NEET-PG 2019 — Biochemistry

Q: Which of the following is true about alpha-1 antitrypsin?

Inhibits elastase. ***Inhibits elastase*** - Alpha-1 antitrypsin (A1AT) primarily serves to **inhibit elastase**, a protease that can damage lung tissue, helping to protect the lungs from destruction [1]. - Deficiency of A1AT leads to **emphysema** and liver disease due to unchecked activity of elastase [1][2]. *Inhibits trypsin* - A1AT specifically does not primarily inhibit **trypsin**, which is involved in protein digestion in the intestine. - Although A1AT affects proteases, its main function is related to **elastase**, not trypsin [1]. *Inhibits trypsinogen activation in pancreas* - A1AT does not have a significant role in the **inhibition of trypsinogen activation** within the pancreas. - Instead, pancreatic enzyme regulation involves other mechanisms that do not involve A1AT's function. *Inhibits chymotrypsin* - A1AT is not known for inhibiting **chymotrypsin**, a serine protease derived from trypsinogen in the gut. - It specifically targets **elastase** and similar enzymes rather than chymotrypsin [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 856-858. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 683-684. [Practice more Biochemistry PYQs on OnCourse]

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. [Practice more Biochemistry PYQs on OnCourse]

Q: In Krebs cycle and Urea cycle the linking amino acid is

Aspartate. ***Aspartate*** - **Aspartate** acts as the crucial amino acid link between the two cycles - In the urea cycle, aspartate condenses with citrulline to form **argininosuccinate** (via argininosuccinate synthetase) - When argininosuccinate is cleaved, it produces **fumarate**, which enters the Krebs cycle - In the Krebs cycle, fumarate is converted to malate, then to **oxaloacetate**, which can be transaminated back to aspartate - This creates the **aspartate-argininosuccinate shunt**, linking both cycles through nitrogen metabolism *Fumarate* - While **fumarate** is a key metabolic intermediate connecting both cycles, it is **not an amino acid** (it's a dicarboxylic acid) - It is produced in the urea cycle from argininosuccinate cleavage and feeds into the Krebs cycle - This is a common distractor since fumarate does link the cycles, but the question specifically asks for an amino acid *Alanine* - **Alanine** participates in the glucose-alanine cycle for nitrogen transport from muscle to liver - It does not directly link the Krebs cycle and urea cycle in the same manner as aspartate *Arginine* - **Arginine** is a urea cycle intermediate that is cleaved by arginase to produce urea and ornithine - While it's an amino acid in the urea cycle, it does not serve as the linking amino acid between the Krebs cycle and urea cycle [Practice more Biochemistry PYQs on OnCourse]

Q: Menkes disease is caused by a deficiency of which protein?

ATP7A (copper-transporting ATPase). ***ATP7A (copper-transporting ATPase)*** - **Menkes disease** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**, which encodes a copper-transporting ATPase. - This protein is essential for **copper absorption** from the intestines and its transport across cell membranes. *ATP7B (Wilson disease protein)* - Mutations in the **ATP7B gene** cause **Wilson disease**, characterized by **copper accumulation** in the liver, brain, and other organs due to impaired copper excretion. - Unlike Menkes disease, Wilson disease involves *too much* copper in tissues, not a deficiency due to poor absorption. *Ceruloplasmin* - **Ceruloplasmin** is a copper-carrying protein that transports copper in the blood and also acts as an oxidase. - While deficiencies in ceruloplasmin can lead to **aceruloplasminemia**, a disorder of iron metabolism, it is not the primary defect in Menkes disease. *Copper-zinc superoxide dismutase* - **Copper-zinc superoxide dismutase (SOD1)** is an enzyme that plays a crucial role in eliminating harmful **reactive oxygen species**. - Mutations in SOD1 are associated with some forms of **amyotrophic lateral sclerosis (ALS)**, not Menkes disease. [Practice more Biochemistry PYQs on OnCourse]

Q: What is the type of cholesterol primarily found in gallstones?

