Protein Structure and Function Practice Questions

Q: Which of these amino acids will migrate slowest to the anode end at physiological pH?

Lysine. ### Explanation The migration of amino acids in an electric field depends on their **net charge** at a specific pH. At physiological pH (~7.4), the charge of an amino acid is determined by its side chain (R-group). **1. Why Lysine is Correct:** Lysine is a **basic amino acid**. At physiological pH (7.4), its side chain amino group is protonated, giving the molecule a **net positive charge**. Since "opposites attract," positively charged molecules (cations) migrate toward the negative electrode (cathode). Because it is moving away from or resisting the **anode (positive electrode)**, it will migrate the slowest toward it compared to neutral or acidic amino acids. **2. Analysis of Incorrect Options:** * **Aspartic Acid (Option A):** This is an **acidic amino acid**. At pH 7.4, its carboxyl side chain loses a proton, giving it a **net negative charge**. It will migrate the **fastest** toward the anode. * **Glycine (Option B) & Valine (Option D):** These are **neutral, non-polar amino acids**. At physiological pH, they exist primarily as zwitterions (net charge near zero). While they don't migrate as fast as Aspartic acid, they lack the positive repulsion of Lysine, thus moving faster toward the anode than Lysine would. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isoelectric Point (pI):** The pH at which an amino acid has no net charge and does not migrate in an electric field. * **Acidic AAs (Aspartate, Glutamate):** pI is low (~3); they are negatively charged at pH 7.4. * **Basic AAs (Lysine, Arginine, Histidine):** pI is high (>7.5–10); they are positively charged at pH 7.4. * **Mnemonic:** **"Anode is Positive" (Panic: Positive Anode, Negative Is Cathode).** Acidic amino acids love the Anode.

Q: Keratin in the nail is exceptionally stronger than other forms of keratin because of which type of bond?

Disulfide bonds. **Explanation:** The exceptional strength and hardness of "hard keratin" found in nails and hair, compared to "soft keratin" in the skin, is primarily due to the high concentration of the sulfur-containing amino acid **Cysteine**. 1. **Why Disulfide Bonds are Correct:** Keratin proteins are organized into intermediate filaments. In nails, these filaments are embedded in a high-sulfur protein matrix. Cysteine residues form strong, covalent **disulfide bonds (S-S)** between adjacent polypeptide chains. This cross-linking creates a rigid, chemically resistant, and mechanically tough structure that accounts for the nail's durability. 2. **Why Incorrect Options are Wrong:** * **Van der Waals forces:** These are weak, short-range intermolecular attractions. While they contribute to protein folding, they are far too weak to provide the structural rigidity characteristic of nails. * **Ionic bonds:** These are electrostatic attractions between charged side chains. Though stronger than Van der Waals forces, they are easily disrupted by changes in pH and are not the primary source of keratin's structural integrity. * **Calcification:** This is a common distractor. Unlike bone or teeth, the hardness of the nail is **not** due to calcium. Nails contain only trace amounts of calcium; their hardness is purely protein-derived (keratinization). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hard vs. Soft Keratin:** Hard keratin (nails/hair) has a higher sulfur content and does not desquamate, whereas soft keratin (stratum corneum) has lower sulfur and undergoes regular shedding. * **Nail Growth:** Nails grow from the **nail matrix**. It takes approximately 6 months for a fingernail and 12–18 months for a toenail to grow out completely. * **Clinical Sign:** Brittle nails or "koilonychia" (spoon-shaped nails) are high-yield associations with **Iron Deficiency Anemia**.

Q: Which of the following contains sulfur?

Insulin. **Explanation:** **Why Insulin is Correct:** Insulin is a peptide hormone consisting of two polypeptide chains (A and B) linked together by **disulfide bonds**. These bonds are formed between the sulfur atoms of **Cysteine** residues. Specifically, insulin contains three disulfide bridges: two inter-chain (connecting the A and B chains) and one intra-chain (within the A chain). These sulfur-sulfur linkages are critical for the structural stability and biological activity of the hormone. **Analysis of Incorrect Options:** * **Creatine:** This is a nitrogenous organic acid derived from amino acids (Arginine, Glycine, and Methionine). While Methionine (a sulfur-containing amino acid) provides the methyl group for its synthesis, the final structure of Creatine itself does not contain sulfur. * **Inulin:** Often confused with insulin due to the name, inulin is a **polysaccharide** (a polymer of fructose). It is a carbohydrate used clinically to measure the Glomerular Filtration Rate (GFR) and contains no sulfur. * **Creatinine:** This is the anhydride form of creatine, produced by the non-enzymatic breakdown of creatine phosphate in muscles. Like creatine, it is a nitrogenous compound but lacks sulfur. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur-containing amino acids:** Cysteine and Methionine. (Note: Only Cysteine can form disulfide bonds). * **Insulin Structure:** Chain A (21 amino acids) and Chain B (30 amino acids). * **C-peptide:** During the conversion of Proinsulin to Insulin, the C-peptide is cleaved. It is a key marker for endogenous insulin production as it is secreted in a 1:1 ratio with insulin. * **Other Sulfur-containing biomolecules:** Biotin, Thiamine (Vitamin B1), Coenzyme A, and Glutathione.

