Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 111: Which amino acid contains an imidazole ring?

A. Tryptophan
B. Arginine
C. Histidine (Correct Answer)
D. Tyrosine

Explanation: **Explanation:** The correct answer is **Histidine**. Amino acids are categorized based on the specific chemical structure of their side chains (R-groups). **1. Why Histidine is correct:** Histidine contains an **imidazole ring**, which is a five-membered heterocyclic structure containing two nitrogen atoms. This ring is unique because its pKa is approximately 6.0, close to physiological pH. This allows histidine to function as an effective proton donor or acceptor (buffer) in the active sites of many enzymes, such as carbonic anhydrase. **2. Why the other options are incorrect:** * **Tryptophan:** Contains an **indole ring** (a benzene ring fused to a pyrrole ring). It is the precursor for serotonin and melatonin. * **Arginine:** Contains a **guanidino group**. It is the most basic amino acid and a precursor for Nitric Oxide (NO). * **Tyrosine:** Contains a **phenol ring** (a hydroxyl group attached to a benzene ring). It is a precursor for catecholamines, thyroid hormones, and melanin. **Clinical Pearls for NEET-PG:** * **Buffering Capacity:** Histidine is the most important amino acid for the buffering action of hemoglobin in the blood. * **Pauly’s Test:** This biochemical test is used specifically to detect the presence of Histidine or Tyrosine. * **FIGLU Test:** Formiminoglutamic acid (FIGLU) is an intermediate of histidine metabolism. Increased urinary excretion of FIGLU is a clinical marker for **Folic Acid deficiency**. * **Essentiality:** Histidine is considered a semi-essential amino acid (essential for growth in children and during pregnancy).

Question 112: Which cellular component is primarily responsible for protein folding?

A. Chaperone (Correct Answer)
B. Endoplasmic reticulum
C. Peroxisome
D. Lysosome

Explanation: **Explanation:** **1. Why Chaperones are the Correct Answer:** Protein folding is the process by which a polypeptide chain acquires its functional 3D conformation. **Chaperones** (and chaperonins) are specialized proteins that facilitate this process. They prevent the aggregation of unfolded or partially folded polypeptide chains by binding to exposed hydrophobic regions. While folding can occur spontaneously, chaperones ensure the process is efficient and error-free, especially under cellular stress (Heat Shock Proteins/HSPs). **2. Why Other Options are Incorrect:** * **Endoplasmic Reticulum (ER):** While the Rough ER is the *site* where secretory and membrane proteins are synthesized and folded, it is not the "component" responsible for the folding itself. The folding within the ER is actually performed by resident chaperones (like BiP/GRP78). * **Peroxisome:** These organelles are involved in long-chain fatty acid oxidation (beta-oxidation), hydrogen peroxide metabolism, and bile acid synthesis, not protein folding. * **Lysosome:** These are the "garbage disposals" of the cell. They contain acid hydrolases meant for protein *degradation* (proteolysis), which is the opposite of folding. **3. High-Yield Clinical Pearls for NEET-PG:** * **HSP70:** A major chaperone that prevents aggregation during translation. * **HSP60 (Chaperonins):** Form a cage-like structure (e.g., GroEL-GroES in bacteria) to fold proteins in isolation. * **Clinical Correlation:** Misfolding of proteins is the hallmark of several neurodegenerative diseases, such as **Alzheimer’s** (Amyloid-beta), **Parkinson’s** (alpha-synuclein), and **Prion diseases** (PrPSc). * **Cystic Fibrosis:** Often results from the premature degradation of a slightly misfolded but functional CFTR protein by the ER quality control system.

Question 113: Keratin in skin is softer than keratin in nail. What is the reason for this difference?

