NEET-PG 2015 - Biochemistry Practice Questions

Q: Which of the following is not a substrate for gluconeogenesis?

Leucine. ***Leucine*** - **Leucine** is an exclusively **ketogenic amino acid**, meaning its breakdown products can only be converted into **ketone bodies** or fatty acids, not glucose. - It does not have a carbon skeleton that can be directly converted into **pyruvate** or **oxaloacetate**, which are key intermediates in gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise or fasting. - It is converted to **pyruvate** by **lactate dehydrogenase**, and pyruvate can then enter the gluconeogenic pathway. *Propionate* - **Propionate** is a fatty acid with an odd number of carbon atoms, primarily derived from the catabolism of odd-chain fatty acids or from bacterial fermentation in the colon. - It can be converted into **succinyl CoA**, an intermediate of the citric acid cycle, which can then be used for gluconeogenesis. *Glycerol* - **Glycerol**, released during the breakdown of triglycerides, is an important substrate for gluconeogenesis. - It is phosphorylated to **glycerol-3-phosphate**, which is then oxidized to **dihydroxyacetone phosphate (DHAP)**, an intermediate in glycolysis and gluconeogenesis.

Q: Which of the following is not a free radical?

Hydrogen peroxide (H2O2). ***Hydrogen peroxide (H₂O₂)*** - **Hydrogen peroxide** is a **reactive oxygen species (ROS)** but is not a free radical because it has **no unpaired electrons** in its outermost shell. - While it can be converted into the highly reactive hydroxyl radical via the **Fenton reaction**, it is stable enough to be transported across membranes. *Superoxide anion (O₂⁻)* - The **superoxide anion (O₂⁻)** is a free radical because it has an **unpaired electron** in its outer shell. - It is one of the primary **reactive oxygen species** formed during cellular metabolism and can damage cellular components. *Nitric oxide (NO·)* - **Nitric oxide** is an important **free radical** with a single **unpaired electron** in its molecular structure. - It functions as a vital signaling molecule in vascular biology, regulating blood pressure and neurotransmission, despite being a free radical. *Hydroxyl radical (·OH)* - The **hydroxyl radical (·OH)** is one of the most reactive and damaging **free radicals** in biological systems. - It has a single **unpaired electron**, making it highly unstable and able to react indiscriminately with virtually all types of biomolecules.

Q: What primarily forms the core of chylomicrons?

Triglycerides. ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.

Q: What is the first purine nucleotide synthesized in de novo purine biosynthesis?

IMP. ***IMP (Inosine Monophosphate)*** - **IMP** is the first complete purine nucleotide synthesized during the **de novo purine biosynthesis pathway**. - It serves as a branch point, from which **AMP** and **GMP** are subsequently synthesized through separate pathways. *AMP (Adenosine Monophosphate)* - **AMP** is a derivative of **IMP**, synthesized by the addition of an amino group from **aspartate** to IMP. - This step occurs after the formation of the complete purine ring structure in IMP. *GMP (Guanosine Monophosphate)* - **GMP** is also derived from **IMP**, through a pathway involving the oxidation of IMP to **XMP** (xanthosine monophosphate) and subsequent amination. - Its synthesis occurs downstream from IMP. *UMP (Uridine Monophosphate)* - **UMP** is a **pyrimidine nucleotide**, not a purine, and is synthesized via a completely different de novo pathway. - Pyrimidine biosynthesis involves forming the ring structure first, then attaching it to ribose-phosphate, unlike purine synthesis which builds the ring on a pre-existing ribose-phosphate.

Q: What are Okazaki fragments?

Short pieces of DNA on the lagging strand.. ***Short pieces of DNA on the lagging strand.*** - **Okazaki fragments** are the short, newly synthesized DNA fragments that are formed on the **lagging strand** during DNA replication. - The lagging strand is synthesized discontinuously because DNA polymerase can only add nucleotides in the **5' to 3' direction**, requiring it to move away from the replication fork as the DNA unwinds. *Long pieces of DNA on the lagging strand.* - The lagging strand is synthesized discontinuously in **short fragments**, not long continuous pieces. - The enzyme **DNA ligase** eventually joins these short fragments together to form a continuous strand. *Short pieces of DNA on the leading strand.* - The **leading strand** is synthesized continuously in one long stretch, moving towards the replication fork. - It does not require the synthesis of short fragments like the lagging strand. *Long pieces of DNA on the leading strand.* - While the leading strand is synthesized in a continuous, long piece, this statement does not accurately describe Okazaki fragments, which are specific to the lagging strand. - The leading strand's continuous synthesis is due to its **3' to 5' template orientation**, allowing DNA polymerase to proceed uninterrupted.

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: Which of the following is not classified as a chaperone protein?

Calbindin. ***Calbindin*** - **Calbindin** is a **calcium-binding protein** that helps regulate intracellular calcium levels, particularly in the brain and intestines. - It does not assist in **protein folding** or assembly like chaperone proteins. *Calnexin* - **Calnexin** is a **chaperone protein** located in the endoplasmic reticulum (ER). - It assists in the proper folding and quality control of newly synthesized **glycoproteins**. *Protein disulfide isomerase* - **Protein disulfide isomerase (PDI)** is an ER enzyme that **catalyzes the formation and rearrangement of disulfide bonds** in newly synthesized proteins, which is crucial for proper folding. - Due to its role in enabling correct protein folding, it is considered a **chaperone-like protein**. *Calreticulin* - **Calreticulin** is another **calcium-binding chaperone protein** found in the endoplasmic reticulum. - It works synergistically with calnexin to ensure the **proper folding of glycoproteins**.

