Protein Structure and Function — MCQs
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Albumin binds with all except?
Ehlers-Danlos syndrome can be caused by a problem with the metabolism of which particular compound?
Which protein precipitates upon heating to 45°C and redissolves upon boiling?
The estimation of 3-methylhistidine in urine is used to study:
Which plasma protein is NOT normally synthesized in the liver?
Hydroxylation of proline requires the following cofactors, except:
What is mucin?
In glutathione, which amino acid acts as the reducing agent?
Which of the following is the commonest protein in mammalian cells?
Which of the following is an example of a conjugated protein?
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 311: Albumin binds with all except?
- A. Steroid
- B. Calcium
- C. FFA
- D. Thyroxine (Correct Answer)
Explanation: ### Explanation The correct answer is **Thyroxine (D)**. While Albumin is the most abundant plasma protein and acts as a versatile transport molecule, it is **not** the primary carrier for thyroxine. Thyroxine (T4) is primarily transported by **Thyroxine-Binding Globulin (TBG)**, which carries about 70% of the hormone. Another 10-15% is carried by **Transthyretin** (Pre-albumin). While Albumin can bind thyroxine with high capacity, it has a very low affinity for it, making it a secondary carrier. **Analysis of Options:** * **A. Steroids:** Albumin is a major non-specific carrier for various steroid hormones (like cortisol and aldosterone), especially when their specific globulins (e.g., CBG) are saturated. * **B. Calcium:** Approximately 40-45% of serum calcium is bound to plasma proteins, and **Albumin** is the primary protein responsible for this binding. This is why "Corrected Calcium" must be calculated in patients with hypoalbuminemia. * **C. FFA (Free Fatty Acids):** Albumin is the definitive carrier for long-chain fatty acids. It possesses specific high-affinity binding sites (Sudlow’s sites) to transport these hydrophobic molecules in the aqueous plasma. **High-Yield NEET-PG Pearls:** 1. **Sudlow’s Site I:** Binds Warfarin, Salicylates, and Phenylbutazone. 2. **Sudlow’s Site II:** Binds Diazepam, Ibuprofen, and Tryptophan. 3. **Bilirubin Transport:** Albumin carries unconjugated bilirubin to the liver. Drugs like Sulfonamides can displace bilirubin from albumin, leading to **Kernicterus** in neonates. 4. **Negative Acute Phase Reactant:** Albumin levels decrease during acute inflammation or infection.
Question 312: Ehlers-Danlos syndrome can be caused by a problem with the metabolism of which particular compound?
- A. Glycogen
- B. Collagen (Correct Answer)
- C. Dopamine
- D. Valine
Explanation: **Explanation:** **Ehlers-Danlos Syndrome (EDS)** is a heterogeneous group of inherited connective tissue disorders characterized by joint hypermobility, skin hyperextensibility, and tissue fragility. The underlying pathophysiology involves defects in the synthesis or processing of **Collagen**, the most abundant structural protein in the human body. * **Why Collagen is Correct:** EDS is caused by mutations in genes encoding fibrillar collagen (such as Type I, III, or V) or enzymes responsible for its post-translational modification (e.g., lysyl hydroxylase or procollagen peptidase). These defects lead to weakened collagen fibrils, resulting in the clinical manifestations of fragile skin and hyperelastic joints. * **Why Incorrect Options are Wrong:** * **Glycogen:** Disorders of glycogen metabolism are termed Glycogen Storage Diseases (e.g., Von Gierke or Pompe disease), primarily affecting the liver and muscles. * **Dopamine:** Abnormalities in dopamine metabolism are linked to neurological conditions like Parkinson’s disease or schizophrenia, not structural connective tissue. * **Valine:** Valine is a branched-chain amino acid. Its metabolic defects lead to Maple Syrup Urine Disease (MSUD). While valine is found in proteins, EDS specifically targets the structural integrity of collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Classical Type (EDS I/II):** Defect in **Type V Collagen**. * **Vascular Type (EDS IV):** Defect in **Type III Collagen** (most severe; risk of arterial or organ rupture). * **Kyphoscoliotic Type (EDS VI):** Deficiency of **Lysyl Hydroxylase** (enzyme required for cross-linking). * **Key Clinical Sign:** "Cigarette paper" or atrophic scarring and "Gorlin sign" (touching the tip of the nose with the tongue).
Question 313: Which protein precipitates upon heating to 45°C and redissolves upon boiling?
