Enzymes Practice Questions

Q: Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

LDH-5. ***LDH-5 isoenzyme most significant in hepatic conditions*** - **LDH-5** is the predominant isoenzyme found in the **liver** and skeletal muscle. - An elevation of **LDH-5** is highly indicative of **hepatocellular damage** or injury. *LDH-1 isoenzyme associated with cardiac tissue* - **LDH-1** is primarily present in the **heart** and red blood cells. - Its elevation suggests conditions like **myocardial infarction** or hemolytic anemia, not liver injury. *LDH-3 isoenzyme typical in respiratory system* - **LDH-3** is found in the **lungs**, kidneys, and other tissues. - While it can be elevated in **pulmonary embolism** or renal disease, it is not specific for liver injury. *LDH-2 isoenzyme linked to erythrocyte metabolism* - **LDH-2** is abundant in **red blood cells** and also found in the heart and kidneys. - Elevations are often seen in conditions involving **hemolysis** or myocardial damage, similar to LDH-1.

Q: What is the cofactor for glutathione peroxidase?

Selenium. ***Selenium*** - **Glutathione peroxidase** is a family of enzymes that protect the body from oxidative damage by reducing hydrogen peroxide and organic hydroperoxides. - **Selenium** is an essential trace element that is incorporated into these enzymes as **selenocysteine**, making it a crucial cofactor for their catalytic activity. *Mg2+* - **Magnesium** is a cofactor for many enzymes, particularly those involved in **ATP hydrolysis and synthesis**, DNA, and RNA metabolism. - It is not directly involved in the catalytic mechanism of glutathione peroxidase. *Mn2+* - **Manganese** is a cofactor for several enzymes, including **superoxide dismutase (SOD2)**, which is involved in antioxidant defense. - However, manganese does not serve as a cofactor for glutathione peroxidase. *Ca2+* - **Calcium** acts as a cofactor for many enzymes, often regulating their activity as a **second messenger** or through direct binding. - It is essential for processes like muscle contraction and neurotransmitter release but is not a cofactor for glutathione peroxidase.

Q: Which of the following flipped pattern of LDH is seen in myocardial infarction?

LDH 1>2. ***LDH 1>2*** - In **myocardial infarction**, the predominant **LDH isoenzyme** released from damaged heart muscle is **LDH-1**. - A **flipped pattern** where **LDH-1 activity exceeds LDH-2 activity** is highly characteristic of acute myocardial infarction. *LDH 3>4* - An increase in **LDH-3** is typically associated with conditions affecting the **lungs, lymph glands, pancreas, and spleen**, rather than acute cardiac events. - This pattern would not indicate specific cardiac damage as seen in myocardial infarction. *LDH 5>4* - Elevated **LDH-5** is usually indicative of **liver disease, skeletal muscle damage, or congestive heart failure**, distinguishing it from myocardial infarction. - This pattern specifically points to damage in tissues rich in the LDH-5 isoenzyme, not the heart. *LDH 2>1* - The **normal pattern** of LDH isoenzymes in healthy individuals is usually **LDH-2 > LDH-1**. - Therefore, this pattern would not suggest myocardial infarction, but rather a normal state or other conditions.

Q: Which enzyme joins two substrates?

Ligase. ***Ligase*** - **Ligases** are a class of enzymes that **catalyze the joining of two large molecules** by forming a new chemical bond, typically with the concomitant hydrolysis of a small pendant chemical group on one of the larger molecules or the coupling of a reaction to the cleavage of pyrophosphate on ATP or similar. - This process often involves the use of **ATP or other energy sources** to form a covalent bond. *Lyase* - **Lyases** are enzymes that **catalyze the breaking of chemical bonds** by means other than hydrolysis (e.g., elimination reactions). - They typically form a new double bond or a ring structure during the bond cleavage. *Synthase* - **Synthases** are a type of **lyase enzyme** that **catalyzes synthesis reactions** without the direct involvement of ATP or other nucleoside triphosphates for energy. - While they synthesize molecules, they don't necessarily "join two substrates" in the same way a ligase does, especially without consuming a high-energy phosphate. *Isomerase* - **Isomerases** catalyze the **rearrangement of atoms within a molecule**, converting a compound into one of its isomers. - They do not join two separate substrates; rather, they alter the structure of a single substrate.

