NEET-PG 2015 - Biochemistry Practice Questions

Q: Which of the following is required in the synthesis of acetylcholine -

Choline. ***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.

Q: What is the respiratory quotient of carbohydrates?

1. ***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.

Q: Epinephrine increases free fatty acid levels by causing which of the following?

Increasing lipolysis. ***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.

Q: Energy source used by brain in later days of starvation is

Ketone bodies. ***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.

Q: Which of the following enzyme activity decreases in fasting?

Phosphofructokinase I. ***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.

Q: Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

Vitamin D. ***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.

Q: Which metabolic pathway is least active during 12 days of fasting?

Glycogenolysis. ***Correct: Glycogenolysis*** - **Glycogenolysis**, the breakdown of glycogen stores, is very active during the **initial hours of fasting** (first 24-48 hours) to maintain blood glucose levels. - However, after **12 days of fasting**, liver and muscle **glycogen stores are completely depleted**, making this pathway **essentially inactive** or the least active of all the metabolic pathways. - Once glycogen is exhausted, this pathway cannot contribute further to energy metabolism. *Incorrect: Gluconeogenesis* - This pathway becomes **increasingly active** during prolonged fasting to **synthesize new glucose** from non-carbohydrate precursors (amino acids, lactate, glycerol). - Essential for maintaining blood glucose for **glucose-dependent tissues** like red blood cells and parts of the brain that haven't fully adapted to ketones. - Remains a **crucial and active pathway** throughout prolonged fasting. *Incorrect: Ketogenesis* - **Ketogenesis** is **highly active** during prolonged fasting, producing **ketone bodies** (acetoacetate, β-hydroxybutyrate) from fatty acids in the liver. - Provides the **primary alternative fuel** for the brain (up to 70% of brain energy needs) and other tissues. - This is a **key metabolic adaptation** to preserve protein and glucose during starvation. *Incorrect: Lipolysis* - **Lipolysis** (breakdown of triglycerides into fatty acids and glycerol) is **highly active** during fasting to mobilize stored energy. - Provides **fatty acids** for direct oxidation by most tissues and **glycerol** as a gluconeogenic substrate. - A **fundamental process** for energy supply during nutrient deprivation.

Q: Pyridoxine is required in -

Transamination. ***Transamination*** - **Pyridoxal phosphate (PLP)**, the active form of pyridoxine (vitamin B6), is an essential **coenzyme for aminotransferases (transaminases)** - Transamination reactions involve the transfer of an **amino group** from an amino acid to a keto acid, which is crucial for amino acid metabolism - This is the classic biochemical function of vitamin B6 and a frequently tested concept *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate - Key cofactors for glycolysis include **NAD+ and ATP**, not vitamin B6 - Does not require pyridoxine as a coenzyme *TCA cycle* - The **TCA cycle (Krebs cycle)** is a central metabolic pathway for energy production - Uses enzymes that require cofactors such as **NAD+, FAD, and Coenzyme A** (derived from pantothenic acid) - Pyridoxine is not directly involved as a coenzyme in TCA cycle reactions *Glycogenesis* - Glycogenesis is the process of synthesizing **glycogen from glucose** - Primarily involves enzymes like **glycogen synthase** and **branching enzyme** - Requires **UTP and glucose-1-phosphate**, not pyridoxine

Q: Pruritus [Itching] associated with Congenital Erythropoietic Porphyria is caused by deficiency of -

Uroporphyrinogen - III synthase. ***Uroporphyrinogen - III synthase*** - Congenital Erythropoietic Porphyria (CEP) is caused by a **deficiency of uroporphyrinogen III synthase**, leading to the accumulation of uroporphyrinogen I and coproporphyrinogen I. - These accumulated **Type I porphyrinogens** are non-functional in heme synthesis and are highly **photoreactive**, causing the characteristic photosensitivity and skin symptoms, including intense pruritus. *5-ALA dehydratase* - Deficiency of **5-ALA dehydratase** (also known as porphobilinogen synthase) is associated with **ALA dehydratase deficiency porphyria (ADP)**, a very rare acute hepatic porphyria. - Symptoms primarily involve **neurovisceral attacks** and do not typically include pruritus or photosensitivity. *Uroporphyrinogen - I synthase* - **Uroporphyrinogen I synthase** is an outdated and incorrect term; the correct enzyme in the heme synthesis pathway is **hydroxymethylbilane synthase (HMB synthase)** or **porphobilinogen deaminase (PBG deaminase)**, which synthesizes HMB. - Deficiency in HMB synthase leads to **acute intermittent porphyria (AIP)**, characterized by acute neurological attacks, not severe pruritus. *HMB synthase* - **HMB synthase** (hydroxymethylbilane synthase), also known as **porphobilinogen deaminase (PBG deaminase)**, is deficient in **acute intermittent porphyria (AIP)**. - AIP is marked by intermittent neurological dysfunction and abdominal pain, with **no significant photosensitivity or pruritus**.

