Protein Structure and Function Practice Questions

Q: Post-translational modification in the form of sulfation of tyrosine residues occurs in which gastrointestinal hormone?

Cholecystokinin (CCK). ### Explanation **Correct Answer: C. Cholecystokinin (CCK)** The correct answer is **Cholecystokinin (CCK)**. Post-translational modification is a critical step for the biological activity of several peptide hormones. For CCK, the **sulfation of the tyrosine residue** located at the **7th position** from the C-terminus is essential. Unlike some other hormones where sulfation is optional, CCK requires this specific modification to bind effectively to the **CCK-A (Alimentary) receptors**. Without this sulfate group, CCK loses its potency in stimulating gallbladder contraction and pancreatic enzyme secretion. **Analysis of Incorrect Options:** * **A. Gastrin:** Gastrin and CCK share the same C-terminal pentapeptide sequence. While Gastrin can also undergo tyrosine sulfation (at the 6th position), it is **not essential** for its biological activity. Gastrin exists in both sulfated (Gastrin II) and non-sulfated (Gastrin I) forms with equal potency. * **B. Somatostatin:** This is a cyclic peptide (14 or 28 amino acids) primarily involved in inhibition. Its activity depends on its cyclic structure (disulfide bonds), not tyrosine sulfation. * **D. Vasoactive Intestinal Peptide (VIP):** A member of the secretin family, VIP relies on its alpha-helical structure for receptor binding; tyrosine sulfation is not a characteristic feature of its activation. **High-Yield Facts for NEET-PG:** * **CCK-A vs. CCK-B Receptors:** CCK-A receptors are selective and require the sulfated tyrosine. CCK-B receptors (found in the brain and stomach) respond equally to both gastrin and CCK. * **Site of Secretion:** CCK is secreted by **I-cells** in the duodenum and jejunum in response to fatty acids and amino acids. * **Functions:** It causes gallbladder contraction, relaxes the Sphincter of Oddi, and is the most potent stimulator of enzyme-rich pancreatic secretion.

Q: Which of the following groups of proteins assist in the folding of other proteins?

Chaperones. **Explanation:** **Correct Answer: D. Chaperones** Protein folding is a critical process where a polypeptide chain assumes its functional 3D conformation. **Chaperones** (also known as Heat Shock Proteins, e.g., HSP70) are specialized proteins that facilitate this by preventing the aggregation of unfolded or partially folded polypeptide chains. They do not carry the information for folding themselves; rather, they provide a protected environment or stabilize hydrophobic regions to ensure the protein reaches its native state efficiently. **Analysis of Incorrect Options:** * **A. Proteases:** These are enzymes that catalyze **proteolysis** (the breakdown of proteins into peptides or amino acids) by peptide bond hydrolysis. They are involved in degradation, not folding. * **B. Proteasomes:** These are large multi-protein complexes responsible for the degradation of damaged or unneeded proteins tagged with **ubiquitin**. They act as the cell’s "garbage disposal" unit. * **C. Templates:** In biochemistry, templates usually refer to DNA or RNA strands used during replication or transcription. Protein folding is generally self-assembling (Anfinsen’s dogma) and does not require a physical template. **High-Yield Clinical Pearls for NEET-PG:** * **HSP70 & HSP60:** These are the most common chaperones. HSP70 binds early during translation, while HSP60 (Chaperonins) forms a cage-like structure for folding. * **Prion Diseases:** Result from the **misfolding** of normal PrPᶜ into the β-sheet-rich PrPˢᶜ, leading to neurodegeneration (e.g., Creutzfeldt-Jakob Disease). * **Ubiquitin-Proteasome Pathway:** Deficits in this pathway are linked to Parkinson’s disease (accumulation of α-synuclein in Lewy bodies). * **Cystic Fibrosis:** Often caused by a mutation (ΔF508) that leads to the misfolding and premature degradation of the CFTR protein.

Q: Which of the following amino acids does NOT contain a hydroxyl group?