Crystalline cholesterol monohydrate.. ***Crystalline cholesterol monohydrate*** - The predominant type of cholesterol found in gallstones is **crystalline cholesterol monohydrate** [1], which reflects the solid form of cholesterol precipitating in bile. - It is often associated with **cholesterol gallstones**, occurring when bile contains excessive cholesterol or insufficient bile salts. *Crystalline Cholesterol dihydrate* - Crystalline cholesterol dihydrate is less commonly associated with gallstones and generally forms in different circumstances, not typical of cholesterol stones. - This type does not represent the main component of gallstones, which primarily consist of monohydrate forms. *Amorphous cholesterol dihydrate* - Amorphous cholesterol dihydrate is not a recognized form typically found in gallstones, as gallstone pathology focuses on crystallized forms. - Amorphous substances are less stable than crystalline forms, making this option unlikely in the context of gallstones. *Amorphous cholesterol monohydrate* - Amorphous cholesterol monohydrate is not the major component found in gallstones; gallstones are more likely to be crystalline in structure. - This form lacks the stable crystalline structure needed to precipitate and form gallstones effectively. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, p. 882. [Practice more Biochemistry PYQs on OnCourse]

23 Previous Year Questions with Answers & Explanations

Questions

Which of the following is true about alpha-1 antitrypsin?

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

In Krebs cycle and Urea cycle the linking amino acid is

Menkes disease is caused by a deficiency of which protein?

What is the type of cholesterol primarily found in gallstones?

Conversion of Norepinephrine to epinephrine is mainly by?

Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

All of the following are examples of Dietary fibre except for which of the following?

Which of the following statements regarding mitochondrial DNA is FALSE?

Q10

In Wilson’s disease, which of the following substances is excreted in lower amounts in the urine?

NEET-PG 2019 - Biochemistry NEET-PG Practice Questions and MCQs

Question 1: Which of the following is true about alpha-1 antitrypsin?

A. Inhibits elastase (Correct Answer)
B. Inhibits trypsin
C. Inhibits chymotrypsin
D. Inhibits trypsinogen activation

Explanation: ***Inhibits elastase*** - Alpha-1 antitrypsin (A1AT) primarily serves to **inhibit elastase**, a protease that can damage lung tissue, helping to protect the lungs from destruction [1]. - Deficiency of A1AT leads to **emphysema** and liver disease due to unchecked activity of elastase [1][2]. *Inhibits trypsin* - A1AT specifically does not primarily inhibit **trypsin**, which is involved in protein digestion in the intestine. - Although A1AT affects proteases, its main function is related to **elastase**, not trypsin [1]. *Inhibits trypsinogen activation in pancreas* - A1AT does not have a significant role in the **inhibition of trypsinogen activation** within the pancreas. - Instead, pancreatic enzyme regulation involves other mechanisms that do not involve A1AT's function. *Inhibits chymotrypsin* - A1AT is not known for inhibiting **chymotrypsin**, a serine protease derived from trypsinogen in the gut. - It specifically targets **elastase** and similar enzymes rather than chymotrypsin [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 856-858. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 683-684.

Question 2: Which of the following is not the source of cytosolic NADPH ?

A. Malic enzyme
B. G6PD
C. Isocitrate dehydrogenase
D. ATP citrate lyase (Correct Answer)

Explanation: ***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.

Question 3: In Krebs cycle and Urea cycle the linking amino acid is

A. Fumarate
B. Alanine
C. Aspartate (Correct Answer)
D. Arginine

Explanation: ***Aspartate*** - **Aspartate** acts as the crucial amino acid link between the two cycles - In the urea cycle, aspartate condenses with citrulline to form **argininosuccinate** (via argininosuccinate synthetase) - When argininosuccinate is cleaved, it produces **fumarate**, which enters the Krebs cycle - In the Krebs cycle, fumarate is converted to malate, then to **oxaloacetate**, which can be transaminated back to aspartate - This creates the **aspartate-argininosuccinate shunt**, linking both cycles through nitrogen metabolism *Fumarate* - While **fumarate** is a key metabolic intermediate connecting both cycles, it is **not an amino acid** (it's a dicarboxylic acid) - It is produced in the urea cycle from argininosuccinate cleavage and feeds into the Krebs cycle - This is a common distractor since fumarate does link the cycles, but the question specifically asks for an amino acid *Alanine* - **Alanine** participates in the glucose-alanine cycle for nitrogen transport from muscle to liver - It does not directly link the Krebs cycle and urea cycle in the same manner as aspartate *Arginine* - **Arginine** is a urea cycle intermediate that is cleaved by arginase to produce urea and ornithine - While it's an amino acid in the urea cycle, it does not serve as the linking amino acid between the Krebs cycle and urea cycle

Question 4: Menkes disease is caused by a deficiency of which protein?

