Enzymes Practice Questions

Q: Which of the following is a co-factor for phosphofructokinase?

Magnesium (Mg²⁺). ***Magnesium (Mg²⁺)*** - **Magnesium** is a critical **cofactor for phosphofructokinase (PFK)**, an enzyme central to glycolysis. - It forms an **ATP-Mg²⁺ complex**, which is the actual substrate for PFK, and also stabilizes the enzyme's structure. *Manganese (Mn²⁺)* - While **manganese** can act as a cofactor for some enzymes, it is **not typically recognized** as a significant cofactor for phosphofructokinase. - It often plays roles in enzymes involved in **oxidative phosphorylation** and **antioxidant defense**. *Iron (Fe²⁺)* - **Iron** is essential for various enzymes, particularly those involved in **electron transport** and oxygen binding (e.g., in hemoglobin and cytochromes). - However, **iron is not a cofactor** for phosphofructokinase. *Zinc (Zn)* - **Zinc** is a critical cofactor for numerous enzymes, particularly those involved in genetic material replication and repair, and immune function. - It is **not a cofactor for phosphofructokinase**, which primarily requires magnesium.

Q: Which of the following enzymes is NAD+ dependent?

Glycerol-3-phosphate dehydrogenase. ***Glycerol-3-phosphate dehydrogenase*** - This enzyme catalyzes the interconversion of **dihydroxyacetone phosphate (DHAP)** and **glycerol-3-phosphate**, using **NAD+** as a cofactor to oxidize glycerol-3-phosphate. - In the **glycerol-phosphate shuttle**, it enables the transfer of reducing equivalents from cytosolic NADH to the mitochondrial electron transport chain. *HMG CoA reductase* - This enzyme is a key regulatory step in **cholesterol biosynthesis** and utilizes **NADPH** as a reducing agent, not NAD+. - It catalyzes the reduction of HMG-CoA to **mevalonate**. *Acyl CoA dehydrogenase* - This enzyme is involved in the first step of **beta-oxidation of fatty acids** and uses **FAD** as a prosthetic group, not NAD+, which is reduced to FADH2. - It catalyzes the formation of a double bond between the alpha and beta carbons of the acyl-CoA. *Succinate dehydrogenase* - This enzyme is part of the **TCA cycle** (Complex II of the electron transport chain) and uses **FAD** as its electron acceptor, converting succinate to fumarate. - It is unique among TCA cycle enzymes as it is membrane-bound and directly links the cycle to the **electron transport chain**.

Q: Heat-stable alkaline phosphatase in pregnancy is primarily derived from which source?

The placenta. ***The placenta*** - The **placenta** is the primary source of heat-stable alkaline phosphatase (HSAP) during pregnancy. This specific isoform is distinct from other alkaline phosphatase isoforms. - Increased levels of HSAP are observed in maternal serum throughout pregnancy, reflecting **placental metabolic activity** and growth. - HSAP levels typically **rise progressively** from the first trimester and peak near term, serving as a marker of **placental function**. *Maternal liver* - The maternal liver produces **liver-specific alkaline phosphatase**, which is **not heat-stable**. - While liver ALP levels may fluctuate slightly in pregnancy, they are not the primary source of the heat-stable form. - Liver ALP is inactivated by heating at 56°C, unlike the placental isoform. *Fetal liver* - The fetal liver produces **alkaline phosphatase**, but this is not released into the maternal circulation in significant amounts as **heat-stable ALP**. - Fetal contribution to maternal serum **heat-stable ALP** is negligible compared to the placenta. - The placental barrier prevents significant transfer of fetal enzymes to maternal blood. *Maternal bone* - Maternal bone produces **bone-specific alkaline phosphatase**, which is also **not heat-stable**. - Bone ALP may increase during pregnancy due to skeletal remodeling, but it represents a different isoform. - Bone ALP can be distinguished from placental ALP by heat stability testing and electrophoresis.

Q: A patient presents 12 hours following a myocardial infarction. Which of the following enzymes will be elevated at this period?

Creatine phosphokinase. ***Creatine phosphokinase*** - **Creatine phosphokinase (CK-MB)** levels begin to rise within **3-6 hours** after an MI, peak at **12-24 hours**, and return to baseline within **48-72 hours**. - At 12 hours post-MI, CK-MB would be significantly elevated and near its peak, making it the most reliable marker for diagnosis at this timepoint. *Lactate dehydrogenase* - **Lactate dehydrogenase (LDH)** levels rise much later, typically around **24-48 hours** after an MI, and peak at **3-6 days**. - At 12 hours post-MI, LDH elevation would be minimal or non-existent. *Myoglobin* - **Myoglobin** is one of the earliest markers to rise, appearing within **1-4 hours** post-MI and peaking around **6-7 hours**. - While elevated at 12 hours, it may already be declining, and it **lacks cardiac specificity** (also released from skeletal muscle injury), making it less reliable than CK-MB for MI diagnosis. *Serum glutamate oxaloacetate transferase (SGOT)* - **SGOT (AST)** levels start to rise around **6-12 hours** post-MI, peak at **18-36 hours**, and return to normal within **3-7 days**. - While it would be beginning to rise at 12 hours, **CK-MB** is more elevated and more specific for cardiac injury at this timepoint.

Q: Which of the following enzymes is not classified as an oxidoreductase?

