Creatinine is formed from -
Coenzyme for phenylalanine hydroxylase is?
Which of the following compounds is not formed with the involvement of glycine?
Which of the following is required in the synthesis of acetylcholine -
What is the respiratory quotient of carbohydrates?
Epinephrine increases free fatty acid levels by causing which of the following?
Energy source used by brain in later days of starvation is
Which of the following enzyme activity decreases in fasting?
Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?
Which of the following statements are true regarding the visual cycle cascade?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 41: Creatinine is formed from -
- A. Creatine (Correct Answer)
- B. Lysine
- C. Leucine
- D. Histidine
Explanation: ***Creatine*** - **Creatinine** is a waste product formed from the non-enzymatic, irreversible degradation of **creatine** and **creatine phosphate**, primarily in muscles. - **Creatine** itself is synthesized endogenously from three amino acids: **glycine, arginine, and methionine** (as S-adenosylmethionine) through a two-step enzymatic process in the kidney and liver. - The amount of creatinine produced daily is relatively constant and directly proportional to an individual's **muscle mass**, making it a useful marker for renal function. *Lysine* - **Lysine** is an **essential amino acid** and a precursor for various compounds like **carnitine** but is not involved in creatinine or creatine formation. - Deficiency can lead to impaired protein synthesis but does not impact creatinine levels. *Leucine* - **Leucine** is another **essential amino acid** and a **branched-chain amino acid (BCAA)** crucial for muscle protein synthesis and repair. - It does not serve as a direct precursor for creatinine or creatine. *Histidine* - **Histidine** is an **essential amino acid** and a precursor for **histamine** and other important compounds, but not creatinine or creatine. - It plays roles in immune response and gastric acid secretion.
Question 42: Coenzyme for phenylalanine hydroxylase is?
- A. Tetrahydrofolate
- B. Pyridoxal phosphate
- C. S-adenosyl methionine
- D. Tetrahydrobiopterin (Correct Answer)
Explanation: ***Tetrahydrobiopterin*** - **Tetrahydrobiopterin (BH4)** is an essential coenzyme for aromatic amino acid hydroxylases, including **phenylalanine hydroxylase (PAH)**. - PAH converts **phenylalanine** to **tyrosine**, and deficiencies in BH4 or PAH itself lead to *phenylketonuria (PKU)*. *Tetrahydrofolate* - **Tetrahydrofolate (THF)** is a coenzyme derived from **folic acid** and is primarily involved in **one-carbon metabolism**, including **purine** and **pyrimidine synthesis**, and various amino acid interconversions. - It does not directly act as a coenzyme for phenylalanine hydroxylase. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, a derivative of **vitamin B6**, is a crucial coenzyme for many enzymes involved in **amino acid metabolism**, particularly in **transamination**, **decarboxylation**, and side-chain cleavage reactions. - It is not the coenzyme for phenylalanine hydroxylase. *S-adenosyl methionine* - **S-adenosyl methionine (SAM)** is a major **methyl donor** in various biochemical reactions, important for the synthesis of **neurotransmitters**, **hormones**, and **phospholipids**. - While essential for many metabolic pathways, it is not involved as a coenzyme for phenylalanine hydroxylase.
Question 43: Which of the following compounds is not formed with the involvement of glycine?
- A. Purines
- B. Glutathione
- C. Thyroxine (Correct Answer)
- D. Heme
Explanation: ***Thyroxine*** - **Thyroxine** (and other thyroid hormones) are derived from the amino acid **tyrosine**. - Their synthesis involves iodination and coupling reactions of tyrosine residues within the protein **thyroglobulin**. *Heme* - **Glycine** is a direct precursor for the initial step in **heme synthesis**. - It condenses with **succinyl CoA** to form α-amino-β-ketoadipate, which then decarboxylates to form δ-aminolevulinate (ALA). *Purines* - **Glycine** contributes part of its structure to the **purine ring**. - Specifically, the **nitrogen at position 7** and the **carbons at positions 4 and 5** of the purine ring are derived from glycine. *Glutathione* - **Glutathione** is a tripeptide composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - **Glycine** is the C-terminal amino acid of glutathione and is essential for its structure and function as an antioxidant.