A. Fewer disulfide bonds (Correct Answer)
B. Fewer salt bridges
C. High sodium content
D. Different affinity for water

Explanation: **Explanation:** Keratin is a fibrous structural protein belonging to the intermediate filament family. The mechanical strength and rigidity of keratin are primarily determined by the degree of **cross-linking between polypeptide chains via disulfide bonds.** **1. Why Option A is Correct:** Disulfide bonds are covalent bonds formed between the sulfhydryl (-SH) groups of **Cysteine** residues. * **Hard Keratin (Nails, Hair):** Contains a high concentration of sulfur-rich cysteine, leading to extensive disulfide cross-linking. This creates a dense, rigid, and chemically resistant matrix. * **Soft Keratin (Skin/Epidermis):** Contains significantly fewer cysteine residues and, consequently, **fewer disulfide bonds**. This allows the protein structure to be more flexible and pliable, which is essential for the skin's elasticity. **2. Why Other Options are Incorrect:** * **B. Fewer salt bridges:** While salt bridges (ionic interactions) contribute to protein stability, they are non-covalent and weaker than disulfide bonds. They do not dictate the fundamental difference between hard and soft keratin. * **C. High sodium content:** Sodium levels relate to electrolyte balance and osmotic pressure but have no structural role in the mechanical hardness of keratin filaments. * **D. Different affinity for water:** While soft keratin is slightly more hygroscopic (water-absorbing) than hard keratin, this is a *result* of the looser protein network, not the primary structural reason for the difference in hardness. **High-Yield Facts for NEET-PG:** * **Amino Acid Precursor:** Cysteine is the key amino acid responsible for the stability of keratin. * **Alpha vs. Beta Keratin:** Mammals have **$\alpha$-keratin** (coiled-coil structure), while birds and reptiles have **$\beta$-keratin** (mostly $\beta$-sheets, found in feathers/scales). * **Clinical Correlation:** In **Scurvy** (Vitamin C deficiency), keratinization is affected (follicular hyperkeratosis) because Vitamin C is necessary for various protein modifications, though the primary defect in Scurvy is collagen synthesis.

Question 114: All of the following represent disorders of protein misfolding, EXCEPT:

A. Alzheimer's disease
B. Tuberculosis (Correct Answer)
C. Cystic fibrosis
D. Creutzfeldt-Jakob disease

Explanation: ### Explanation The core concept tested here is the pathophysiology of **proteopathies**—diseases caused by proteins failing to fold into their correct three-dimensional shapes, leading to loss of function or toxic aggregation. **Why Tuberculosis is the Correct Answer:** **Tuberculosis (TB)** is an infectious disease caused by the bacterium *Mycobacterium tuberculosis*. It is not a primary disorder of protein folding. Its pathogenesis involves bacterial invasion, intracellular survival within macrophages, and the subsequent host immune response (granuloma formation). **Analysis of Incorrect Options (Misfolding Disorders):** * **Alzheimer’s Disease:** Characterized by the misfolding of **Amyloid-β** (forming extracellular plaques) and **Tau protein** (forming intracellular neurofibrillary tangles). * **Cystic Fibrosis:** Most commonly caused by the **ΔF508 mutation** in the CFTR gene. This mutation leads to the misfolding of the CFTR protein in the endoplasmic reticulum, which is then recognized by the "quality control" system and degraded before reaching the cell membrane. * **Creutzfeldt-Jakob Disease (CJD):** A classic **Prion disease**. It involves the conformational change of normal cellular prion protein ($PrP^C$) into the pathological, $\beta$-sheet-rich isoform ($PrP^{Sc}$), which is resistant to proteolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones (Heat Shock Proteins):** These are specialized proteins (e.g., HSP70) that assist in correct protein folding and prevent aggregation. * **Prion Diseases:** These are unique because the misfolded protein itself acts as an infectious agent (e.g., Kuru, Mad Cow Disease). * **Other Misfolding Examples:** Parkinson’s (α-synuclein), Huntington’s (Huntingtin), and Sickle Cell Anemia (Hemoglobin S polymerization). * **Amyloidosis:** A condition where various misfolded proteins form insoluble fibrils with a **cross-β sheet** structure, staining positive with **Congo Red** (showing apple-green birefringence).

