NEET-PG 2015 - Biochemistry Practice Questions

Q: Which is an inhibitor of ferrochelatase ?

Lead. ***Lead*** - **Lead** is a potent environmental toxin that directly inhibits the enzyme **ferrochelatase**, preventing the insertion of **iron** into **protoporphyrin IX** to form heme. - This inhibition leads to the accumulation of **protoporphyrin IX** and can cause **anemia** due to impaired **heme synthesis**. *Mercury* - While **mercury** is a heavy metal and neurotoxin, its primary mechanism of toxicity does not involve direct inhibition of **ferrochelatase**. - Its effects are more commonly associated with protein denaturation and enzyme inactivation through binding with **sulfhydryl groups**. *Iron* - **Iron** is a substrate for **ferrochelatase**, not an inhibitor. **Ferrochelatase** catalyzes the insertion of **iron** into **protoporphyrin IX** to complete the synthesis of **heme**. - Deficiencies or excesses of **iron** can affect **heme synthesis**, but **iron** itself does not inhibit the enzyme in a toxic manner. *Arsenic* - **Arsenic** is a metalloid that is toxic through various mechanisms, including interference with cellular respiration and DNA repair. - However, **arsenic** is not known to be a direct inhibitor of **ferrochelatase** in the same way **lead** is.

Q: Which method is used to separate a mixture of lipids?

Chromatography. ***Chromatography*** - **Chromatography** (e.g., thin-layer chromatography, gas chromatography, high-performance liquid chromatography) is widely used to separate lipids based on differences in their **polarity**, **molecular weight**, or **solubility** in various solvents. - This method allows for the isolation and identification of different lipid classes and individual lipid species from a complex mixture. *Electrophoresis* - **Electrophoresis** separates molecules based on their **charge** and **size** in an electric field, making it more commonly used for proteins and nucleic acids. - Lipids are generally **uncharged** or have very low charge, which makes them poorly suited for separation by standard electrophoretic methods without modification. *Isoelectric focusing* - **Isoelectric focusing** is a type of electrophoresis that separates molecules based on their **isoelectric point (pI)**, which is the pH at which a molecule has no net charge. - This technique is primarily used for **proteins** and **peptides**, as lipids typically lack ionizable groups necessary for establishing a distinct pI. *PAGE* - **PAGE** (Polyacrylamide Gel Electrophoresis) is a common method used to separate **proteins** and **nucleic acids** based on their size and charge. - Lipids are **hydrophobic** and do not readily migrate through an aqueous polyacrylamide gel matrix, making PAGE unsuitable for their direct separation.

Q: Chymotrypsinogen is activated into chymotrypsin by:

Trypsin. ***Activation of Chymotrypsinogen by Trypsin*** - **Trypsin** is the primary enzyme responsible for the activation of **chymotrypsinogen** into its active form, **chymotrypsin**, by cleaving a specific peptide bond. - This activation is part of a cascade of proteolytic enzyme activations in the **pancreatic juice**, crucial for protein digestion in the small intestine. *Pepsin* - **Pepsin** is a protease active in the **stomach**, requiring an acidic environment for its activity, and is involved in the initial breakdown of proteins. - It does not play a role in the activation of pancreatic zymogens like chymotrypsinogen; its primary function is protein digestion in the gastric lumen. *Renin* - **Renin** is an enzyme primarily involved in the **renin-angiotensin-aldosterone system** (RAAS), which regulates blood pressure and fluid balance. - Its action involves cleaving **angiotensinogen** to form angiotensin I, and it has no role in the activation of digestive enzymes like chymotrypsinogen. *HCl* - **Hydrochloric acid (HCl)** is produced in the stomach and is essential for providing the acidic environment required for **pepsin's activity** and for denaturing proteins. - While HCl is crucial for digestion, it does not directly activate chymotrypsinogen; this activation is an enzymatic process carried out by another protease.

Q: Which gene is responsible for the production of COX type 3?

