Biochemistry
10 questionsWhich of the following amino acids is not involved in the production of creatine?
Catecholamines are synthesized from?
Which amino acids accumulate in maple syrup urine disease?
What is the primary enzymatic source of ammonia production in urine?
What does salvage purine synthesis refer to?
Which protein hormone is often referred to as the 'guardian angel against obesity' due to its role in regulating metabolism?
Which of the following molecular interactions are found in the structure of DNA?
Hereditary orotic aciduria Type-I is due to deficiency of?
Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?
In eukaryotic cells, where does the majority of functional RNA activity occur?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 281: Which of the following amino acids is not involved in the production of creatine?
- A. Glycine
- B. Methionine
- C. Alanine (Correct Answer)
- D. Arginine
Explanation: ***Alanine*** - **Alanine** is not directly involved as a precursor for **creatine synthesis**. It can be converted to pyruvate and enter the gluconeogenic pathway. - The primary amino acids involved in **creatine synthesis** are arginine, glycine, and methionine. *Glycine* - **Glycine** is a direct precursor for creatine, reacting with arginine in the first step of its synthesis to form **guanidinoacetate**. - This reaction is catalyzed by **arginine:glycine amidinotransferase (AGAT)**. *Methionine* - **Methionine**, in the form of **S-adenosylmethionine (SAM)**, acts as the methyl donor in the second step of creatine synthesis. - It methylates guanidinoacetate to form **creatine**, a reaction catalyzed by **guanidinoacetate methyltransferase (GAMT)**. *Arginine* - **Arginine** donates its guanidino group to glycine, forming **guanidinoacetate**, the initial intermediate in creatine synthesis. - This is the first committed step in the **creatine biosynthesis pathway**.
Question 282: Catecholamines are synthesized from?
- A. Tyrosine (Correct Answer)
- B. Histidine
- C. Methionine
- D. Tryptophan
Explanation: ***Tyrosine*** - **Tyrosine** is the direct precursor amino acid for the synthesis of all **catecholamines**, including **dopamine**, **norepinephrine**, and **epinephrine**. - The synthesis pathway begins with the conversion of tyrosine to **L-DOPA** by tyrosine hydroxylase, followed by subsequent enzymatic steps. *Methionine* - **Methionine** is an essential amino acid primarily involved in **protein synthesis** and as a precursor for S-adenosylmethionine (SAM), a key methyl donor in various metabolic reactions. - It is not a direct precursor for the synthesis of **catecholamines**. *Histidine* - **Histidine** is the precursor for the synthesis of **histamine**, a neurotransmitter and inflammatory mediator. - It is not involved in the biosynthesis pathway of **catecholamines**. *Tryptophan* - **Tryptophan** is the precursor for the synthesis of **serotonin** and **melatonin**, important neurotransmitters and hormones. - It does not play a role in the synthesis of **catecholamines**.
Question 283: Which amino acids accumulate in maple syrup urine disease?
- A. Valine
- B. Leucine
- C. Isoleucine
- D. All branched-chain amino acids (Correct Answer)
Explanation: ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.
Question 284: What is the primary enzymatic source of ammonia production in urine?
- A. Glutaminase (Correct Answer)
- B. Urease
- C. Glutamate dehydrogenase
- D. Arginase
Explanation: ***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.
Question 285: What does salvage purine synthesis refer to?
- A. Synthesis of purine nucleotides from purine bases (Correct Answer)
- B. Synthesis of purine nucleotides from ribose-5-phosphate
- C. Synthesis of purine nucleotides from simple precursors (de novo synthesis)
- D. Synthesis of purine nucleotides from degraded RNA
Explanation: ***Synthesis of purine nucleotides from purine bases*** - **Salvage pathways** recycle pre-existing purine or pyrimidine bases (from nucleic acid degradation) by re-attaching them to a **ribose phosphate** to form a new nucleotide. - This process is energy-efficient as it bypasses several steps of the de novo synthesis pathway, utilizing enzymes like **adenine phosphoribosyltransferase (APRT)** and **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. *Synthesis of purine nucleotides from ribose-5-phosphate.* - While **ribose-5-phosphate** is a precursor, the complete synthesis from this molecule is part of the **de novo pathway**, which starts with PRPP (phosphoribosyl pyrophosphate) formation from ribose-5-phosphate. - This option does not specify the direct reuse of a pre-formed purine base, which is the hallmark of salvage. *Synthesis of purine nucleotides from simple precursors (de novo synthesis).* - **De novo synthesis** is the creation of nucleotides from scratch using simple metabolic precursors like amino acids (glycine, aspartate, glutamine), CO2, and THF derivatives. - This contrasts with salvage pathways, which recycle existing bases. *Synthesis of purine nucleotides from degraded RNA.* - Degraded RNA breaks down into **nucleotides**, which can then be further broken down into **purine bases** and ribose phosphates. - The direct synthesis of purine nucleotides from *degraded RNA* involves recovering the individual bases or nucleosides, then converting them to nucleotides via salvage, not directly using the entire degraded RNA.
