Protein Structure and Function Practice Questions

Q: Which of the following signals directs a protein to the lysosomes?

Attached carbohydrate with terminal mannose-6-phosphate. ### Explanation **Correct Answer: C. Attached carbohydrate with terminal mannose-6-phosphate** The targeting of acid hydrolases (enzymes) to the **lysosome** is a post-translational modification process occurring in the Golgi apparatus. Enzymes destined for lysosomes are tagged with **Mannose-6-Phosphate (M6P)**. Specific M6P receptors in the Trans-Golgi Network (TGN) recognize this tag and package these proteins into clathrin-coated vesicles for transport to the late endosomes/lysosomes. **Analysis of Incorrect Options:** * **A. KDEL Sequence:** This tetrapeptide (Lys-Asp-Glu-Leu) is a retrieval signal for soluble proteins belonging to the **Endoplasmic Reticulum (ER)**. It ensures that ER-resident proteins that escape to the Golgi are captured and sent back to the ER via COPI vesicles. * **B. Dolichol Phosphate:** This is a long-chain lipid molecule located in the ER membrane. It acts as a membrane-bound carrier for the assembly of the core oligosaccharide during **N-linked glycosylation**, but it does not serve as a targeting signal. * **D. Terminal Mannose:** While mannose is a component of many glycoproteins, it is the **phosphorylation** at the 6th position (M6P) that specifically signals lysosomal trafficking. Terminal mannose residues on circulating proteins are often recognized by receptors on macrophages for clearance. **Clinical Pearls for NEET-PG:** * **I-Cell Disease (Mucolipidosis II):** Caused by a deficiency of **N-acetylglucosamine-1-phosphotransferase**. Without this enzyme, M6P tags cannot be added. Consequently, lysosomal enzymes are constitutively secreted into the extracellular space rather than being directed to lysosomes, leading to inclusion bodies and severe developmental symptoms. * **Vesicle Transport:** Remember the "Coat" proteins: **COPI** (Retrograde: Golgi to ER), **COPII** (Anterograde: ER to Golgi), and **Clathrin** (TGN to Lysosomes/Plasma membrane).

Q: What is the largest protein in the human body?

Titin. **Explanation:** **Correct Answer: A. Titin** Titin (also known as connectin) is the largest known protein in the human body, consisting of approximately 27,000 to 33,000 amino acids depending on the isoform. It is a giant elastic protein found in the sarcomere of muscle fibers. It spans half the length of a sarcomere, extending from the Z-disc to the M-line. Its primary function is to act as a molecular spring, providing passive elasticity to muscles and ensuring the structural integrity of the sarcomere by centering the thick filaments during contraction. **Why incorrect options are wrong:** * **B. Myosin:** While myosin is a large protein complex (the "thick filament"), it is significantly smaller than titin. It is a hexamer composed of two heavy chains and four light chains. * **C. Actin:** Actin is a globular protein (G-actin) that polymerizes to form thin filaments (F-actin). It is much smaller than both titin and myosin. * **D. Troponin:** Troponin is a small regulatory protein complex (consisting of subunits T, I, and C) associated with the thin filament. It is involved in calcium-mediated muscle contraction. **High-Yield Clinical Pearls for NEET-PG:** * **Gene:** Titin is encoded by the *TTN* gene, which contains the largest number of exons in a single gene. * **Clinical Correlation:** Mutations in the *TTN* gene are a leading cause of **Dilated Cardiomyopathy (DCM)** and certain skeletal myopathies. * **Function:** Titin is responsible for the "resting tension" of muscle; it prevents the sarcomere from overstretching. * **Comparison:** If Titin is the largest protein, **Glucagon** or **Insulin** are often cited among the smallest functional proteins/hormones (though technically peptides).

Q: Which amino acid is present in Glutathione peroxidase?