Q: The predominant isozyme of LDH in lung is:

LD-3. ***LD-3*** - **LD-3** is the predominant **LDH isozyme** found in the **lungs**, spleen, pancreas, and lymph nodes. - Its elevation often suggests conditions affecting these organs, such as pulmonary embolism or pancreatitis. *LD-1* - **LD-1** is primarily associated with the **heart** and **red blood cells**. - Elevated levels are typically seen in conditions like myocardial infarction and hemolytic anemia. *LD-2* - **LD-2** is also found in the **heart** and **red blood cells**, though typically in lower concentrations than LD-1 in the heart. - It is often elevated after an MI, but typically LD-1 is elevated higher than LD-2 after an MI. *LD-5* - **LD-5** is predominantly found in the **liver** and **skeletal muscle**. - Its increase is indicative of liver damage or muscle injury, such as hepatitis or muscular dystrophy.

Q: Bile acids consist of all of the following except -

Bilirubin. ***Bilirubin*** - **Bilirubin** is a pigment formed from the breakdown of **heme**, not a bile acid. - It is excreted in bile but does not aid in **lipid digestion** or **absorption**. *Lithocholic acid* - **Lithocholic acid** is a **secondary bile acid** formed in the colon by bacterial dehydroxylation of chenodeoxycholic acid. - It is still considered a bile acid, despite its secondary nature. *Deoxycholic acid* - **Deoxycholic acid** is a **secondary bile acid** formed by bacterial action on cholic acid in the colon. - Like other bile acids, it plays a role in **fat digestion** and **absorption**. *Chenodeoxycholic acid* - **Chenodeoxycholic acid** is a **primary bile acid** synthesized in the liver from cholesterol. - It is one of the main bile acids directly involved in **emulsifying dietary fats**.

Q: Which of the following elements have antioxidant properties?

All of the options. ***All of the options*** - **Selenium**, **copper**, and **zinc** all possess antioxidant properties, directly or indirectly, by being cofactors for various antioxidant enzymes or by directly scavenging free radicals. - These elements play crucial roles in maintaining **cellular redox balance** and protecting against **oxidative stress**. *Selenium* - It is a vital component of **glutathione peroxidase**, a key enzyme in the body's antioxidant defense system, which converts harmful **hydrogen peroxide** into water. - Selenium also contributes to the function of **thioredoxin reductases**, enzymes involved in regulating **redox signaling**. *Copper* - Copper is an essential cofactor for **superoxide dismutase (SOD1 and SOD3)**, an enzyme that catalyzes the dismutation of **superoxide radicals** into oxygen and hydrogen peroxide. - However, excessive copper can also act as a **pro-oxidant**, highlighting the importance of proper balance. *Zinc* - Zinc is another crucial cofactor for **superoxide dismutase (SOD1)** and is involved in protecting cells from **oxidative damage**. - It also stabilizes cell membranes, making them less susceptible to **oxidative damage**, and plays a role in regulating the expression of genes involved in **antioxidant defense**.

Question 1

Which of the following is not a substrate for gluconeogenesis?

Accepted Answer

Leucine

Answer

Lactate

Answer

Propionate

Answer

Glycerol

Question 2

Which of the following is not a free radical?

Accepted Answer

Hydrogen peroxide (H2O2)

Answer

Superoxide anion

Answer

Nitric oxide (NO·)

Answer

Hydroxyl radical (.OH)

Question 3

What primarily forms the core of chylomicrons?

Accepted Answer

Triglycerides

Answer

Triglycerides and Cholesterol together

Answer

Free fatty acids

Answer

Triglyceride, Cholesterol and Phospholipids

Question 4

What is the first purine nucleotide synthesized in de novo purine biosynthesis?

Accepted Answer

IMP

Answer

AMP

Answer

GMP

Answer

UMP

Question 5

What are Okazaki fragments?

Accepted Answer

Short pieces of DNA on the lagging strand.

Answer

Long pieces of DNA on the lagging strand.

Answer

Short pieces of DNA on the leading strand.

Answer

Long pieces of DNA on the leading strand.

Question 6

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

Accepted Answer

Glycogenolysis

Answer

Gluconeogenesis

Answer

Ketogenesis

Answer

Lipolysis

Question 7

Which of the following is not classified as a chaperone protein?

Accepted Answer

Calbindin

Answer

Calnexin

Answer

Protein disulfide isomerase

Answer

Calreticulin

Question 8

The predominant isozyme of LDH in lung is:

Accepted Answer

LD-3

Answer

LD-2

Answer

LD-5

Answer

LD-1

Question 9

Bile acids consist of all of the following except -

Accepted Answer

Bilirubin

Answer

Lithocholic acid

Answer

Deoxycholic acid

Answer

Chenodeoxycholic acid

Question 10

Which of the following elements have antioxidant properties?

Accepted Answer

All of the options

Answer

Copper

Answer

Zinc

Answer

Selenium

NEET-PG 2015 — Biochemistry