Question 44: Which of the following is required in the synthesis of acetylcholine -
- A. Inositol
- B. Carnitine
- C. Glycine
- D. Choline (Correct Answer)
Explanation: ***Choline*** - **Choline** is a **precursor** to acetylcholine, providing the **choline moiety** for its synthesis. - The enzyme **choline acetyltransferase** combines choline with acetyl-CoA (which provides the acetyl group) to form acetylcholine. *Inositol* - **Inositol** is a sugar alcohol that plays a role in **cell signaling** and as a secondary messenger in various pathways. - It is not directly involved as a substrate in the synthesis of acetylcholine. *Carnitine* - **Carnitine** is essential for the **transport of fatty acids** into the mitochondria for beta-oxidation. - It does not participate in the enzymatic reactions that produce acetylcholine. *Glycine* - **Glycine** is an amino acid that functions as an **inhibitory neurotransmitter** in the central nervous system. - While it is a neurotransmitter, it is not a component or precursor for acetylcholine synthesis.
Question 45: What is the respiratory quotient of carbohydrates?
- A. 0.5
- B. 0.8
- C. 0.75
- D. 1 (Correct Answer)
Explanation: ***Option: 1 (Correct Answer)*** - The **respiratory quotient (RQ)** is the ratio of **carbon dioxide produced to oxygen consumed** during metabolism. - For carbohydrates, complete oxidation yields equal moles of CO2 and O2, resulting in an **RQ of 1.0**. - Example: C6H12O6 + 6O2 → 6CO2 + 6H2O, giving RQ = 6CO2/6O2 = 1.0 - This value reflects that carbohydrates are highly oxygenated molecules, requiring less external oxygen for their oxidation relative to the CO2 produced. *Option: 0.5* - An RQ of 0.5 is not observed for any major macronutrient during complete oxidation. - This value would imply significantly lower CO2 production relative to O2 consumption, which doesn't match any physiological substrate metabolism. *Option: 0.8* - An RQ of approximately 0.8 is characteristic of a **mixed diet** or the average value sometimes cited for **protein metabolism**. - Protein RQ typically ranges from 0.8-0.85, as proteins require more oxygen for their oxidation compared to the CO2 produced. - The exact RQ can vary depending on the specific amino acids being metabolized. *Option: 0.75* - An RQ around 0.7-0.75 may represent **fat-predominant metabolism** or a mixed diet with fats and carbohydrates. - Pure **fat metabolism** has an RQ of approximately **0.7**, as fats require substantial oxygen for oxidation due to their lower oxygen content relative to carbon and hydrogen. - Fats contain many C-H bonds and few C-O bonds, necessitating more oxygen for complete combustion.
Question 46: Epinephrine increases free fatty acid levels by causing which of the following?
- A. Increasing fatty acid synthesis
- B. Increasing lipolysis (Correct Answer)
- C. Increasing cholesterol catabolism
- D. None of the options
Explanation: ***Increasing lipolysis*** - Epinephrine activates **hormone-sensitive lipase** in adipose tissue through a **cAMP-dependent mechanism**, leading to the breakdown of stored triglycerides into free fatty acids and glycerol. - This process, known as **lipolysis**, directly increases the release of free fatty acids into the bloodstream. *Increased fatty acid synthesis* - **Fatty acid synthesis** is a process that builds fatty acids, which would decrease, not increase, free fatty acid levels in the blood. - Epinephrine's primary action is to mobilize energy reserves, which involves breaking down stored fats rather than synthesizing new ones. *Increasing cholesterol catabolism* - While cholesterol metabolism is important, epinephrine does not directly or significantly increase **cholesterol catabolism** as a primary mechanism for raising free fatty acid levels. - The catabolism of cholesterol primarily involves its conversion to bile acids and steroid hormones, which is distinct from fatty acid release. *None of the options* - This option is incorrect because increasing lipolysis is a direct and well-established mechanism by which epinephrine raises free fatty acid levels.
Question 47: Energy source used by brain in later days of starvation is
- A. Glucose
- B. Ketone bodies (Correct Answer)
- C. Glycogen
- D. Fatty acids
Explanation: ***Ketone bodies*** - During **prolonged starvation**, the liver produces **ketone bodies** (acetoacetate and β-hydroxybutyrate) from fatty acid breakdown. - The brain adapts to utilize these ketone bodies as a primary energy source, reducing its reliance on **glucose**. *Glucose* - While **glucose** is the primary energy source for the brain under normal conditions, its availability diminishes significantly during prolonged starvation. - The brain attempts to conserve glucose for essential functions by switching to alternative fuels. *Glycogen* - The brain stores very limited amounts of **glycogen**, which are rapidly depleted within minutes of glucose deprivation. - It is not a sustainable or significant energy source during extended periods of starvation. *Fatty acids* - **Fatty acids** cannot directly cross the **blood-brain barrier** to a significant extent, thus they are not a direct fuel source for brain cells. - They are, however, used by the liver to synthesize ketone bodies, which then serve as brain fuel.
