NEET-PG 2012 - Biochemistry Practice Questions

Q: Which of the following statements is true regarding medium chain fatty acids?

All of the options are true. ***All of the options are true*** - This option is correct because medium-chain fatty acids (MCFAs) possess unique metabolic properties that differentiate them from long-chain fatty acids (LCFAs), making all listed statements accurate. - Their shorter chain length allows for distinct digestion, absorption, and metabolic fates, which are beneficial in various clinical contexts. *Do not require pancreatic lipase for digestion* - MCFAs have **shorter carbon chains** (typically 6-12 carbons) and are more hydrophilic than LCFAs. - This property allows them to be digested by **lingual and gastric lipases** to a greater extent, reducing the reliance on pancreatic lipase. *Absorb directly into portal circulation* - Unlike LCFAs, which are re-esterified into triglycerides, packaged into **chylomicrons**, and absorbed into the lymphatic system, MCFAs are absorbed directly into the **portal vein**. - This bypasses the lymphatic system and directly transports them to the liver, making them a rapid energy source. *Are less likely to be deposited in adipose tissue compared to long-chain fatty acids* - MCFAs are **rapidly oxidized** in the liver for energy via beta-oxidation and are less likely to be stored as triglycerides in adipose tissue. - They are also not efficiently utilized for the synthesis of complex lipids or stored fat due to their unique metabolic pathway and preference for oxidation.

Q: Which of the following types of bonds is considered the weakest?

Van der Waals. ***Van der Waals*** - **Van der Waals forces** are very **weak, short-range attractive forces** that arise from transient fluctuations in electron distribution, creating fleeting dipoles. - They are crucial for phenomena like **protein folding** and **molecular recognition**, but are easily overcome. *Covalent* - **Covalent bonds** involve the **sharing of electron pairs** between atoms, resulting in very strong and stable connections. - They require a significant amount of energy to break, making them fundamental to the structure of most organic and biological molecules. *Hydrogen* - **Hydrogen bonds** are **intermolecular forces** that occur when a hydrogen atom covalently bonded to a highly electronegative atom (like **oxygen** or **nitrogen**) is attracted to another electronegative atom. - While weaker than covalent bonds, they are significantly stronger than Van der Waals forces and play critical roles in **DNA structure** and **water properties**. *Electrostatic* - **Electrostatic interactions** (also known as **ionic bonds** or salt bridges) occur between oppositely charged ions or polar molecules. - These forces can be quite strong, especially in a non-polar environment, and are important for **protein stability** and **enzyme-substrate binding**.

Q: Acute intermittent porphyria is due to deficiency of?

Porphobilinogen deaminase. ***Porphobilinogen deaminase*** - **Acute intermittent porphyria (AIP)** results from a deficiency in **porphobilinogen deaminase** (also known as hydroxymethylbilane synthase). - This enzyme deficiency leads to the accumulation of **aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**, which are neurotoxic and cause the characteristic symptoms of AIP. *Uroporphyrinogen III synthase* - A deficiency in **uroporphyrinogen III synthase** causes **congenital erythropoietic porphyria (Günther disease)**, which is characterized by severe photosensitivity and hemolytic anemia. - This enzyme defect leads to the accumulation of uroporphyrinogen I and coproporphyrinogen I, not the ALA and PBG associated with AIP. *Ferrochelatase* - Deficiency in **ferrochelatase** causes **erythropoietic protoporphyria (EPP)**, which presents with photosensitivity and chronic liver disease due to the accumulation of **protoporphyrin**. - This condition does not cause the acute neurological attacks seen in AIP. *ALA synthase* - **ALA synthase** is the **rate-limiting enzyme** in heme synthesis; while its activity is crucial, a congenital *deficiency* is not the cause of AIP. - Instead, the *upregulation* of ALA synthase activity in AIP (due to the PBG deaminase block) contributes to the accumulation of ALA and PBG.

Q: Major source of energy for brain in fasting/starvation?

