NEET-PG 2015

1414 Questions — Page 32 of 142

Anatomy

1 questions

Q311

Which type of glial cell is derived from mesodermal origin?

Biochemistry

9 questions

Q311

Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

Q312

Abnormal proteins which are bound to ubiquitin are degraded in -

Q313

Pyridoxine is required in -

Q314

Prolyl hydroxylase requires which cofactor?

Q315

Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

Q316

Transport of lipids from the intestine to other tissues is by -

Q317

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

Q318

Which method is used to separate a mixture of lipids?

Q319

Which of the following is not a metabolic product of the urea cycle?

NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs

Question 311: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

A. Vitamin K
B. Vitamin D (Correct Answer)
C. Vitamin A
D. Biotin

Explanation: ***Vitamin D*** - The skin synthesizes vitamin D (specifically **cholecalciferol**) upon exposure to **ultraviolet B (UVB) radiation** from sunlight. - This endogenous production can be sufficient to meet physiological needs under adequate sun exposure, making it conditionally non-essential in the diet. *Vitamin K* - While **intestinal bacteria synthesize some vitamin K (K2)**, it is generally not in sufficient quantities to meet all physiological needs, especially for blood clotting. - Dietary intake of **vitamin K1 (phylloquinone)** from leafy green vegetables is critical. *Vitamin A* - **Vitamin A (retinol)** is obtained primarily from the diet, either directly from animal sources or from carotenoid precursors (like **beta-carotene**) in plants. - The body cannot synthesize vitamin A de novo; it relies on dietary intake and conversion from precursors. *Biotin* - Although the **gut microbiota can synthesize biotin**, the amount produced is generally considered insufficient to meet the body's requirements. - Therefore, biotin is primarily obtained through dietary intake, functioning as a coenzyme in various metabolic reactions.

Question 312: Abnormal proteins which are bound to ubiquitin are degraded in -

A. Proteasomes (Correct Answer)
B. Golgi apparatus
C. Smooth ER
D. Lysosomes

Explanation: ***Proteasomes*** - **Proteasomes** are multi-subunit protein complexes responsible for degrading **ubiquitin-tagged proteins**. - This degradation is a tightly regulated process essential for cell cycle control, gene expression, and immune response. *Golgi apparatus* - The **Golgi apparatus** primarily functions in modifying, sorting, and packaging proteins and lipids synthesized in the Endoplasmic Reticulum. - It does not directly participate in the degradation of **ubiquitin-bound proteins**. *Smooth ER* - The **smooth endoplasmic reticulum (SER)** is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. - It lacks ribosomes and is not directly implicated in the degradation of misfolded proteins tagged with ubiquitin. *Lysosomes* - **Lysosomes** are organelles containing various hydrolytic enzymes that break down waste materials and cellular debris, as well as foreign invaders like bacteria. - While they degrade proteins, they primarily target **extracellular proteins** taken up by endocytosis or cellular components via **autophagy**, not specifically ubiquitin-bound proteins.

Question 313: Pyridoxine is required in -

A. Glycolysis
B. TCA cycle
C. Glycogenesis
D. Transamination (Correct Answer)

Explanation: ***Transamination*** - **Pyridoxal phosphate (PLP)**, the active form of pyridoxine (vitamin B6), is an essential **coenzyme for aminotransferases (transaminases)** - Transamination reactions involve the transfer of an **amino group** from an amino acid to a keto acid, which is crucial for amino acid metabolism - This is the classic biochemical function of vitamin B6 and a frequently tested concept *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate - Key cofactors for glycolysis include **NAD+ and ATP**, not vitamin B6 - Does not require pyridoxine as a coenzyme *TCA cycle* - The **TCA cycle (Krebs cycle)** is a central metabolic pathway for energy production - Uses enzymes that require cofactors such as **NAD+, FAD, and Coenzyme A** (derived from pantothenic acid) - Pyridoxine is not directly involved as a coenzyme in TCA cycle reactions *Glycogenesis* - Glycogenesis is the process of synthesizing **glycogen from glucose** - Primarily involves enzymes like **glycogen synthase** and **branching enzyme** - Requires **UTP and glucose-1-phosphate**, not pyridoxine

Question 314: Prolyl hydroxylase requires which cofactor?

A. Vitamin C (Correct Answer)
B. Iron (Fe²⁺)
C. Molybdenum
D. Vitamin K1

Explanation: ***Vitamin C*** - **Prolyl hydroxylase** is an enzyme critical for the hydroxylation of proline residues during **collagen synthesis**. - **Vitamin C** (ascorbic acid) acts as an essential **cofactor**, reducing the ferric iron of the enzyme back to its ferrous state after each catalytic cycle, enabling continued activity. - The enzyme requires both **iron (Fe²⁺)** as a metal cofactor and **vitamin C** to maintain the iron in its reduced state. *Iron (Fe²⁺)* - While **iron** is indeed required by prolyl hydroxylase as a **metal cofactor**, the question asks for the cofactor, which specifically refers to **vitamin C**. - Iron functions as part of the enzyme's active site, but vitamin C is the reducing agent that keeps iron functional. - Vitamin C deficiency (scurvy) leads to defective collagen synthesis despite adequate iron. *Molybdenum* - **Molybdenum** is a cofactor for several human enzymes, including **xanthine oxidase** and **sulfite oxidase**. - However, it plays no direct role in the activity of prolyl hydroxylase. *Vitamin K1* - **Vitamin K1** is a crucial cofactor for **gamma-glutamyl carboxylase**, an enzyme involved in the carboxylation of glutamic acid residues in clotting factors. - It is not involved in the hydroxylation of proline by prolyl hydroxylase.