Q: Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

Vitamin D. ***Correct Answer: Vitamin D*** - **Vitamin D** (specifically its active form, **calcitriol** or **1,25-dihydroxyvitamin D₃**) is the **most classically recognized** fat-soluble vitamin that functions as a **steroid hormone** - It binds to the **vitamin D receptor (VDR)**, which is a member of the **nuclear receptor superfamily** - This VDR-calcitriol complex acts as a transcription factor, regulating gene expression involved in **calcium and phosphate homeostasis**, bone metabolism, skeletal development, and immune function - The mechanism is analogous to classic steroid hormones like cortisol, estrogen, and testosterone *Incorrect: Vitamin A* - **Vitamin A** (as **retinoic acid**) also interacts with nuclear receptors (**retinoic acid receptors - RARs** and **retinoid X receptors - RXRs**) to regulate gene transcription - However, Vitamin A is **most classically associated** with vision (rhodopsin in retinal photoreceptors), epithelial cell differentiation, embryonic development, and immune function - While it does have nuclear receptor-mediated actions, **Vitamin D is more prominently described** as having steroid hormone-like activity in standard medical education *Incorrect: Vitamin K* - **Vitamin K** functions primarily as a **cofactor for γ-glutamyl carboxylase**, an enzyme that catalyzes post-translational modification of glutamate residues to γ-carboxyglutamate (Gla) - Essential for the synthesis of **clotting factors** (II, VII, IX, X, protein C, protein S) and bone proteins (osteocalcin) - Does **not** act through nuclear receptors or function as a steroid hormone *Incorrect: Vitamin E* - **Vitamin E** (α-tocopherol) is a powerful **lipid-soluble antioxidant** that protects cell membranes from oxidative damage by scavenging free radicals - Functions primarily through its **antioxidant properties**, not through nuclear receptor binding - Does **not** have steroid hormone-like actions

Question 1

Which of the following is required in the synthesis of acetylcholine -

Accepted Answer

Choline

Answer

Inositol

Answer

Carnitine

Answer

Glycine

Question 2

What is the respiratory quotient of carbohydrates?

Accepted Answer

1

Answer

0.5

Answer

0.8

Answer

0.75

Question 3

Epinephrine increases free fatty acid levels by causing which of the following?

Accepted Answer

Increasing lipolysis

Answer

Increasing fatty acid synthesis

Answer

Increasing cholesterol catabolism

Answer

None of the options

Question 4

Energy source used by brain in later days of starvation is

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

Question 5

Which of the following enzyme activity decreases in fasting?

Accepted Answer

Phosphofructokinase I

Answer

Hormone sensitive lipase

Answer

Glycogen phosphorylase

Answer

Acetyl CoA Carboxylase

Question 6

Which of the following vitamins can be synthesized in the body in sufficient quantities to meet physiological needs?

Accepted Answer

Vitamin D

Answer

Vitamin K

Answer

Vitamin A

Answer

Biotin

Question 7

Which metabolic pathway is least active during 12 days of fasting?

Accepted Answer

Glycogenolysis

Answer

Gluconeogenesis

Answer

Ketogenesis

Answer

Lipolysis

Question 8

Pyridoxine is required in -

Accepted Answer

Transamination

Answer

Glycolysis

Answer

TCA cycle

Answer

Glycogenesis

Question 9

Pruritus [Itching] associated with Congenital Erythropoietic Porphyria is caused by deficiency of -

Accepted Answer

Uroporphyrinogen - III synthase

Answer

Uroporphyrinogen - I synthase

Answer

5-ALA dehydratase

Answer

HMB synthase

Question 10

Which fat-soluble vitamin is most classically known for its steroid hormone-like action through nuclear receptors?

Accepted Answer

Vitamin D

Answer

Vitamin K

Answer

Vitamin A

Answer

Vitamin E

NEET-PG 2015 — Biochemistry