Protein Structure and Function Practice Questions

Q: Aspartame contains which of the following amino acids?

Phenylalanine. **Explanation:** **Aspartame** is a low-calorie artificial sweetener widely used in the food industry. Chemically, it is a **methyl ester of a dipeptide**, consisting of two amino acids: **L-aspartic acid** and **L-phenylalanine**. 1. **Why Phenylalanine is Correct:** Upon ingestion, aspartame is broken down in the GI tract into its constituent parts: aspartate, phenylalanine, and methanol. Because it releases phenylalanine, it is a significant clinical concern for patients with **Phenylketonuria (PKU)**. These individuals lack the enzyme *phenylalanine hydroxylase* and cannot metabolize this amino acid, leading to toxic accumulation and intellectual disability. This is why products containing aspartame must carry a warning label for "Phenylketonurics." 2. **Why Other Options are Incorrect:** * **Histidine, Tryptophan, and Tyrosine:** These are not components of the aspartame molecule. While they are essential or semi-essential amino acids involved in various metabolic pathways (e.g., Tryptophan is a precursor for Serotonin; Tyrosine for Catecholamines), they do not form the chemical structure of this specific sweetener. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Aspartyl-phenylalanine-1-methyl ester. * **Caloric Value:** It provides 4 kcal/g (same as proteins), but because it is ~200 times sweeter than sucrose, it is used in negligible amounts. * **Heat Lability:** Aspartame is **heat-unstable**; it loses its sweetness upon heating, making it unsuitable for baking. * **Methanol Toxicity:** A minor byproduct of aspartame metabolism is methanol, though the amount produced in dietary doses is generally considered sub-toxic.

Q: Which among the following is the shortest peptide?

Angiotensin-III. To answer this question, one must know the specific amino acid counts of common biologically active peptides, particularly those in the Renin-Angiotensin-Aldosterone System (RAAS). ### **Explanation of the Correct Answer** **Angiotensin-III** is a **heptapeptide**, meaning it consists of **7 amino acids**. It is formed by the action of aminopeptidase on Angiotensin-II (which removes one amino acid from the N-terminus). While less potent than Angiotensin-II in terms of vasoconstriction, it is equally potent in stimulating aldosterone secretion. Among the given options, it has the fewest number of amino acids. ### **Analysis of Incorrect Options** * **Angiotensin-I (Option D):** This is a **decapeptide (10 amino acids)**. It is the inactive precursor formed when Renin acts on Angiotensinogen. * **Angiotensin-II (Option C):** This is an **octapeptide (8 amino acids)**. It is formed from Angiotensin-I by the action of Angiotensin-Converting Enzyme (ACE), which removes two C-terminal amino acids. * **Vasopressin (Option B):** Also known as Antidiuretic Hormone (ADH), it is a **nonapeptide (9 amino acids)** produced in the hypothalamus and stored in the posterior pituitary. ### **High-Yield Clinical Pearls for NEET-PG** * **RAAS Cascade Summary:** Angiotensinogen (452 aa) → Angiotensin I (10 aa) → Angiotensin II (8 aa) → Angiotensin III (7 aa). * **Glutathione:** A very common high-yield **tripeptide** (3 aa: Glutamate, Cysteine, Glycine). * **TRH (Thyrotropin-Releasing Hormone):** One of the shortest peptide hormones (3 aa). * **Oxytocin:** Like Vasopressin, it is also a **nonapeptide** (9 aa). * **Insulin:** A 51-amino acid polypeptide consisting of two chains (A chain: 21, B chain: 30) linked by disulfide bridges.

Q: Which is the 21st amino acid?

Selenocysteine. **Explanation:** **Selenocysteine (Sec)** is recognized as the **21st amino acid** because it is incorporated into proteins during translation, unlike other non-standard amino acids that are formed via post-translational modification. * **The Mechanism:** It is unique because it is encoded by the **UGA codon**, which normally acts as a "Stop" codon. However, in the presence of a specific mRNA structure called the **SECIS element** (Selenocysteine Insertion Sequence), the ribosome incorporates Selenocysteine instead of terminating translation. It contains **selenium** in place of the sulfur atom found in cysteine. **Analysis of Incorrect Options:** * **A. Alanine:** One of the 20 standard amino acids encoded by the universal genetic code. * **B. Cystine:** This is a post-translational product formed by the oxidation of two cysteine residues (disulfide bond). It is not considered a "genetically encoded" amino acid. * **C. Arginine:** A standard basic amino acid involved in the urea cycle and nitric oxide synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **The 22nd Amino Acid:** **Pyrrolysine** is considered the 22nd amino acid (found in some archaea and bacteria, encoded by the UAG codon). * **Key Selenoproteins:** Human enzymes containing Selenocysteine include **Glutathione Peroxidase** (antioxidant), **Thioredoxin Reductase**, and **Deiodinases** (converts T4 to T3). * **tRNA Specificity:** Selenocysteine has its own specialized tRNA (tRNA^Sec), which is first charged with Serine and then enzymatically converted to Selenocysteine.

