NEET-PG 2013 — Biochemistry
133 Previous Year Questions with Answers & Explanations
What is the major apolipoprotein of chylomicrons?
Which vitamin is required for transfer of 1-carbon unit?
During angiogenesis, what factors are responsible for the recruitment of pericytes and periendothelial cells?
Which of the following is NOT present in DNA?
Number of ATP molecules formed per turn of the citric acid cycle is
Which of the following amino acids is found in keratin?
Which form of tetrahydrofolate (THF) serves as the central intermediate in the one-carbon pool and is most directly involved in thymidine synthesis?
What is the process of Hofmann elimination in organic chemistry?
What is the primary reason for the detergent action of bile salts?
Glucagon stimulates
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 1: What is the major apolipoprotein of chylomicrons?
- A. B-48 (Correct Answer)
- B. B-100
- C. Apo-C
- D. Apo-E
Explanation: ***B-48*** - **Apolipoprotein B-48** is exclusively produced in the intestine and is the **major structural apolipoprotein** found only on **chylomicrons**. - It is critical for the **assembly and secretion of chylomicrons** from intestinal cells into the lymphatic system. - ApoB-48 represents the N-terminal 48% of ApoB-100 and lacks the LDL receptor-binding domain. *B-100* - **Apolipoprotein B-100** is synthesized in the liver and is the primary structural apolipoprotein of **VLDL, IDL, and LDL**. - It acts as the **ligand for the LDL receptor**, facilitating the uptake of cholesterol into cells. *Apo-C* - **Apolipoprotein C (Apo-C)** proteins (e.g., ApoC-II, ApoC-III) are exchangeable apolipoproteins found on several lipoproteins, including chylomicrons, VLDL, and HDL. - **ApoC-II activates lipoprotein lipase**, which hydrolyzes triglycerides, but these are **not structural proteins** and are present in smaller quantities. *Apo-E* - **Apolipoprotein E (Apo-E)** is acquired by chylomicrons in circulation and is important for **chylomicron remnant clearance** by the liver. - While present on chylomicrons, it is **not the major structural apolipoprotein** – that role belongs to ApoB-48.
Question 2: Which vitamin is required for transfer of 1-carbon unit?
- A. Niacin
- B. Vitamin B12
- C. Vitamin A
- D. Folic acid (Correct Answer)
Explanation: ***Folic acid*** - **Folic acid** (vitamin B9) is essential for the transfer of **one-carbon units**, particularly as **tetrahydrofolate (THF)**. - These one-carbon units are critical in metabolic processes such as **DNA synthesis**, **amino acid metabolism**, and **neurotransmitter synthesis**. *Vitamin A* - **Vitamin A** (retinol) is primarily involved in **vision**, **immune function**, and **cell differentiation**. - It does not play a direct role in the transfer of one-carbon units. *Vitamin B12* - **Vitamin B12** (cobalamin) is involved in two main reactions: the conversion of **methylmalonyl-CoA to succinyl-CoA** and the transfer of a **methyl group from N5-methyl THF to homocysteine** to form methionine. - While it works with folate, it does not directly transfer one-carbon units in the same way as folic acid. *Niacin* - **Niacin** (vitamin B3) is a precursor to **NAD+ and NADP+**, which are crucial coenzymes in **redox reactions** and energy metabolism. - It is not involved in the transfer of one-carbon units.
Question 3: During angiogenesis, what factors are responsible for the recruitment of pericytes and periendothelial cells?
