NEET-PG 2020 - Biochemistry Practice Questions

Q: A child was fed on a staple diet of maize for a long time. Which of the following vitamin may get deficient in his body?

Niacin (Vitamin B3). ***Niacin (Vitamin B3)*** - Maize is deficient in tryptophan (an amino acid that can be converted to **niacin**) and contains niacin in a bound form (**niacytin**) that is not bioavailable. - A staple diet of maize without adequate supplementation can lead to **pellagra**, characterized by the classic triad of dermatitis, diarrhea, and dementia (3 D's). - This is particularly common in populations relying heavily on untreated maize as a staple food. *Thiamine (Vitamin B1)* - While polishing rice can remove thiamine, maize itself is not primarily associated with **thiamine deficiency** as a staple. - **Beriberi** (thiamine deficiency) presents with neurological and cardiovascular symptoms (wet and dry beriberi), distinct from pellagra. *Vitamin B6 (Pyridoxine)* - Deficiency of **pyridoxine** is uncommon with maize-based diets unless there are other contributing factors like drug interactions (e.g., isoniazid). - Symptoms include peripheral neuropathy, seborrheic dermatitis, and sideroblastic anemia, which are not directly linked to a maize staple diet. *Cobalamin (Vitamin B12)* - **Vitamin B12** is found primarily in animal products, so a vegetarian or vegan diet poses a risk for deficiency, not specifically a maize-based diet. - Deficiency leads to megaloblastic anemia and neurological damage (subacute combined degeneration), unrelated to maize's nutritional profile.

Q: Which of the following is true about non-competitive inhibition?

Km remains same, Vmax decreases. ***Km remains same, Vmax decreases*** - In **non-competitive inhibition**, the inhibitor binds to an allosteric site on the enzyme, altering its conformation, thereby **reducing its catalytic efficiency**. - This binding does not affect the **enzyme's affinity for the substrate (Km remains the same)**, but it **reduces the maximum reaction rate (Vmax decreases)** because fewer enzyme molecules are able to perform catalysis per unit time. *Km increases, Vmax remains same* - This describes **competitive inhibition**, where the inhibitor competes with the substrate for the enzyme's active site. - While it **increases the apparent Km** (more substrate needed to reach half Vmax), **Vmax remains unchanged** as high substrate concentrations can overcome the inhibition. *Km decreases, Vmax increases* - This scenario would imply an activation rather than inhibition, where both enzyme affinity and catalytic efficiency are enhanced. - This is not characteristic of any standard **enzyme inhibition mechanism**. *Km increases, Vmax increases* - This combination is not observed in any typical **enzyme inhibition pattern**. - An increase in **Vmax** implies enhanced catalytic activity, while an increase in **Km** suggests reduced substrate affinity, which are contradictory effects for a single inhibitor.

Q: Which of the following binds to Tyrosine Kinase receptor?

Insulin. ***Insulin*** - **Insulin** is a classic example of a hormone that binds to and activates a **tyrosine kinase receptor**, leading to a cascade of intracellular signaling events for glucose uptake and metabolism. - The **insulin receptor** is a heterodimeric protein with intrinsic tyrosine kinase activity that phosphorylates itself and other proteins upon insulin binding. *Glucagon* - **Glucagon** primarily acts on **G protein-coupled receptors (GPCRs)**, specifically the glucagon receptor, to increase cyclic AMP (cAMP) and activate protein kinase A. - Its main roles are to stimulate **glycogenolysis** and **gluconeogenesis** in the liver. *Prolactin* - **Prolactin** binds to a receptor that is a member of the **cytokine receptor superfamily**, which lacks intrinsic enzyme activity. - Upon ligand binding, these receptors associate with and activate **Janus kinases (JAKs)**, leading to the JAK-STAT signaling pathway. *Growth Hormone* - **Growth hormone (GH)** also binds to a receptor belonging to the **cytokine receptor superfamily** (similar to prolactin), which then associates with and activates **JAKs**. - This activation subsequently initiates the **JAK-STAT signaling pathway**, mediating its diverse growth-promoting and metabolic effects.

Q: What is the major source of cyanocobalamin (vitamin B12)?

