Rate limiting step in pyrimidine synthesis?
Hereditary orotic aciduria Type-I is due to deficiency of?
Sweaty feet odor in urine is seen in which condition?
What does salvage purine synthesis refer to?
Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
What is the role of Anandamide in the human body?
Clinical effect of vitamin D is reduced by ?
What is the primary effect of moderate alcohol consumption on cholesterol levels?
What is the number of variable regions present on each light and heavy chain of an antibody?
Which immunoglobulin is known to be heat-labile?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 111: Rate limiting step in pyrimidine synthesis?
- A. Aspartate transcarbamoylase (ATCase)
- B. Dihydroorotate dehydrogenase
- C. Dihydro-orotase
- D. Carbamoyl phosphate synthase-II (Correct Answer)
Explanation: ***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**
Question 112: 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 113: Sweaty feet odor in urine is seen in which condition?
- A. Phenylketonuria
- B. Isovaleric acidemia (Correct Answer)
- C. Alkaptonuria
- D. Maple syrup urine disease
Explanation: ***Isovaleric acidemia*** - This condition is characterized by a distinctive "sweaty feet" odor in body fluids, including urine, due to the accumulation of **isovaleric acid**. - It results from a deficiency in the enzyme **isovaleryl-CoA dehydrogenase**, which is crucial for leucine metabolism. *Phenylketonuria* - Patients with **phenylketonuria (PKU)** typically have a "mousy" or "musty" odor in their urine, not a sweaty feet smell. - This is due to the accumulation of **phenylalanine** and its metabolites. *Maple syrup urine disease* - This metabolic disorder is named for the characteristic sweet, maple syrup-like odor of the urine, which is distinctly different from a sweaty feet odor. - It is caused by a defect in the metabolism of **branched-chain amino acids (leucine, isoleucine, and valine)**. *Alkaptonuria* - This condition is known for urine that turns **dark brown or black** upon standing or when exposed to air, due to the oxidation of **homogentisic acid**. - It does not produce a sweaty feet odor.
Question 114: 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 115: Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
- A. RBC
- B. Leukocytes
- C. Liver (Correct Answer)
- D. Brain
Explanation: ***Liver*** - The **liver** is capable of both *de novo* synthesis and the salvage pathway of purine nucleotides, but it primarily utilizes the **de novo pathway** due to its high metabolic capacity and central role in biosynthesis for the entire body. - While salvage pathways exist, the liver's robust *de novo* synthesis allows it to readily produce purines from simple precursors, making it less reliant on salvaging pre-formed bases. *Brain* - The **brain** relies heavily on the **salvage pathway** for purine nucleotide synthesis because it has a limited capacity for *de novo* purine synthesis. - This dependency makes the brain particularly vulnerable to deficiencies in salvage enzymes, such as in **Lesch-Nyhan syndrome** where HGPRT deficiency leads to severe neurological dysfunction. *RBC* - **Red blood cells (RBCs)** are anucleated and lack the machinery for *de novo* purine synthesis, making them entirely dependent on the **salvage pathway** to maintain their purine nucleotide pool. - They salvage pre-formed purine bases and nucleosides from the plasma to synthesize necessary adenine and guanine nucleotides. *Leukocytes* - **Leukocytes**, particularly lymphocytes, have a high turn-over rate and metabolic activity, and they primarily rely on the **salvage pathway** for purine nucleotide synthesis. - The **immune system's rapid proliferation** and response demand efficient nucleotide synthesis, and the salvage pathway offers a quick and energy-efficient way to achieve this.
Question 116: What is the role of Anandamide in the human body?
- A. Opioid
- B. D2 blocker
- C. Cannabinoid neurotransmitter (Correct Answer)
- D. CCK1 antagonist
Explanation: ***Cannabinoid neurotransmitter*** - **Anandamide** is an **endogenous cannabinoid neurotransmitter** that binds to **CB1** and **CB2 receptors**. - It plays a role in **pain modulation**, **appetite stimulation**, and **memory regulation**. *Opioid* - **Opioids** bind to **opioid receptors** (mu, delta, kappa) and are known for their **analgesic** and **euphoric effects**. - Examples include **morphine** and **endorphins**, which are chemically distinct from anandamide and have different receptor targets. *CK 1 antagonist* - This option refers to a **cholecystokinin 1 (CCK1) receptor antagonist**, which would block the effects of **CCK**. - **CCK** is a hormone involved in **digestion** and **satiety**, and its role is unrelated to anandamide. *D2 blocker* - A **D2 blocker** is an agent that antagonizes the **dopamine D2 receptor**. - These are typically **antipsychotic medications** that modulate **dopamine pathways** in the brain, unrelated to the function of anandamide.
Question 117: Clinical effect of vitamin D is reduced by ?