Question 115: All of the following hormones are glycoproteins, EXCEPT?

A. TSH
B. FSH
C. LH
D. GH (Correct Answer)

Explanation: **Explanation:** The correct answer is **GH (Growth Hormone)**. **1. Why GH is the correct answer:** Hormones are chemically classified into proteins/peptides, steroids, or amino acid derivatives. **Glycoproteins** are a specific subset of proteins that contain oligosaccharide chains (carbohydrates) covalently attached to amino acid side-chains. * **Growth Hormone (GH)** and **Prolactin** are pure **polypeptide hormones** (single-chain proteins) secreted by the acidophil cells of the anterior pituitary. They do not contain carbohydrate moieties. **2. Why the other options are incorrect:** Options A, B, and C (**TSH, FSH, and LH**) along with **hCG** (Human Chorionic Gonadotropin) form the family of **Glycoprotein Hormones**. * They are dimeric, consisting of two subunits: **Alpha (α) and Beta (β)**. * The **α-subunit** is identical in all four hormones. * The **β-subunit** is unique to each and confers biological and immunological specificity. * These hormones require glycosylation for proper folding, stability, and biological activity. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Acidophils vs. Basophils:** Remember the mnemonic **"GPA"** (Growth Hormone & Prolactin are produced by Acidophils) and **"B-FLAT"** (Basophils produce FSH, LH, ACTH, and TSH). Note that while ACTH is from basophils, it is a polypeptide, not a glycoprotein. * **Common α-subunit:** Because TSH, FSH, LH, and hCG share the same α-subunit, extremely high levels of hCG (as seen in molar pregnancies) can cross-react with TSH receptors, leading to hyperthyroidism. * **GH Structure:** Human GH is a single polypeptide chain of 191 amino acids with two intramolecular disulfide bridges.

Question 116: Keratin of skin and nail differ because of which type of bond?

A. Disulfide bond (Correct Answer)
B. Covalent bond
C. Van der Waals bond
D. Hydrogen bond

Explanation: **Explanation:** Keratin is a fibrous structural protein that forms the primary constituent of skin, hair, and nails. The fundamental difference in the mechanical properties (rigidity vs. flexibility) of various keratins is determined by the **degree of cross-linking via disulfide bonds.** 1. **Why Disulfide bond is correct:** Keratins are classified into "soft" keratins (found in the skin's stratum corneum) and "hard" keratins (found in hair and nails). Hard keratins contain a significantly higher concentration of the sulfur-containing amino acid **Cysteine**. These cysteine residues form inter-chain **disulfide bridges** (covalent linkages between sulfur atoms), creating a dense, rigid, and chemically resistant matrix. The more disulfide bonds present, the harder the structure. 2. **Why other options are incorrect:** * **Covalent bond:** While a disulfide bond is a type of covalent bond, it is the *specific* type of covalent cross-linkage that differentiates these structures. "Covalent bond" is too broad a term. * **Van der Waals bonds:** These are weak intermolecular forces present in all proteins; they contribute to stability but do not account for the structural hardness of nails. * **Hydrogen bonds:** These stabilize the alpha-helices within the keratin polypeptide chains. While essential for the secondary structure, they are easily broken (e.g., by water or heat) and do not provide the permanent rigidity seen in nails. **Clinical Pearls for NEET-PG:** * **Amino Acid Composition:** Keratin is exceptionally rich in **Cysteine**. * **Secondary Structure:** Keratin in mammals is primarily **$\alpha$-keratin** (right-handed $\alpha$-helix). * **Clinical Correlation:** In **Kwashiorkor**, protein deficiency leads to poor disulfide bond formation, resulting in brittle hair and "flaky paint" dermatosis. * **Perming/Straightening:** Hair styling involves chemically breaking and reforming these disulfide bonds to change hair shape.