Lysine. **Explanation:** The presence of a hydroxyl (-OH) group in an amino acid's side chain is a critical structural feature that determines its polarity, post-translational modifications, and catalytic functions. **Why Lysine is the Correct Answer:** Lysine is a **basic (positively charged)** amino acid. Its side chain consists of a four-carbon aliphatic chain ending in a primary **amino group (-NH₂)**. It does not contain a hydroxyl group. In biochemistry, lysine is primarily known for its role in forming "Schiff bases" and undergoing post-translational modifications like acetylation and methylation (especially in histones). **Analysis of Incorrect Options:** * **Serine:** Contains a primary hydroxyl group attached to a methyl group. It is the simplest hydroxy-amino acid. * **Threonine:** Contains a secondary hydroxyl group. It is an essential amino acid with two chiral centers. * **Tyrosine:** Contains a phenolic hydroxyl group. It is a precursor for catecholamines, thyroid hormones, and melanin. **High-Yield NEET-PG Clinical Pearls:** 1. **Phosphorylation:** The hydroxyl groups of **Serine, Threonine, and Tyrosine** are the primary sites for phosphorylation by protein kinases, a key mechanism in signal transduction. 2. **O-linked Glycosylation:** Occurs at the -OH group of Serine or Threonine. 3. **Collagen Synthesis:** While Lysine lacks an -OH group initially, it is post-translationally modified to **Hydroxylysine** by the enzyme *Lysyl hydroxylase* (requires Vitamin C as a cofactor). Deficiency leads to Scurvy. 4. **Catalytic Triad:** Serine is a crucial component of the "catalytic triad" in serine proteases (e.g., Trypsin, Chymotrypsin).

Q: Histone proteins are rich in which of the following amino acids?

Lysine and Arginine. **Explanation:** Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. **1. Why Lysine and Arginine are correct:** The fundamental interaction between histones and DNA is based on **electrostatic attraction**. DNA is negatively charged due to its phosphate backbone. To bind tightly to DNA, histone proteins must carry a strong positive charge. **Lysine and Arginine** are basic amino acids that are positively charged at physiological pH. This allows histones to act as a "spool" around which the negatively charged DNA can wrap, facilitating efficient DNA condensation. **2. Analysis of Incorrect Options:** * **Option A & C (Histidine):** While Histidine is technically a basic amino acid, its pKa is close to physiological pH (~6.0). This means it is not consistently or strongly positively charged in the cellular environment compared to Lysine and Arginine. Therefore, it is not a primary constituent of histones. * **Option D (Valine):** Valine is a branched-chain non-polar (hydrophobic) amino acid. It lacks the necessary charge to interact with the DNA backbone. **High-Yield Clinical Pearls for NEET-PG:** * **Nucleosome Core:** Consists of an octamer of histone proteins: two each of **H2A, H2B, H3, and H4**. * **Linker Histone:** **H1** is the "linker" histone that resides outside the nucleosome core and helps stabilize the 30nm chromatin fiber. * **Epigenetics:** Histone tails undergo post-translational modifications (Acetylation, Methylation). **Acetylation** (by HATs) neutralizes the positive charge on Lysine, weakening the histone-DNA bond and leading to **Euchromatin** (transcriptionally active). * **Protamines:** In sperm, histones are replaced by protamines, which are even richer in Arginine for tighter DNA packing.

Q: Intermediate filaments in glial cells are?