Q: Which of the following is NOT true about selenocysteine?

It is synthesized from cysteine and methionine.. **Explanation:** Selenocysteine (Sec) is a unique amino acid that contains selenium instead of the sulfur found in cysteine. It is essential for the catalytic activity of several human enzymes. **Why Option D is correct (The False Statement):** Selenocysteine is **not** synthesized from cysteine or methionine. Instead, it is synthesized while attached to its unique tRNA (tRNA^Sec). The process begins with the charging of tRNA^Sec with **Serine**. This seryl-tRNA is then enzymatically converted to selenocysteinyl-tRNA using selenophosphate as the selenium donor. Thus, **Serine** is the carbon skeleton precursor, not cysteine. **Analysis of Incorrect Options:** * **Option A:** It is officially recognized as the **21st amino acid** because it is incorporated into proteins during translation, unlike post-translationally modified amino acids (like hydroxyproline). * **Option B:** It is encoded by the **UGA codon**, which normally functions as a "Stop" codon. For UGA to code for selenocysteine, a specific mRNA secondary structure called the **SECIS element** (Selenocysteine Insertion Sequence) must be present in the 3' untranslated region. * **Option C:** It is a critical component of the active site of **Glutathione Peroxidase**, an enzyme that protects cells from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzymes containing Sec:** Glutathione peroxidase, Thioredoxin reductase, and Deiodinases (converting T4 to T3). * **Selenium Deficiency:** Can lead to **Keshan disease** (cardiomyopathy). * **The 22nd Amino Acid:** Pyrrolysine (found in some archaea, coded by UAG). * **Translation Factor:** A specialized elongation factor (**eEFSec**) is required for its insertion.

Q: Which of the following sequence is directed in a retrograde manner to the ER in COP-I vesicles?

KDEL. ### Explanation **Concept Overview:** In cellular biology, proteins are synthesized in the Endoplasmic Reticulum (ER) and transported to the Golgi apparatus. However, some proteins are "ER-resident" and must be returned if they escape. This process is called **retrograde transport**. **Why KDEL is Correct:** The **KDEL sequence** (Lysine-Aspartic Acid-Glutamic Acid-Leucine) is a specific C-terminal retrieval signal found on soluble ER-resident proteins (e.g., BiP, Protein Disulfide Isomerase). * **Mechanism:** When these proteins reach the Golgi, they bind to **KDEL receptors**. * **Vesicle Type:** This receptor-ligand complex is packaged into **COP-I coated vesicles**, which move in a **retrograde** direction (Golgi → ER) to return the protein to its functional home. **Analysis of Incorrect Options:** * **B, C, and D (KDAL, DALK, KDUL):** These are incorrect amino acid sequences. The retrieval system is highly sequence-specific. Even a single amino acid substitution (like Alanine for Glutamic Acid) prevents the KDEL receptor from recognizing the protein, leading to its accidental secretion from the cell. **High-Yield Clinical Pearls for NEET-PG:** * **COP-I vs. COP-II:** Remember **"Two (II) steps forward, One (I) step back."** COP-II is for anterograde transport (ER → Golgi); COP-I is for retrograde transport (Golgi → ER). * **Clathrin:** Involved in transport from the Trans-Golgi Network to endosomes/lysosomes and receptor-mediated endocytosis from the plasma membrane. * **I-Cell Disease:** A high-yield pathology related to protein trafficking where a defect in phosphotransferase prevents the "Mannose-6-Phosphate" tag, causing lysosomal enzymes to be secreted extracellularly instead of being directed to lysosomes.

Q: Which of the following amino acids is both polar and ionic?