Ketone bodies. ***Ketone bodies*** - During **prolonged fasting or starvation**, the body depletes its **glycogen stores** and begins to break down fatty acids. The liver converts these fatty acids into **ketone bodies**, such as **acetoacetate and beta-hydroxybutyrate**. - These **ketone bodies** can cross the **blood-brain barrier** and be used by the brain as an alternative energy source when glucose becomes scarce, preventing protein breakdown for gluconeogenesis. *Glucose* - While **glucose** is the primary and preferred energy source for the brain under normal physiological conditions, its availability significantly decreases during **prolonged fasting or starvation**. - The brain requires a continuous supply of glucose, but in states of severe caloric restriction, the body must conserve glucose for other critical functions and adapt by using alternative fuels. *Glycogen* - **Glycogen** is a stored form of glucose found predominantly in the **liver and muscles**. - The brain itself has minimal **glycogen stores**, which are rapidly depleted during fasting, and thus cannot be a major long-term energy source. *Fatty acids* - **Fatty acids** are a major energy source for many tissues in the body, especially during fasting, but they **cannot directly cross the blood-brain barrier** in significant amounts to fuel the brain. - Instead, **fatty acids** are metabolized into **ketone bodies** in the liver, which then serve as the brain's alternative fuel.

Q: Which of the following processes does not occur in mitochondria?

Glycogenolysis. ***Glycogenolysis*** - **Glycogenolysis** is the breakdown of **glycogen** into glucose, which primarily occurs in the **cytosol** of cells, mainly in the liver and muscles. - This process is crucial for maintaining blood glucose levels and providing energy during periods of fasting or increased demand, and it does not take place within the mitochondria. *Fatty acid oxidation* - **Fatty acid oxidation**, also known as beta-oxidation, is a mitochondrial process that breaks down fatty acids into **acetyl-CoA** for energy production. - This occurs extensively within the mitochondrial matrix, producing ATP. *Electron transport chain* - The **electron transport chain** is located in the **inner mitochondrial membrane** and is the final stage of aerobic respiration, producing the majority of ATP. - It involves a series of protein complexes that transfer electrons to oxygen, creating a proton gradient for ATP synthesis. *Citric acid cycle (Kreb's cycle)* - The **citric acid cycle**, or **Krebs cycle**, is a central metabolic pathway that occurs in the **mitochondrial matrix**. - It oxidizes acetyl-CoA, derived from carbohydrates, fats, and proteins, to produce ATP, NADH, and FADH2.

Q: Repetitive chains of glucosamine with uronic acid are seen in ?

Heparan sulphate. ***Heparan sulphate*** - Heparan sulfate is a **linear polysaccharide** composed of repeating disaccharide units of either **glucosamine** and **glucuronic acid** or **glucosamine** and **iduronic acid** (a uronic acid). - It plays crucial roles in various biological processes, including cell adhesion, cell growth, and coagulation, primarily due to its ability to bind to a wide range of proteins. *N-acetylneuraminic acid* - **N-acetylneuraminic acid** is a type of **sialic acid**, a nine-carbon sugar acid, and is not a repeating chain of glucosamine and uronic acid. - It is often found at the **non-reducing ends of glycan chains** on cell surfaces and in secreted glycoproteins, contributing to cell recognition and signaling. *Galactosaminoglycan* - This term is a general descriptor for a group of glycosaminoglycans that contain **N-acetylgalactosamine** as one of their repeating units, such as **chondroitin sulfate** and **dermatan sulfate**. - While they are repeating disaccharide units, they feature **galactosamine** (or its N-acetylated form) instead of glucosamine and are also linked to uronic acids. *None of the options* - This option is incorrect because **heparan sulfate** accurately describes a polysaccharide with repeating units of glucosamine and uronic acid.

Q: Mutation in GLUT-2 causes which syndrome?