GFAP. The enriched explanation with citations is as follows: **Explanation:** **Correct Answer: C. GFAP (Glial Fibrillary Acidic Protein)** Intermediate filaments are key structural components of the cytoskeleton. In the central nervous system, **GFAP** is the specific type III intermediate filament expressed by **astrocytes** (the most abundant glial cells) [1]. It provides mechanical strength to the cells and supports the blood-brain barrier. GFAP is a highly specific marker used in immunohistochemistry to identify astrocytes and tumors derived from them. **Analysis of Incorrect Options:** * **A. Keratin:** These are intermediate filaments found in **epithelial cells** (e.g., skin, hair, nails). They provide structural integrity to the epidermis. * **B. Desmin:** This is a type III intermediate filament found in **muscle cells** (skeletal, cardiac, and smooth muscle). It links myofibrils to the sarcolemma. * **D. Bone:** This is an incorrect classification. Bone is a specialized connective tissue composed of an organic matrix (mostly Type I collagen) and inorganic minerals (hydroxyapatite), not an intermediate filament. **High-Yield Clinical Pearls for NEET-PG:** 1. **Glioblastoma Multiforme (GBM):** This highly malignant brain tumor is characteristically **GFAP positive** on staining [2]. 2. **Vimentin:** Another intermediate filament found in cells of **mesenchymal origin** (fibroblasts, endothelium, macrophages). 3. **Neurofilaments:** These are the specific intermediate filaments found in **neurons**, not glial cells. 4. **Alexander Disease:** A rare genetic disorder caused by mutations in the GFAP gene, leading to the accumulation of **Rosenthal fibers** (eosinophilic inclusions in astrocytes).

Q: Which of the following is NOT involved in stabilizing the tertiary structure of a protein?

Peptide bond. ### Explanation The **tertiary structure** of a protein refers to its three-dimensional spatial arrangement, where a single polypeptide chain folds into a compact, functional shape. **Why "Peptide Bond" is the Correct Answer:** The **peptide bond** is a strong covalent bond that links amino acids together in a linear sequence. It is the fundamental force responsible for the **primary structure** of a protein. While it provides the backbone, it does not participate in the folding or stabilization of the tertiary structure. Tertiary structure is primarily stabilized by interactions between the **R-groups (side chains)** of amino acids, rather than the peptide backbone itself. **Analysis of Incorrect Options:** * **Hydrogen Bonds:** These occur between polar side chains (e.g., Serine, Threonine) and are vital for stabilizing the folded 3D conformation. * **Hydrophobic Interactions:** This is the **primary driving force** for protein folding. Non-polar side chains (e.g., Valine, Leucine) cluster in the interior of the protein to avoid water, stabilizing the core. * **Van der Waals Forces:** These are weak, short-range attractions between non-polar side chains that contribute to the tight packing of the protein’s interior. **High-Yield Clinical Pearls for NEET-PG:** * **Disulfide Bonds:** These are the only **covalent** bonds involved in stabilizing tertiary and quaternary structures (formed between two Cysteine residues). * **Denaturation:** Agents like heat or urea disrupt tertiary structures by breaking hydrogen and hydrophobic bonds, but they **do not** break peptide bonds (primary structure remains intact). * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in the correct folding of proteins into their tertiary structures.

Q: Genetic analysis of a patient showed a loss of function mutation in a gene that leads to accumulation of misfolded proteins. Which of the following biochemical processes is most likely affected?

Ubiquitination. ### Explanation **Correct Option: B. Ubiquitination** The accumulation of misfolded proteins is a hallmark of a defective **Ubiquitin-Proteasome Pathway (UPP)**. In cells, misfolded or damaged proteins are tagged with a small regulatory protein called **Ubiquitin**. This process, mediated by enzymes (E1, E2, and E3 ligases), marks the protein for destruction by the **26S Proteasome** (the cell's "garbage disposal"). If ubiquitination is impaired, these toxic, misfolded proteins aggregate, leading to cellular dysfunction and diseases such as Parkinson’s and Alzheimer’s. **Analysis of Incorrect Options:** * **A. Glucuronidation:** This is a Phase II detoxification reaction in the liver (mediated by UDP-glucuronosyltransferase) that makes hydrophobic substances more water-soluble for excretion. It is not involved in protein folding or degradation. * **C. Acetylation:** This is a post-translational modification (e.g., histone acetylation) that primarily regulates gene expression by altering DNA-protein interactions. * **D. Phosphorylation:** This involves the addition of a phosphate group by kinases to activate or deactivate enzymes and signaling molecules. While it regulates protein function, it is not the primary pathway for clearing misfolded proteins. **NEET-PG High-Yield Pearls:** * **Chaperones (Heat Shock Proteins):** These are the primary molecules responsible for helping proteins fold correctly in the first place. * **Prion Diseases:** These occur when misfolded proteins (PrPSc) induce normal proteins to misfold, resisting proteolysis. * **Bortezomib:** A high-yield clinical drug that acts as a **Proteasome Inhibitor**, used in the treatment of Multiple Myeloma to induce apoptosis by allowing misfolded proteins to accumulate in cancer cells.

