Enzymes — MCQs
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Which of the following is the UNIQUE characteristic feature of glutamate dehydrogenase that distinguishes it from other dehydrogenases?
Which enzyme is primarily responsible for the transfer of hydrogen ions in oxidation-reduction reactions?
Which of the following is a constitutive enzyme?
Which of the following statements about glucokinase is true?
What is the primary function of the enzyme tyrosinase?
Which enzyme is considered a marker for the endoplasmic reticulum?
Hepatomegaly with hypoglycemia occurs in deficiency of
Which of the following enzymes is not a free radical scavenger?
Which of the following statements about acid phosphatase is correct?
Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?
Enzymes Indian Medical PG Practice Questions and MCQs
Question 491: Which of the following is the UNIQUE characteristic feature of glutamate dehydrogenase that distinguishes it from other dehydrogenases?
- A. Can use both NAD+ and NADP+ as coenzymes (Correct Answer)
- B. Liver mitochondrial enzyme, catalyzing reversible oxidative deamination
- C. Catalyzes the conversion of glutamate to alpha-ketoglutarate and ammonia
- D. Activated by ADP and inhibited by GTP
Explanation: ***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.
Question 492: Which enzyme is primarily responsible for the transfer of hydrogen ions in oxidation-reduction reactions?
- A. Hydratase
- B. Peroxidase
- C. Oxidase
- D. Dehydrogenase (Correct Answer)
Explanation: ***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.
Question 493: Which of the following is a constitutive enzyme?
- A. Hexokinase (Correct Answer)
- B. Glucokinase
- C. Cyclooxygenase-2
- D. β-galactosidase
Explanation: ***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.
Question 494: Which of the following statements about glucokinase is true?
- A. It is present in all tissues.
- B. It is an enzyme that is always active.
- C. It has a high Km for glucose. (Correct Answer)
- D. It is inhibited by glucose 6-phosphate.
Explanation: ***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.
Question 495: What is the primary function of the enzyme tyrosinase?
- A. Synthesis of melanin (Correct Answer)
- B. Synthesis of norepinephrine
- C. Synthesis of dopamine
- D. Synthesis of thyroxine
Explanation: ***Synthesis of melanin*** - **Tyrosinase** is a copper-containing enzyme that catalyzes the hydroxylation of **tyrosine** to DOPA and the oxidation of DOPA to dopaquinone. - These steps are crucial for the biosynthesis of **melanin**, the primary pigment responsible for skin, hair, and eye color. *Synthesis of norepinephrine* - **Norepinephrine** synthesis involves a series of enzymatic steps starting from **tyrosine**, but tyrosinase is not directly involved in its formation. - The conversion of **dopamine** to norepinephrine is catalyzed by **dopamine β-hydroxylase**. *Synthesis of dopamine* - **Dopamine** is synthesized from **L-DOPA** by the enzyme **DOPA decarboxylase**. - While DOPA is an intermediate in melanin synthesis, **tyrosinase** is not the primary enzyme for dopamine production, although it can produce DOPA from tyrosine. *Synthesis of thyroxine* - **Thyroxine (T4)** is a thyroid hormone synthesized from **tyrosine residues** on **thyroglobulin** by the enzyme **thyroid peroxidase**. - This process is distinct from tyrosinase's role in melanin synthesis.
Question 496: Which enzyme is considered a marker for the endoplasmic reticulum?
- A. Catalase
- B. LDH
- C. Glucose-6-phosphatase (Correct Answer)
- D. Acid phosphatase
Explanation: ***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.