Selenocysteine. **Explanation:** The correct answer is **Selenocysteine**. **1. Why Selenocysteine is correct:** Glutathione peroxidase (GPx) is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide. It belongs to a unique class of proteins called **selenoproteins**. The active site of this enzyme contains **Selenocysteine**, often referred to as the "21st amino acid." Unlike other post-translational modifications, selenocysteine is incorporated into the polypeptide chain during translation, encoded by the **UGA stop codon** (with the help of a specific SECIS element). The selenium atom in selenocysteine is essential for the enzyme's catalytic activity, providing higher nucleophilic strength than sulfur. **2. Why other options are incorrect:** * **Alanine:** A simple non-polar amino acid that does not possess the redox-active properties required for the catalytic center of antioxidant enzymes. * **Cysteine:** While structurally similar to selenocysteine (containing sulfur instead of selenium), it is not the primary amino acid in Glutathione peroxidase. However, it is found in the related enzyme *Glutathione Reductase*. * **Serine:** Although selenocysteine is biosynthetically derived from serine (attached to tRNA), serine itself lacks the selenium atom necessary for GPx function. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Selenium Deficiency:** Leads to **Keshan Disease** (endemic cardiomyopathy), primarily due to the decreased activity of Glutathione peroxidase. * **Cofactor:** Glutathione peroxidase requires **Reduced Glutathione (GSH)** as a hydrogen donor. * **Other Selenoproteins:** **Thioredoxin reductase** and **Deiodinase** (which converts T4 to T3) also contain Selenocysteine. * **Key Enzyme Pair:** While GPx reduces $H_2O_2$, **Glutathione Reductase** regenerates GSH using **NADPH** (derived from the HMP Shunt).

Q: Which of the following amino acids would most likely be found on the surface of a protein molecule?

Arginine. ### Explanation The distribution of amino acids in a protein is primarily governed by the **Hydrophobic Effect**. In an aqueous environment (like the cytosol or extracellular fluid), proteins fold such that **hydrophilic (polar/charged)** side chains are exposed on the surface to interact with water, while **hydrophobic (non-polar)** side chains are buried in the interior core to avoid water. **Why Arginine is Correct:** Arginine is a **positively charged, basic amino acid**. Its side chain contains a guanidino group that is highly polar and carries a formal charge at physiological pH. Because it is strongly hydrophilic, it preferentially localizes on the protein surface where it can form hydrogen bonds and ionic interactions (salt bridges) with the aqueous environment or other molecules. **Why the Other Options are Incorrect:** * **Alanine (A), Isoleucine (C), and Leucine (D):** These are all **non-polar, aliphatic amino acids**. They possess hydrophobic side chains that "shun" water. During protein folding, these residues are sequestered into the hydrophobic core to stabilize the protein's tertiary structure through Van der Waals forces. --- ### High-Yield NEET-PG Pearls: * **Surface Amino Acids:** Usually charged (Arg, Lys, Asp, Glu) or polar uncharged (Ser, Thr, Asn, Gln). * **Core Amino Acids:** Usually non-polar (Val, Leu, Ile, Phe, Trp, Met). * **The Exception:** **Glycine** is small and can be found anywhere; **Proline** is often found at "bends" or "turns" on the surface because it disrupts alpha-helices. * **Clinical Correlation:** In **Sickle Cell Anemia**, a hydrophilic Glutamate (surface) is replaced by a hydrophobic Valine. This creates a "sticky patch" on the surface, causing hemoglobin polymerization.

Q: Osteogenesis imperfecta has an abnormality in which type of collagen?

Collagen 1. **Explanation:** **Osteogenesis Imperfecta (OI)**, also known as "Brittle Bone Disease," is a genetic disorder characterized by increased bone fragility. The fundamental defect lies in the synthesis of **Type 1 Collagen**, which is the primary structural protein in bone, skin, tendons, and sclera. Most cases are autosomal dominant and result from mutations in the **COL1A1** or **COL1A2** genes. These mutations lead to either a quantitative deficiency or a qualitative defect (structural abnormality) in the collagen triple helix, resulting in weak osteoid and frequent fractures. **Analysis of Options:** * **Option D (Correct): Type 1 Collagen** is the "bone" collagen (Mnemonic: Type **One** is in B**one**). It provides tensile strength to the organic matrix. * **Option B: Type 2 Collagen** is primarily found in hyaline **cartilage** and vitreous humor (Mnemonic: Type **Two** is in Car-**two**-lage). * **Option A: Type 3 Collagen** (Reticulin) is found in extensible connective tissues like blood vessels, skin, and fetal tissues. It is deficient in the Vascular type of Ehlers-Danlos Syndrome. * **Option C: Type 4 Collagen** is a major component of the **Basement Membrane** (Mnemonic: Type **Four** is on the **Floor**). Defects here lead to Alport Syndrome. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Triad:** Fragile bones (multiple fractures), **Blue Sclera** (due to thinning of collagen allowing uveal pigment to show through), and **Early Otosclerosis** (hearing loss). 2. **Dental Findings:** Dentinogenesis imperfecta (translucent, weak teeth). 3. **Sillence Classification:** Type I is the most common and mildest; Type II is the most severe (perinatal lethal). 4. **Radiology:** Look for "Popcorn calcifications" at metaphyses and "Codfish vertebrae" (biconcave).