- A. Bence Jones protein (Correct Answer)
- B. Gamma globulin
- C. Albumin
- D. Myosin
Explanation: **Explanation:** **Bence Jones proteins (BJP)** are monoclonal immunoglobulin light chains (either kappa or lambda) produced in excess by neoplastic plasma cells. They possess a unique **thermosolubility characteristic** that distinguishes them from other proteins: 1. **Precipitation:** They begin to coagulate and precipitate when heated to **40°C–60°C** (typically around 45°C). 2. **Redissolution:** Unlike most proteins that denature permanently, BJPs **redissolve upon boiling (100°C)**. 3. **Re-precipitation:** They reappear as the urine cools back down to the 40°C–60°C range. **Analysis of Incorrect Options:** * **B. Gamma globulin:** These are full-sized immunoglobulins. While they may precipitate with heat, they do not redissolve upon boiling; they undergo irreversible denaturation. * **C. Albumin:** This is the most common protein found in urine (proteinuria). Albumin coagulates upon heating but remains a solid precipitate at 100°C. * **D. Myosin:** A structural muscle protein that is not typically found in urine and does not exhibit this specific reversible thermal solubility. **Clinical Pearls for NEET-PG:** * **Disease Association:** Bence Jones proteinuria is a hallmark of **Multiple Myeloma** (found in ~50-80% of cases) and is also seen in Waldenström macroglobulinemia. * **Diagnostic Note:** Standard urine dipsticks primarily detect Albumin and often give a **false-negative** result for Bence Jones proteins. Detection requires the **Sulfosalicylic Acid (SSA) test**, heat precipitation test, or the gold standard: **Urine Protein Electrophoresis (UPEP)** showing an 'M-spike'. * **Renal Impact:** These light chains are nephrotoxic and can lead to "Myeloma Kidney" (cast nephropathy).
Question 314: The estimation of 3-methylhistidine in urine is used to study:
- A. Status of folate in the body
- B. Renal disease
- C. Skeletal muscle turnover (Correct Answer)
- D. Protein absorption in the gut
Explanation: **Explanation:** **3-methylhistidine (3-MH)** is a unique amino acid derivative formed by the post-translational methylation of specific histidine residues in **actin and myosin**, the primary contractile proteins of skeletal muscle. **Why Option C is correct:** When muscle proteins are degraded (catabolized), 3-methylhistidine is released into the bloodstream. Unlike other amino acids, 3-MH cannot be re-utilized for protein synthesis or further metabolized; it is excreted unchanged in the urine. Therefore, the urinary excretion rate of 3-methylhistidine serves as a specific and sensitive **biochemical marker for skeletal muscle protein turnover** and myofibrillar protein breakdown. **Why other options are incorrect:** * **Option A (Folate status):** Folate status is typically assessed using serum folate levels or the **FIGLU (Formiminoglutamic acid) excretion test**, where FIGLU accumulates in the urine during folate deficiency. * **Option B (Renal disease):** While 3-MH is excreted by the kidneys, it is not a diagnostic marker for renal disease. Standard markers include Serum Creatinine, Urea, and Cystatin C. * **Option D (Protein absorption):** Protein absorption is generally assessed via fecal nitrogen studies or D-xylose tests (for general malabsorption), not by specific methylated amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** 3-MH is found exclusively in actin and myosin. * **Clinical Utility:** Elevated levels are seen in hypercatabolic states like severe trauma, sepsis, starvation, and muscular dystrophy. * **Dietary Note:** To ensure accuracy, patients should follow a **meat-free diet** for 3 days prior to the test, as exogenous 3-MH from consumed animal muscle can falsely elevate results.
Question 315: Which plasma protein is NOT normally synthesized in the liver?
- A. Angiotensin
- B. Angiotensin-converting enzyme (Correct Answer)
- C. C-reactive protein
- D. Albumin
Explanation: **Explanation** The liver is the primary factory for plasma proteins, synthesizing nearly all of them except for immunoglobulins (produced by plasma cells) and specific enzymes produced by vascular endothelium. **1. Why Angiotensin-converting enzyme (ACE) is the correct answer:** ACE is a glycoprotein primarily synthesized and located on the **luminal surface of vascular endothelial cells**, particularly within the **pulmonary capillaries**. While it is found in various tissues (kidneys, heart), the lungs are the major site of conversion for Angiotensin I to Angiotensin II. Because it is an endothelial product rather than a hepatic one, it is the correct exception. **2. Analysis of Incorrect Options:** * **Angiotensin (Angiotensinogen):** Angiotensinogen is the precursor protein of the RAAS pathway. It is synthesized and secreted constitutively into the plasma exclusively by the **liver**. * **C-reactive protein (CRP):** This is a classic "Acute Phase Reactant." Its synthesis is induced in the **hepatocytes** in response to inflammatory cytokines like IL-6. * **Albumin:** This is the most abundant plasma protein and is synthesized solely by the **liver**. It serves as a marker of hepatic synthetic function. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Synthesis:** All plasma proteins are synthesized in the liver EXCEPT Gamma-globulins (Plasma cells), ACE (Endothelium), and von Willebrand Factor (Endothelium/Megakaryocytes). * **ACE Inhibitors:** Drugs like Enalapril and Ramipril target this enzyme to treat hypertension and heart failure. * **Negative Acute Phase Reactants:** Remember that while CRP goes up during inflammation, **Albumin and Transferrin** levels decrease (Negative APRs).