Q: Which of the following hormones are synthesized as peptide precursors?

All of the above. **Explanation:** The synthesis of protein and peptide hormones follows a universal pathway: they are initially synthesized on ribosomes as large, inactive molecules called **Pre-prohormones**. These undergo post-translational modifications in the endoplasmic reticulum (removal of the signal peptide) to become **Prohormones**, and are further cleaved in the Golgi apparatus or secretory vesicles into the final active **Hormone**. * **Insulin (Option A):** Synthesized as *Preproinsulin*. The signal sequence is removed to form *Proinsulin*. In the secretory granules, Proinsulin is cleaved into active Insulin and C-peptide. * **Parathyroid Hormone (Option B):** Synthesized as *Prepro-PTH* (115 amino acids), which is cleaved to *Pro-PTH* (90 amino acids) and finally to the active PTH (84 amino acids). * **Renin (Option C):** Although often discussed as an enzyme, Renin is a peptide hormone synthesized as *Preprorenin*. It is processed into *Prorenin* and then into active Renin in the juxtaglomerular cells of the kidney. Since all three substances are peptide-based and follow this biosynthetic pathway, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **C-Peptide:** Since C-peptide and Insulin are secreted in equimolar amounts from Proinsulin, C-peptide levels are used as a marker of endogenous insulin production (useful in distinguishing Type 1 vs. Type 2 Diabetes or Factitious Hypoglycemia). * **Storage:** Unlike steroid hormones (which are synthesized on demand), peptide hormones are stored in **secretory vesicles** and released via exocytosis in response to a stimulus (usually involving Calcium influx). * **Receptors:** Peptide hormones are water-soluble and cannot cross the lipid bilayer; they bind to **cell surface receptors** (GPCRs or Enzyme-linked receptors).

Q: Which of the following is an example of post-translational modification?

Proline to Hydroxyproline. **Explanation:** **1. Why Proline to Hydroxyproline is Correct:** Post-translational modification (PTM) refers to the covalent modification of proteins after they have been synthesized by ribosomes. The conversion of **Proline to Hydroxyproline** is a classic example of PTM occurring during collagen synthesis. This hydroxylation is catalyzed by the enzyme **prolyl hydroxylase** within the endoplasmic reticulum. Hydroxyproline is essential for the thermal stability of the collagen triple helix, as it provides additional hydrogen bonding. **2. Analysis of Incorrect Options:** * **Options A & C (Glutamate to Glutamine / Aspartate to Asparagine):** These are simple amidation reactions involving free amino acids or metabolic intermediates. While Glutamine and Asparagine are standard amino acids encoded by the genetic code, their interconversion is part of general nitrogen metabolism (e.g., urea cycle or ammonia transport), not a modification of a protein chain. * **Option D (Leucine to Isoleucine):** These are distinct, essential branched-chain amino acids. One cannot be converted into the other as a modification within a protein structure; they are incorporated into proteins as separate entities based on specific mRNA codons. **3. NEET-PG High-Yield Clinical Pearls:** * **Vitamin C Requirement:** Prolyl and lysyl hydroxylases require **Vitamin C (Ascorbic acid)** as a co-factor to keep the enzyme's iron in the reduced ($Fe^{2+}$) state. * **Scurvy:** Deficiency of Vitamin C leads to defective hydroxylation, resulting in unstable collagen, which manifests clinically as bleeding gums, poor wound healing, and petechiae. * **Other PTM Examples:** Gamma-carboxylation of Glutamate (Vitamin K dependent), phosphorylation, glycosylation, and proteolytic cleavage (e.g., Proinsulin to Insulin).