COX I gene. ***COX I gene*** - COX-3 is an **alternatively spliced variant** of the **COX-1 gene** (specifically, a splice variant of the COX-1 mRNA that retains intron 1). - While it was initially thought to be a distinct gene, research has shown that it arises from the same genetic locus as COX-1. *COX 2 gene* - The COX-2 gene encodes for the **inducible cyclooxygenase enzyme**, which is responsible for prostaglandin synthesis during inflammation. - It is a separate gene from COX-1 and has distinct regulatory mechanisms and physiological roles. *COX 3 gene* - There is currently **no distinct gene in humans** specifically identified as "COX-3". - COX-3 refers to a protein isoform derived from the COX-1 gene, not a separate genetic locus. *None of the above* - This option is incorrect because COX-3 is indeed derived from the **COX-1 gene** through alternative splicing. - The existence of COX-3 as a distinct protein product has been demonstrated, although its precise physiological role in humans is still under investigation.

Q: Creatinine is formed from -

Creatine. ***Creatine*** - **Creatinine** is a waste product formed from the non-enzymatic, irreversible degradation of **creatine** and **creatine phosphate**, primarily in muscles. - **Creatine** itself is synthesized endogenously from three amino acids: **glycine, arginine, and methionine** (as S-adenosylmethionine) through a two-step enzymatic process in the kidney and liver. - The amount of creatinine produced daily is relatively constant and directly proportional to an individual's **muscle mass**, making it a useful marker for renal function. *Lysine* - **Lysine** is an **essential amino acid** and a precursor for various compounds like **carnitine** but is not involved in creatinine or creatine formation. - Deficiency can lead to impaired protein synthesis but does not impact creatinine levels. *Leucine* - **Leucine** is another **essential amino acid** and a **branched-chain amino acid (BCAA)** crucial for muscle protein synthesis and repair. - It does not serve as a direct precursor for creatinine or creatine. *Histidine* - **Histidine** is an **essential amino acid** and a precursor for **histamine** and other important compounds, but not creatinine or creatine. - It plays roles in immune response and gastric acid secretion.

Q: What type of bond is involved in the side chain linkage of proteoglycans?

Covalent. ***Covalent*** - Proteoglycans are formed by **glycosaminoglycan (GAG)** chains that are covalently linked to a protein core. - Specifically, an **O-glycosidic bond** forms between a xylose residue on the GAG chain and a serine residue on the core protein. *Hydrogen bond* - **Hydrogen bonds** are weaker intermolecular forces that stabilize protein secondary structures and interactions between water molecules. - They are not strong enough to form the primary structural linkage between the GAG chains and the core protein in proteoglycans. *Electrostatic bond* - **Electrostatic bonds**, or ionic bonds, involve attraction between oppositely charged ions. While proteoglycans have many charged groups, these bonds are not the primary linkage connecting the GAG chains to the protein core. - They contribute to the overall structure and interactions of proteoglycans with other molecules but do not form the main side chain linkage. *Van-der Waal's force* - **Van der Waals forces** are weak, short-range intermolecular forces that arise from temporary fluctuations in electron distribution. - These forces play a role in tertiary and quaternary protein structure and molecular packing, but they are far too weak to establish the covalent attachments of GAG chains to the proteoglycan core protein.

Q: In the context of energy metabolism, which coenzyme is niacin a precursor to?

NAD. ***NAD*** - Niacin (vitamin B3) is a direct precursor to **nicotinamide adenine dinucleotide (NAD/NAD+)**. - NAD is the crucial coenzyme in **energy metabolism**, primarily involved in **catabolic pathways** such as glycolysis, TCA cycle, and electron transport chain. - Functions as an **electron carrier** in redox reactions, accepting electrons during oxidation of fuel molecules. *Thiamine pyrophosphate (TPP)* - **Thiamine (vitamin B1)** is the precursor to TPP, not niacin. - TPP plays a vital role in **carbohydrate metabolism**, particularly in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes. *NADP* - While niacin is also a precursor to **NADP/NADPH**, this coenzyme is primarily used in **anabolic (biosynthetic) pathways**, not energy metabolism. - NADP functions in reductive biosynthesis (fatty acid synthesis, cholesterol synthesis) and **oxidative stress protection** via the pentose phosphate pathway. - The question specifically asks about **energy metabolism**, making NAD the correct answer as it participates in catabolic, energy-producing reactions. *Flavin adenine dinucleotide (FAD)* - **Riboflavin (vitamin B2)** is the precursor to FAD, not niacin. - FAD is a coenzyme involved in various metabolic reactions, especially in the **TCA cycle** and **electron transport chain**, acting as an electron acceptor.