- A. VEGF & PDGF
- B. Angiopoietins, TGF & PDGF (Correct Answer)
- C. VEGF, IL-2, IL-6
- D. TGF, VEGF & PDGF
Explanation: ***Angiopoietins, TGF & PDGF*** - **Angiopoietins** are crucial for the stabilization of blood vessels and recruitment of **pericytes**, enhancing vessel maturation [1]. - **TGF (Transforming Growth Factor)** and **PDGF (Platelet-Derived Growth Factor)** also play significant roles in the recruitment and proliferation of **pericytes** and periendothelial cells during angiogenesis [1]. *VEGF & PDGF* - While **VEGF (Vascular Endothelial Growth Factor)** is important for endothelial cell migration and proliferation, it does not directly recruit **pericytes** alone. - This combination lacks **angiopoietins**, which are key for the stabilization of newly formed blood vessels [1]. *VEGF, IL-2, IL-6* - **IL-2** and **IL-6** are primarily associated with immune responses and do not directly contribute to pericyte recruitment during angiogenesis. - **VEGF** alone supports endothelial cells but does not effectively recruit **pericytes** without the cooperation of other factors. *TGF, VEGF & PDGF* - Although both **TGF** and **PDGF** are involved in pericyte recruitment [1], the absence of **angiopoietins** limits the effectiveness of this combination for the recruitment process. - **VEGF** alone does not facilitate direct recruitment of **pericytes**, as it mainly focuses on endothelial cells. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 115-116.
Question 4: Which of the following is NOT present in DNA?
- A. Uracil (Correct Answer)
- B. Thymine
- C. Adenine
- D. Cytosine
Explanation: ***Uracil*** - **Uracil** is a pyrimidine nitrogenous base that replaces **thymine** in RNA, meaning it is not found in the structure of DNA. - In RNA, uracil pairs with **adenine** through two hydrogen bonds. *Thymine* - **Thymine** is a pyrimidine found in DNA, where it pairs with **adenine**. - It is replaced by **uracil** in RNA. *Cytosine* - **Cytosine** is a pyrimidine that is a fundamental component of both **DNA** and **RNA**. - In DNA, cytosine pairs with **guanine**. *Adenine* - **Adenine** is a purine that is a fundamental component of both **DNA** and **RNA**. - In DNA, adenine pairs with **thymine**, and in RNA, it pairs with **uracil**.
Question 5: Number of ATP molecules formed per turn of the citric acid cycle is
- A. 5
- B. 7
- C. 15
- D. 10 (Correct Answer)
Explanation: ***10*** - Each turn of the citric acid cycle directly produces **1 GTP** molecule (which is equivalent to 1 ATP). - Additionally, it generates **3 NADH** and **1 FADH2**, which upon oxidative phosphorylation yield approximately **2.5 ATP per NADH** and **1.5 ATP per FADH2**. - Total calculation: (3 × 2.5) + (1 × 1.5) + 1 (from GTP) = 7.5 + 1.5 + 1 = **10 ATP equivalents** per turn. *5* - This number is **too low** and does not account for the significant energy yield from the **NADH** and **FADH2** molecules produced during the cycle. - It likely only considers a partial or incorrect calculation of the ATP equivalents generated. *7* - This value is **insufficient** as it underestimates the total ATP generated when considering the contributions from both **direct substrate-level phosphorylation (GTP)** and the **electron transport chain**. - It may arise from an incomplete understanding of the ATP yield from NADH and FADH2. *15* - This number is **too high** for the ATP equivalents produced per turn of the citric acid cycle. - Such a value would imply a higher energy yield from the electron carriers or direct ATP production than is biologically accurate.
Question 6: Which of the following amino acids is found in keratin?
- A. Lysine
- B. Histidine
- C. Arginine
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - **Keratin** is a fibrous structural protein that forms the main component of hair, skin, and nails - All three amino acids listed - **Histidine, Lysine, and Arginine** - are indeed found in keratin's composition - **Lysine** and **Arginine** are basic amino acids that contribute to keratin's structural stability and are involved in ionic interactions - **Histidine** is also present and plays a role in the protein's functional and structural aspects - While keratin is particularly rich in **cysteine** (which forms disulfide bonds responsible for its strength), it also contains significant amounts of these other amino acids - The complete amino acid composition of keratin includes all of these and many other amino acids working together to provide its characteristic properties *Why individual options alone are incomplete* - Selecting only **Histidine**, **Lysine**, or **Arginine** individually would be incorrect because it would imply the other amino acids are NOT found in keratin - Since the question asks which amino acid "is found" in keratin and all three ARE present, the correct answer must acknowledge all of them
Question 7: Which form of tetrahydrofolate (THF) serves as the central intermediate in the one-carbon pool and is most directly involved in thymidine synthesis?
