species**.

Question 1

Which of the following enzymes directly catalyzes the decomposition of hydrogen peroxide to water and oxygen without requiring an additional reducing substrate?

Accepted Answer

Catalase

Answer

Superoxide dismutase

Answer

Glutathione peroxidase

Answer

Peroxiredoxin

Question 2

What is the significance of the 'Flipping effect' in clinical biochemistry?

Accepted Answer

LDH 1 > LDH 2 in myocardial infarction

Answer

LDH 2 > LDH 1 in liver disease

Answer

LDH 2 > LDH 3 in hemolysis

Answer

LDH 3 > LDH 2 in other conditions

Question 3

Which enzyme is primarily responsible for the transfer of hydrogen ions in oxidation-reduction reactions?

Accepted Answer

Dehydrogenase

Answer

Hydratase

Answer

Peroxidase

Answer

Oxidase

Question 4

Which of the following is a constitutive enzyme?

Accepted Answer

Hexokinase

Answer

Glucokinase

Answer

Cyclooxygenase-2

Answer

β-galactosidase

Question 5

Which of the following statements about glucokinase is true?

Accepted Answer

It has a high Km for glucose.

Answer

It is present in all tissues.

Answer

It is an enzyme that is always active.

Answer

It is inhibited by glucose 6-phosphate.

Question 6

Which of the following is the UNIQUE characteristic feature of glutamate dehydrogenase that distinguishes it from other dehydrogenases?

Accepted Answer

Can use both NAD+ and NADP+ as coenzymes

Answer

Liver mitochondrial enzyme, catalyzing reversible oxidative deamination

Answer

Catalyzes the conversion of glutamate to alpha-ketoglutarate and ammonia

Answer

Activated by ADP and inhibited by GTP

Question 7

Which of the following is a serine protease?

Accepted Answer

Chymotrypsin

Answer

Caspases

Answer

Carboxypeptidase

Answer

Pepsin

Question 8

What is the primary mechanism of action of 5-alpha reductase?

Accepted Answer

Reduction of C4-C5 double bond

Answer

Breakage of C-N bond

Answer

Breakage of amide bond

Answer

Breakage of N-N bond

Question 9

Which enzyme is considered a marker for the endoplasmic reticulum?

Accepted Answer

Glucose-6-phosphatase

Answer

Catalase

Answer

LDH

Answer

Acid phosphatase

Question 10

Which of the following enzymes is not a free radical scavenger?

Accepted Answer

Xanthine oxidase

Answer

Glutathione peroxidase

Answer

Superoxide dismutase

Answer

Catalase

Enzymes — MCQs

Enzymes — MCQs

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