Enzymes Practice Questions

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: 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 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 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 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: 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**.

Q: Hepatomegaly with hypoglycemia occurs in deficiency of

Glucose-6-phosphatase. ***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.

Q: Which of the following statements about acid phosphatase is correct?

Erythrocyte isoform is inhibited by cupric ions. ***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.

Q: Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?

Malonate. ***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.

Question 1

Which of the following is a constitutive enzyme?

Accepted Answer

Hexokinase

Answer

Glucokinase

Answer

Cyclooxygenase-2

Answer

β-galactosidase

Question 2

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 3

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 4

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 5

Which of the following is a serine protease?

Accepted Answer

Chymotrypsin

Answer

Caspases

Answer

Carboxypeptidase

Answer

Pepsin

Question 6

Which enzyme is considered a marker for the endoplasmic reticulum?

Accepted Answer

Glucose-6-phosphatase

Answer

Catalase

Answer

LDH

Answer

Acid phosphatase

Question 7

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

Accepted Answer

Xanthine oxidase

Answer

Glutathione peroxidase

Answer

Superoxide dismutase

Answer

Catalase

Question 8

Hepatomegaly with hypoglycemia occurs in deficiency of

Accepted Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Hepatic phosphorylase

Question 9

Which of the following statements about acid phosphatase is correct?

Accepted Answer

Erythrocyte isoform is inhibited by cupric ions

Answer

Acts at pH 8-9

Answer

Prostate isoform is tartrate resistant

Answer

None of the options

Question 10

Which of the following is a specific inhibitor of succinate dehydrogenase, an enzyme crucial in the Krebs cycle?

Accepted Answer

Malonate

Answer

Fluoroacetate

Answer

Arsenite

Answer

Fluoride

Enzymes — MCQs

Enzymes — MCQs

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