Question 286: Which protein hormone is often referred to as the 'guardian angel against obesity' due to its role in regulating metabolism?
- A. Adiponectin (Correct Answer)
- B. Fibronectin
- C. High-Density Lipoprotein (HDL)
- D. Insulin
Explanation: ***Adiponectin*** - **Adiponectin** is a hormone secreted by **adipose tissue** that plays a crucial role in regulating glucose and fatty acid metabolism, increasing **insulin sensitivity**, and decreasing inflammation. - Its levels are inversely correlated with body fat percentage; individuals with obesity tend to have lower adiponectin levels, leading to its nickname as the 'guardian angel against obesity'. *Fibronectin* - **Fibronectin** is a glycoprotein involved in cell adhesion, growth, migration, and differentiation, and is a key component of the **extracellular matrix**. - It does not primarily function in metabolic regulation or body weight control, unlike adiponectin. *High-Density Lipoprotein (HDL)* - **HDL** is a type of lipoprotein that transports cholesterol from peripheral tissues back to the liver, a process known as **reverse cholesterol transport**. - While beneficial for cardiovascular health, HDL is a lipid-carrying particle, not a protein hormone, and its primary role is not in metabolic regulation or direct obesity prevention. *Insulin* - **Insulin** is a peptide hormone produced by the pancreas that regulates carbohydrate and fat metabolism, primarily by facilitating glucose uptake from the blood into cells. - While essential for metabolism, high levels of insulin in the context of insulin resistance can contribute to obesity, rather than act against it.
Question 287: Which of the following molecular interactions are found in the structure of DNA?
- A. Hydrogen bond
- B. Glycosidic bond
- C. Covalent interactions
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - All three types of molecular interactions listed are present in DNA structure, making this the correct answer. - **Hydrogen bonds** hold together the two strands of the DNA double helix, forming between complementary base pairs (A-T with 2 hydrogen bonds, G-C with 3 hydrogen bonds). - **Glycosidic bonds** (N-glycosidic bonds) link the nitrogenous bases to the C1' carbon of the deoxyribose sugar in each nucleotide. - **Covalent interactions** (phosphodiester bonds) form the strong, stable sugar-phosphate backbone by linking the 3' hydroxyl group of one sugar to the 5' phosphate group of the next. *Hydrogen bond* - This is a **true statement** - hydrogen bonds are essential structural components of DNA. - However, this option alone is **incomplete** as DNA structure also contains glycosidic bonds and covalent phosphodiester bonds. - If only hydrogen bonds were present, there would be no nucleotides or backbone structure. *Glycosidic bond* - This is a **true statement** - glycosidic bonds are present in every nucleotide of DNA. - However, this option alone is **incomplete** as DNA also requires hydrogen bonds for base pairing and phosphodiester bonds for the backbone. - Without other bonds, individual nucleotides could not form a functional double helix. *Covalent interactions* - This is a **true statement** - covalent phosphodiester bonds form the DNA backbone within each strand. - However, this option alone is **incomplete** as it doesn't account for glycosidic bonds (nucleotide formation) or hydrogen bonds (strand pairing). - While the strongest bonds in DNA, they alone cannot create the complete double helix structure.
Question 288: Hereditary orotic aciduria Type-I is due to deficiency of?