Q: A young boy presents with failure to thrive. Biochemical analysis of a duodenal aspirate after a meal reveals a deficiency of enteropeptidase (enterokinase). Which one of the following digestive enzymes would be affected by this deficiency?

Trypsin. ***Trypsin*** - Enteropeptidase (enterokinase) is crucial for activating **trypsinogen** into its active form, **trypsin**. Without active trypsin, the entire cascade of pancreatic protease activation is disrupted. - Trypsin then activates other pancreatic proteases like chymotrypsin, elastase, and carboxypeptidases, all of which are essential for **protein digestion** in the small intestine. *Amylase* - **Amylase** is a carbohydrate-digesting enzyme, primarily involved in breaking down starch. Its activity is independent of enteropeptidase. - **Pancreatic amylase** is secreted in its active form and does not require proteolytic cleavage by trypsin for activation. *Pepsin* - **Pepsin** is an enzyme found in the stomach that initiates protein digestion. It is activated by **hydrochloric acid** from its inactive precursor, pepsinogen. - Its activity is entirely independent of enteropeptidase, which functions in the duodenum. *Lactase* - **Lactase** is a brush border enzyme located in the small intestine that digests the disaccharide **lactose** into glucose and galactose. - Its production and activity are genetically regulated and not dependent on the protein-digesting enzymes or enteropeptidase.

Q: Inactive precursors of enzymes are known as:

Proenzymes. ***Proenzymes*** - **Proenzymes**, also known as **zymogens**, are inactive precursor forms of enzymes that require a biochemical change (e.g., proteolytic cleavage) to become active. - This mechanism allows for the **controlled activation** of enzymes, preventing premature or inappropriate enzymatic activity. *Apoenzymes* - An **apoenzyme** is the protein component of an enzyme that requires a **non-protein cofactor** (like a metal ion or coenzyme) to become active. - It describes the enzyme without its essential cofactor, making it inactive until the cofactor binds. *Coenzymes* - **Coenzymes** are small, non-protein organic molecules that bind to apoenzymes to assist in catalysis. - They often function as **carriers of electrons, atoms, or functional groups** during enzymatic reactions. *Holoenzymes* - A **holoenzyme** is the catalytically active form of an enzyme, consisting of an **apoenzyme** (protein part) combined with its essential **cofactor** (e.g., coenzyme or metal ion). - It represents the complete and functional enzyme complex.

Q: Which enzyme is primarily evaluated for diagnosing myopathies?

Creatine kinase (CK). ***Creatine kinase (CK)*** - **Creatine kinase** is an enzyme found predominantly in **muscle cells** (skeletal and cardiac) and to a lesser extent in the brain. - Elevated levels in the blood are indicative of **muscle damage** or breakdown, making it the primary marker for diagnosing and monitoring myopathies. *Gamma-glutamyl transferase (GGT)* - GGT is an enzyme primarily found in the **liver** and **bile ducts**. - Elevated GGT levels are typically associated with **liver disease** or **bile duct obstruction**, not muscle disorders. *Lactic dehydrogenase (LDH)* - **Lactic dehydrogenase** is an enzyme found in many body tissues, including the heart, liver, kidneys, skeletal muscle, and red blood cells. - While LDH can be elevated in muscle damage, its lack of specificity to muscle makes it a less precise diagnostic indicator for myopathies compared to CK. *Alanine aminotransferase (ALT)* - **Alanine aminotransferase** is an enzyme found predominantly in the **liver**. - Elevated ALT levels are a key indicator of **hepatocellular damage** or liver disease, not primary muscle disorders.

Q: Which of the following minerals is a component of glutathione peroxidase, a major antioxidant enzyme?

