Which of the following binds to Tyrosine Kinase receptor?
What is the major source of cyanocobalamin (vitamin B12)?
Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?
When the insulin:glucagon ratio is decreased, which enzyme is active?
NEET-PG 2020 - Biochemistry NEET-PG Practice Questions and MCQs
Question 21: Which of the following binds to Tyrosine Kinase receptor?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. Prolactin
- D. Growth Hormone
Explanation: ***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.
Question 22: What is the major source of cyanocobalamin (vitamin B12)?
- A. Sunlight
- B. Plant-only diet
- C. Animal-only diet (Correct Answer)
- D. None of the options
Explanation: ***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.
Question 23: Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?
- A. Glycine
- B. Tryptophan (Correct Answer)
- C. Arginine
- D. Lysine
Explanation: ***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.
Question 24: When the insulin:glucagon ratio is decreased, which enzyme is active?
- A. Glucokinase
- B. Phosphofructokinase
- C. Hexokinase
- D. Glucose-6-phosphatase (Correct Answer)
Explanation: ***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.