Question 117: Glutathione is composed of which amino acids?

A. Glycine
B. Cysteine
C. Glutamate
D. All of the above (Correct Answer)

Explanation: **Explanation:** Glutathione (GSH) is a vital **tripeptide** found in high concentrations in almost all mammalian cells. It is composed of three specific amino acids: **L-Glutamate, L-Cysteine, and Glycine.** The unique structural feature of Glutathione is the linkage between the amino acids. Unlike typical proteins, the amino group of Cysteine is linked to the **gamma-carboxyl group** of Glutamate (rather than the alpha-carboxyl), forming a **$\gamma$-glutamyl linkage**. This makes it resistant to degradation by most intracellular peptidases. * **Why Option D is correct:** Glutathione is chemically defined as **$\gamma$-L-Glutamyl-L-cysteinyl-glycine**. Therefore, all three amino acids (Glycine, Cysteine, and Glutamate) are essential structural components. * **Why individual options (A, B, C) are "wrong":** While Glycine, Cysteine, and Glutamate are indeed part of the molecule, selecting any single one would be incomplete. In the context of a multiple-choice question, "All of the above" is the most accurate description of its composition. **High-Yield Clinical Pearls for NEET-PG:** 1. **The Active Site:** The **thiol (-SH) group of Cysteine** is the functional part of the molecule, responsible for its antioxidant properties and redox reactions. 2. **Function:** It acts as a major intracellular antioxidant, neutralizing free radicals and maintaining Vitamins C and E in their reduced forms. 3. **Enzymatic Role:** It is a cofactor for **Glutathione Peroxidase**, which neutralizes hydrogen peroxide ($H_2O_2$) into water, protecting RBCs from hemolysis. 4. **Clinical Correlation:** In **G6PD deficiency**, a lack of NADPH leads to a failure in regenerating reduced glutathione, resulting in oxidative stress and Heinz body formation.

Question 118: Which one of the following is a heterocyclic amino acid?

A. Leucine
B. Methionine
C. Alanine
D. Proline (Correct Answer)

Explanation: **Explanation:** The correct answer is **Proline**. **1. Why Proline is the Correct Answer:** A heterocyclic compound is a cyclic structure that contains at least one atom other than carbon (such as Nitrogen, Oxygen, or Sulfur) within the ring. Proline is unique among the 20 standard amino acids because its side chain cyclizes back onto its own amino group. This forms a five-membered **pyrrolidine ring**, making it a heterocyclic amino acid. Technically, proline is an **imino acid** because it contains a secondary amino group (NH) rather than a primary one ($NH_2$). **2. Analysis of Incorrect Options:** * **A. Leucine:** An aliphatic, branched-chain amino acid (BCAA) with a non-cyclic hydrocarbon side chain. * **B. Methionine:** A sulfur-containing amino acid with a linear (non-cyclic) thioether side chain. * **C. Alanine:** The simplest chiral amino acid with a methyl group side chain; it is aliphatic and non-cyclic. **3. High-Yield Clinical Pearls for NEET-PG:** * **Helix Breaker:** Due to its rigid cyclic structure, Proline creates "kinks" in polypeptide chains and is known as a **$\alpha$-helix breaker**. It is rarely found in $\alpha$-helices but is abundant in **$\beta$-turns**. * **Collagen Synthesis:** Proline and its derivative, **Hydroxyproline**, are essential for the stability of the collagen triple helix. Hydroxylation of proline requires **Vitamin C** (Ascorbic acid); deficiency leads to Scurvy. * **Ninhydrin Test:** While most amino acids give a purple/Ruhemann's purple color with ninhydrin, Proline (and Hydroxyproline) gives a characteristic **yellow color** due to its imino group. * **Other Cyclic Amino Acids:** Do not confuse "heterocyclic" with "aromatic." Phenylalanine and Tyrosine are carbocyclic (rings made only of carbon), while **Tryptophan** (Indole ring) and **Histidine** (Imidazole ring) are also heterocyclic.

