What is essential for the transfer of fatty acid across the mitochondrial membrane?
Krabbe's disease is due to deficiency of ?
Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?
Which of the following statements about LDL is false?
Concentration of which is inversely related to the risk of coronary heart disease?
Apolipoprotein E is rich in
Which type of bond is primarily responsible for the primary structure of a protein?
Which amino acid has two chiral centers?
Which of the following is a neutral amino acid?
Amino acid with aliphatic side chain is?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 71: What is essential for the transfer of fatty acid across the mitochondrial membrane?
- A. Creatinine
- B. Carnitine (Correct Answer)
- C. Biotin
- D. Creatine
Explanation: ***Carnitine*** - **Carnitine** is crucial for transporting **long-chain fatty acids** into the mitochondrial matrix for **beta-oxidation**. - It forms **acylcarnitine** by esterifying with fatty acids, allowing passage through the inner mitochondrial membrane via the **carnitine-acylcarnitine translocase**. *Creatinine* - **Creatinine** is a waste product formed from the breakdown of **creatine phosphate** in muscles and is excreted by the kidneys. - It serves as a marker for **kidney function** and has no role in fatty acid transport. *Biotin* - **Biotin** is a vitamin cofactor essential for **carboxylase enzymes**, including acetyl-CoA carboxylase in **fatty acid synthesis**. - While involved in lipid metabolism, it plays no role in the transport of fatty acids across mitochondrial membranes. *Creatine* - **Creatine** is a nitrogenous organic acid that helps supply energy to cells, primarily muscle, by facilitating the regeneration of **ATP**. - It plays no direct role in the facilitated transport of fatty acids across the mitochondrial membrane.
Question 72: Krabbe's disease is due to deficiency of ?
- A. Sphingomyelinase
- B. Beta galactocerebrosidase (Correct Answer)
- C. Hexosaminidase
- D. Arylsulfatase
Explanation: ***Beta galactocerebrosidase*** - Krabbe's disease is specifically caused by a deficiency of **beta-galactocerebrosidase**, leading to the accumulation of toxic substances in the brain [1]. - This disease predominantly affects the **myelin sheath**, resulting in severe neurological deterioration [1]. *Arylsulfatase* - Deficiency of **arylsulfatase** is associated with **metachromatic leukodystrophy**, not Krabbe's disease. - Symptoms and pathology differ significantly, primarily affecting **sulfatides** rather than galactocerebrosides. *Sphingomyelinase* - A deficiency of **sphingomyelinase** is linked to **Niemann-Pick disease**, characterized by splenomegaly and liver involvement. - This condition does not involve the same neurological deterioration seen in Krabbe's disease. *Hexosaminidase* - Hexosaminidase deficiency is associated with **Tay-Sachs disease**, primarily affecting the **GM2 gangliosides** [2]. - This results in different clinical manifestations, such as **cherry-red spots** and progressive neurodegeneration, rather than the symptoms of Krabbe's disease [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1304-1305. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 161.
Question 73: Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?
- A. VLDL
- B. Chylomicron
- C. Lp (a)
- D. LDL (Correct Answer)
Explanation: ***LDL*** - **Low-density lipoprotein (LDL)** is commonly referred to as "bad" cholesterol because elevated levels are the **primary driver** of atherosclerotic plaque buildup in arterial walls. - LDL particles transport cholesterol from the liver to peripheral tissues; when present in excess, they infiltrate the arterial intima and undergo oxidative modification, triggering inflammatory responses that lead to atherosclerosis. - **Clinical significance:** LDL cholesterol is the primary target of lipid-lowering therapy in cardiovascular disease prevention. *VLDL* - **Very low-density lipoprotein (VLDL)** primarily transports endogenously synthesized **triglycerides** from the liver to peripheral tissues. - While elevated VLDL levels do contribute to cardiovascular risk (particularly through conversion to small, dense LDL particles), it is not the primary lipoprotein targeted in cardiovascular risk assessment. *Chylomicron* - **Chylomicrons** transport **dietary lipids** (triglycerides and cholesterol) from the intestines to tissues after meals. - They are rapidly cleared from circulation (half-life of 5-10 minutes) and are typically not present during fasting, making their contribution to chronic atherosclerotic plaque formation minimal. *Lp(a)* - **Lipoprotein(a) [Lp(a)]** is structurally similar to LDL but contains an additional apolipoprotein(a) molecule, which has homology to plasminogen and may interfere with fibrinolysis. - While Lp(a) is an independent cardiovascular risk factor, it is less commonly measured in routine clinical practice, and **LDL remains the cornerstone lipoprotein** for cardiovascular risk stratification and management.