Q: Fumarate is formed from which amino acid?

Tyrosine. ***Tyrosine*** - **Tyrosine** is a **glucogenic and ketogenic amino acid** that is catabolized to acetoacetate and fumarate. - **Fumarate** then enters the **citric acid cycle (Krebs cycle)**, whereas acetoacetate is a ketone body. *Methionine* - **Methionine** is an **essential amino acid** and a precursor for **S-adenosylmethionine (SAM)**, a methyl donor in many reactions. - Its catabolism produces **succinyl CoA**, not fumarate, through a series of steps via propionyl CoA. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** that is exclusively **glucogenic**. - Its catabolism ultimately leads to the formation of **succinyl CoA**, which can enter the citric acid cycle. *Histidine* - **Histidine** is an **essential amino acid** that is catabolized to **formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which can eventually be deaminated to α-ketoglutarate, a citric acid cycle intermediate, but not directly fumarate.

Q: Which coenzyme is not required in the formation of glutamate?

Thiamine pyrophosphate. ***Thiamine pyrophosphate*** - **Thiamine pyrophosphate (TPP)** is a coenzyme derived from **vitamin B1** that is essential for reactions involving decarboxylation, such as those catalyzed by pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. - The formation of glutamate primarily involves transamination or reductive amination, which do not require TPP. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, derived from **vitamin B6**, is a crucial coenzyme for **transamination reactions**, which are a major pathway for glutamate synthesis (e.g., from alpha-ketoglutarate). - It also plays a role in decarboxylation and deamination reactions of amino acids. *Niacin* - **Niacin (vitamin B3)** is a precursor for **NAD+** and **NADP+**, which are essential coenzymes in many metabolic pathways. - **NADPH**, derived from NADP+, is required as a reductant in the **reductive amination** of **alpha-ketoglutarate** to form glutamate, catalyzed by glutamate dehydrogenase. *None of the above* - This option is incorrect because **thiamine pyrophosphate** is indeed not required for the formation of glutamate. - The other two coenzymes, **pyridoxal phosphate** and **niacin (as NAD(P)H)**, are involved in glutamate synthesis.

Question 1

Which is an inhibitor of ferrochelatase ?

Accepted Answer

Lead

Answer

Mercury

Answer

Iron

Answer

Arsenic

Question 2

Which method is used to separate a mixture of lipids?

Accepted Answer

Chromatography

Answer

Electrophoresis

Answer

Isoelectric focusing

Answer

PAGE

Question 3

Chymotrypsinogen is activated into chymotrypsin by:

Accepted Answer

Trypsin

Answer

Pepsin

Answer

Renin

Answer

HCl

Question 4

Which gene is responsible for the production of COX type 3?

Accepted Answer

COX I gene

Answer

COX 3 gene

Answer

COX 2 gene

Answer

None of the above

Question 5

What is a key similarity between the processes of replication and transcription?

Accepted Answer

Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.

Answer

Use RNA primers for initiation.

Answer

Use ribonucleotides as precursors.

Answer

Are semi-conservative events.

Question 6

Creatinine is formed from -

Accepted Answer

Creatine

Answer

Lysine

Answer

Leucine

Answer

Histidine

Question 7

What type of bond is involved in the side chain linkage of proteoglycans?

Accepted Answer

Covalent

Answer

Hydrogen bond

Answer

Electrostatic bond

Answer

Van-der Waal's force

Question 8

In the context of energy metabolism, which coenzyme is niacin a precursor to?

Accepted Answer

NAD

Answer

Thiamine pyrophosphate (TPP)

Answer

NADP

Answer

Flavin adenine dinucleotide (FAD)

Question 9

Fumarate is formed from which amino acid?

Accepted Answer

Tyrosine

Answer

Methionine

Answer

Valine

Answer

Histidine

Question 10

Which coenzyme is not required in the formation of glutamate?

Accepted Answer

Thiamine pyrophosphate

Answer

None of the above

Answer

Pyridoxal phosphate

Answer

Niacin

NEET-PG 2015 — Biochemistry