Question 497: Hepatomegaly with hypoglycemia occurs in deficiency of
- A. Branching enzyme
- B. Debranching enzyme
- C. Hepatic phosphorylase
- D. Glucose-6-phosphatase (Correct Answer)
Explanation: ***Acid maltase*** - Deficiency of acid maltase, also known as Pompe disease, leads to significant muscle and liver glycogen accumulation, which can cause an **AST/ALT ratio > 2** due to hepatocellular injury [1,2]. - It predominantly causes **myopathy** and hepatomegaly, making this enzyme deficiency clinically relevant in this context [1]. *Glucose-6-phosphotase* - This deficiency leads to **von Gierke disease**, characterized by severe hypoglycemia and increased lactate, but not specifically an AST/ALT ratio > 2 [1]. - It results in **glycogen accumulation in the liver**, affecting glucose metabolism but not primarily liver enzymes [1]. *Branching enzyme* - Branching enzyme deficiency results in **Andersen disease**, which is characterized by long unbranched glycogen chains, and predominantly causes **hepatic dysfunction** without a specific AST/ALT ratio > 2. - Clinical manifestations include **cirrhosis** and splenomegaly, but not elevation of liver transaminases in the described ratio. *Liver phosphorylase* - Liver phosphorylase deficiency results in **Cori disease**, which presents with hypoglycemic episodes and hepatomegaly, but not necessarily an AST/ALT ratio above 2 [1]. - The enzyme affects glycogen mobilization, and clinical features do not consistently include liver enzyme elevation as described. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 164-167. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 850-851.
Question 498: Which of the following enzymes is not a free radical scavenger?
- A. Glutathione peroxidase
- B. Superoxide dismutase
- C. Catalase
- D. Xanthine oxidase (Correct Answer)
Explanation: ***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 499: Which of the following statements about acid phosphatase is correct?
- A. Acts at pH 8-9
- B. Prostate isoform is tartrate resistant
- C. Erythrocyte isoform is inhibited by cupric ions (Correct Answer)
- D. None of the options
Explanation: ***Erythrocyte isoform is inhibited by cupric ions*** - The **erythrocyte isoform** of acid phosphatase, often referred to as **red cell acid phosphatase (ACP1)**, is known to be inhibited by **cupric ions (Cu2+)**. This characteristic is used in some forensic and biochemical applications. - This isoform is also important in forensic analysis for genetic typing from bloodstains, where its activity can be distinguished based on inhibitors and electrophoretic patterns. *Acts at pH 8-9* - Acid phosphatase, by definition, functions optimally in an **acidic environment**, typically with an optimal pH ranging from **4.5 to 5.5**. - An enzyme acting at pH 8-9 would be an **alkaline phosphatase**, not an acid phosphatase. *Prostate isoform is tartrate resistant* - The **prostate-specific acid phosphatase (PSAP)**, an isoform of acid phosphatase, is notably **inhibited by L-tartrate**. This property is used diagnostically to differentiate PSAP from other acid phosphatase isoforms. - Therefore, the statement that it is tartrate resistant is incorrect; it is actually **tartrate sensitive**. *None of the options* - This option is incorrect because the statement regarding the **erythrocyte isoform being inhibited by cupric ions** is factually accurate.
Question 500: Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?
- A. Fluoroacetate
- B. Arsenite
- C. Malonate (Correct Answer)
- D. Fluoride
Explanation: ***Malonate*** - **Malonate** is a **competitive inhibitor** of **succinate dehydrogenase** because its structure is very similar to that of succinate, allowing it to bind to the enzyme's active site but preventing the catalytic reaction. - This enzyme, also known as Complex II, is vital for both the **Krebs cycle** and the **electron transport chain**, linking these two metabolic pathways. *Fluoroacetate* - **Fluoroacetate** is an inhibitor of **aconitase**, an enzyme in the Krebs cycle that converts citrate to isocitrate. - It is metabolically converted to **fluorocitrate**, which then acts as a potent inhibitor of aconitase. *Arsenite* - **Arsenite** inhibits enzymes that require **lipoic acid** as a coenzyme, such as the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**. - Its mechanism involves binding to the sulfhydryl groups of dihydrolipoyl transacetylase, preventing its function. *Fluoride* - **Fluoride** is known to inhibit **enolase**, an enzyme involved in **glycolysis**. - Its inhibitory action is typically enhanced in the presence of phosphate.
Practice by Chapter
Enzyme Classification and Nomenclature
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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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