Question 48: Which of the following enzyme activity decreases in fasting?
- A. Hormone sensitive lipase
- B. Glycogen phosphorylase
- C. Acetyl CoA Carboxylase
- D. Phosphofructokinase I (Correct Answer)
Explanation: ***Phosphofructokinase I*** - **Phosphofructokinase I (PFK-1)** activity **decreases** during fasting due to **decreased insulin-to-glucagon ratio**, which reduces **fructose-2,6-bisphosphate (F-2,6-BP)** levels, a powerful allosteric activator of PFK-1. - This reduction in activity slows down **glycolysis**, conserving glucose for critical tissues like the brain and redirecting metabolism toward **gluconeogenesis**. - **PFK-1 is the rate-limiting enzyme of glycolysis**, making its regulation particularly significant in the fasted state. *Hormone sensitive lipase* - **Hormone sensitive lipase (HSL)** activity **increases** during fasting due to elevated **glucagon** and **epinephrine** levels, which stimulate its phosphorylation via **protein kinase A (PKA)**. - This increased activity promotes the breakdown of stored **triglycerides** in adipose tissue, releasing **fatty acids** for β-oxidation and energy production. *Glycogen phosphorylase* - **Glycogen phosphorylase** activity **increases** during fasting, primarily stimulated by **glucagon** and **epinephrine**, leading to the breakdown of **glycogen** stores. - This enzyme is crucial for **glycogenolysis**, providing glucose to maintain blood sugar levels when dietary intake is absent. *Acetyl CoA Carboxylase* - **Acetyl CoA Carboxylase (ACC)** activity also **decreases** during fasting, as it is inhibited by **phosphorylation** mediated by **AMP-activated protein kinase (AMPK)** and **protein kinase A (PKA)**. - This reduction in activity inhibits **fatty acid synthesis**, shifting metabolism towards fatty acid **oxidation** for energy production. - **Note:** While ACC activity does decrease during fasting, **PFK-1** is considered the primary answer as it represents the key regulatory point for **glucose metabolism** (glycolysis vs. gluconeogenesis), which is the central metabolic shift during fasting.
Question 49: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?
- A. Vitamin K
- B. Vitamin D (Correct Answer)
- C. Vitamin A
- D. Biotin
Explanation: ***Vitamin D*** - The skin synthesizes vitamin D (specifically **cholecalciferol**) upon exposure to **ultraviolet B (UVB) radiation** from sunlight. - This endogenous production can be sufficient to meet physiological needs under adequate sun exposure, making it conditionally non-essential in the diet. *Vitamin K* - While **intestinal bacteria synthesize some vitamin K (K2)**, it is generally not in sufficient quantities to meet all physiological needs, especially for blood clotting. - Dietary intake of **vitamin K1 (phylloquinone)** from leafy green vegetables is critical. *Vitamin A* - **Vitamin A (retinol)** is obtained primarily from the diet, either directly from animal sources or from carotenoid precursors (like **beta-carotene**) in plants. - The body cannot synthesize vitamin A de novo; it relies on dietary intake and conversion from precursors. *Biotin* - Although the **gut microbiota can synthesize biotin**, the amount produced is generally considered insufficient to meet the body's requirements. - Therefore, biotin is primarily obtained through dietary intake, functioning as a coenzyme in various metabolic reactions.
Question 50: Which of the following statements are true regarding the visual cycle cascade?
- A. All of the options are true (Correct Answer)
- B. Light causes isomerization of 11-cis-retinal to all-trans-retinal
- C. Retinal is involved in the visual cycle
- D. Involves a conformational change in opsin
Explanation: ***All of the statements are true*** The visual cycle cascade involves multiple interconnected events in phototransduction: **Light causes isomerization of 11-cis-retinal to all-trans-retinal** - This is the **primary photochemical event** that initiates vision - Light absorption causes the **cis-trans isomerization** in less than a picosecond - This conformational change is the only light-dependent step in the entire cascade **Retinal is involved in the visual cycle** - **11-cis-retinal** serves as the chromophore bound to opsin forming rhodopsin - After isomerization to **all-trans-retinal**, it must be converted back to 11-cis-retinal - This regeneration occurs through the **retinoid cycle** involving RPE cells **Involves a conformational change in opsin** - The isomerization of retinal triggers **conformational changes in opsin** - This converts rhodopsin to **metarhodopsin II** (the active form) - Activated opsin then activates **transducin** (G-protein), amplifying the signal and leading to hyperpolarization of photoreceptor cells All three statements accurately describe essential components of the visual cycle cascade.