Animal-only diet. ***Animal-only diet*** - **Cyanocobalamin (vitamin B12)** is primarily found in **animal products** such as meat, fish, poultry, eggs, and dairy. - It is synthesized by microorganisms and accumulates in animal tissues, making animal-derived foods the most reliable dietary source for humans. *Sunlight* - **Sunlight** is the primary source of **vitamin D** synthesis in the skin, not vitamin B12. - Exposure to UV radiation from the sun does not contribute to the production or uptake of cyanocobalamin. *Plant-only diet* - A **plant-only diet (vegan diet)** is deficient in naturally occurring **vitamin B12**, as plants do not synthesize this vitamin. - Vegans often require **fortified foods** or **supplements** to meet their B12 needs. *None of the options* - This option is incorrect because an **animal-only diet** is identified as a major source of cyanocobalamin. - The question has a correct answer among the provided choices.

Q: Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?

Tryptophan. ***Tryptophan*** - **Tryptophan** contains an **indole ring**, which is a **chromophore** that strongly absorbs UV light at 280 nm. - Increased tryptophan content in a protein directly correlates with a higher **UV absorbance** at this wavelength. *Glycine* - **Glycine** is the simplest amino acid, with only a **hydrogen atom** as its side chain. - It does not contain any aromatic rings or other groups that absorb UV light at 280 nm, so replacing alanine with glycine would not increase UV absorbance. *Arginine* - **Arginine** is a basic amino acid with a **guanidinium group** in its side chain. - While it has a slightly complex side chain, it does not possess any **aromatic rings** that absorb significantly at 280 nm. *Lysine* - **Lysine** is another basic amino acid with a long **aliphatic chain** and an **amino group** at the end. - Similar to arginine, lysine lacks the necessary **aromatic chromophores** to contribute to UV absorbance at 280 nm.

Q: When the insulin:glucagon ratio is decreased, which enzyme is active?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - A decreased insulin:glucagon ratio indicates a **fasting state** or **catabolic state**, promoting glucose production and release rather than storage. - **Glucose-6-phosphatase** is the key enzyme that enables glucose release from the liver by removing the phosphate group from glucose-6-phosphate, producing free glucose that can exit hepatocytes. - This enzyme is active during both **gluconeogenesis** and **glycogenolysis** and is only present in liver, kidney, and intestinal cells. *Glucokinase* - **Glucokinase** is active in the **fed state** when insulin levels are high and the insulin:glucagon ratio is increased. - It phosphorylates glucose to trap it in hepatocytes for glycogen synthesis and metabolism, which is the opposite of what occurs during fasting. *Phosphofructokinase* - **Phosphofructokinase (PFK-1)** is the rate-limiting enzyme of **glycolysis**, active when glucose needs to be broken down for energy. - It is stimulated by high insulin:glucagon ratios and inhibited during fasting when gluconeogenesis (the reverse pathway) is active. *Hexokinase* - **Hexokinase** phosphorylates glucose in peripheral tissues for intracellular utilization. - During a low insulin:glucagon ratio, the priority is glucose **release** from the liver, not glucose **uptake** and phosphorylation in tissues.

Question 1

A child was fed on a staple diet of maize for a long time. Which of the following vitamin may get deficient in his body?

Accepted Answer

Niacin (Vitamin B3)

Answer

Thiamine (Vitamin B1)

Answer

Vitamin B6 (Pyridoxine)

Answer

Cobalamin (Vitamin B12)

Question 2

Which of the following is true about non-competitive inhibition?

Accepted Answer

Km remains same, Vmax decreases

Answer

Km increases, Vmax remains same

Answer

Km decreases, Vmax increases

Answer

Km increases, Vmax increases

Question 3

Which of the following binds to Tyrosine Kinase receptor?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Prolactin

Answer

Growth Hormone

Question 4

What is the major source of cyanocobalamin (vitamin B12)?

Accepted Answer

Animal-only diet

Answer

Sunlight

Answer

Plant-only diet

Answer

None of the options

Question 5

Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?

Accepted Answer

Tryptophan

Answer

Glycine

Answer

Arginine

Answer

Lysine

Question 6

When the insulin:glucagon ratio is decreased, which enzyme is active?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucokinase

Answer

Phosphofructokinase

Answer

Hexokinase

NEET-PG 2020 — Biochemistry