Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 171: Which of the following amino acids does NOT absorb light at 280 nm?

A. Phenylalanine
B. Tyrosine
C. Tryptophan
D. Methionine (Correct Answer)

Explanation: **Explanation:** The ability of proteins to absorb ultraviolet (UV) light at **280 nm** is primarily due to the presence of **aromatic amino acids**. These amino acids contain conjugated double bonds in their ring structures (delocalized pi electrons), which can be excited by UV radiation. **1. Why Methionine is the Correct Answer:** Methionine is a sulfur-containing aliphatic amino acid. It lacks an aromatic ring structure and, therefore, does not show significant absorbance at 280 nm. While it can absorb light at much lower wavelengths (around 200-210 nm), it is "silent" at the standard 280 nm protein quantification mark. **2. Analysis of Incorrect Options:** * **Tryptophan (C):** This is the strongest absorber of UV light at 280 nm due to its bulky indole ring. It contributes the most to the molar extinction coefficient of a protein. * **Tyrosine (B):** Contains a phenolic ring and is the second most significant contributor to absorbance at 280 nm. * **Phenylalanine (A):** While phenylalanine is an aromatic amino acid, its absorption peak is actually closer to **257 nm**. However, it still exhibits some absorbance in the UV range, unlike Methionine. **High-Yield NEET-PG Pearls:** * **Spectrophotometry:** The concentration of a protein in a solution is commonly measured at 280 nm using the **Beer-Lambert Law**. * **Order of Absorbance (at 280nm):** Tryptophan > Tyrosine > Phenylalanine. * **Ninhydrin Test:** All alpha-amino acids (including Methionine) react with ninhydrin to give a **purple/Ruhemann's purple** color, except Proline and Hydroxyproline, which give a **yellow** color. * **Sulfur-containing amino acids:** Methionine and Cysteine. Only Cysteine can form disulfide bridges.

Question 172: Which of the following is the most abundant protein in the animal kingdom?

A. Keratin
B. Elastin
C. Collagen (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Correct Option: C (Collagen)** Collagen is the most abundant protein in the animal kingdom, accounting for approximately **25% to 35% of the total protein content** in mammals. It is a structural fibrous protein found primarily in the extracellular matrix of connective tissues, including skin, bones, tendons, cartilage, and teeth. Its abundance is due to its critical role in providing tensile strength and structural integrity to almost every organ system. **Why Incorrect Options are Wrong:** * **A. Keratin:** While keratin is a major structural protein found in the epidermis, hair, and nails, its distribution is limited to epithelial cells and their derivatives. It is not as globally abundant as collagen. * **B. Elastin:** Elastin provides elasticity to tissues like the aorta and lungs. Although vital, it is found in much smaller quantities compared to the ubiquitous collagen framework. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Collagen is a triple helix composed of three polypeptide chains. Every third amino acid is **Glycine** (the smallest amino acid, allowing tight packing). * **Post-translational Modification:** Hydroxylation of Proline and Lysine requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy** due to defective collagen cross-linking. * **Types to Remember:** * Type I: Bone, Skin, Tendon (90% of body collagen). * Type II: Cartilage. * Type III: Reticulin (Blood vessels, fetal skin). * Type IV: Basement membrane. * **Global Context:** While Collagen is the most abundant in the **animal kingdom**, **RuBisCO** is the most abundant protein in the **entire biosphere**.

Question 173: What is the most abundant type of collagen in the human body?

