Which of the following enzymes is classified as a serine protease?
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?
Which transporter is responsible for the transport of glucose in the pancreas?
Amino acid with aliphatic side chain is?
Serotonin is derived from -
Fructose intolerance is due to deficiency of which enzyme?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 41: Which of the following enzymes is classified as a serine protease?
- A. Pepsin
- B. Trypsin (Correct Answer)
- C. Carboxypeptidase
- D. None of the options
Explanation: ***Trypsin*** - **Trypsin** is a digestive enzyme belonging to the **serine protease** family, characterized by a crucial **serine residue** in its active site. - It plays a vital role in protein digestion in the small intestine, cleaving peptide bonds on the carboxyl side of **lysine** or **arginine** residues. *Pepsin* - **Pepsin** is an aspartic protease, meaning it utilizes an **aspartate residue** in its active site for catalysis. - It primarily functions in the stomach, digesting proteins into smaller peptides in an **acidic environment**. *Carboxypeptidase* - **Carboxypeptidase** is a **metalloexopeptidase** that contains a zinc ion in its active site. - It removes amino acids one by one from the **carboxyl-terminal** end of polypeptide chains. *None of the options* - This option is incorrect because **trypsin** is indeed a well-known example of a serine protease.
Question 42: 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 43: 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 44: 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 45: 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 46: 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 47: Which transporter is responsible for the transport of glucose in the pancreas?
- A. GLUT 1
- B. GLUT 2 (Correct Answer)
- C. GLUT 3
- D. GLUT 4
Explanation: ***GLUT 2*** - **GLUT2** is a **low-affinity, high-capacity** glucose transporter primarily found in the **pancreatic beta cells**, liver, small intestine, and kidneys. - In pancreatic beta cells, GLUT2 allows rapid entry of glucose for metabolism, leading to **insulin secretion** in response to elevated blood glucose levels. *GLUT 1* - **GLUT1** is a **ubiquitous glucose transporter** found in most tissues, including red blood cells and the blood-brain barrier. - It has a high affinity for glucose, ensuring **basal glucose uptake** even at low concentrations. *GLUT 3* - **GLUT3** is a **high-affinity glucose transporter** concentrated in **neurons** and the brain. - Its efficient glucose uptake is critical for the constant and high energy demands of the central nervous system. *GLUT 4* - **GLUT4** is an **insulin-dependent glucose transporter** primarily found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**. - Insulin stimulates the translocation of GLUT4 to the cell membrane, facilitating glucose uptake from the blood after a meal.
Question 48: 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.
Question 49: Serotonin is derived from -
- A. Tyrosine
- B. Tryptophan (Correct Answer)
- C. Phenylalanine
- D. Methionine
Explanation: ***Tryptophan*** - **Serotonin**, also known as 5-hydroxytryptamine (5-HT), is synthesized from the essential amino acid **tryptophan** through a two-step enzymatic pathway. - Tryptophan is first hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan (5-HTP), which is then decarboxylated by L-amino acid decarboxylase to form serotonin. *Tyrosine* - **Tyrosine** is a precursor for the synthesis of **catecholamines** (dopamine, norepinephrine, and epinephrine) and thyroid hormones. - It is not involved in the synthesis pathway for serotonin. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is hydroxylated to form **tyrosine**. - Therefore, it is indirectly involved in catecholamine synthesis but not in serotonin synthesis. *Methionine* - **Methionine** is an essential amino acid primarily known for its role in protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl group donor in many biological reactions. - It does not serve as a direct precursor for serotonin.
Question 50: Fructose intolerance is due to deficiency of which enzyme?
- A. Aldolase B (Correct Answer)
- B. Aldolase A
- C. Fructokinase
- D. Triokinase
Explanation: ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.