Arginine. **Explanation:** Amino acids are classified based on the properties of their R-groups (side chains) at physiological pH (~7.4). **1. Why Arginine is Correct:** Arginine is a **basic amino acid**. It contains a guanidino group in its side chain which remains protonated at physiological pH, giving it a positive charge. Because it carries a charge, it is **ionic**. Additionally, all ionic amino acids are inherently **polar** because their charges allow them to form strong hydrogen bonds and electrostatic interactions with water. **2. Analysis of Incorrect Options:** * **Asparagine (B) & Glutamine (C):** These are **polar but non-ionic (uncharged)**. They contain amide groups in their side chains. While these groups are polar and can participate in hydrogen bonding, they do not gain or lose protons at physiological pH, meaning they carry no net charge. * **Leucine (D):** This is a **non-polar, hydrophobic** amino acid. Its side chain consists of a branched hydrocarbon (isobutyl group), which does not interact favorably with water and carries no charge. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Basic" Trio:** Arginine, Lysine, and Histidine are the three basic amino acids. Arginine is the most basic (highest pKa). * **Urea Cycle:** Arginine is a key intermediate in the urea cycle and is the immediate precursor of **Nitric Oxide (NO)** and **Urea**. * **Histones:** Arginine and Lysine are found in high concentrations in histones. Their positive charge allows them to bind strongly to the negatively charged phosphate backbone of DNA. * **Ketogenic vs. Glucogenic:** Leucine is one of the two purely ketogenic amino acids (the other being Lysine).

Q: Crystallization and storage of insulin requires which ion?

Zn++. **Explanation:** **Why Zn++ is the correct answer:** Insulin is synthesized in the pancreatic β-cells as a monomer, but for efficient storage within secretory granules, it must be stabilized. **Zinc (Zn++)** plays a critical role in this process. Two zinc ions coordinate with six insulin molecules to form a stable **hexameric structure**. This hexamerization allows for the dense packing and crystallization of insulin, protecting it from degradation and maintaining a concentrated reservoir ready for secretion. Once released into the bloodstream, the hexamer dissociates back into active monomers. **Analysis of Incorrect Options:** * **Mn++ (Manganese):** Primarily acts as a cofactor for enzymes like pyruvate carboxylase and superoxide dismutase; it has no structural role in insulin storage. * **Cu++ (Copper):** Essential for enzymes like cytochrome c oxidase and lysyl oxidase, but it is not involved in the hexameric stabilization of insulin. * **Ca++ (Calcium):** While calcium is crucial for the **exocytosis** (release) of insulin granules from the β-cell, it is not the ion responsible for the crystallization and internal storage structure of the hormone itself. **High-Yield Clinical Pearls for NEET-PG:** * **Hexameric vs. Monomeric:** Pharmaceutical insulin analogs are modified based on this principle. **Rapid-acting analogs** (Lispro, Aspart) are designed to resist hexamer formation for faster absorption, while **long-acting analogs** (Glargine) promote precipitation/crystallization at the injection site. * **C-Peptide:** Remember that insulin and C-peptide are secreted in equimolar amounts; C-peptide levels are used to differentiate endogenous insulin production from exogenous administration. * **Zinc Deficiency:** Can lead to impaired glucose tolerance due to decreased insulin storage efficiency.

Q: Which of the following is an example of a chromoprotein?

Hemoglobin. **Explanation:** **Chromoproteins** are a type of **conjugated protein** where the protein molecule is combined with a non-protein prosthetic group that is a colored pigment. 1. **Why Hemoglobin is Correct:** Hemoglobin is the classic example of a chromoprotein. It consists of a protein part (**globin**) and a prosthetic group called **heme** (an iron-porphyrin complex). The heme group imparts the characteristic red color to the protein, which is essential for its function in oxygen transport. 2. **Analysis of Incorrect Options:** * **Sturine (Option B):** This is a **protamine**, which is a type of simple protein. Protamines are basic, low-molecular-weight proteins found associated with DNA in the sperm of some fish (like sturgeon). * **Nuclein (Option C):** This is a **nucleoprotein**. It is a conjugated protein where the prosthetic group is a nucleic acid (DNA or RNA), not a pigment. * **Gliadin (Option D):** This is a **prolamine**, a type of simple protein found in wheat (gluten). It is soluble in 70-80% ethanol and is notable for its high proline and glutamine content. **High-Yield Clinical Pearls for NEET-PG:** * **Other Chromoproteins:** Myoglobin (red), Cytochromes (red/brown), Rhodopsin (visual purple), and Flavoproteins (yellow). * **Classification Tip:** Always distinguish between **Simple proteins** (only amino acids, e.g., Albumin, Globulin) and **Conjugated proteins** (Protein + Prosthetic group). * **Clinical Correlation:** Gliadin is the primary protein fraction responsible for the inflammatory response in **Celiac Disease**.

Q: What is the approximate half-life of plasma albumin?

