NEET-PG 2012 - Biochemistry Practice Questions

Q: Which of the following enzymes is not involved in the urea cycle?

CPS-II. ***CPS-II*** - Carbamoyl phosphate synthetase II is involved in **pyrimidine synthesis**, not the urea cycle. - It uses **glutamine** as a nitrogen donor and is located in the **cytosol**. *CPS-I* - Carbamoyl phosphate synthetase I is the **rate-limiting enzyme** of the urea cycle. - It catalyzes the formation of **carbamoyl phosphate** from **ammonia**, CO2, and ATP in the mitochondria. *Arginase* - Arginase is the **final enzyme** in the urea cycle, converting **arginine** to **ornithine** and **urea**. - This reaction occurs in the cytosol and releases urea for excretion. *Argininosuccinate lyase* - Argininosuccinate lyase catalyzes the cleavage of **argininosuccinate** into **fumarate** and **arginine**. - This is a key step in regenerating arginine for the final step of the urea cycle.

Q: Urea & Kreb's cycle are linked at?

Fumarate. ***Fumarate*** - **Fumarate** is a key intermediate produced during the **urea cycle** when argininosuccinate is cleaved into arginine and fumarate. - This fumarate then enters the **Krebs cycle** (citric acid cycle) as an intermediate to be converted into malate and then oxaloacetate, thus linking the two cycles. *Arginine* - **Arginine** is an amino acid that participates in the urea cycle, serving as a precursor for the formation of urea. - While arginine is a part of the urea cycle, it does not directly enter the Krebs cycle or serve as its linking metabolite. *Ornithine* - **Ornithine** is another amino acid central to the urea cycle, being regenerated at the end of the cycle to combine with carbamoyl phosphate. - It is a carrier molecule for the nitrogen atoms, but it does not directly link to the Krebs cycle. *Oxaloacetate* - **Oxaloacetate** is a central intermediate in the Krebs cycle, and it can be a precursor for intermediates in the urea cycle (e.g., through aspartate). - However, it is not the direct molecule that links the two cycles in the direction of the urea cycle feeding into the Krebs cycle.

Q: Ochronosis is due to accumulation of ?

Homogentisic acid. ***Homogentisic acid*** - Ochronosis is a condition caused by the excessive accumulation of **homogentisic acid** in connective tissues. - This accumulation occurs due to a deficiency of the enzyme **homogentisate 1,2-dioxygenase** (HGD), which is responsible for breaking down homogentisic acid in the tyrosine degradation pathway. *Phenylpyruvate* - **Phenylpyruvate** accumulates in **phenylketonuria (PKU)**, a different metabolic disorder where there's a deficiency in phenylalanine hydroxylase. - PKU leads to intellectual disability and other neurological problems, not the characteristic pigmentation and arthritis of ochronosis. *Xanthurenate* - **Xanthurenate** is an abnormal metabolite of tryptophan metabolism that can accumulate in **vitamin B6 deficiency**. - Its accumulation is associated with increased urinary excretion and is not directly implicated in the pathology of ochronosis. *Glyoxylate* - **Glyoxylate** is involved in various metabolic pathways, and its accumulation (or accumulation of its metabolic products like oxalate) is associated with **primary hyperoxaluria**. - Primary hyperoxaluria leads to kidney stones and kidney failure, a condition distinct from ochronosis.

Q: Which will activate carbamoyl phosphate synthase I?

N-acetyl glutamate. **N-acetyl glutamate** - **N-acetyl glutamate** is an **allosteric activator** of **carbamoyl phosphate synthase I (CPS I)**, which is the mitochondrial enzyme that catalyzes the first committed step of the **urea cycle**. - Its synthesis is stimulated by high levels of **arginine**, linking nitrogen load to urea production. *Acetyl-CoA* - Acetyl-CoA is a common **substrate** and **product** in various metabolic pathways, but it is not a direct activator of CPS I. - It is a precursor for the synthesis of **N-acetyl glutamate**, but does not activate CPS I directly. *Ornithine* - **Ornithine** is a key intermediate of the **urea cycle**, but it does not directly activate CPS I. - It combines with carbamoyl phosphate (the product of CPS I) in the second step of the urea cycle to form citrulline. *ATP* - **ATP** is a **substrate** used by CPS I to provide energy for the synthesis of carbamoyl phosphate. - While essential for the reaction, ATP itself does not act as an allosteric activator of the enzyme.

Q: What is the role of nonsense codons in protein synthesis?