A. ATP7B (Wilson disease protein)
B. Ceruloplasmin
C. Copper-zinc superoxide dismutase
D. ATP7A (copper-transporting ATPase) (Correct Answer)

Explanation: ***ATP7A (copper-transporting ATPase)*** - **Menkes disease** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**, which encodes a copper-transporting ATPase. - This protein is essential for **copper absorption** from the intestines and its transport across cell membranes. *ATP7B (Wilson disease protein)* - Mutations in the **ATP7B gene** cause **Wilson disease**, characterized by **copper accumulation** in the liver, brain, and other organs due to impaired copper excretion. - Unlike Menkes disease, Wilson disease involves *too much* copper in tissues, not a deficiency due to poor absorption. *Ceruloplasmin* - **Ceruloplasmin** is a copper-carrying protein that transports copper in the blood and also acts as an oxidase. - While deficiencies in ceruloplasmin can lead to **aceruloplasminemia**, a disorder of iron metabolism, it is not the primary defect in Menkes disease. *Copper-zinc superoxide dismutase* - **Copper-zinc superoxide dismutase (SOD1)** is an enzyme that plays a crucial role in eliminating harmful **reactive oxygen species**. - Mutations in SOD1 are associated with some forms of **amyotrophic lateral sclerosis (ALS)**, not Menkes disease.

Question 5: What is the type of cholesterol primarily found in gallstones?

A. Amorphous cholesterol monohydrate.
B. Amorphous cholesterol dihydrate.
C. Crystalline cholesterol dihydrate.
D. Crystalline cholesterol monohydrate. (Correct Answer)

Explanation: ***Crystalline cholesterol monohydrate*** - The predominant type of cholesterol found in gallstones is **crystalline cholesterol monohydrate** [1], which reflects the solid form of cholesterol precipitating in bile. - It is often associated with **cholesterol gallstones**, occurring when bile contains excessive cholesterol or insufficient bile salts. *Crystalline Cholesterol dihydrate* - Crystalline cholesterol dihydrate is less commonly associated with gallstones and generally forms in different circumstances, not typical of cholesterol stones. - This type does not represent the main component of gallstones, which primarily consist of monohydrate forms. *Amorphous cholesterol dihydrate* - Amorphous cholesterol dihydrate is not a recognized form typically found in gallstones, as gallstone pathology focuses on crystallized forms. - Amorphous substances are less stable than crystalline forms, making this option unlikely in the context of gallstones. *Amorphous cholesterol monohydrate* - Amorphous cholesterol monohydrate is not the major component found in gallstones; gallstones are more likely to be crystalline in structure. - This form lacks the stable crystalline structure needed to precipitate and form gallstones effectively. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, p. 882.

Question 6: Conversion of Norepinephrine to epinephrine is mainly by?

A. S-adenosyl methionine (Correct Answer)
B. Arginine
C. Phenylalanine
D. Dehydrogenase

Explanation: **S-adenosyl methionine (SAM)** - SAM acts as the **methyl donor** in the enzymatic conversion of **norepinephrine to epinephrine** by phenylethanolamine N-methyltransferase (PNMT). - This **methylation reaction** adds a methyl group to the nitrogen atom of norepinephrine, forming epinephrine. *Arginine* - Arginine is a precursor for **nitric oxide (NO)** synthesis, an important signaling molecule, and is also involved in the **urea cycle**. - It is not directly involved in the methylation of norepinephrine to epinephrine. *Phenylalanine* - Phenylalanine is an **essential amino acid** and a precursor for the synthesis of **tyrosine**, which is subsequently converted to **catecholamines** like dopamine, norepinephrine, and epinephrine. - However, it does not directly facilitate the final conversion step from norepinephrine to epinephrine. *Dehydrogenase* - Dehydrogenases are enzymes that catalyze **redox reactions** by removing hydrogen atoms from a substrate. - These enzymes are crucial in many metabolic pathways, but they are not involved in the **methylation reaction** that converts norepinephrine to epinephrine.

Question 7: Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

A. succinate
B. α-ketoglutarate (Correct Answer)
C. malate
D. fumarate

Explanation: ***a keto glutarate*** - **Hyperammonemia** leads to the depletion of **α-ketoglutarate** through its amination to form **glutamate** by glutamate dehydrogenase and subsequently glutamine by glutamine synthetase. - The removal of **α-ketoglutarate** from the TCA cycle impairs its ability to produce energy and essential intermediates, contributing to neurological dysfunction in hyperammonemia. *succinate* - **Succinate** is an intermediate in the TCA cycle, but its depletion is not the primary mechanism by which hyperammonemia inhibits the cycle. - The direct consumption of **α-ketoglutarate** for ammonia detoxification is the more direct and significant impact. *malate* - **Malate** is another intermediate in the TCA cycle but is downstream from **α-ketoglutarate**. - Its depletion is a consequence of overall TCA cycle inhibition, not the initial cause mediated by hyperammonemia. *fumarate* - **Fumarate** is also a TCA cycle intermediate and is produced after succinate. - Its levels would be affected by the overall inhibition of the cycle, but it is not the direct target or substrate for ammonia detoxification that depletes the cycle.