Q: Defect in collagen formation is seen in which condition?

Scurvy. **Explanation:** **Scurvy** is the correct answer because it is caused by a deficiency of **Vitamin C (Ascorbic acid)**, which is a mandatory co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the post-translational hydroxylation of proline and lysine residues in the pro-alpha chains of collagen. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. Without Vitamin C, collagen fibers are under-hydroxylated, leading to poor cross-linking, reduced tensile strength, and fragile connective tissues (manifesting as bleeding gums and poor wound healing). **Why other options are incorrect:** * **Hunter’s Syndrome:** This is a **Mucopolysaccharidosis (Type II)** caused by a deficiency of Iduronate-2-sulfatase. It involves a defect in the degradation of Glycosaminoglycans (GAGs), specifically dermatan and heparan sulfate, not collagen formation. * **Marfan’s Syndrome:** This is an autosomal dominant disorder caused by a mutation in the **FBN1 gene**, which encodes **Fibrillin-1**. Fibrillin-1 is a major component of microfibrils that act as a scaffold for **elastin**, not collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Collagen Synthesis:** Hydroxylation occurs in the **Endoplasmic Reticulum (ER)** and is a post-translational modification. * **Cross-linking:** The final step of collagen stabilization (covalent cross-linking) occurs extracellularly and is catalyzed by the copper-dependent enzyme **Lysyl oxidase**. * **Osteogenesis Imperfecta:** Caused by defects in Type I collagen. * **Ehlers-Danlos Syndrome:** A heterogeneous group of disorders; the vascular type (Type IV) involves defects in Type III collagen.

Q: Which type of collagen is predominantly found in the brain?

Type XXV. **Explanation:** The correct answer is **Type XXV collagen**, also known as **CLAC-P** (Collagen-like Alzheimer amyloid plaque component). Unlike the common fibrillar collagens found in connective tissue, Type XXV is a specialized **membrane-associated collagen with interrupted triple helices (MACIT)**. It is primarily expressed in neurons within the central nervous system. Its physiological role involves synapse formation and maintenance; however, it is most notable for binding to amyloid-beta peptides, promoting their aggregation into the insoluble plaques characteristic of Alzheimer’s disease. **Analysis of Incorrect Options:** * **Type IV:** This is the main structural component of the **basal lamina** (basement membrane). While present in the brain, it is restricted to the blood-brain barrier (cerebral microvasculature) rather than the neural parenchyma itself. * **Type VII:** This forms **anchoring fibrils** that connect the epidermis to the dermis. It is found in the dermo-epidermal junction, not the brain. * **Type XXVI:** This is a specialized collagen primarily expressed in the **testis and ovary**, involved in reproductive system development. **Clinical Pearls for NEET-PG:** * **High-Yield Fact:** Type XXV collagen is the only collagen specifically linked to the pathogenesis of **Alzheimer’s Disease** due to its presence in senile plaques [1]. * **Collagen Distribution:** Remember **"Type One is in Bone"** and **"Type Two is in Cartilage (Car-two-lage)."** * **Brain Stroma:** The brain is unique because it lacks a traditional collagenous stroma; its structural integrity is instead maintained by **glial cells** (astrocytes). Collagen in the CNS is largely limited to the meninges and blood vessel walls, with Type XXV being the notable neuronal exception.

Q: Which of the following amino acids can be phosphorylated?