Termination of protein synthesis. ***Termination of protein synthesis*** - **Nonsense codons**, also known as **stop codons** (UAA, UAG, UGA), signal the end of translation. - When a ribosome encounters a nonsense codon, it binds **release factors** instead of an aminoacyl-tRNA, leading to the dissociation of the polypeptide chain. *Elongation of the polypeptide chain* - **Elongation** involves the sequential addition of amino acids to the growing polypeptide chain, guided by sense codons. - Nonsense codons do not code for any amino acid and thus do not contribute to chain elongation. *Pre-translational modification of proteins* - **Pre-translational modifications** refer to events like protein folding and disulfide bond formation that occur as the polypeptide is being synthesized. - Nonsense codons are involved in halting the synthesis, not in modifying the protein. *Initiation of protein synthesis* - **Initiation** of protein synthesis begins at the **start codon** (AUG), which codes for methionine. - Nonsense codons are distinct from the start codon and fulfill a different role in the translation process.

Q: Which of the following is not the source of cytosolic NADPH ?

ATP citrate lyase. ***ATP citrate lyase*** - **ATP citrate lyase** is an enzyme involved in the synthesis of **acetyl-CoA** from citrate in the cytosol, which is then used for **fatty acid synthesis**. It does not generate NADPH. - While the **acetyl-CoA** produced is used in pathways that require NADPH, ATP citrate lyase itself does not directly produce NADPH. *Isocitrate dehydrogenase* - Cytosolic **isocitrate dehydrogenase** catalyzes the oxidative decarboxylation of **isocitrate** to alpha-ketoglutarate, producing **NADPH**. - This reaction is an important source of **cytosolic NADPH**, especially in non-photosynthetic tissues. *Malic enzyme* - **Malic enzyme** catalyzes the oxidative decarboxylation of **malate** to pyruvate, simultaneously reducing **NADP+ to NADPH**. - This enzyme is a significant source of **cytosolic NADPH** in various tissues, contributing to fatty acid synthesis and other reductive processes. *G6PD* - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme in the **pentose phosphate pathway** (PPP). - It catalyzes the first step of the PPP, converting **glucose-6-phosphate** to 6-phosphogluconolactone and producing **NADPH** as a crucial coenzyme.

Q: Which amino acid requires ascorbic acid for its formation in the body?

Hydroxyproline. ***Hydroxyproline*** - **Ascorbic acid (Vitamin C)** is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes - These enzymes catalyze the **post-translational hydroxylation** of proline and lysine residues within collagen chains to form hydroxyproline and hydroxylysine - This hydroxylation is crucial for **stabilization of the collagen triple helix** structure - Hydroxyproline is formed by **modification of proline after incorporation into collagen**, not as a free amino acid - **Scurvy** (Vitamin C deficiency) results in defective collagen due to inadequate hydroxyproline formation *Lysine* - Lysine is an **essential amino acid** obtained from diet - Does not require ascorbic acid for its synthesis or formation - While lysine residues in collagen can be hydroxylated (forming hydroxylysine), the question asks about the amino acid whose formation requires Vitamin C *Cysteine* - Cysteine is a **sulfur-containing amino acid** synthesized from methionine via transsulfuration pathway - Its synthesis does not involve ascorbic acid *Proline* - Proline is a **non-essential amino acid** synthesized from glutamate - **Proline synthesis does not require ascorbic acid** - Proline serves as the precursor that gets hydroxylated to hydroxyproline within collagen

Q: Which of the following requires vitamin B12?

Conversion of homocysteine to methionine. ***Homocysteine to methionine*** - The conversion of **homocysteine to methionine** is catalyzed by **methionine synthase**, an enzyme that requires **vitamin B12** (cobalamin) as a cofactor. - **Vitamin B12** facilitates the transfer of a methyl group from **methyltetrahydrofolate** to homocysteine, forming methionine. *Conversion of serine to lysine* - The metabolism of **serine to lysine** involves multiple steps and different enzymes, but it does not directly require **vitamin B12**. - Lysine is an **essential amino acid** and is primarily obtained from dietary sources or synthesized through complex pathways. *Conversion of serine to glycine* - The conversion of **serine to glycine** is catalyzed by **serine hydroxymethyltransferase**, which requires **tetrahydrofolate (THF)** as a cofactor, not vitamin B12. - This reaction generates **5,10-methylenetetrahydrofolate**, an important one-carbon donor. *Conversion of glutamine to glutamate* - The conversion of **glutamine to glutamate** is primarily catalyzed by **glutaminase**, an enzyme that does not require **vitamin B12**. - This reaction involves the removal of an **ammonia group** from glutamine to form glutamate.