Selenium. ***Correct: Selenium*** - **Selenium** is an essential component of **glutathione peroxidase (GPx)**, one of the body's most important antioxidant enzyme systems - It is incorporated as **selenocysteine** at the active site of the enzyme - Glutathione peroxidase **neutralizes hydrogen peroxide and organic hydroperoxides**, protecting cells from oxidative damage - Selenium deficiency leads to **Keshan disease** (cardiomyopathy) due to increased oxidative stress *Incorrect: Copper* - Copper is a cofactor for **superoxide dismutase (Cu-Zn SOD)** and **ceruloplasmin**, which have antioxidant properties - However, copper is NOT a component of glutathione peroxidase *Incorrect: Zinc* - Zinc is a component of **superoxide dismutase (Cu-Zn SOD)** and helps prevent lipid peroxidation - However, zinc is NOT a component of glutathione peroxidase *Incorrect: Manganese* - Manganese is a cofactor for **manganese superoxide dismutase (Mn-SOD)** in mitochondria - However, manganese is NOT a component of glutathione peroxidase

Q: In which of the following conditions does the level of creatine kinase-1 increase?

Brain Ischemia. ***Brain Ischemia*** - **Creatine kinase-1 (CK-BB)**, or brain type CK, is predominantly found in the **brain**, gastrointestinal tract, and smooth muscle. - An increase in its level in the **serum** primarily indicates damage to the brain, such as following an **ischemic stroke** or severe head injury. *Myocardial ischemia* - **Myocardial ischemia** primarily causes an elevation in **creatine kinase-2 (CK-MB)**, the cardiac-specific isoform of creatine kinase. - While total CK may be elevated, **CK-MB** is the hallmark marker for cardiac muscle damage, not CK-BB. *Kidney damage* - **Kidney damage** is typically assessed by markers like **creatinine** and **blood urea nitrogen (BUN)**. - Creatine kinase isoforms are not specific indicators of kidney injury, although severe muscle damage (and thus CK elevation) can lead to secondary kidney problems like **rhabdomyolysis-induced acute kidney injury**. *Electrical cardioversion* - **Electrical cardioversion** can cause minor skeletal muscle damage and, less commonly, some myocardial stunning. - This typically leads to a transient increase in **creatine kinase-3 (CK-MM)**, the skeletal muscle isoform, or potentially CK-MB if there is minor myocardial injury, but not CK-BB.

Question 1

Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

Accepted Answer

LDH-5

Answer

LDH-3

Answer

LDH-1

Answer

LDH-2

Question 2

The catalytic serine (Ser195) of chymotrypsin is replaced with proline. Which of the following will occur?

Accepted Answer

Chymotrypsin retains binding ability but loses catalytic activity

Answer

Chymotrypsin retains catalytic activity but loses binding ability

Answer

Chymotrypsin retains both binding and catalytic activity

Answer

Chymotrypsin loses both binding and catalytic activity

Question 3

What is the cofactor for glutathione peroxidase?

Accepted Answer

Selenium

Answer

Mg2+

Answer

Mn2+

Answer

Ca2+

Question 4

Which of the following flipped pattern of LDH is seen in myocardial infarction?

Accepted Answer

LDH 1>2

Answer

LDH 5>4

Answer

LDH 3>4

Answer

LDH 2>1

Question 5

Which enzyme joins two substrates?

Accepted Answer

Ligase

Answer

Synthase

Answer

Lyase

Answer

Isomerase

Question 6

A young boy presents with failure to thrive. Biochemical analysis of a duodenal aspirate after a meal reveals a deficiency of enteropeptidase (enterokinase). Which one of the following digestive enzymes would be affected by this deficiency?

Accepted Answer

Trypsin

Answer

Amylase

Answer

Pepsin

Answer

Lactase

Question 7

Inactive precursors of enzymes are known as:

Accepted Answer

Proenzymes

Answer

Apoenzymes

Answer

Coenzymes

Answer

Holoenzymes

Question 8

Which enzyme is primarily evaluated for diagnosing myopathies?

Accepted Answer

Creatine kinase (CK)

Answer

Gamma-glutamyl transferase (GGT)

Answer

Lactic dehydrogenase (LDH)

Answer

Alanine aminotransferase (ALT)

Question 9

Which of the following minerals is a component of glutathione peroxidase, a major antioxidant enzyme?

Accepted Answer

Selenium

Answer

Copper

Answer

Manganese

Answer

Zinc

Question 10

In which of the following conditions does the level of creatine kinase-1 increase?

Accepted Answer

Brain Ischemia

Answer

Myocardial ischemia

Answer

Kidney damage

Answer

Electrical cardioversion

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?