Serine. **Explanation:** **1. Why Serine is Correct:** Phosphorylation is a critical post-translational modification (PTM) where a phosphate group is added to an amino acid residue by enzymes called **kinases**. For this reaction to occur, the amino acid must possess a free **hydroxyl (-OH) group** in its side chain. **Serine**, along with **Threonine** and **Tyrosine**, contains this essential hydroxyl group, making them the primary targets for phosphorylation in eukaryotic cells. This process acts as a molecular "on/off" switch, regulating enzyme activity and signal transduction pathways. **2. Why the Other Options are Incorrect:** * **Cysteine (A):** Contains a **sulfhydryl (-SH) group**. While it is involved in forming disulfide bonds and can undergo palmitoylation, it is not a standard target for phosphorylation. * **Leucine (B):** An aliphatic, branched-chain amino acid with a non-polar hydrocarbon side chain. It lacks the functional group required for phosphate attachment. * **Methionine (C):** A sulfur-containing amino acid (thioether). Like Leucine, it lacks a hydroxyl group and cannot be phosphorylated. **3. Clinical Pearls & High-Yield Facts:** * **The "Big Three":** Always remember **Serine, Threonine, and Tyrosine** as the amino acids that undergo phosphorylation. * **Enzymes:** Phosphorylation is catalyzed by **Kinases** (using ATP), while the removal of phosphate is catalyzed by **Phosphatases**. * **Clinical Relevance:** Dysregulation of protein phosphorylation (especially by Tyrosine Kinases) is a hallmark of many cancers. Drugs like **Imatinib** (a tyrosine kinase inhibitor) are used in treating Chronic Myeloid Leukemia (CML). * **Other PTMs:** * **N-glycosylation** occurs on Asparagine. * **O-glycosylation** occurs on Serine or Threonine. * **Hydroxylation** (important for collagen) occurs on Proline and Lysine (requires Vitamin C).

Q: What is the number of amino acids in the A chain and B chain of insulin?

21, 30. **Explanation:** Insulin is a peptide hormone synthesized in the beta cells of the islets of Langerhans in the pancreas. It is initially produced as a single-chain precursor called **Preproinsulin**, which undergoes post-translational modifications to become active insulin. **1. Why Option D is Correct:** Mature human insulin consists of **51 amino acids** arranged in two polypeptide chains: * **A chain (Acidic):** Contains **21 amino acids**. * **B chain (Basic):** Contains **30 amino acids**. These two chains are linked together by two interchain disulfide bridges (A7-B7 and A20-B19). There is also one intrachain disulfide bridge within the A chain (A6-A11). **2. Why Other Options are Incorrect:** * **Option A (30, 21):** This reverses the order. In biochemistry nomenclature, the A chain is always the shorter chain (21) and the B chain is the longer one (30). * **Options B & C:** These numbers do not correspond to the structural components of human insulin. While different species (like bovine or porcine insulin) have variations in specific amino acid sequences, the total count of 21 and 30 remains highly conserved across mammals. **3. High-Yield Clinical Pearls for NEET-PG:** * **Proinsulin:** Consists of A chain, B chain, and a **C-peptide** (Connecting peptide). * **C-peptide:** It contains **31 amino acids**. It is cleaved from proinsulin to form active insulin and is secreted in equimolar amounts with insulin. * **Clinical Utility:** Measuring C-peptide levels helps distinguish between Type 1 Diabetes (low C-peptide) and Type 2 Diabetes (normal/high C-peptide), and identifies factitious hypoglycemia (exogenous insulin lacks C-peptide). * **Zinc:** Insulin is stored in pancreatic granules as a **hexamer** coordinated with zinc ions.

Q: Which of the following proteins contains a triple helix structure?

Collagen. **Explanation:** **Collagen** is the correct answer because it is characterized by a unique **triple helix** quaternary structure (also known as a tropocollagen molecule). This structure consists of three polypeptide alpha-chains wound around each other. The stability of this helix is maintained by hydrogen bonds and the repetitive amino acid sequence **Gly-X-Y**, where Glycine (the smallest amino acid) fits into the restricted space of the helix core, and X and Y are typically Proline and Hydroxyproline. **Analysis of Incorrect Options:** * **Cystine (A):** This is a sulfur-containing amino acid formed by the oxidation of two cysteine molecules joined by a disulfide bond. It does not form a triple helix. * **Pectin (C):** This is a complex structural polysaccharide (carbohydrate) found in the primary cell walls of terrestrial plants. It is not a protein. * **DNA (D):** While DNA is a helical structure, it is a **double helix** composed of two polynucleotide strands, not a triple-stranded protein. **Clinical Pearls for NEET-PG:** * **Most Abundant Protein:** Collagen is the most abundant protein in the human body (approx. 25-30%). * **Vitamin C Role:** Vitamin C is a vital cofactor for the **hydroxylation** of proline and lysine residues; deficiency leads to **Scurvy** due to defective triple helix stabilization. * **Types to Remember:** * Type I: Bone, Skin, Tendon (90% of body collagen). * Type II: Cartilage. * Type III: Reticulin (Blood vessels, granulation tissue) [1]. * Type IV: Basement membrane ("4 is on the floor"). * **Genetic Disorders:** Osteogenesis Imperfecta (Type I defect) and Ehlers-Danlos Syndrome (Type III or V defects) are high-yield associations [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 154-156.

