NEET-PG 2013 - Biochemistry Practice Questions

Q: What is the primary enzymatic source of ammonia production in urine?

Glutaminase. ***Glutaminase*** - This enzyme catalyzes the **hydrolysis of glutamine** located predominantly in the cells of the **renal tubules**, producing **ammonia** (NH3) and glutamate. - This process is crucial for **acid-base balance**, as the ammonia can bind with excess hydrogen ions to form ammonium (NH4+), which is then excreted in the urine. *Urease* - This enzyme breaks down **urea into ammonia and carbon dioxide**, primarily produced by **bacteria**, not human renal cells, and contributes to ammonia in urine in cases of **urinary tract infections**. - While it produces ammonia, it is not the primary enzymatic source within the healthy human kidney for **acid-base regulation**. *Glutamate dehydrogenase* - This enzyme converts **glutamate into alpha-ketoglutarate and ammonia**, but its contribution to urinary ammonia production is secondary to glutaminase in the kidney. - Its primary role is in **oxidative deamination** and the interconversion of glutamate and alpha-ketoglutarate, acting in both mitochondrial and cytosolic compartments. *Arginase* - This enzyme is involved in the **urea cycle**, converting **arginine into urea and ornithine** in the liver. - While it is important for the detoxification of ammonia by converting it into urea for excretion, it **does not produce ammonia** in the kidney for urinary excretion.

Q: Which of the following statements about vitamin E is false?

Water soluble vitamin. ***Water soluble vitamin*** - This statement is false because **vitamin E** is a **fat-soluble vitamin**, meaning it dissolves in lipids and is stored in the body's fatty tissues and liver. - Its absorption requires the presence of dietary fat and bile salts, unlike water-soluble vitamins. *Act as antioxidant* - **Vitamin E** is a powerful **lipid-soluble antioxidant**, protecting cell membranes from damage by scavenging **free radicals**. - It helps prevent **oxidative damage** to polyunsaturated fatty acids within cell membranes. *Chemically tocopherol* - The most biologically active form of **vitamin E** is **alpha-tocopherol**, although vitamin E encompasses a group of eight fat-soluble compounds, including tocopherols and tocotrienols. - This term correctly identifies the chemical nature of the prominent forms of vitamin E. *Prevent lipid peroxidation of cell membrane* - As an **antioxidant**, **vitamin E** specifically interferes with the **propagation of lipid peroxidation**, a chain reaction that damages cell membranes by oxidizing their lipid components. - It donates an electron to **free radicals**, thereby neutralizing them and protecting the integrity of the cell membrane.

Q: Fructose intolerance is due to deficiency of which enzyme?

Aldolase B. ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.

Q: Which of the following statements about gluconeogenesis is correct?

Occurs mainly in the liver. ***Occurs mainly in the liver*** - The **liver** is the primary site for **gluconeogenesis**, responsible for maintaining blood glucose levels during fasting. - The kidneys also contribute, especially during prolonged fasting, but to a lesser extent. *It uses exactly the same enzymes as glycolysis in reverse* - While gluconeogenesis shares some enzymes with glycolysis, there are **three irreversible steps in glycolysis** that require different enzymes in gluconeogenesis to bypass them. - Key bypass enzymes include **pyruvate carboxylase**, **phosphoenolpyruvate carboxykinase (PEPCK)**, **fructose-1,6-bisphosphatase**, and **glucose-6-phosphatase**. *It only occurs during fed state when insulin levels are high* - **Gluconeogenesis is activated during fasting or starvation** when blood glucose levels are low, and it is largely **inhibited by high insulin levels**. - Its purpose is to produce new glucose to prevent hypoglycemia, not to store excess glucose. *Fatty acids are the primary substrate for gluconeogenesis* - **Fatty acids cannot be directly converted to glucose** in significant amounts in humans because they are broken down into acetyl-CoA, which cannot be used for net glucose synthesis. - Primary substrates include **lactate**, **amino acids** (from protein breakdown), and **glycerol** (from triglyceride breakdown).

Q: Trypsinogen is converted to trypsin by?

Removal of specific amino acids from trypsinogen. ***Removal of specific amino acids from trypsinogen*** - Trypsinogen is an **inactive zymogen** that is activated by the enzymatic cleavage of a **short N-terminal peptide**. - This cleavage event, primarily catalyzed by **enteropeptidase** (or trypsin itself), transforms trypsinogen into active **trypsin**, a process known as **proteolytic activation**. *Combination of 2 molecules of trypsinogen* - The activation of trypsinogen to trypsin is a **unimolecular conformational change** followed by proteolytic cleavage, not a combination reaction between two zymogen molecules. - While trypsin can activate other trypsinogen molecules, the initial activation does not involve the physical combination of two zymogen molecules. *Phosphorylation* - **Phosphorylation** is a common regulatory mechanism in proteins but is not the primary method for activating inactive trypsinogen. - Trypsinogen activation relies on a **proteolytic cleavage event**, rather than the addition of a phosphate group. *Addition of alkyl group* - The addition of an **alkyl group** is not a known mechanism for the physiological activation of trypsinogen. - Enzymatic activation typically involves **hydrolysis of peptide bonds** or other specific post-translational modifications.

Q: Rate limiting step in pyrimidine synthesis?