20 days. **Explanation:** Albumin is the most abundant plasma protein, synthesized exclusively by the liver. The correct answer is **20 days** (Option B), which represents the typical physiological half-life of albumin in a healthy adult. **Why 20 days is correct:** Albumin’s long half-life is primarily due to its interaction with the **neonatal Fc receptor (FcRn)**. Instead of being degraded after endocytosis by vascular endothelial cells, albumin binds to FcRn in acidic endosomes and is recycled back into the circulation. This mechanism prevents rapid catabolism and maintains high serum concentrations (3.5–5.0 g/dL). **Why other options are incorrect:** * **7 days (Option A):** This is too short for albumin but is closer to the half-life of other plasma proteins like Transferrin (approx. 8 days) or Prealbumin (Transthyretin), which has a very short half-life of ~2 days. * **60–90 days (Options C & D):** These durations are far too long for plasma proteins. For context, 90–120 days is the average lifespan of a Red Blood Cell (RBC), not a soluble protein. **High-Yield Clinical Pearls for NEET-PG:** * **Marker of Nutrition:** Because of its long half-life (20 days), albumin is a **poor marker** for acute nutritional changes. **Prealbumin** (half-life: 2 days) is the preferred marker for monitoring acute nutritional support. * **Functions:** Albumin provides 80% of the **plasma oncotic pressure** and acts as a non-specific transport protein for bilirubin, fatty acids, calcium, and drugs (e.g., warfarin, phenytoin). * **Clinical Correlation:** Hypoalbuminemia leads to edema (due to decreased oncotic pressure) and is a hallmark of chronic liver disease, nephrotic syndrome, and protein-energy malnutrition (Kwashiorkor).

Q: Selenocysteine is structurally similar to which other amino acid?

Cysteine. **Explanation:** **Selenocysteine (Sec)**, often referred to as the **21st amino acid**, is an analogue of the proteinogenic amino acid **Cysteine**. 1. **Why Cysteine is Correct:** Structurally, Selenocysteine is identical to Cysteine, with one critical substitution: the sulfur atom in the thiol group (-SH) of Cysteine is replaced by a **selenium atom**, forming a **selenol group (-SeH)**. This substitution lowers the pKa of the side chain, making Selenocysteine a more efficient nucleophile and a potent catalyst in redox reactions. 2. **Why Other Options are Incorrect:** * **Arginine and Lysine:** These are basic, positively charged amino acids with long, complex side chains (guanidino and amino groups, respectively) that bear no structural resemblance to the simple three-carbon structure of Selenocysteine. * **Alanine:** While Alanine is the structural "backbone" of many amino acids, it lacks the reactive functional group (sulfur or selenium) found in Cysteine and Selenocysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Coding:** Unlike other non-standard amino acids, Selenocysteine is incorporated during translation via the **UGA stop codon**. This requires a specific mRNA secondary structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **tRNA:** It is synthesized while attached to its unique tRNA, **tRNA[Ser]Sec**. * **Key Selenoproteins:** Important human enzymes containing Selenocysteine include **Glutathione peroxidase** (antioxidant defense), **Thioredoxin reductase**, and **Deiodinase** (converts T4 to T3). * **Deficiency:** Low selenium levels can lead to **Keshan disease** (cardiomyopathy).

Question 1

Aspartame contains which of the following amino acids?

Accepted Answer

Phenylalanine

Answer

Histidine

Answer

Tryptophan

Answer

Tyrosine

Question 2

Which among the following is the shortest peptide?

Accepted Answer

Angiotensin-III

Answer

Vasopressin

Answer

Angiotensin-II

Answer

Angiotensin-I

Question 3

Which is the 21st amino acid?

Accepted Answer

Selenocysteine

Answer

Alanine

Answer

Cystine

Answer

Arginine

Question 4

Which of the following is NOT true about selenocysteine?

Accepted Answer

It is synthesized from cysteine and methionine.

Answer

It is considered the 21st amino acid.

Answer

It is coded by the UGA codon.

Answer

It occurs in glutathione peroxidase.

Question 5

Which of the following sequence is directed in a retrograde manner to the ER in COP-I vesicles?

Accepted Answer

KDEL

Answer

KDAL

Answer

DALK

Answer

KDUL

Question 6

Which of the following amino acids is both polar and ionic?

Accepted Answer

Arginine

Answer

Asparagine

Answer

Glutamine

Answer

Leucine

Question 7

Crystallization and storage of insulin requires which ion?

Accepted Answer

Zn++

Answer

Mn++

Answer

Cu++

Answer

Ca++

Question 8

Which of the following is an example of a chromoprotein?

Accepted Answer

Hemoglobin

Answer

Sturine

Answer

Nuclein

Answer

Gliadin

Question 9

What is the approximate half-life of plasma albumin?

Accepted Answer

20 days

Answer

7 days

Answer

60 days

Answer

90 days

Question 10

Selenocysteine is structurally similar to which other amino acid?

Accepted Answer

Cysteine

Answer

Arginine

Answer

Alanine

Answer

Lysine

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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