Protein Structure and Function Practice Questions

Q: What is the correct sequence of vesicle transport starting from the Endoplasmic Reticulum (ER)?

ER - Cis Golgi - Trans Golgi - Cell membrane. ### Explanation **1. Why Option B is Correct:** The secretory pathway follows a unidirectional flow known as **anterograde transport**. Proteins are synthesized in the **Rough Endoplasmic Reticulum (RER)** and packaged into COPII-coated vesicles. These vesicles first enter the **Cis-Golgi network** (the receiving side facing the ER). As proteins undergo post-translational modifications (like glycosylation), they move through the medial cisternae to the **Trans-Golgi network** (the shipping side). From here, they are sorted into secretory vesicles that fuse with the **cell membrane** for exocytosis or incorporation. **2. Why Other Options are Incorrect:** * **Option A & C:** These suggest that vesicles move from Trans to Cis or bypass the Golgi polarity. In physiological conditions, the Cis-face always receives material from the ER, while the Trans-face exports it. * **Option D:** This reverses the initial step. The ER is the site of protein synthesis; the Golgi acts only as a processing and "post-office" center. Transport cannot begin at the Golgi. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Vesicle Coats:** * **COPII:** Anterograde transport (ER → Golgi). *Mnemonic: "Two (II) steps forward."* * **COPI:** Retrograde transport (Golgi → ER). *Mnemonic: "One (I) step back."* * **Clathrin:** Transport from Trans-Golgi to lysosomes and endocytosis from the plasma membrane. * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating mannose residues (in the Golgi) leads to failure of protein trafficking to lysosomes, causing enzymes to be secreted extracellularly instead. * **Golgi Marker:** **Nucleoside diphosphatase** is the characteristic biochemical marker for the Golgi apparatus.

Q: In N-linked glycosylation, a core oligosaccharide is transferred to the polypeptide chain after first being attached to which molecule?

Dolichol. ### Explanation **Correct Option: D. Dolichol** N-linked glycosylation is a critical post-translational modification that occurs in the **Rough Endoplasmic Reticulum (RER)**. The process begins with the assembly of a core oligosaccharide (GlcNAc, mannose, and glucose) on a specialized long-chain polyisoprenoid lipid called **Dolichol pyrophosphate**, which is embedded in the ER membrane. Once the 14-sugar precursor is fully assembled, it is transferred en bloc to the amide nitrogen of an **asparagine (Asn)** residue within the consensus sequence Asn-X-Ser/Thr. **Analysis of Incorrect Options:** * **A. Serine:** Serine (and Threonine) residues are the attachment sites for **O-linked glycosylation**, which typically occurs in the Golgi apparatus. Unlike N-linked, O-linked sugars are added one by one directly to the protein, not via a lipid carrier. * **B. Phosphatidyl choline:** This is a major structural phospholipid of cell membranes but does not serve as a carrier for oligosaccharide synthesis. * **C. Cholesterol:** While cholesterol is a vital component of animal cell membranes and a precursor for steroid hormones, it plays no role in the glycosylation pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Tunicamycin:** An antibiotic that inhibits the first step of N-linked glycosylation (transfer of GlcNAc-P to dolichol), effectively blocking the entire process. * **I-Cell Disease:** Caused by a deficiency in the enzyme *N-acetylglucosaminyl-1-phosphotransferase*, leading to a failure to tag lysosomal enzymes with **Mannose-6-Phosphate**. This results in enzymes being secreted extracellularly rather than reaching lysosomes. * **Sequence:** N-linked glycosylation occurs on the **Asn-X-Ser/Thr** motif (where X is any amino acid except proline).

Q: What is true about denatured proteins?

