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?
Serotonin is derived from -
Catecholamines are synthesized from?
Which of the following amino acids is not involved in the production of creatine?
Ninhydrin test is used for?
Which enzyme catalyzes oxidative deamination?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 91: 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 92: 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 93: 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 94: 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 95: 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 96: 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 97: Catecholamines are synthesized from?
- A. Tyrosine (Correct Answer)
- B. Histidine
- C. Methionine
- D. Tryptophan
Explanation: ***Tyrosine*** - **Tyrosine** is the direct precursor amino acid for the synthesis of all **catecholamines**, including **dopamine**, **norepinephrine**, and **epinephrine**. - The synthesis pathway begins with the conversion of tyrosine to **L-DOPA** by tyrosine hydroxylase, followed by subsequent enzymatic steps. *Methionine* - **Methionine** is an essential amino acid primarily involved in **protein synthesis** and as a precursor for S-adenosylmethionine (SAM), a key methyl donor in various metabolic reactions. - It is not a direct precursor for the synthesis of **catecholamines**. *Histidine* - **Histidine** is the precursor for the synthesis of **histamine**, a neurotransmitter and inflammatory mediator. - It is not involved in the biosynthesis pathway of **catecholamines**. *Tryptophan* - **Tryptophan** is the precursor for the synthesis of **serotonin** and **melatonin**, important neurotransmitters and hormones. - It does not play a role in the synthesis of **catecholamines**.
Question 98: Which of the following amino acids is not involved in the production of creatine?
- A. Glycine
- B. Methionine
- C. Alanine (Correct Answer)
- D. Arginine
Explanation: ***Alanine*** - **Alanine** is not directly involved as a precursor for **creatine synthesis**. It can be converted to pyruvate and enter the gluconeogenic pathway. - The primary amino acids involved in **creatine synthesis** are arginine, glycine, and methionine. *Glycine* - **Glycine** is a direct precursor for creatine, reacting with arginine in the first step of its synthesis to form **guanidinoacetate**. - This reaction is catalyzed by **arginine:glycine amidinotransferase (AGAT)**. *Methionine* - **Methionine**, in the form of **S-adenosylmethionine (SAM)**, acts as the methyl donor in the second step of creatine synthesis. - It methylates guanidinoacetate to form **creatine**, a reaction catalyzed by **guanidinoacetate methyltransferase (GAMT)**. *Arginine* - **Arginine** donates its guanidino group to glycine, forming **guanidinoacetate**, the initial intermediate in creatine synthesis. - This is the first committed step in the **creatine biosynthesis pathway**.
Question 99: Ninhydrin test is used for?
- A. Bile salts
- B. Amino acids (Correct Answer)
- C. Nucleic acid
- D. Lipids
Explanation: ***Amino acids*** - The **ninhydrin test** is a chemical test used to detect the presence of **amino acids** and primary and secondary amines. - It produces a **purple-blue color** when it reacts with most amino acids, due to the formation of a colored complex called Ruhemann's purple. *Bile salts* - The detection of **bile salts** typically involves tests like Hay's test or Pettenkofer's test, which are distinct from the ninhydrin reaction. - These tests rely on the physical or chemical properties of bile salts, such as changes in surface tension or specific color reactions with sulfuric acid. *Nucleic acid* - **Nucleic acids** (DNA and RNA) are detected using specific tests like the **diphenylamine test** (for DNA) or orcinol test (for RNA). - These tests target the deoxyribose or ribose sugars present in their structures and result in different color changes compared to ninhydrin. *Lipids* - **Lipids** are typically identified using tests that exploit their nonpolar nature, such as the **emulsion test** or solubility tests in organic solvents. - Their detection does not involve ninhydrin, as they lack the primary or secondary amine groups that react with this reagent.
Question 100: Which enzyme catalyzes oxidative deamination?
- A. Glutaminase
- B. Glutamine synthase
- C. Glutamate dehydrogenase (Correct Answer)
- D. None of the options
Explanation: ***Glutamate dehydrogenase*** - This enzyme catalyzes the conversion of **glutamate** to **α-ketoglutarate** and ammonia (NH₃), which is an oxidative deamination reaction. - It utilizes **NAD⁺ or NADP⁺** as a coenzyme to remove hydrogen atoms during the oxidation process. - Plays a crucial role in both **amino acid catabolism** and anabolism. *Glutaminase* - This enzyme hydrolyzes **glutamine** to glutamate and ammonia, which is a **hydrolytic deamidation** reaction, not an oxidative deamination. - It does not involve the oxidation of the substrate or require NAD⁺/NADP⁺ as cofactors. *Glutamine synthase* - This enzyme synthesizes **glutamine** from glutamate and ammonia, using ATP, which is a **biosynthetic** reaction, not a catabolic deamination. - It is involved in **ammonia detoxification** and amino acid synthesis, functioning in the opposite direction of deamination. *None of the options* - This option is incorrect because **glutamate dehydrogenase** is a valid correct answer. - Glutamate dehydrogenase is the primary enzyme responsible for oxidative deamination in human metabolism.