Question 119: Flexibility of protein depends on which amino acid?

A. Glycine (Correct Answer)
B. Tryptophan
C. Phenylalanine
D. Histidine

Explanation: **Explanation:** The flexibility of a protein is primarily determined by the size and nature of the side chains (R-groups) of its constituent amino acids. **Why Glycine is the correct answer:** Glycine is the simplest amino acid, having only a single **hydrogen atom** as its side chain. Because this side chain is so small, it imposes minimal steric hindrance. This allows the polypeptide backbone at the glycine position to rotate more freely and adopt a wider range of dihedral angles (phi and psi) on the **Ramachandran plot**. Consequently, glycine acts as a "hinge" or "flexibility provider," allowing proteins to form tight turns (like Beta-turns) and complex folds that larger amino acids would physically block. **Why the other options are incorrect:** * **Tryptophan & Phenylalanine:** These are large, bulky **aromatic amino acids**. Their massive side chains create significant steric hindrance, restricting the rotation of the peptide bond and decreasing local protein flexibility. * **Histidine:** While smaller than tryptophan, histidine still possesses a relatively rigid imidazole ring side chain, which provides significantly less conformational freedom compared to glycine. **High-Yield Clinical Pearls for NEET-PG:** * **The "Helix Breaker":** While Glycine provides flexibility, **Proline** provides rigidity. Proline is an imino acid that creates a "kink" in the chain and is known as a helix breaker. * **Collagen Structure:** Glycine is essential for the triple helix of collagen. It occurs at every third position (**Gly-X-Y**) because only glycine is small enough to fit into the crowded central core of the triple helix. * **Ramachandran Plot:** Glycine is the only amino acid that can occupy regions of the plot that are "disallowed" for other amino acids due to its lack of a beta-carbon.

Question 120: Post-translational modification is seen with which of the following drugs?

A. Streptomycin (Correct Answer)
B. Penicillin
C. Sulphonamides
D. Cephalosporins

Explanation: **Explanation:** The correct answer is **Streptomycin**. **Mechanism and Post-translational Modification:** Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis. Streptomycin, an aminoglycoside, acts by binding to the **30S ribosomal subunit** of bacteria. This binding induces a conformational change that causes **misreading of mRNA**. Consequently, incorrect amino acids are incorporated into the polypeptide chain, leading to the synthesis of "non-functional" or "erroneous" proteins. In the context of biochemistry and pharmacology, the interference with the fidelity of protein synthesis and the subsequent alterations to the protein product are categorized under the broader umbrella of affecting protein maturation and modification. **Analysis of Incorrect Options:** * **Penicillin & Cephalosporins (B & D):** These are Beta-lactam antibiotics. Their primary mechanism is the inhibition of bacterial cell wall synthesis by binding to Penicillin-Binding Proteins (PBPs) and preventing the cross-linking of peptidoglycan. They do not directly interfere with protein synthesis or post-translational modifications. * **Sulphonamides (C):** These are antimetabolites that inhibit the enzyme dihydropteroate synthase. They interfere with folic acid synthesis, which eventually inhibits nucleic acid (DNA/RNA) synthesis, rather than protein modification. **Clinical Pearls for NEET-PG:** * **Aminoglycoside Toxicity:** Remember the triad of **Ototoxicity** (irreversible), **Nephrotoxicity** (reversible), and **Neuromuscular blockade**. * **Resistance Mechanism:** The most common mechanism of bacterial resistance to aminoglycosides is the production of **plasmid-mediated modifying enzymes** (acetylation, adenylation, or phosphorylation of the drug). * **Other PTM Examples:** Common biological PTMs include hydroxylation of proline/lysine (requires Vitamin C), gamma-carboxylation of glutamate (requires Vitamin K), and glycosylation.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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