Question 315: Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

A. Vitamin K
B. Vitamin D (Correct Answer)
C. Vitamin A
D. Vitamin E

Explanation: ***Correct Answer: Vitamin D*** - **Vitamin D** (specifically its active form, **calcitriol** or **1,25-dihydroxyvitamin D₃**) is the **most classically recognized** fat-soluble vitamin that functions as a **steroid hormone** - It binds to the **vitamin D receptor (VDR)**, which is a member of the **nuclear receptor superfamily** - This VDR-calcitriol complex acts as a transcription factor, regulating gene expression involved in **calcium and phosphate homeostasis**, bone metabolism, skeletal development, and immune function - The mechanism is analogous to classic steroid hormones like cortisol, estrogen, and testosterone *Incorrect: Vitamin A* - **Vitamin A** (as **retinoic acid**) also interacts with nuclear receptors (**retinoic acid receptors - RARs** and **retinoid X receptors - RXRs**) to regulate gene transcription - However, Vitamin A is **most classically associated** with vision (rhodopsin in retinal photoreceptors), epithelial cell differentiation, embryonic development, and immune function - While it does have nuclear receptor-mediated actions, **Vitamin D is more prominently described** as having steroid hormone-like activity in standard medical education *Incorrect: Vitamin K* - **Vitamin K** functions primarily as a **cofactor for γ-glutamyl carboxylase**, an enzyme that catalyzes post-translational modification of glutamate residues to γ-carboxyglutamate (Gla) - Essential for the synthesis of **clotting factors** (II, VII, IX, X, protein C, protein S) and bone proteins (osteocalcin) - Does **not** act through nuclear receptors or function as a steroid hormone *Incorrect: Vitamin E* - **Vitamin E** (α-tocopherol) is a powerful **lipid-soluble antioxidant** that protects cell membranes from oxidative damage by scavenging free radicals - Functions primarily through its **antioxidant properties**, not through nuclear receptor binding - Does **not** have steroid hormone-like actions

Question 316: Transport of lipids from the intestine to other tissues is by -

A. Chylomicrons (Correct Answer)
B. LDL
C. HDL
D. VLDL

Explanation: ***Chylomicrons*** - **Chylomicrons** are the **largest lipoprotein particles** that transport **dietary (exogenous) lipids** from the **intestine** to peripheral tissues - They are synthesized in **intestinal enterocytes** after fat absorption and enter the bloodstream via the **lymphatic system (thoracic duct)** - They carry **triglycerides (85-95%), cholesterol, phospholipids, and fat-soluble vitamins** (A, D, E, K) - **Apolipoprotein B-48** is the characteristic structural protein of chylomicrons - After delivering triglycerides to tissues (via lipoprotein lipase), chylomicron remnants are taken up by the **liver** *LDL (Low-Density Lipoprotein)* - LDL transports **cholesterol from the liver to peripheral tissues** (not from intestine) - It carries **endogenous cholesterol**, not dietary lipids from the intestine - Often called "**bad cholesterol**" due to its role in atherosclerosis - Contains **Apolipoprotein B-100** *HDL (High-Density Lipoprotein)* - HDL performs **reverse cholesterol transport** - moving excess cholesterol from peripheral tissues **back to the liver** - It does **not transport lipids from the intestine** to tissues - Called "**good cholesterol**" for its protective cardiovascular role - Contains **Apolipoprotein A-I and A-II** *VLDL (Very-Low-Density Lipoprotein)* - VLDL is synthesized in the **liver** (not intestine) and transports **endogenous triglycerides** to peripheral tissues - It carries lipids **from the liver**, not from the intestine - VLDL is converted to IDL and then LDL after losing triglycerides - Contains **Apolipoprotein B-100**

Question 317: In the context of energy metabolism, which coenzyme is niacin a precursor to?

A. Thiamine pyrophosphate (TPP)
B. NADP
C. NAD (Correct Answer)
D. Flavin adenine dinucleotide (FAD)

Explanation: ***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.

Question 318: Which method is used to separate a mixture of lipids?

A. Electrophoresis
B. Chromatography (Correct Answer)
C. Isoelectric focusing
D. PAGE

Explanation: ***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.

Question 319: Which of the following is not a metabolic product of the urea cycle?

A. Citrulline
B. Arginine
C. Alanine (Correct Answer)
D. Ornithine

Explanation: ***Alanine*** - **Alanine** is an amino acid primarily involved in the **glucose-alanine cycle** for glucose production and ammonia transport, not as a direct metabolic product within the urea cycle. - While it plays a role in nitrogen metabolism, it is not synthesized or directly consumed as an intermediate in the reactions that convert ammonia to urea. *Citrulline* - **Citrulline** is a key intermediate formed during the second step of the urea cycle when **ornithine carbamoyltransferase** combines carbamoyl phosphate with ornithine. - It is then transported out of the mitochondrion into the cytosol to continue the cycle. *Ornithine* - **Ornithine** is an amino acid that acts as a **catalytic intermediate** in the urea cycle, being regenerated at the end of the cycle to combine with carbamoyl phosphate. - It does not directly contribute a nitrogen atom to urea but is essential for the cycle's continuation. *Arginine* - **Arginine** is an amino acid that is a direct precursor to urea in the penultimate step of the urea cycle, where **arginase** cleaves it into urea and ornithine. - It provides one of the nitrogen atoms and the carbon atom for the formation of urea.