Question 316: Hydroxylation of proline requires the following cofactors, except:
- A. Fe2+
- B. O2
- C. Ascorbic Acid
- D. Succinate (Correct Answer)
Explanation: **Explanation:** The hydroxylation of proline is a critical post-translational modification occurring in the endoplasmic reticulum during **collagen synthesis**. This reaction is catalyzed by the enzyme **Prolyl hydroxylase**. **Why Succinate is the correct answer:** Succinate is a **product** of the reaction, not a cofactor. During the hydroxylation process, **α-ketoglutarate** (the actual substrate/cofactor) undergoes oxidative decarboxylation to be converted into succinate and CO2. Therefore, succinate does not facilitate the reaction; it is the result of it. **Why the other options are incorrect:** * **Fe2+ (Ferrous iron):** This is a vital cofactor located at the enzyme's active site. It must remain in the reduced (ferrous) state for the enzyme to function. * **O2 (Molecular Oxygen):** One atom of oxygen is incorporated into the proline residue (forming hydroxyproline), while the other is incorporated into α-ketoglutarate. * **Ascorbic Acid (Vitamin C):** It acts as a reducing agent to maintain iron in the **Fe2+ state**. If iron is oxidized to Fe3+, the enzyme becomes inactive; Vitamin C reduces it back to Fe2+. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective prolyl and lysyl hydroxylation. This results in unstable collagen triple helices, leading to fragile blood vessels, easy bruising, and poor wound healing. * **Hydroxyproline** is a specific marker for collagen; its urinary excretion levels are used to estimate the rate of **bone resorption** (collagen breakdown). * **Requirement Summary:** Prolyl hydroxylase requires: **Fe2+, O2, Ascorbate, and α-ketoglutarate.**
Question 317: What is mucin?
- A. Nucleoprotein
- B. Glycoprotein (Correct Answer)
- C. Phosphoprotein
- D. Chromoprotein
Explanation: ### Explanation **Correct Answer: B. Glycoprotein** **Why it is correct:** Mucins are high-molecular-weight **glycoproteins** produced by epithelial tissues. They are the primary constituents of mucus, providing lubrication and acting as a chemical barrier. Structurally, they consist of a protein core with extensive **O-linked glycosylation** (carbohydrates attached to Serine or Threonine residues). The carbohydrate content in mucins is very high, often exceeding 50% of the total weight, which allows them to retain water and form gel-like structures. **Why other options are incorrect:** * **A. Nucleoprotein:** These are proteins conjugated with nucleic acids (DNA or RNA). Examples include histones, chromatin, and ribosomes. * **C. Phosphoprotein:** These are proteins containing phosphoric acid groups esterified to amino acid residues (usually Serine, Threonine, or Tyrosine). Examples include **Casein** (milk) and **Vitellin** (egg yolk). * **D. Chromoprotein:** These are proteins conjugated with a pigment or a colored prosthetic group. Examples include **Hemoglobin** (heme), **Cytochromes**, and **Flavoproteins**. **High-Yield Clinical Pearls for NEET-PG:** * **O-linked Glycosylation:** Unlike N-linked glycosylation (which occurs in the ER), O-linked glycosylation of mucins occurs exclusively in the **Golgi apparatus**. * **Tumor Markers:** Overexpression or altered glycosylation of mucins is seen in adenocarcinomas. **CA 125** (Ovarian cancer) and **CA 19-9** (Pancreatic cancer) are clinically significant mucin-type glycoproteins. * **Cystic Fibrosis:** This condition involves the production of dehydrated, hyperviscous mucus due to defective chloride transport, leading to obstructive lung disease and pancreatic insufficiency.
Question 318: In glutathione, which amino acid acts as the reducing agent?