Question 74: Which of the following statements about LDL is false?
- A. More dense than chylomicron
- B. Transports maximum amount of lipid (Correct Answer)
- C. Contains maximum cholesterol
- D. Smaller than VLDL
Explanation: ***Transports maximum amount of lipid*** - This statement is false because **chylomicrons**, not LDL, are primarily responsible for transporting the **maximum amount of dietary lipids** (triglycerides) from the intestines to various tissues. - While LDL does transport lipids, its primary role is to deliver **cholesterol** to cells, and it contains a lower proportion of triglyceride compared to chylomicrons and VLDL. *More dense than chylomicron* - This statement is true; **LDL is denser than chylomicrons** because it has a higher protein-to-lipid ratio. - **Chylomicrons** are the least dense lipoproteins due to their very high triglyceride content. *Smaller than VLDL* - This statement is true; **LDL is smaller than VLDL** (Very Low-Density Lipoprotein). - VLDL particles are larger and contain more triglycerides, which are gradually removed, leading to the formation of smaller LDL particles. *Contains maximum cholesterol* - This statement is true; **LDL contains the highest proportion of cholesterol** (specifically, **cholesterol esters**) among the lipoproteins. - This characteristic makes LDL the primary carrier for delivering cholesterol to peripheral tissues.
Question 75: Concentration of which is inversely related to the risk of coronary heart disease?
- A. VLDL
- B. LDL
- C. HDL (Correct Answer)
- D. None of the options
Explanation: ***HDL*** - **High-density lipoprotein (HDL)** is known as "good cholesterol" because it helps remove excess cholesterol from the arteries and transport it back to the liver for excretion. - Higher levels of HDL are generally associated with a **lower risk of coronary heart disease (CHD)**, hence the inverse relationship. *VLDL* - **Very low-density lipoprotein (VLDL)** carries triglycerides and cholesterol and is considered an independent risk factor for CHD when present in high concentrations. - High VLDL levels are associated with an **increased risk of CHD**, not an inverse relationship. *LDL* - **Low-density lipoprotein (LDL)** is often referred to as "bad cholesterol" because it contributes to plaque buildup in arteries (**atherosclerosis**). - High levels of LDL are strongly associated with an **increased risk of CHD**, indicating a direct, not inverse, relationship. *None of the options* - This option is incorrect because HDL clearly demonstrates an **inverse relationship** with the risk of coronary heart disease.
Question 76: Apolipoprotein E is rich in
- A. Methionine
- B. Lysine
- C. Histidine
- D. Arginine (Correct Answer)
Explanation: ***Arginine*** - **Apolipoprotein E (apoE)** is notably rich in **basic amino acids**, with **arginine** being particularly abundant. - The high content of **positively charged arginine residues** is critical for apoE's ability to bind to negatively charged lipid surfaces and interact with receptors such as the **LDL receptor** and **LDL receptor-related protein (LRP)**. - This arginine-rich composition is a defining characteristic of apoE and is essential for its role in **lipid metabolism** and **receptor-mediated lipoprotein uptake**. *Lysine* - While apoE does contain **lysine** (another basic amino acid), it is **arginine** that is particularly abundant and functionally emphasized. - Both lysine and arginine contribute positive charges, but **arginine residues** are specifically highlighted in apoE's **receptor binding domains** and are more characteristic of this apolipoprotein. *Histidine* - **Histidine** is also a **basic amino acid**, but it is not present in the same high proportions as **arginine** in apoE. - Its pKa (~6.0) is closer to physiological pH, meaning its charge state can vary, making it less consistently positive than arginine or lysine in biological contexts. - Histidine is not a defining feature of apoE's amino acid composition. *Methionine* - **Methionine** is a **sulfur-containing, nonpolar amino acid**, not a basic amino acid. - It does not contribute to the positive charge characteristic of apoE. - Its role in proteins is typically structural or as the initiator of protein synthesis (as the first amino acid), but it is not relevant to apoE's receptor-binding properties.
Question 77: Which type of bond is primarily responsible for the primary structure of a protein?