Q: Which of the following is not required for wound healing?

Sodium. **Explanation:** Wound healing is a complex physiological process involving inflammation, tissue formation, and remodeling. It relies heavily on specific micronutrients and amino acids for collagen synthesis and cross-linking. **Why Sodium is the correct answer:** Sodium is the primary extracellular cation responsible for maintaining osmotic pressure and fluid balance. While essential for overall homeostasis, it does not play a direct biochemical role in the synthesis of connective tissue, collagen maturation, or the enzymatic reactions required for wound repair. Therefore, it is not considered a specific requirement for the healing process. **Why the other options are incorrect:** * **Vitamin C (Ascorbic Acid):** It is a mandatory cofactor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes hydroxylate proline and lysine residues, a step crucial for the stabilization of the collagen triple helix. Deficiency leads to Scurvy and poor wound healing. * **Copper:** It is a cofactor for **Lysyl oxidase**, the enzyme responsible for creating covalent cross-links between collagen fibrils. This process provides the wound with tensile strength. * **Methionine:** As a sulfur-containing amino acid, methionine is essential for protein synthesis. It also serves as a precursor to **Cysteine**, which is vital for the structure of skin proteins and connective tissue. **High-Yield Clinical Pearls for NEET-PG:** * **Zinc:** Often tested alongside these; it is a cofactor for RNA and DNA polymerase and is essential for cell proliferation and epithelialization. * **Vitamin A:** Enhances epithelization and helps reverse the inhibitory effects of glucocorticoids on wound healing. * **Tensile Strength:** A wound reaches approximately 70-80% of original skin strength by 3 months, but never returns to 100%.

Q: Albumin serves as a transporter of all the following molecules, except:

Iron. **Explanation:** The correct answer is **Iron (Option D)**. **Why Iron is the correct answer:** While albumin is the most abundant plasma protein and a versatile "taxi" for many substances, it does not transport iron. Iron is highly reactive and can induce oxidative stress via the Fenton reaction; therefore, it requires a specific, high-affinity transport protein called **Transferrin**. Once inside cells, iron is stored by **Ferritin**. **Analysis of incorrect options:** * **Bilirubin (Option A):** Albumin is the primary carrier for unconjugated (indirect) bilirubin, transporting it from peripheral tissues to the liver for conjugation. * **Free Fatty Acids (Option B):** Long-chain fatty acids are hydrophobic and cannot travel freely in the blood. Albumin has multiple high-affinity binding sites specifically for their transport. * **Thyroxine (Option C):** While most thyroid hormones are carried by Thyroxine-Binding Globulin (TBG) and Transthyretin, albumin serves as a high-capacity, low-affinity secondary transporter for $T_4$ and $T_3$. **High-Yield NEET-PG Pearls:** * **Albumin’s Functions:** It provides 80% of the **Plasma Colloid Oncotic Pressure** (essential for preventing edema) and acts as a pH buffer. * **Ligands:** Albumin also transports calcium (40% of serum Ca is protein-bound), drugs (Warfarin, Aspirin, Phenytoin), and copper (though **Ceruloplasmin** is the primary copper carrier). * **Clinical Correlation:** In **Analbuminemia**, patients show surprisingly mild symptoms because other globulins compensatorily increase to take over transport functions. * **Negative Acute Phase Reactant:** Albumin levels decrease during inflammation, surgery, or trauma.

Q: Which of the following is an optically inactive amino acid?

Glycine. ### Explanation **Correct Answer: B. Glycine** **Concept:** Optical activity in amino acids depends on the presence of a **chiral center** (an asymmetric carbon atom). A carbon is chiral when it is bonded to four different chemical groups. All standard amino acids have a central alpha-carbon ($\alpha$-C) bonded to: 1. An amino group ($-NH_2$) 2. A carboxyl group ($-COOH$) 3. A hydrogen atom ($-H$) 4. A variable side chain ($-R$ group) In **Glycine**, the side chain ($-R$ group) is simply another **hydrogen atom**. Because the $\alpha$-carbon is bonded to two identical hydrogen atoms, it lacks asymmetry. Therefore, Glycine is the only **achiral** and **optically inactive** amino acid. **Analysis of Incorrect Options:** * **A. Proline:** Contains a unique cyclic pyrrolidine side chain. The $\alpha$-carbon is bonded to four different groups, making it chiral and optically active. * **C. Lysine:** Has a long aliphatic side chain ending in an amino group. Its $\alpha$-carbon is asymmetric. * **D. Leucine:** A branched-chain amino acid with an asymmetric $\alpha$-carbon, thus it is optically active. --- ### High-Yield Facts for NEET-PG: * **Smallest Amino Acid:** Glycine is the simplest and smallest amino acid, allowing it to fit into tight spaces in protein structures (e.g., the triple helix of **Collagen**). * **Configuration:** All amino acids found in human proteins are of the **L-configuration**. * **Exceptions to Chirality:** While Glycine has no chiral center, **Isoleucine and Threonine** are unique because they possess **two** chiral centers. * **Helix Breaker:** Both Glycine and Proline act as "helix breakers" and are frequently found in **beta-bends** (turns) of protein secondary structures.