- A. Orotate phosphoribosyl transferase
- B. UMP synthase (Correct Answer)
- C. Orotic acid decarboxylase
- D. All of the options
Explanation: ***UMP synthase*** - Hereditary orotic aciduria Type-I is caused by a deficiency of the **bifunctional enzyme UMP synthase** (also called UMP synthase complex). - UMP synthase catalyzes two sequential reactions in the *de novo* pyrimidine synthesis pathway: 1. **OPRT activity**: Converts orotate → orotidine 5'-monophosphate (OMP) 2. **ODC activity**: Converts OMP → uridine 5'-monophosphate (UMP) - This is the **most precise and complete answer** as it identifies the actual enzyme complex that is deficient. - **Clinical features**: Megaloblastic anemia, growth retardation, immunodeficiency; responds to oral uridine supplementation. *Orotate phosphoribosyl transferase* - This represents only **one of the two catalytic activities** of the UMP synthase enzyme (the first step). - While this activity is indeed deficient in Type-I orotic aciduria, naming only this activity is **incomplete** because the enzyme has two functions. - This would be a **partial answer** rather than the complete enzyme name. *Orotic acid decarboxylase* - This represents only **the second catalytic activity** of the UMP synthase enzyme (converts OMP to UMP). - Like OPRT, this activity is also deficient, but naming only this component is **incomplete**. - **Type II orotic aciduria** (extremely rare) involves isolated ODC deficiency without OPRT deficiency. *All of the options* - While technically both OPRT and ODC activities are affected in Type-I orotic aciduria, the **standard nomenclature** refers to the deficient enzyme as **"UMP synthase"** - the name of the complete bifunctional enzyme. - In medical terminology and examination context, we identify enzyme deficiencies by the **name of the enzyme complex**, not by listing all its individual catalytic activities. - Therefore, **"UMP synthase"** is the single most accurate and complete answer.
Question 289: Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?
- A. DNA polymerase delta
- B. DNA polymerase alpha
- C. DNA polymerase gamma (Correct Answer)
- D. DNA polymerase beta
Explanation: ***DNA polymerase gamma*** - **DNA polymerase gamma** is the sole DNA polymerase responsible for replicating and repairing the mitochondrial DNA in eukaryotic cells. - It consists of a large catalytic subunit and two smaller accessory subunits that provide **proofreading** and processivity functions. *DNA polymerase alpha* - **DNA polymerase alpha** is primarily involved in initiating DNA replication on both the leading and lagging strands of nuclear DNA. - It forms a complex with **primase** to synthesize short RNA primers followed by a short stretch of DNA. *DNA polymerase delta* - **DNA polymerase delta** is a key enzyme in nuclear DNA replication, primarily responsible for the **elongation of the lagging strand**. - It also plays a significant role in various DNA repair pathways, including **nucleotide excision repair**. *DNA polymerase beta* - **DNA polymerase beta** is mainly involved in **DNA repair processes**, specifically **base excision repair (BER)** in the nucleus. - It has a low processivity and lacks **proofreading activity**, making it unsuitable for bulk DNA replication.
Question 290: In eukaryotic cells, where does the majority of functional RNA activity occur?
- A. Nucleus
- B. Ribosome
- C. Cytoplasm (Correct Answer)
- D. None of the options
Explanation: ***Cytoplasm*** - The **cytoplasm** is the cellular compartment where the **majority of functional RNA activity** occurs, including **translation** (protein synthesis) involving mRNA, tRNA, and rRNA. - **Ribosomes** (the sites of translation) are located in the cytoplasm, either free-floating or bound to the endoplasmic reticulum. - Many types of **regulatory RNAs** such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) exert their functions in the cytoplasm by targeting mRNAs for degradation or translational repression. - **mRNA degradation** and **RNA interference pathways** primarily operate in the cytoplasm. - The question asks for the broader location rather than the specific molecular machinery, making cytoplasm the most comprehensive answer. *Nucleus* - While RNA is **transcribed** from DNA and **processed** (capping, polyadenylation, splicing) in the nucleus, these are preparatory steps. - The nucleus is primarily the site of **RNA synthesis**, not where most RNA performs its functional roles. - Only a small fraction of functional RNA activity (like rRNA processing in the nucleolus) occurs here compared to the cytoplasm. *Ribosome* - While **ribosomes are the specific sites of translation** and are composed of rRNA and proteins, they represent molecular machinery rather than a cellular location. - Ribosomes themselves are located **within the cytoplasm**, making cytoplasm the more inclusive answer for where RNA activity occurs. - The question asks "where" in terms of cellular compartment, not which molecular complex. *None of the options* - This is incorrect as the cytoplasm is indeed the primary site where the majority of functional RNA activities occur in eukaryotic cells.