A. Type I (Correct Answer)
B. Type III
C. Type IV
D. Type VI

Explanation: **Explanation:** **Type I Collagen** is the most abundant protein in the human body, accounting for approximately **90%** of the total collagen content. It is a fibrillar collagen characterized by its high tensile strength, making it the primary structural component of "hard" and "tough" tissues such as **bone (organic matrix), tendons, ligaments, skin, and the cornea.** **Analysis of Options:** * **Type III (Option B):** Also known as **Reticulin**, these fibers form a supportive meshwork in distensible organs like the liver, spleen, blood vessels, and skin. It is the first collagen synthesized during wound healing before being replaced by Type I. * **Type IV (Option C):** This is a non-fibrillar collagen that forms a 2D network. It is the key structural component of the **Basal Lamina** (basement membrane) and the lens of the eye. * **Type VI (Option D):** This is a microfibrillar collagen found in the extracellular matrix of skeletal muscle and cartilage, but it exists in much smaller quantities compared to Type I. **NEET-PG High-Yield Pearls:** * **Mnemonic for Locations:** * Type **I**: **B**one (and skin/tendon) * Type **II**: **C**artilage (hyaline) * Type **III**: **R**eticular fibers (blood vessels) * Type **IV**: **B**asement membrane ("Under the floor") * **Clinical Correlation:** Mutations in Type I collagen lead to **Osteogenesis Imperfecta** (brittle bone disease), while mutations in Type IV lead to **Alport Syndrome** (hereditary nephritis and deafness). * **Structure:** Collagen is characterized by a triple helix rich in **Glycine** (every 3rd residue), Proline, and Hydroxyproline. Vitamin C is essential for the hydroxylation of these residues.

Question 174: Folding of nascent polypeptide chains is the function of which of the following?

A. Chaperones (Correct Answer)
B. Proteasome
C. Heat shock proteins
D. Ribosomes

Explanation: **Explanation:** **1. Why Chaperones are correct:** Protein folding is a critical process where a nascent (newly synthesized) polypeptide chain assumes its functional 3D conformation. **Chaperones** (also known as molecular chaperones) are specialized proteins that facilitate this process. They prevent the aggregation of unfolded or partially folded polypeptide chains by binding to exposed hydrophobic regions, ensuring they reach their native state efficiently. **2. Why other options are incorrect:** * **Proteasomes:** These are protein complexes responsible for the **degradation** of damaged or unneeded proteins (typically tagged with ubiquitin), not their folding. * **Heat Shock Proteins (HSPs):** While many HSPs (like HSP70) *act* as chaperones, "Chaperones" is the broader, more accurate functional category for the folding of nascent chains. HSPs are specifically upregulated during cellular stress to refold denatured proteins. * **Ribosomes:** These are the cellular machinery responsible for **translation** (protein synthesis), not the subsequent folding of the polypeptide chain. **3. High-Yield Clinical Pearls for NEET-PG:** * **ATP-Dependence:** Most chaperone-mediated folding (e.g., the Chaperonin system/HSP60) requires energy from ATP hydrolysis. * **Prion Diseases:** These occur due to the **misfolding** of normal PrP proteins into beta-sheet rich pathological forms, resisting degradation. * **Alzheimer’s Disease:** Characterized by the accumulation of misfolded amyloid-beta plaques and tau tangles. * **Cystic Fibrosis:** The most common mutation (ΔF508) leads to misfolding of the CFTR protein, which is then recognized and degraded by the proteasome before reaching the cell membrane.

Question 175: All of the following proteins are synthesized by the liver EXCEPT:

A. Haptoglobin
B. Antithrombin-III
C. Von Willebrand factor (Correct Answer)
D. Hemopexin

Explanation: ### Explanation The liver is the primary site for the synthesis of most plasma proteins, including those involved in coagulation and transport. However, **Von Willebrand factor (vWF)** is a notable exception. **1. Why Von Willebrand factor (vWF) is the correct answer:** vWF is a large multimeric glycoprotein essential for platelet adhesion and stabilization of Factor VIII. It is synthesized and stored in two specific locations: * **Endothelial cells:** Stored in **Weibel-Palade bodies**. * **Megakaryocytes:** Stored in the **α-granules of platelets**. Because it is produced by the vascular endothelium and megakaryocytes rather than hepatocytes, it is the correct answer. **2. Why the other options are incorrect:** * **Haptoglobin (A):** An acute-phase reactant synthesized by the liver that binds free hemoglobin to prevent oxidative damage and iron loss. * **Antithrombin-III (B):** A potent natural anticoagulant synthesized by the liver that inactivates thrombin and Factor Xa. * **Hemopexin (D):** A transport protein synthesized by the liver that binds free heme with high affinity, transporting it to the liver for recycling. **3. High-Yield Clinical Pearls for NEET-PG:** * **Liver Failure:** In end-stage liver disease, levels of almost all clotting factors drop, **except for Factor VIII and vWF**, as they are produced by the endothelium. * **Acute Phase Reactants:** Most are synthesized by the liver in response to IL-6 (e.g., CRP, Fibrinogen, Haptoglobin). * **Albumin:** The most abundant plasma protein, synthesized exclusively by the liver; it is a marker of the liver's synthetic function. * **Gamma-globulins:** These are the only major class of plasma proteins **not** synthesized by the liver (produced by plasma cells).

Question 176: Which of the following is an aromatic amino acid?

A. Serine
B. Histidine
C. Leucine
D. Tryptophan (Correct Answer)

Explanation: **Explanation:** Amino acids are categorized based on the chemical properties of their side chains (R-groups). **Aromatic amino acids** contain a benzene ring or a related cyclic structure with conjugated double bonds. **Why Tryptophan is Correct:** Tryptophan is one of the three primary aromatic amino acids (along with Phenylalanine and Tyrosine). It contains an **indole ring** (a benzene ring fused to a pyrrole ring). Due to its complex aromatic structure, it is the largest amino acid and is responsible for the highest absorbance of UV light at 280 nm, a property used to quantify proteins in the lab. **Analysis of Incorrect Options:** * **A. Serine:** This is a **polar, uncharged** amino acid containing a hydroxyl (-OH) group. It is a common site for O-linked glycosylation and phosphorylation. * **B. Histidine:** While Histidine contains an imidazole ring (which has aromatic character), it is primarily classified as a **basic (positively charged)** amino acid in the context of the NEET-PG syllabus. * **C. Leucine:** This is a **non-polar, branched-chain amino acid (BCAA)**. It is purely ketogenic and plays a vital role in muscle protein synthesis. **High-Yield Facts for NEET-PG:** * **Precursor Functions:** Tryptophan is the precursor for **Serotonin, Melatonin, and Niacin (Vitamin B3)**. * **Essentiality:** Tryptophan and Phenylalanine are essential; Tyrosine is semi-essential (derived from Phenylalanine). * **Clinical Correlation:** **Hartnup disease** is caused by a defect in the transport of neutral amino acids (primarily Tryptophan) in the gut and kidneys, leading to pellagra-like symptoms. * **UV Absorption:** Order of UV absorbance at 280nm: **Tryptophan > Tyrosine > Phenylalanine.**

Question 177: How many prosthetic groups are present in a hemoglobin molecule?

A. 1
B. 2
C. 3
D. 4 (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Hemoglobin (HbA) is a **heterotetrameric** protein consisting of four polypeptide globin chains (two alpha and two beta chains). Each individual globin chain is non-covalently associated with one **Heme** group. Since a prosthetic group is defined as a non-protein component tightly bound to a protein that is essential for its biological activity, and each hemoglobin molecule contains four heme groups, the total number of prosthetic groups is **4**. Each heme group contains one iron atom ($Fe^{2+}$), allowing one molecule of hemoglobin to bind up to four molecules of oxygen ($O_2$). **2. Why Other Options are Incorrect:** * **Option A (1):** A single heme group is found in **Myoglobin**, which is a monomer. * **Option B (2):** This would imply a dimeric structure; while hemoglobin can dissociate into $\alpha\beta$ dimers under certain conditions, the functional physiological unit is a tetramer. * **Option C (3):** There is no physiological form of hemoglobin that functions as a trimer. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Heme Composition:** Heme is a complex of **Protoporphyrin IX** and ferrous iron ($Fe^{2+}$). * **Binding Site:** Iron is held in the center of the protoporphyrin ring by four nitrogen atoms. It forms a 5th bond with the **Proximal Histidine (F8)** and a 6th coordinate bond with **Oxygen**. * **Cooperativity:** The presence of four prosthetic groups allows for **positive cooperativity**, represented by the Sigmoid-shaped oxygen dissociation curve (ODC). * **Methemoglobinemia:** If the iron in the prosthetic group is oxidized to the ferric state ($Fe^{3+}$), it cannot bind oxygen, leading to cyanosis.