Glucokinase. ***Glucokinase*** - **Glucokinase** is a **transferase** enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. - Its function is primarily in **glucose metabolism** and **insulin secretion**, not in oxidation or reduction reactions. *Catalase* - **Catalase** is an **oxidoreductase** that catalyzes the decomposition of **hydrogen peroxide** into water and oxygen. - This reaction involves the **oxidation and reduction** of substrates, fitting the definition of an oxidoreductase. *Alcohol dehydrogenase* - **Alcohol dehydrogenase** is an **oxidoreductase** that catalyzes the interconversion between alcohols and aldehydes or ketones with the concomitant reduction and oxidation of **NAD+** to **NADH**. - This enzyme is crucial in **detoxifying alcohol** by oxidizing it and is a classic example of an oxidoreductase. *Peroxidase* - **Peroxidase** is an **oxidoreductase** that catalyzes the oxidation of a substrate by **hydrogen peroxide**. - Peroxidases work by using hydrogen peroxide to accept electrons from another molecule, thereby **oxidizing** that molecule.

Q: Which of the following enzymes is not considered a rate-limiting enzyme in metabolic pathways?

Succinate dehydrogenase. ***Succinate dehydrogenase*** - **Succinate dehydrogenase** is an enzyme of the **Krebs cycle** and **electron transport chain** but is not considered a primary rate-limiting enzyme. - Its activity is generally high, and it operates close to its maximum velocity under most physiological conditions, thus not typically controlling the overall flux of its respective pathways in a rate-limiting manner. *HMG CoA reductase* - **HMG CoA reductase** is the **rate-limiting enzyme in cholesterol biosynthesis**. - It is a key target for **statins**, drugs that lower cholesterol by inhibiting this enzyme. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is the **rate-limiting enzyme in glycolysis**. - Its activity is tightly regulated by **allosteric modulators** like ATP, AMP, and citrate to control the flux of glucose metabolism. *Acetyl CoA carboxylase* - **Acetyl CoA carboxylase (ACC)** is the **rate-limiting enzyme in fatty acid synthesis**. - It catalyzes the irreversible carboxylation of acetyl-CoA to **malonyl-CoA**, a crucial committed step in the pathway.

Q: Which enzyme requires zinc as a cofactor?

Carbonic anhydrase. ***Carbonic anhydrase*** - **Carbonic anhydrase** is a critical enzyme that rapidly interconverts carbon dioxide and water into carbonic acid, which then dissociates into a proton and a bicarbonate ion. - This enzyme contains a **zinc ion** in its active site, which is essential for its catalytic activity, particularly in binding and activating water for the hydration of CO2. *Lactate dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of pyruvate to lactate, a key step in anaerobic glycolysis. - LDH primarily relies on **NAD+ or NADH** as cofactors and does not require zinc. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** is an antioxidant enzyme that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides to water using glutathione. - Most mammalian glutathione peroxidases are **selenium-dependent enzymes**, incorporating selenocysteine at their active site, rather than zinc. *Hexokinase* - **Hexokinase** is an enzyme that phosphorylates hexoses, most notably glucose, to glucose-6-phosphate, the first step in glycolysis. - This enzyme requires **magnesium (Mg2+)** as a cofactor for its activity, as it forms a complex with ATP, facilitating the transfer of the phosphate group.

Question 1

Which of the following is a co-factor for phosphofructokinase?

Accepted Answer

Magnesium (Mg²⁺)

Answer

Manganese (Mn²⁺)

Answer

Iron (Fe²⁺)

Answer

Zinc (Zn)

Question 2

Which of the following enzymes is NAD+ dependent?

Accepted Answer

Glycerol-3-phosphate dehydrogenase

Answer

HMG CoA reductase

Answer

Acyl CoA dehydrogenase

Answer

Succinate dehydrogenase

Question 3

Which of the following statements about cofactors and metalloenzymes is true?

Accepted Answer

Coenzymes are loosely bound cofactors that transiently bind to enzymes, while prosthetic groups are tightly bound cofactors

Answer

The most common cofactors are metal ions

Answer

Enzymes that require metal ion cofactors are termed as metalloenzymes

Answer

Cofactors bind in a transient, dissociable manner to enzymes

Question 4

Heat-stable alkaline phosphatase in pregnancy is primarily derived from which source?

Accepted Answer

The placenta

Answer

Maternal liver

Answer

Fetal liver

Answer

Maternal bone

Question 5

A patient presents 12 hours following a myocardial infarction. Which of the following enzymes will be elevated at this period?

Accepted Answer

Creatine phosphokinase

Answer

Lactate dehydrogenase

Answer

Myoglobin

Answer

Serum glutamate oxaloacetate transferase (SGOT)

Question 6

Which of the following enzymes is not classified as an oxidoreductase?

Accepted Answer

Glucokinase

Answer

Alcohol dehydrogenase

Answer

Catalase

Answer

Peroxidase

Question 7

Which of the following enzymes is not considered a rate-limiting enzyme in metabolic pathways?

Accepted Answer

Succinate dehydrogenase

Answer

HMG CoA reductase

Answer

Phosphofructokinase

Answer

Acetyl CoA carboxylase

Question 8

Which enzyme requires zinc as a cofactor?

Accepted Answer

Carbonic anhydrase

Answer

Lactate dehydrogenase

Answer

Glutathione peroxidase

Answer

Hexokinase

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?