- A. None of the options
- B. Methylene THF (Correct Answer)
- C. Methyl THF
- D. Formyl THF
Explanation: ***Methylene THF*** - **N5,N10-Methylene tetrahydrofolate** is the direct **one-carbon donor** for the methylation of deoxyuridylate (dUMP) to deoxythymidylate (dTMP) by the enzyme **thymidylate synthase**, which is crucial for **DNA synthesis**. - During this reaction, **methylene THF** is oxidized to **dihydrofolate (DHF)**, requiring reduction by **dihydrofolate reductase** (DHFR) to regenerate THF. *Methyl THF* - **N5-Methyl tetrahydrofolate** is primarily involved in the **methionine synthase** reaction to convert **homocysteine to methionine**, requiring **vitamin B12**. - It is not directly involved in thymidine synthesis but plays a role in remethylation pathways and the **folate trap** when B12 is deficient. *None of the options* - This option is incorrect because **methylene THF** is indeed a direct participant in thymidine synthesis, serving as the critical one-carbon donor. - The roles of various THF forms in one-carbon metabolism are well-defined, and one of them is specifically responsible for this function. *Formyl THF* - **N10-Formyl tetrahydrofolate** is essential for the synthesis of **purine nucleotides** by donating formyl groups at two steps in the purine ring formation. - While important for nucleic acid synthesis, it does not directly contribute the methyl group needed for thymidine synthesis from dUMP.
Question 8: What is the process of Hofmann elimination in organic chemistry?
- A. E1 elimination reaction favoring tertiary substrates
- B. E2 elimination reaction producing the least substituted alkene (Correct Answer)
- C. SN1 substitution reaction with carbocation intermediate
- D. SN2 substitution reaction with inversion of configuration
Explanation: ***E2 elimination reaction producing the least substituted alkene*** - **Hofmann elimination** is a type of **E2 elimination** reaction where a **quaternary ammonium salt** is heated in the presence of a strong base. - Unlike most E2 reactions which follow **Zaitsev's rule** (producing the most substituted alkene), Hofmann elimination follows the **Hofmann rule**, leading to the formation of the **least substituted (least stable) alkene**. *SN1 substitution reaction with carbocation intermediate* - **SN1 reactions** involve the formation of a **carbocation intermediate** and are typically substitution reactions, not elimination. - Hofmann elimination is an elimination reaction and does not proceed through a carbocation intermediate. *E1 elimination reaction favoring tertiary substrates* - **E1 reactions** are a two-step process involving a **carbocation intermediate** and generally favor **tertiary substrates** and produce the **Zaitsev product**. - Hofmann elimination is a concerted, one-step E2 mechanism and does not involve carbocations. *SN2 substitution reaction with inversion of configuration* - **SN2 reactions** are **bimolecular nucleophilic substitution** reactions that occur in a single step with **inversion of configuration** at the carbon center. - Hofmann elimination is an elimination process resulting in an alkene, not a substitution product, and does not involve inversion of configuration at a stereocenter.
Question 9: What is the primary reason for the detergent action of bile salts?
- A. Hydrophobic properties
- B. Acts as a zwitterion
- C. Amphipathic nature (Correct Answer)
- D. None of the options
Explanation: ***Amphipathic nature*** - Bile salts are **amphipathic molecules**, meaning they have both **hydrophilic (water-loving)** and **hydrophobic (water-fearing)** regions. - This dual nature allows them to emulsify fats by surrounding lipid droplets with their hydrophobic ends dissolving in the fat and their hydrophilic ends facing the aqueous environment, stabilizing the emulsion. *Hydrophobic properties* - While bile salts do possess **hydrophobic regions**, these alone are not sufficient for detergent action. - The ability to interact with both oil and water phases simultaneously is crucial for their role in **emulsification**. *Acts as a zwitterion* - A zwitterion is a molecule with both a **positive and negative charge**, but an overall neutral charge. - This property is not the primary mechanism behind the **detergent action** of bile salts, which relies more on their ability to solubilize fats. *None of the options* - The **amphipathic nature** is indeed the primary reason for the detergent action; therefore, this option is incorrect.
Question 10: Glucagon stimulates
- A. Gluconeogenesis (Correct Answer)
- B. Glycogenesis
- C. Fatty acid synthesis
- D. Glycolysis
Explanation: ***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.