Question 178: Which of the following is/are basic amino acid(s)?

A. Arginine
B. Lysine
C. Histidine
D. All of these (Correct Answer)

Explanation: **Explanation:** Amino acids are classified based on the chemical nature of their side chains (R-groups). **Basic amino acids** are those that possess a side chain containing a nitrogenous base, which can accept a proton ($H^+$), giving them a net positive charge at physiological pH (7.4). 1. **Arginine (Arg):** Contains a **guanidino group**. It is the most basic amino acid because its side chain remains protonated under almost all physiological conditions. 2. **Lysine (Lys):** Contains an **$\epsilon$-amino group** ($NH_3^+$) on its aliphatic side chain. 3. **Histidine (His):** Contains an **imidazole ring**. Interestingly, its pKa is close to physiological pH (~6.0), meaning it can function as both an acid and a base, making it a versatile component in enzyme active sites. Since all three options—Arginine, Lysine, and Histidine—possess these basic side chains, **Option D (All of these)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Histones:** These proteins are exceptionally rich in Arginine and Lysine. Their positive charge allows them to bind tightly to the negatively charged phosphate backbone of DNA, facilitating DNA packaging. * **Urea Cycle:** Arginine is a key intermediate in the urea cycle; its cleavage by the enzyme arginase produces urea and ornithine. * **Hemoglobin:** Histidine residues play a critical role in the "Bohr Effect" and buffering capacity of hemoglobin. * **Essentiality:** Lysine and Histidine are nutritionally essential amino acids, while Arginine is considered semi-essential (required during periods of rapid growth).

Question 179: What is the axial periodicity of collagen protein?

A. 67 nm (Correct Answer)
B. 107 nm
C. 207 nm
D. 307 nm

Explanation: **Explanation:** The correct answer is **67 nm**. Collagen is a fibrous protein characterized by a unique triple-helical structure. The fundamental unit of collagen is **tropocollagen**. In the extracellular matrix, these tropocollagen molecules align in a staggered, "head-to-tail" parallel fashion to form fibrils. The **axial periodicity** (also known as **D-spacing** or **D-period**) refers to the repeating banding pattern seen under an electron microscope. This occurs because adjacent tropocollagen molecules are displaced longitudinally by approximately **67 nm** (about 234 amino acids). This staggered arrangement creates "gap" and "overlap" regions, which give collagen its characteristic striated appearance and provide high tensile strength. **Analysis of Incorrect Options:** * **B, C, and D (107 nm, 207 nm, 307 nm):** These values do not correspond to any recognized structural intervals in human fibrillar collagen. While a single tropocollagen molecule is approximately **300 nm** long, the repeating period in a fibril remains 67 nm due to the specific 1/4th molecule overlap. **High-Yield Clinical Pearls for NEET-PG:** * **Amino Acid Composition:** Collagen is rich in **Glycine** (occurs at every 3rd position, Gly-X-Y), Proline, and Hydroxyproline. * **Post-translational Modification:** Hydroxylation of proline and lysine requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy** due to defective collagen cross-linking. * **Copper's Role:** The enzyme **Lysyl oxidase**, which facilitates the covalent cross-linking of collagen fibers, is copper-dependent. * **Type I Collagen:** Most abundant; found in bone, skin, and tendons (Mnemonic: "B**one**" for Type **One**).

Question 180: What is the largest protein in the human body?

A. Titin (Correct Answer)
B. Myosin
C. Actin
D. Troponin

Explanation: **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).

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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