Fanconi-Bickel syndrome. ***Fanconi-Bickel syndrome*** - This syndrome is caused by a **mutation in the GLUT-2 gene**, leading to dysfunctional glucose transport in the liver, kidneys, and intestines. - Key features include **hepatorenal glycogen accumulation**, **renal tubulopathy** (Fanconi syndrome), and **impaired glucose and galactose utilization**. *Dandy-Walker syndrome* - This is a **congenital brain malformation** involving the cerebellum and fourth ventricle. - It is often associated with hydrocephalus, but not directly linked to glucose transporter defects. *Beckwith-Wiedemann syndrome* - This is an **overgrowth disorder** characterized by a high risk of childhood cancer and congenital anomalies. - It is primarily caused by genetic abnormalities on **chromosome 11p15.5** and is unrelated to GLUT-2 mutations. *Menke's disease* - This is a rare X-linked recessive disorder of **copper metabolism**, leading to severe neurological degeneration. - It results from mutations in the **ATP7A gene**, which encodes a copper-transporting ATPase.

Q: What is the most sensitive biochemical marker for a 7-day old myocardial infarction?

Troponin I/T. ***Troponin I/T*** - **Cardiac troponins (I and T)** are highly sensitive and specific biomarkers for **myocardial injury**. - While they rise quickly after an MI, they also remain elevated for an extended period, typically **7 to 10-14 days**, making them ideal for detecting a 7-day-old event. *CPK MB* - **Creatine phosphokinase MB (CPK-MB)** is a well-known cardiac marker, but its elevation is more transient, usually returning to normal within **2-3 days** post-MI. - Therefore, it would likely be undetectable or near baseline 7 days after the event, making it insensitive for this duration. *LDH* - **Lactate dehydrogenase (LDH)** used to be used as a cardiac marker, but it is **non-specific** and found in various tissues. - While it can remain elevated for an extended period after an MI (up to 10-14 days), its lack of specificity makes other markers, particularly troponins, much more reliable for confirming myocardial damage. *Myoglobin* - **Myoglobin** is one of the earliest markers to rise after myocardial injury, but it is also **rapidly cleared** from the bloodstream, usually within 24 hours. - Due to its short half-life, myoglobin would not be elevated 7 days after an MI, making it unsuitable for detecting such a remote event.

Q: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

2 ATP, 2 NADH. **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.

Q: Which enzyme is required for cutting the DNA strand during synthesis?

Topoisomerase. ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.

Question 1

Which of the following statements is true regarding medium chain fatty acids?

Accepted Answer

All of the options are true

Answer

Do not require pancreatic lipase for digestion

Answer

Absorb directly into portal circulation

Answer

Are less likely to be deposited in adipose tissue compared to long-chain fatty acids

Question 2

Which of the following types of bonds is considered the weakest?

Accepted Answer

Van der Waals

Answer

Covalent

Answer

Hydrogen

Answer

Electrostatic

Question 3

Acute intermittent porphyria is due to deficiency of?

Accepted Answer

Porphobilinogen deaminase

Answer

Uroporphyrinogen III synthase

Answer

Ferrochelatase

Answer

ALA synthase

Question 4

Major source of energy for brain in fasting/starvation?

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

Question 5

Which of the following processes does not occur in mitochondria?

Accepted Answer

Glycogenolysis

Answer

Fatty acid oxidation

Answer

Electron transport chain

Answer

Citric acid cycle (Kreb's cycle)

Question 6

Repetitive chains of glucosamine with uronic acid are seen in ?

Accepted Answer

Heparan sulphate

Answer

None of the options

Answer

N-acetylneuraminic acid

Answer

Galactosaminoglycan

Question 7

Mutation in GLUT-2 causes which syndrome?

Accepted Answer

Fanconi-Bickel syndrome

Answer

Dandy walker syndrome

Answer

Beckwith-Wiedemann syndrome

Answer

Menke's disease

Question 8

What is the most sensitive biochemical marker for a 7-day old myocardial infarction?

Accepted Answer

Troponin I/T

Answer

CPK MB

Answer

LDH

Answer

Myoglobin

Question 9

What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

Accepted Answer

2 ATP, 2 NADH

Answer

4 ATP, 2 NADH

Answer

4 ATP, 4 NADH

Answer

2 ATP, 4 NADH

Question 10

Which enzyme is required for cutting the DNA strand during synthesis?

Accepted Answer

Topoisomerase

Answer

DNA polymerase

Answer

DNA ligase

Answer

Helicase

NEET-PG 2012 — Biochemistry