- A. Simultaneous ingestion of lactose
- B. Simultaneous ingestion of phytates (Correct Answer)
- C. None of the options
- D. Acidic environment
Explanation: ***Simultaneous ingestion of phytates*** - **Phytates (phytic acid)** found in whole grains, nuts, seeds, and legumes can **reduce the clinical effect of vitamin D** through multiple mechanisms - Phytates **chelate calcium** and form insoluble calcium-phytate complexes, reducing calcium absorption - Since **vitamin D and calcium metabolism are closely linked**, impaired calcium absorption indirectly reduces vitamin D efficacy - Phytates can also **directly bind to vitamin D** in the gastrointestinal tract, reducing its bioavailability - Studies show that **high phytate intake increases vitamin D requirements** and can impair vitamin D status *Simultaneous ingestion of lactose* - Lactose does **not reduce** vitamin D absorption or efficacy - In fact, **dairy products are commonly fortified** with vitamin D, and the presence of lactose does not interfere with its beneficial effects - Lactose may actually **enhance calcium absorption**, which works synergistically with vitamin D *Acidic environment* - Vitamin D is a **fat-soluble vitamin** absorbed primarily in the small intestine - An acidic environment (stomach acid) is **not known to inhibit** vitamin D absorption - The absorption process occurs in the **alkaline environment of the small intestine** where fat-soluble vitamins are absorbed with dietary fats *None of the options* - This is **incorrect** as phytates do reduce the clinical effect of vitamin D through calcium chelation and direct binding mechanisms
Question 118: What is the primary effect of moderate alcohol consumption on cholesterol levels?
- A. Total cholesterol
- B. Low-Density Lipoprotein (LDL)
- C. Very Low-Density Lipoprotein (VLDL)
- D. High-Density Lipoprotein (HDL) (Correct Answer)
Explanation: ***High-Density Lipoprotein (HDL)*** - Moderate alcohol consumption is known to **increase HDL cholesterol** levels. - HDL cholesterol helps in the **reverse cholesterol transport**, removing excess cholesterol from tissues and transporting it back to the liver for excretion. *Total cholesterol* - The effect of moderate alcohol on **total cholesterol** is less consistent and may vary, as it is a sum of HDL, LDL, and 20% of VLDL. - While HDL increases, other components might remain unchanged or show minimal variation, thus not making it the primary and direct effect. *Low-Density Lipoprotein (LDL)* - Moderate alcohol consumption generally has **little to no significant effect** on **LDL cholesterol** levels. - Some studies suggest a slight decrease or no change, but it is not the primary lipid affected. *Very Low-Density Lipoprotein (VLDL)* - There is generally **no significant direct effect** of moderate alcohol consumption on **VLDL cholesterol** levels. - Excessive alcohol intake, however, can elevate triglycerides, which are the main component of VLDL particles.
Question 119: What is the number of variable regions present on each light and heavy chain of an antibody?
- A. 1 (Correct Answer)
- B. 2
- C. 3
- D. 4
Explanation: ***1*** - Each **light chain** and **heavy chain** within an antibody molecule contains **one variable region (V domain)**. - These variable regions are crucial for **antigen binding specificity**, as they combine to form the antigen-binding site. - The variable domain is located at the **N-terminal end** of each chain. *2* - While a complete antibody molecule has **two antigen-binding sites** (bivalent), each formed by pairing of VH and VL domains, individual chains possess only **one variable region each**. - The number '2' refers to the total number of identical binding sites on the intact antibody, not the number of variable regions per chain. *3* - The number **3** does not correspond to the number of variable regions on individual chains. - This might be confused with the **three complementarity-determining regions (CDRs)** present within each variable domain (CDR1, CDR2, CDR3), which are hypervariable loops that directly contact the antigen. *4* - The number **4** is incorrect for variable regions. - This number corresponds to the total number of **polypeptide chains** in a complete IgG antibody (2 heavy + 2 light chains), or the number of **constant domains** in some heavy chain isotypes (IgM, IgE have 4 CH domains).
Question 120: Which immunoglobulin is known to be heat-labile?
- A. IgA
- B. IgG
- C. IgM (Correct Answer)
- D. IgE
Explanation: ***IgM*** - **IgM** is known for its **heat lability** and is readily denatured at 56°C within a few minutes. - This characteristic is due to its **pentameric structure** held together by disulfide bonds and J chains, which are sensitive to thermal denaturation. - Heat lability of IgM is clinically important in complement fixation tests and other laboratory assays where heat inactivation is performed. - IgM is the first antibody produced in primary immune response and its heat sensitivity distinguishes it from other immunoglobulins. *IgA* - **IgA** exists in monomeric (serum) and dimeric (secretory) forms and shows moderate stability to heat. - Secretory IgA is relatively stable as it needs to function in harsh mucosal environments, though not as heat-resistant as IgG. - Does not exhibit the pronounced heat lability characteristic of IgM. *IgG* - **IgG** is the most stable immunoglobulin and is highly resistant to heat denaturation. - Can withstand temperatures up to 60-70°C without significant loss of activity. - Its monomeric structure with strong intramolecular bonds provides exceptional thermal stability. - Most abundant antibody in serum and has the longest half-life. *IgE* - **IgE** is actually quite stable to heat and can withstand 56°C for extended periods. - While it has a short half-life in serum (2-3 days), this is due to receptor binding rather than heat instability. - Important in type I hypersensitivity reactions and parasitic infections. - Does not show the characteristic heat lability that defines IgM.