Q: What is true regarding ubiquitin?

Involved in protein destruction. **Explanation:** Ubiquitin is a small, highly conserved regulatory protein (76 amino acids) that plays a pivotal role in the **Ubiquitin-Proteasome Pathway (UPP)**, the primary mechanism for non-lysosomal protein degradation in eukaryotic cells. **Why Option B is Correct:** Proteins destined for destruction are tagged with a chain of ubiquitin molecules (polyubiquitination) through a three-step enzymatic process (E1, E2, and E3 ligases). These tagged proteins are then recognized and degraded by the **26S Proteasome** complex into small peptides. This process is essential for removing misfolded proteins and regulating the cell cycle. **Analysis of Incorrect Options:** * **Option A:** Uric acid is the end product of purine metabolism in humans, not ubiquitin. * **Option C:** Ubiquitin is found only in **eukaryotes** (hence the name "ubiquitous" in eukaryotic cells). Prokaryotes use different systems, such as pupylation, for protein degradation. * **Option D:** Ubiquitin is involved in the *catabolism* (destruction) of proteins, whereas ribosomes and tRNAs are the primary machinery for protein synthesis (anabolism). **High-Yield Clinical Pearls for NEET-PG:** * **ATP-Dependence:** The attachment of ubiquitin to a substrate protein requires ATP. * **Bortezomib:** A proteasome inhibitor used clinically in the treatment of **Multiple Myeloma**. * **Neurodegeneration:** Failure of the ubiquitin system leads to the accumulation of protein aggregates, a hallmark of diseases like **Parkinson’s** (Lewy bodies) and **Alzheimer’s**. * **Nobel Prize:** Aaron Ciechanover, Avram Hershko, and Irwin Rose won the 2004 Nobel Prize in Chemistry for discovering ubiquitin-mediated protein degradation.

Question 1

Which of the following signals directs a protein to the lysosomes?

Accepted Answer

Attached carbohydrate with terminal mannose-6-phosphate

Answer

A lys-asp-glu-leu (KDEL) sequence in the protein

Answer

Dolichol phosphate

Answer

Attached carbohydrate with terminal mannose

Question 2

What is the largest protein in the human body?

Accepted Answer

Titin

Answer

Myosin

Answer

Actin

Answer

Troponin

Question 3

Which amino acid is present in Glutathione peroxidase?

Accepted Answer

Selenocysteine

Answer

Alanine

Answer

Cysteine

Answer

Serine

Question 4

Which of the following amino acids would most likely be found on the surface of a protein molecule?

Accepted Answer

Arginine

Answer

Alanine

Answer

Isoleucine

Answer

Leucine

Question 5

Osteogenesis imperfecta has an abnormality in which type of collagen?

Accepted Answer

Collagen 1

Answer

Collagen 3

Answer

Collagen 2

Answer

Collagen 4

Question 6

Which of the following is not required for wound healing?

Accepted Answer

Sodium

Answer

Vitamin C

Answer

Copper

Answer

Methionine

Question 7

Albumin serves as a transporter of all the following molecules, except:

Accepted Answer

Iron

Answer

Bilirubin

Answer

Free fatty acids

Answer

Thyroxine

Question 8

Which of the following is a non-polar amino acid?

Accepted Answer

Alanine

Answer

Tryptophan

Answer

Isoleucine

Answer

Lysine

Question 9

Which of the following is an optically inactive amino acid?

Accepted Answer

Glycine

Answer

Proline

Answer

Lysine

Answer

Leucine

Question 10

What is true regarding ubiquitin?

Accepted Answer

Involved in protein destruction

Answer

Product of purine metabolism

Answer

Present in prokaryotes

Answer

Involved in protein synthesis

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

On this page

Practice by Chapter

Want unlimited practice?