Q: Which of the following micronutrient deficiencies can lead to anemia?

Copper. ***Copper*** - **Copper** is essential for **iron metabolism** and red blood cell formation; its deficiency can lead to **sideroblastic anemia** (often with microcytic or normocytic features) that may be accompanied by neutropenia. - Copper is required for **ceruloplasmin** function, which is necessary for iron mobilization from stores and incorporation into hemoglobin. - It also plays a role in the function of **superoxide dismutase** and **cytochrome c oxidase**, enzymes involved in antioxidant defense and energy production. *Molybdenum* - **Molybdenum** is a cofactor for several enzymes, including **xanthine oxidase** and **sulfite oxidase**, crucial for purine metabolism and detoxification. - While essential, its deficiency does not typically lead to **anemia** in humans. *Selenium* - **Selenium** is a component of selenoproteins, such as **glutathione peroxidase**, which protect cells from oxidative damage. - Deficiency is associated with conditions like **Keshan disease** (cardiomyopathy) but not primary anemia. *Fluorine* - **Fluorine** (as fluoride) is primarily known for its role in **bone and tooth mineralization**, protecting against dental caries. - It does not directly participate in **hematopoiesis** or iron metabolism, and its deficiency is not linked to anemia.

Q: How do enzymes function in biochemical reactions?

Decrease in activation energy. ***Decrease in activation energy*** - Enzymes act as **biological catalysts** by providing an alternative reaction pathway with a lower **transition state energy**. - This reduction in the **activation energy** allows a higher proportion of reactant molecules to overcome the energy barrier and react, thereby increasing the reaction rate. *Increase in activation energy* - This statement is incorrect as increasing activation energy would slow down the reaction rate, which is contrary to the function of enzymes. - Enzymes are designed to accelerate reactions, not inhibit them, by making them energetically more favorable to proceed. *Shift equilibrium constant* - Enzymes catalyze both the forward and reverse reactions equally, meaning they accelerate the rate at which equilibrium is reached but **do not alter the equilibrium constant (Keq)** of a reaction. - The equilibrium constant is determined by the difference in free energy between reactants and products, which enzymes do not change. *Provide energy to the reaction* - This statement is incorrect because enzymes do **not provide energy** to reactions; they only lower the activation energy barrier. - Enzymes facilitate reactions by stabilizing the transition state, not by adding energy to the system, which would violate thermodynamic principles.

Question 1

Which of the following enzymes is not involved in the urea cycle?

Accepted Answer

CPS-II

Answer

Arginase

Answer

Argininosuccinate lyase

Answer

CPS-I

Question 2

Urea & Kreb's cycle are linked at?

Accepted Answer

Fumarate

Answer

Arginine

Answer

Ornithine

Answer

Oxaloacetate

Question 3

Ochronosis is due to accumulation of ?

Accepted Answer

Homogentisic acid

Answer

Phenylpyruvate

Answer

Xanthurenate

Answer

Glyoxylate

Question 4

Which will activate carbamoyl phosphate synthase I?

Accepted Answer

N-acetyl glutamate

Answer

ATP

Answer

Acetyl-CoA

Answer

Ornithine

Question 5

What is the role of nonsense codons in protein synthesis?

Accepted Answer

Termination of protein synthesis

Answer

Elongation of the polypeptide chain

Answer

Pre-translational modification of proteins

Answer

Initiation of protein synthesis

Question 6

Which of the following is not the source of cytosolic NADPH ?

Accepted Answer

ATP citrate lyase

Answer

Malic enzyme

Answer

G6PD

Answer

Isocitrate dehydrogenase

Question 7

Which amino acid requires ascorbic acid for its formation in the body?

Accepted Answer

Hydroxyproline

Answer

Lysine

Answer

Cysteine

Answer

Proline

Question 8

Which of the following requires vitamin B12?

Accepted Answer

Conversion of homocysteine to methionine

Answer

Conversion of serine to lysine

Answer

Conversion of serine to glycine

Answer

Conversion of glutamine to glutamate

Question 9

Which of the following micronutrient deficiencies can lead to anemia?

Accepted Answer

Copper

Answer

Molybdenum

Answer

Fluorine

Answer

Selenium

Question 10

How do enzymes function in biochemical reactions?

Accepted Answer

Decrease in activation energy

Answer

Increase in activation energy

Answer

Shift equilibrium constant

Answer

Provide energy to the reaction

NEET-PG 2012 — Biochemistry