Biologically inactive. **Explanation:** **Denaturation** is the process where a protein loses its native three-dimensional conformation (secondary, tertiary, and quaternary structures) due to external stress such as heat, extreme pH, or organic solvents. 1. **Why Option A is correct:** The biological function of a protein is strictly dependent on its specific 3D shape (conformation). When a protein is denatured, it unfolds, losing its active sites and specific binding capabilities. Consequently, it becomes **biologically inactive** (e.g., enzymes lose catalytic activity, antibodies cannot bind antigens). 2. **Why Option B is incorrect:** Denaturation typically exposes the internal hydrophobic (water-fearing) amino acid residues to the surface. This leads to aggregation and precipitation, making denatured proteins **insoluble** in water (e.g., the coagulation of egg white when boiled). 3. **Why Options C and D are incorrect:** Denaturation is not strong enough to break covalent bonds. The **primary structure** (the linear sequence of amino acids) and the **peptide bonds** remain intact. Only the non-covalent interactions (hydrogen bonds, ionic bonds, and hydrophobic interactions) are disrupted. **High-Yield NEET-PG Pearls:** * **Primary Structure:** The only level of protein structure that remains unaffected by denaturation. * **Renaturation:** Some proteins can regain their native state if the denaturing agent is removed (e.g., Ribonuclease), proving that the primary structure determines the folding pattern. * **Chaperones:** These are specialized proteins (Heat Shock Proteins) that assist in the correct folding of proteins and prevent denaturation under stress. * **Clinical Correlation:** Prion diseases and Alzheimer’s involve protein misfolding, which is a pathological deviation from the native functional state.

Q: Which of the following is structurally related to insulin-like growth factors I and II?

Proinsulin. **Explanation:** The correct answer is **Proinsulin**. **Why Proinsulin is correct:** Insulin-like growth factors (IGF-I and IGF-II) are polypeptide hormones that share significant structural homology with **proinsulin** [1]. Proinsulin consists of three chains: the A-chain, the B-chain, and a connecting C-peptide. Similarly, IGFs are single-chain polypeptides that contain domains analogous to the A and B chains of insulin, linked by a connecting peptide (C-domain) [1]. Unlike proinsulin, however, the C-domain in IGFs is not cleaved during processing, meaning IGFs remain as single-chain molecules that structurally resemble the precursor form of insulin [1]. **Why the other options are incorrect:** * **Preproinsulin (A):** This is the initial translation product that includes a signal peptide. The signal peptide is cleaved in the endoplasmic reticulum to form proinsulin; therefore, the mature IGF structure specifically mirrors the proinsulin stage. * **Insulin (C):** Mature insulin consists of two separate polypeptide chains (A and B) held together by disulfide bonds after the C-peptide has been removed [2]. Because IGFs retain their connecting peptide, they are more closely related to the precursor (proinsulin) than the mature hormone [1]. * **C-peptide (D):** While IGFs have a "C-domain," it shows very little sequence homology with the actual human C-peptide found in proinsulin. **High-Yield NEET-PG Pearls:** * **IGF-I (Somatomedin C):** Primarily produced in the liver under the influence of Growth Hormone (GH) [2]. It mediates most of the growth-promoting effects of GH. * **Receptor Homology:** The IGF-I receptor is a tyrosine kinase receptor, structurally very similar to the insulin receptor [3]. * **Clinical Correlation:** In cases of **Islet Cell Tumors (Non-islet cell tumor hypoglycemia - NICTH)**, tumors may secrete "Big IGF-II," which can bind to insulin receptors and cause profound hypoglycemia.

Q: Which of the following post-translational modifications can occur multiple times during a protein's life span?

Phosphorylation. **Explanation:** The correct answer is **Phosphorylation**. **Why Phosphorylation is Correct:** Phosphorylation is a **reversible** and **dynamic** post-translational modification (PTM). It involves the addition of a phosphate group (usually to Serine, Threonine, or Tyrosine residues) by enzymes called **kinases** and its removal by **phosphatases**. This "molecular switch" allows a single protein molecule to be turned "on" or "off" multiple times throughout its lifespan to regulate metabolic pathways, signal transduction, and the cell cycle. **Why the Other Options are Incorrect:** * **Disulfide bond formation (A):** This typically occurs once during the initial folding of secretory proteins in the Rough Endoplasmic Reticulum (RER) to stabilize the tertiary or quaternary structure. * **Gamma-carboxylation (B):** This occurs once during the maturation of clotting factors (II, VII, IX, X) in the liver. It is a permanent modification required for these proteins to bind calcium. * **Glycosylation (C):** The addition of carbohydrate chains occurs during protein synthesis and maturation in the RER and Golgi apparatus. While the sugar chain may be trimmed, the initial glycosylation event is not a repetitive regulatory cycle like phosphorylation. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Enzymes:** Kinases add phosphate (using ATP); Phosphatases remove it. * **Amino Acids:** In eukaryotes, phosphorylation occurs most commonly on **Serine**, followed by Threonine and Tyrosine. * **Vitamin K Connection:** Gamma-carboxylation of Glutamate residues requires **Vitamin K** as a cofactor; this is the target of Warfarin. * **Glycosylation Site:** **N-linked** glycosylation occurs on Asparagine; **O-linked** occurs on Serine or Threonine. * **Disease Link:** Abnormal phosphorylation of **Tau proteins** is a hallmark of Alzheimer’s disease.