Q: Trimming and further modification of N-linked glycoproteins takes place at which cellular organelle?

Golgi apparatus. **Explanation:** The synthesis of N-linked glycoproteins is a highly coordinated process occurring in two distinct stages across the Endomembrane system. **1. Why Golgi Apparatus is Correct:** While the initial core glycosylation begins in the Rough Endoplasmic Reticulum (RER), the **Golgi apparatus** is the primary site for the **trimming** of mannose residues and the **further modification** (addition of galactose, sialic acid, and fucose). This "post-translational processing" occurs as proteins move from the *cis* to the *trans* Golgi cisternae, resulting in complex or hybrid oligosaccharide chains. **2. Analysis of Incorrect Options:** * **Endoplasmic Reticulum (ER):** This is the site of **initial** N-glycosylation. A pre-formed 14-sugar precursor (GlcNAc, mannose, and glucose) is transferred from a **Dolichol** lipid carrier to the Asparagine residue of the protein. Only minor trimming of glucose occurs here. * **Peroxisomes:** These are involved in long-chain fatty acid oxidation (VLCFA) and hydrogen peroxide metabolism, not protein glycosylation. * **Lysosomes:** These organelles are responsible for the **degradation** of glycoproteins via acid hydrolases, not their synthesis or modification. **3. NEET-PG High-Yield Pearls:** * **N-linked glycosylation** occurs on the **Asparagine** residue (Asn-X-Ser/Thr motif). * **O-linked glycosylation** occurs exclusively in the **Golgi** on Serine or Threonine residues. * **I-Cell Disease:** A high-yield clinical correlate where a deficiency in *phosphotransferase* in the Golgi fails to phosphorylate mannose residues. This prevents proteins from being targeted to lysosomes, leading to inclusion bodies and skeletal deformities. * **Tunicamycin** is an antibiotic that inhibits N-linked glycosylation by blocking the synthesis of the Dolichol-PP-GlcNAc intermediate.

Question 1

Which of these amino acids will migrate slowest to the anode end at physiological pH?

Accepted Answer

Lysine

Answer

Aspartic acid

Answer

Glycine

Answer

Valine

Question 2

Keratin in the nail is exceptionally stronger than other forms of keratin because of which type of bond?

Accepted Answer

Disulfide bonds

Answer

Van der Waals forces

Answer

Ionic bonds

Answer

Calcification

Question 3

Which of the following contains sulfur?

Accepted Answer

Insulin

Answer

Creatine

Answer

Inulin

Answer

Creatinine

Question 4

Defect in collagen formation is seen in which condition?

Accepted Answer

Scurvy

Answer

Hunter's syndrome

Answer

Marfan's syndrome

Answer

All of the above

Question 5

Which type of collagen is predominantly found in the brain?

Accepted Answer

Type XXV

Answer

Type IV

Answer

Type VII

Answer

Type XXVI

Question 6

Which of the following amino acids can be phosphorylated?

Accepted Answer

Serine

Answer

Cysteine

Answer

Leucine

Answer

Methionine

Question 7

What is the number of amino acids in the A chain and B chain of insulin?

Accepted Answer

21, 30

Answer

30, 21

Answer

28, 32

Answer

32, 28

Question 8

Which of the following proteins contains a triple helix structure?

Accepted Answer

Collagen

Answer

Cystine

Answer

Pectin

Answer

DNA

Question 9

Trimming and further modification of N-linked glycoproteins takes place at which cellular organelle?

Accepted Answer

Golgi apparatus

Answer

Endoplasmic reticulum

Answer

Peroxisomes

Answer

Lysosomes

Question 10

Defect in collagen formation is seen in which of the following conditions?

Accepted Answer

Scurvy

Answer

Hunter's syndrome

Answer

Marfan's syndrome

Answer

Osteogenesis imperfecta

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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