- A. Glutamic acid
- B. Cysteine (Correct Answer)
- C. Glycine
- D. Alanine
Explanation: **Explanation:** Glutathione (GSH) is a tripeptide composed of **γ-glutamyl-cysteinyl-glycine**. It serves as the body’s premier endogenous antioxidant and a key component of the redox buffer system. **Why Cysteine is correct:** The functional core of glutathione is the **thiol (-SH) group** located on the side chain of the **Cysteine** residue. This sulfhydryl group acts as a donor of reducing equivalents (electrons/hydrogen). When neutralizing reactive oxygen species (ROS) like hydrogen peroxide, two molecules of reduced glutathione (GSH) are oxidized to form a disulfide-linked dimer, glutathione disulfide (GSSG). Cysteine is the only amino acid among the three that contains sulfur, making it the active site for all redox reactions. **Why other options are incorrect:** * **Glutamic acid:** It provides the N-terminal of the peptide. Notably, it forms a unique **gamma-glutamyl bond** (using the side-chain carboxyl group) which protects the molecule from degradation by ordinary peptidases. It does not participate in redox transfer. * **Glycine:** It forms the C-terminal of the tripeptide and provides structural stability but lacks a reactive side chain for antioxidant activity. * **Alanine:** This amino acid is not a constituent of the glutathione tripeptide. **High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Peroxidase:** The enzyme that utilizes GSH to neutralize $H_2O_2$; it requires **Selenium** as a cofactor. * **Glutathione Reductase:** Regenerates GSH from GSSG using **NADPH** (primarily sourced from the HMP Shunt). * **G6PD Deficiency:** Leads to hemolysis because the lack of NADPH prevents the regeneration of reduced glutathione, leaving RBCs vulnerable to oxidative stress (Heinz bodies). * **Acetaminophen (Paracetamol) Poisoning:** Depletes glutathione stores; **N-acetylcysteine (NAC)** is the antidote as it acts as a precursor for cysteine synthesis.
Question 319: Which of the following is the commonest protein in mammalian cells?
- A. Actin (Correct Answer)
- B. Tubulin
- C. Keratin
- D. None of the above
Explanation: **Explanation:** **Actin** is the correct answer because it is the most abundant protein in most eukaryotic (mammalian) cells. In muscle cells, it comprises about 10% of the total protein weight, and even in non-muscle cells, it makes up approximately 1–5%. It exists in two forms: **G-actin** (globular monomer) and **F-actin** (filamentous polymer). Actin is a critical component of the cytoskeleton, essential for maintaining cell shape, enabling cell motility, and facilitating cytokinesis and intracellular transport. **Analysis of Incorrect Options:** * **Tubulin:** While tubulin is a major cytoskeletal component forming microtubules (essential for mitosis and ciliary movement), its total concentration in the cell is significantly lower than that of actin. * **Keratin:** This is an intermediate filament found specifically in epithelial cells. While it is abundant in the epidermis (skin), it is not the "commonest" protein when considering all mammalian cell types collectively. * **Note on Collagen:** Students often confuse this question with "What is the most abundant protein in the **human body**?" The answer to that is **Collagen** (extracellular). However, for the most abundant protein **within the cell**, the answer is **Actin**. **High-Yield Clinical Pearls for NEET-PG:** * **Microfilaments:** Actin filaments are the thinnest (7 nm) components of the cytoskeleton. * **Muscle Contraction:** Actin (thin filament) interacts with Myosin (thick filament) in a calcium-dependent manner. * **Wiskott-Aldrich Syndrome:** Caused by a defect in the WASP protein, which regulates actin polymerization in hematopoietic cells. * **Drug Correlation:** **Cytochalasins** inhibit actin polymerization, while **Phalloidin** (from death cap mushrooms) stabilizes filaments and prevents depolymerization.
Question 320: Which of the following is an example of a conjugated protein?
- A. Heme
- B. Chylomicron (Correct Answer)
- C. Albumin
- D. Histone
Explanation: **Explanation:** **1. Why Chylomicron is the Correct Answer:** Conjugated proteins are defined as proteins that contain a non-protein component (called a **prosthetic group**) attached to the polypeptide chain [2]. **Chylomicrons** are a classic example of **Lipoproteins** [1]. They consist of a protein part (Apolipoproteins like Apo B-48, C-II, and E) conjugated with lipids (primarily triglycerides, along with cholesterol and phospholipids) [3]. Other examples of conjugated proteins include Glycoproteins (mucin), Phosphoproteins (casein), and Metalloproteins (ferritin). **2. Analysis of Incorrect Options:** * **A. Heme:** This is a common distractor. Heme is the **prosthetic group** itself (an iron-porphyrin complex), not the protein. The conjugated protein would be Hemoglobin (Heme + Globin). * **C. Albumin:** This is a **Simple Protein**. Simple proteins yield only amino acids upon hydrolysis and do not contain a non-protein prosthetic group. * **D. Histone:** These are also **Simple Proteins**. They are basic proteins rich in Arginine and Lysine that help in DNA packaging, but the protein molecule itself does not contain a non-amino acid component. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chylomicrons** are the largest and least dense lipoproteins; they transport **exogenous (dietary) lipids** from the intestines to peripheral tissues [4]. * **Apo B-48** is the unique structural marker for chylomicrons (formed via mRNA editing in the intestine). * **Milky appearance of plasma** post-meals is due to the presence of chylomicrons. * **Classification Tip:** Remember that "Proteoglycans" and "Nucleoproteins" are also high-yield examples of conjugated proteins frequently tested in exams.
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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