Q: Ubiquitin is a key molecule in protein degradation. Ubiquitinated proteins are degraded by which cellular component?

Proteasome. ### Explanation **Correct Answer: C. Proteasome** The **Ubiquitin-Proteasome Pathway (UPP)** is the primary mechanism for the targeted degradation of short-lived, misfolded, or damaged intracellular proteins. 1. **Ubiquitination:** The process begins when a small regulatory protein called **Ubiquitin** is covalently attached to a target protein (tagging). 2. **Recognition:** Poly-ubiquitinated proteins are specifically recognized by the **26S Proteasome**, a large, barrel-shaped multi-enzyme complex. 3. **Degradation:** The proteasome acts as a "cellular garbage disposal," unfolding the protein and breaking it down into small peptides, while the ubiquitin molecules are recycled. --- ### Why the other options are incorrect: * **A. Chaperones:** These are proteins that assist in the **folding** or refolding of proteins. They prevent aggregation but do not degrade proteins. * **B. Protease:** While proteasomes contain protease activity, "protease" is a general term for any enzyme that breaks down proteins (e.g., pepsin, trypsin). The specific cellular component responsible for the ubiquitin-mediated pathway is the proteasome. * **C. Caspases:** These are specialized cysteine proteases involved specifically in **Apoptosis** (programmed cell death), not the routine degradation of ubiquitinated proteins. --- ### NEET-PG High-Yield Pearls: * **Energy Requirement:** Unlike lysosomal degradation, the Ubiquitin-Proteasome pathway is **ATP-dependent**. * **Clinical Correlation:** **Bortezomib** is a proteasome inhibitor used in the treatment of **Multiple Myeloma**. It works by preventing the degradation of pro-apoptotic factors, leading to cancer cell death. * **Parkinson’s Disease:** Mutations in the *Parkin* gene (an E3 ubiquitin ligase) lead to the accumulation of misfolded proteins, contributing to neurodegeneration.

Q: In osteogenesis imperfecta, the defect occurs in which type of collagen?

Type 1 collagen.. **Explanation:** **Osteogenesis Imperfecta (OI)**, also known as "Brittle Bone Disease," is a genetic disorder characterized by increased bone fragility and low bone mass. 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 (approx. 90%) result from autosomal dominant mutations in the **COL1A1** or **COL1A2** genes, leading to either a quantitative deficiency or a qualitative defect in the pro-collagen triple helix. **Analysis of Options:** * **Type 1 Collagen (Correct):** Found in "B-O-N-E" (mnemonic). It provides tensile strength to the organic matrix (osteoid). Defects lead to the classic triad of OI: fragile bones, blue sclera, and early-onset hearing loss. * **Type 2 Collagen (Incorrect):** Primarily found in **hyaline cartilage** and vitreous humor. Defects are associated with skeletal dysplasias like *Achondrogenesis* and *Stickler Syndrome*. * **Type 4 Collagen (Incorrect):** A major component of the **basal lamina** (basement membrane). Mutations here lead to *Alport Syndrome* (nephritis and sensorineural deafness). * **Type 6 Collagen (Incorrect):** Involved in anchoring muscle fibers to the extracellular matrix. Mutations are associated with *Ulrich Congenital Muscular Dystrophy*. **High-Yield Clinical Pearls for NEET-PG:** * **Blue Sclera:** Caused by thinning of the collagen layers, allowing the underlying choroidal veins to show through. * **Dentinogenesis Imperfecta:** Often co-exists, presenting as translucent, brownish-blue teeth. * **Classification:** The **Sillence Classification** is used to grade severity (Type II is the most severe/lethal perinatally). * **Radiology:** Look for "Popcorn calcifications" near growth plates and "Codfish vertebrae" (biconcave) due to spinal compression fractures.