Question 8: All of the following are examples of Dietary fibre except for which of the following?

A. Starch (Correct Answer)
B. Pectin
C. Lignin
D. Cellulose

Explanation: ***Correct: Starch*** - **Starch** is a **polysaccharide** that serves as a **storage carbohydrate** in plants and is readily digestible by human enzymes, breaking down into glucose. - While it's a carbohydrate found in plant foods, it does not fit the definition of dietary fibre which is generally resistant to human digestive enzymes. *Incorrect: Pectin* - **Pectin** is a type of **soluble dietary fibre** found in fruits, particularly apples and citrus. - It forms a gel in water, contributing to satiety and helping to **lower cholesterol** and **regulate blood sugar**. *Incorrect: Lignin* - **Lignin** is a **non-carbohydrate dietary fibre** that provides structural support in plants. - It is an **insoluble fibre** and is resistant to breakdown by digestive enzymes, aiding in bulk formation in stool. *Incorrect: Cellulose* - **Cellulose** is a major component of **plant cell walls** and is a type of **insoluble dietary fibre**. - It adds bulk to stool, promoting regularity and preventing constipation.

Question 9: Which of the following statements regarding mitochondrial DNA is FALSE?

A. Double stranded
B. Inherited from mother
C. High mutation rate
D. All respiratory proteins are synthesized within the mitochondria (Correct Answer)

Explanation: ***All respiratory proteins are synthesized within the mitochondria.*** - While mitochondrial DNA (mtDNA) encodes some proteins essential for the **electron transport chain** (respiratory proteins), not all respiratory proteins are synthesized within the mitochondria. - Many crucial respiratory proteins are encoded by **nuclear DNA** and imported into the mitochondria from the cytoplasm. *Double stranded* - **Mitochondrial DNA (mtDNA)** is a **double-stranded**, circular molecule, similar to bacterial chromosomes. - This structure provides stability and allows for efficient replication within the organelle. *Inherited from mother* - Mitochondria and their DNA are exclusively inherited from the **mother** during fertilization, as sperm primarily contributes nuclear DNA. - This **maternal inheritance pattern** is a key feature of mtDNA and is used in tracing ancestry. *High mutation rate* - mtDNA has a significantly **higher mutation rate** compared to nuclear DNA due to several factors, including lack of robust repair mechanisms and exposure to reactive oxygen species. - This contributes to the rapid evolution of mtDNA and its use in **population genetics** studies.

Question 10: In Wilson’s disease, which of the following substances is excreted in lower amounts in the urine?

A. Phosphotyrosine
B. Methyl- Histidine (Correct Answer)
C. Phosphorus
D. Serine

Explanation: **Important Note:** In Wilson's disease with Fanconi syndrome (proximal renal tubular dysfunction), most substances show **increased** urinary excretion, not decreased. This question appears to test knowledge of what is NOT characteristically elevated. ***Methyl-Histidine*** (Most appropriate answer) - **Methyl-histidine** (3-methylhistidine) is a marker of **muscle protein breakdown** and is not directly affected by the renal tubular dysfunction in Wilson's disease - Unlike phosphate, amino acids, and glucose which are pathologically increased in urine due to Fanconi syndrome, methyl-histidine excretion remains **normal** or is unrelated to copper-induced renal damage - This represents the substance least affected by Wilson's disease pathology *Phosphorus* (Actually INCREASED, not decreased) - **Medically incorrect as stated**: Phosphorus (phosphate) is actually **INCREASED** in urine in Wilson's disease, not decreased - Fanconi syndrome causes **renal phosphate wasting**, leading to hyperphosphaturia and resultant hypophosphatemia - This is a characteristic feature of proximal tubular dysfunction *Serine* (INCREASED due to aminoaciduria) - **Serine** and other amino acids show **generalized aminoaciduria** in Wilson's disease due to impaired proximal tubular reabsorption - Urinary serine is **elevated**, not decreased *Phosphotyrosine* - A phosphorylated amino acid involved in cell signaling, not routinely measured clinically - Not characteristically implicated in Wilson's disease urinary patterns