- A. Hydrogen bond
- B. Disulfide bond
- C. Peptide bond (Correct Answer)
- D. Electrostatic bond
Explanation: ***Peptide bond*** - The **primary structure** of a protein is defined by the unique linear sequence of **amino acids** linked together by **peptide bonds**. - These are **amide bonds** formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water. *Hydrogen bond* - **Hydrogen bonds** are crucial for the **secondary structure** (e.g., alpha-helices and beta-sheets) and **tertiary/quaternary structures** of proteins, stabilizing their 3D folds. - They involve interactions between polar atoms, not the direct linkage of amino acids in the primary sequence. *Disulfide bond* - **Disulfide bonds** are **covalent bonds** formed between the sulfur atoms of two **cysteine residues**, contributing to the **tertiary** and sometimes **quaternary structure** stability. - They are not involved in forming the linear sequence of amino acids, which is the primary structure. *Electrostatic bond* - **Electrostatic bonds**, or **ionic bonds**, occur between oppositely charged amino acid side chains and are important for **tertiary** and **quaternary structure** stability. - They do not form the backbone of the protein's primary sequence.
Question 78: Which amino acid has two chiral centers?
- A. Threonine (Correct Answer)
- B. Tyrosine
- C. Tryptophan
- D. Phenylalanine
Explanation: ***Threonine*** - Threonine is unique among the standard 20 amino acids because it possesses **two chiral centers**: one at the **alpha-carbon** and another at the **beta-carbon**. - The presence of two chiral centers means that threonine can exist as **four stereoisomers** (2^n, where n is the number of chiral centers). *Tryptophan* - Tryptophan has only **one chiral center**, which is the **alpha-carbon** bonded to the amino group, carboxyl group, hydrogen atom, and the side chain. - Its side chain, an **indole ring**, does not contain an additional chiral center. *Tyrosine* - Tyrosine, like most amino acids, possesses only **one chiral center** at its **alpha-carbon**. - The aromatic ring system (phenol group) in its side chain does not introduce another chiral center. *Phenylalanine* - Phenylalanine also has only **one chiral center** located at its **alpha-carbon**. - Its benzyl side chain, consisting of a methylene group and a benzene ring, is not chiral.
Question 79: Which of the following is a neutral amino acid?
- A. Aspartate
- B. Arginine
- C. Glycine (Correct Answer)
- D. Histidine
Explanation: ***Glycine*** - **Glycine** has a hydrogen atom as its side chain, making it the **simplest amino acid** and electrically neutral at physiological pH. - Its **nonpolar side chain** contributes to its neutral charge and allows it to fit into various protein structures. *Aspartate* - **Aspartate** is an **acidic amino acid** with a carboxyl group in its side chain. - This **carboxyl group** can lose a proton, giving aspartate a net negative charge at physiological pH. *Arginine* - **Arginine** is a **basic amino acid** characterized by a guanidinium group in its side chain. - The **guanidinium group** contains multiple nitrogen atoms that can accept protons, making arginine positively charged at physiological pH. *Histidine* - **Histidine** is classified as a **basic amino acid** due to the imidazole ring in its side chain. - The **imidazole ring** has a pKa close to physiological pH, allowing it to be protonated and positively charged, but it is not neutral.
Question 80: Amino acid with aliphatic side chain is?
- A. Serine
- B. Leucine (Correct Answer)
- C. Threonine
- D. Aspartate
Explanation: ***Leucine*** - Leucine has an **isobutyl group** (-CH2CH(CH3)2) as its side chain, making it a **nonpolar aliphatic amino acid**. - **Aliphatic amino acids** (glycine, alanine, valine, leucine, isoleucine, proline) have side chains consisting of only carbon and hydrogen atoms in straight or branched chains, with **no polar functional groups**. - These amino acids are **hydrophobic** and typically found in the interior of proteins. *Serine* - Serine has a **hydroxyl group** (-OH) in its side chain (-CH2OH), classifying it as a **polar uncharged amino acid**, not an aliphatic amino acid. - The hydroxyl group makes the side chain **hydrophilic** and capable of hydrogen bonding. - The presence of the polar functional group distinguishes it from aliphatic amino acids. *Threonine* - Threonine also contains a **hydroxyl group** (-OH) in its side chain (-CH(OH)CH3), making it a **polar uncharged amino acid**, not an aliphatic amino acid. - Like serine, the hydroxyl group provides **polarity and hydrogen bonding capacity**. - This functional group places it in a different classification from aliphatic amino acids. *Aspartate* - Aspartate has a **carboxyl group** (-COOH) in its side chain (-CH2COOH), making it an **acidic (negatively charged) amino acid**. - At physiological pH, this group is deprotonated (COO⁻), making aspartate **negatively charged**. - This clearly distinguishes it from nonpolar aliphatic amino acids.