Q: Which of the following is NOT a hydrophilic amino acid?

Proline. ### Explanation The classification of amino acids based on their side-chain polarity is a high-yield topic for NEET-PG. Amino acids are categorized as **hydrophilic (polar)** or **hydrophobic (non-polar)** based on their ability to interact with water. **Why Proline is the Correct Answer:** Proline is a **non-polar, hydrophobic** amino acid. Structurally, it is unique because its side chain forms a cyclic structure by bonding back to the amino group of the backbone, making it an **imino acid**. This rigid, aliphatic ring structure does not interact favorably with water, placing it firmly in the hydrophobic category. **Analysis of Incorrect Options:** * **Cysteine (A):** It is a polar, uncharged amino acid. Its thiol (-SH) group can participate in hydrogen bonding and is critical for forming disulfide bridges, making it hydrophilic. * **Glycine (C):** Although it has only a hydrogen atom as a side chain, it is generally classified as a polar/hydrophilic amino acid in most medical biochemistry texts (like Harper’s) because its tiny side chain does not offer enough hydrophobic character to outweigh the polarity of the amino and carboxyl groups. * **Serine (D):** It is a classic polar amino acid. The presence of a hydroxyl (-OH) group allows it to form hydrogen bonds easily with water. **High-Yield Clinical Pearls for NEET-PG:** 1. **Proline’s Role:** Due to its rigid ring, Proline acts as a **"helix breaker"** and is frequently found in the turns of beta-sheets. 2. **Collagen Connection:** Proline and Lysine undergo post-translational **hydroxylation** (requiring Vitamin C) to stabilize the collagen triple helix. 3. **Glycine Fact:** It is the only **achiral** amino acid (no asymmetric carbon) and is the smallest amino acid, essential for the tight packing of collagen (Gly-X-Y motif).

Question 1

Which of the following proteins are involved in protein folding?

Accepted Answer

Protein disulfide isomerase

Answer

GLUT-1

Answer

Calnexin

Answer

Cytochrome 450

Question 2

What is the correct sequence of vesicle transport starting from the Endoplasmic Reticulum (ER)?

Accepted Answer

ER - Cis Golgi - Trans Golgi - Cell membrane

Answer

ER - Trans Golgi - Cis Golgi - Lysosomes

Answer

ER - Lysosomes - Trans Golgi - Cis Golgi

Answer

Cis Golgi - ER - Trans Golgi - Cell membrane

Question 3

In N-linked glycosylation, a core oligosaccharide is transferred to the polypeptide chain after first being attached to which molecule?

Accepted Answer

Dolichol

Answer

Serine

Answer

Phosphatidyl choline

Answer

Cholesterol

Question 4

Which of the following are intracellular events occurring in fibroblasts during the synthesis of collagen?

Accepted Answer

Formation of triple helix

Answer

Hydrolysis of procollagen to collagen

Answer

Glycosylation of proline

Answer

Formation of covalent cross-link between fibrils

Question 5

What is true about denatured proteins?

Accepted Answer

Biologically inactive

Answer

Soluble in water

Answer

Primary structure of amino acid sequence is disrupted

Answer

Peptide bonds are hydrolysed

Question 6

Which of the following is structurally related to insulin-like growth factors I and II?

Accepted Answer

Proinsulin

Answer

Preproinsulin

Answer

Insulin

Answer

C peptide

Question 7

Which of the following post-translational modifications can occur multiple times during a protein's life span?

Accepted Answer

Phosphorylation

Answer

Disulfide bond formation

Answer

Gamma-carboxylation

Answer

Glycosylation

Question 8

Ubiquitin is a key molecule in protein degradation. Ubiquitinated proteins are degraded by which cellular component?

Accepted Answer

Proteasome

Answer

Chaperone

Answer

Protease

Answer

Caspase

Question 9

In osteogenesis imperfecta, the defect occurs in which type of collagen?

Accepted Answer

Type 1 collagen.

Answer

Type 2 collagen.

Answer

Type 4 collagen.

Answer

Type 6 collagen.

Question 10

Which of the following is NOT a hydrophilic amino acid?

Accepted Answer

Proline

Answer

Cysteine

Answer

Glycine

Answer

Serine

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

On this page

Practice by Chapter

Want unlimited practice?