Carbamoyl phosphate synthase-II. ***Carbamoyl phosphate synthetase II (CPS-II)*** - **CPS-II** is the **committed and rate-limiting enzyme** in **de novo pyrimidine synthesis** in **mammals (including humans)** - It catalyzes the formation of **carbamoyl phosphate** from glutamine, CO₂, and 2 ATP in the **cytoplasm** - This is the **first committed step** and the main **regulatory checkpoint**, inhibited by UTP (feedback inhibition) and activated by PRPP and ATP - CPS-II is part of the **CAD complex** (carbamoyl phosphate synthetase, aspartate transcarbamoylase, dihydroorotase) in mammals *Aspartate transcarbamoylase (ATCase)* - ATCase catalyzes the **second step**: condensation of carbamoyl phosphate with aspartate to form carbamoyl aspartate - While ATCase is the **rate-limiting step in bacteria** (E. coli), in **mammals** it is part of the CAD complex and **not the primary regulatory step** - This option is incorrect for human/mammalian biochemistry tested in NEET PG *Dihydro-orotase* - The **third enzyme** in the pathway, converting carbamoyl aspartate to dihydroorotate - Part of the CAD complex in mammals but **not the rate-limiting step** *Dihydroorotate dehydrogenase* - Catalyzes the **fourth step**: oxidation of dihydroorotate to orotate - Located on the **outer surface of the inner mitochondrial membrane** (only mitochondrial enzyme in the pathway) - Important enzyme but **not rate-limiting**

Q: Which type of RNA contains codons for specific amino acids?

Messenger RNA (mRNA). ***Messenger RNA (mRNA)*** - **mRNA** carries the genetic information from **DNA** in the nucleus to the **ribosomes** in the cytoplasm. - This information is encoded in sequences of three nucleotides called **codons**, each specifying a particular amino acid. *Transfer RNA (tRNA)* - **tRNA** molecules are responsible for **carrying specific amino acids** to the ribosome during protein synthesis. - Each **tRNA** has an **anticodon** that base-pairs with a complementary **codon** on the **mRNA** strand. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in **RNA splicing**, a process that removes introns from pre-mRNA. - It forms part of the **spliceosome** complex, which is crucial for mature mRNA formation but does not contain codons itself. *Ribosomal RNA (rRNA)* - **rRNA** is a major component of **ribosomes**, the cellular machinery responsible for protein synthesis. - While it plays a critical structural and catalytic role in translation, it does not carry genetic code in the form of codons.

Q: Which of the following statements regarding collagen synthesis is incorrect?

Hydroxylation of proline occurs in Golgi apparatus. ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.

Q: Glucagon stimulates

Gluconeogenesis. ***Gluconeogenesis*** - **Glucagon** is a hormone that primarily acts to raise **blood glucose levels** by stimulating the production of glucose from non-carbohydrate sources. - This process, **gluconeogenesis**, occurs mainly in the liver and is initiated by glucagon to counteract hypoglycemia. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from glucose and is primarily stimulated by insulin when blood glucose levels are high. - Glucagon's role is to *inhibit* glycogen synthesis and instead promote glycogen breakdown. *Fatty acid synthesis* - **Fatty acid synthesis** is an anabolic process that primarily occurs when there is an excess of energy and glucose, often stimulated by **insulin**. - Glucagon generally has an **inhibitory effect** on fatty acid synthesis, as its main goal is to mobilize energy stores, not create them. *Glycolysis* - **Glycolysis** is the breakdown of glucose to produce energy, and it is stimulated when glucose is abundant and energy is needed. - Glucagon primarily acts to *inhibit* glycolysis in the liver, thereby conserving glucose for use by other tissues and promoting its release into the bloodstream.

Q: Carboxypeptidase contains which mineral?

Zinc. ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.

Question 1

What is the primary enzymatic source of ammonia production in urine?

Accepted Answer

Glutaminase

Answer

Urease

Answer

Glutamate dehydrogenase

Answer

Arginase

Question 2

Which of the following statements about vitamin E is false?

Accepted Answer

Water soluble vitamin

Answer

Act as antioxidant

Answer

Chemically tocopherol

Answer

Prevent lipid peroxidation of cell membrane

Question 3

Fructose intolerance is due to deficiency of which enzyme?

Accepted Answer

Aldolase B

Answer

Aldolase A

Answer

Fructokinase

Answer

Triokinase

Question 4

Which of the following statements about gluconeogenesis is correct?

Accepted Answer

Occurs mainly in the liver

Answer

It uses exactly the same enzymes as glycolysis in reverse

Answer

It only occurs during fed state when insulin levels are high

Answer

Fatty acids are the primary substrate for gluconeogenesis

Question 5

Trypsinogen is converted to trypsin by?

Accepted Answer

Removal of specific amino acids from trypsinogen

Answer

Combination of 2 molecules of trypsinogen

Answer

Phosphorylation

Answer

Addition of alkyl group

Question 6

Rate limiting step in pyrimidine synthesis?

Accepted Answer

Carbamoyl phosphate synthase-II

Answer

Aspartate transcarbamoylase (ATCase)

Answer

Dihydroorotate dehydrogenase

Answer

Dihydro-orotase

Question 7

Which type of RNA contains codons for specific amino acids?

Accepted Answer

Messenger RNA (mRNA)

Answer

Transfer RNA (tRNA)

Answer

Small nuclear RNA (snRNA)

Answer

Ribosomal RNA (rRNA)

Question 8

Which of the following statements regarding collagen synthesis is incorrect?

Accepted Answer

Hydroxylation of proline occurs in Golgi apparatus

Answer

Hydroxylation of lysine occurs in ER

Answer

Synthesized in ribosomes as preprocollagen

Answer

Triple helix assembly occurs in ER

Question 9

Glucagon stimulates

Accepted Answer

Gluconeogenesis

Answer

Glycogenesis

Answer

Fatty acid synthesis

Answer

Glycolysis

Question 10

Carboxypeptidase contains which mineral?

Accepted Answer

Zinc

Answer

Copper

Answer

Iron

Answer

None of the options

NEET-PG 2013 — Biochemistry