Q: Folding of a protein chain is due to which type of bond?

Hydrogen bond. **Explanation:** Protein folding is the process by which a linear polypeptide chain acquires its functional three-dimensional conformation. The primary driver for the formation of **secondary structures** (like alpha-helices and beta-pleated sheets) is the formation of **Hydrogen bonds** between the carbonyl oxygen (-CO) and the amide hydrogen (-NH) of the peptide backbone. While hydrophobic interactions drive the initial "molten globule" collapse, hydrogen bonding provides the structural stability and specificity required for the final folded state. **Analysis of Options:** * **A. Amide bond:** Also known as a peptide bond, this is a strong covalent bond that links amino acids together. It defines the **primary structure** (sequence) but does not cause the folding into higher-order structures. * **C. Phosphodiester bond:** These bonds form the backbone of nucleic acids (DNA and RNA), linking the 3' carbon of one sugar to the 5' carbon of another. They are not involved in protein structure. * **D. Disulphide bond:** These are strong covalent bonds between cysteine residues that stabilize the **tertiary and quaternary structures**. While they are crucial for stability, they are considered "cross-links" rather than the primary force responsible for the folding process itself. **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones (Heat Shock Proteins):** Specialized proteins (e.g., HSP70) that prevent misfolding and facilitate the correct folding of nascent polypeptides. * **Prion Diseases:** Caused by the misfolding of normal PrP proteins into beta-sheet rich pathological forms (e.g., Creutzfeldt-Jakob Disease). * **Alzheimer’s Disease:** Characterized by the accumulation of misfolded Amyloid-beta proteins. * **Denaturation:** The loss of secondary, tertiary, and quaternary structure (breaking of H-bonds) without breaking the primary peptide bonds.

Question 1

Post-translational modification in the form of sulfation of tyrosine residues occurs in which gastrointestinal hormone?

Accepted Answer

Cholecystokinin (CCK)

Answer

Gastrin

Answer

Somatostatin

Answer

Vasoactive Intestinal Peptide (VIP)

Question 2

Which of the following groups of proteins assist in the folding of other proteins?

Accepted Answer

Chaperones

Answer

Proteases

Answer

Proteosomes

Answer

Templates

Question 3

Which of the following amino acids does NOT contain a hydroxyl group?

Accepted Answer

Lysine

Answer

Serine

Answer

Threonine

Answer

Tyrosine

Question 4

Histone proteins are rich in which of the following amino acids?

Accepted Answer

Lysine and Arginine

Answer

Histidine and lysine

Answer

Arginine and Histidine

Answer

Histidine and Valine

Question 5

Intermediate filaments in glial cells are?

Accepted Answer

GFAP

Answer

Keratin

Answer

Desmin

Answer

Bone

Question 6

Which of the following is NOT involved in stabilizing the tertiary structure of a protein?

Accepted Answer

Peptide bond

Answer

Hydrogen bond

Answer

Hydrophobic interaction

Answer

Van der Waals forces

Question 7

Genetic analysis of a patient showed a loss of function mutation in a gene that leads to accumulation of misfolded proteins. Which of the following biochemical processes is most likely affected?

Accepted Answer

Ubiquitination

Answer

Glucuronidation

Answer

Acetylation

Answer

Phosphorylation

Question 8

Which of the following hormones are synthesized as peptide precursors?

Accepted Answer

All of the above

Answer

Insulin

Answer

Parathyroid hormone (PTH)

Answer

Renin

Question 9

Which of the following is an example of post-translational modification?

Accepted Answer

Proline to Hydroxyproline

Answer

Glutamate to Glutamine

Answer

Aspartate to Asparagine

Answer

Leucine to Isoleucine

Question 10

Folding of a protein chain is due to which type of bond?

Accepted Answer

Hydrogen bond

Answer

Amide bond

Answer

Phosphodiester bond

Answer

Disulphide bond

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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