Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 331: What are the characteristics of a glycoprotein?

A. A protein linked by a glycosidic bond. (Correct Answer)
B. A core protein.
C. Carbohydrate portion contains long sugar residues.
D. None of the above.

Explanation: ### Explanation **1. Why Option A is Correct:** Glycoproteins are conjugated proteins where the prosthetic group is a carbohydrate. The defining feature of a glycoprotein is the **covalent attachment** of carbohydrate chains (glycans) to the polypeptide backbone via a **glycosidic bond**. * **N-glycosidic linkage:** The sugar is attached to the amide nitrogen of **Asparagine**. * **O-glycosidic linkage:** The sugar is attached to the hydroxyl group of **Serine** or **Threonine**. **2. Why Other Options are Incorrect:** * **Option B (Core protein):** While glycoproteins contain a protein component, the term "core protein" specifically refers to the central protein molecule in **Proteoglycans**, to which long glycosaminoglycan (GAG) chains are attached. * **Option C (Long sugar residues):** This is a characteristic of Proteoglycans, which contain long, unbranched, repeating disaccharide units (GAGs). In contrast, glycoproteins typically contain **short, often branched, oligosaccharide chains** (usually 2–15 sugar units) without repeating serial units. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Function:** Most plasma proteins (except Albumin) are glycoproteins. They serve as cell surface receptors, antibodies (Immunoglobulins), and hormones (e.g., TSH, HCG). * **Synthesis:** The protein part is synthesized on the Rough ER; glycosylation begins in the ER and is completed in the **Golgi apparatus**. * **I-Cell Disease:** A high-yield clinical correlate where a defect in phosphotransferase prevents the tagging of glycoproteins with Mannose-6-Phosphate, leading to lysosomal storage issues. * **Distinction:** Remember that in Proteoglycans, the carbohydrate content is much higher (up to 95%) than the protein content, whereas in Glycoproteins, the protein content usually predominates.

Question 332: What describes tautomerization?

A. Shift of hydrogen (Correct Answer)
B. Shift of carbon
C. Shift of both hydrogen and carbon
D. None of the above

Explanation: **Explanation:** **Tautomerization** is a fundamental biochemical process defined as a form of structural isomerism where a single chemical compound tends to exist in two or more interconvertible structures. This interconversion specifically involves the **relocation of a hydrogen atom (proton)** accompanied by a switch of a single bond and an adjacent double bond. 1. **Why Option A is Correct:** In biochemistry, the most common example is the **Keto-Enol tautomerism**. In this process, a hydrogen atom moves from the alpha-carbon to the carbonyl oxygen. This shift is crucial in metabolic pathways, such as glycolysis, where it facilitates the interconversion of isomers (e.g., during the reaction catalyzed by triosephosphate isomerase). 2. **Why Options B and C are Incorrect:** Tautomerization is strictly a "proton shift." It does not involve the breaking of the carbon skeleton or the migration of carbon atoms. A shift of carbon atoms would result in a different type of structural rearrangement (skeletal isomerism), which requires significantly more energy and different enzymatic mechanisms. 3. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Nucleic Acids:** Tautomerization of nitrogenous bases (e.g., Uracil, Thymine, Guanine) is a major cause of **spontaneous mutations**. If a base exists in its rare "imino" or "enol" form during DNA replication, it can lead to mismatched base pairing (e.g., Enol-Guanine pairing with Thymine instead of Cytosine). * **Enzymatic Catalysis:** Many enzymes utilize "acid-base catalysis" to stabilize tautomeric intermediates, lowering the activation energy of metabolic reactions. * **Key Example:** The conversion of Pyruvate (keto form) to Enol-pyruvate is a classic biochemical representation of this hydrogen shift.

Question 333: Which of the following contains sulfur?

A. Creatine
B. Insulin (Correct Answer)
C. Inulin
D. Creatinine

Explanation: **Explanation:** **Why Insulin is Correct:** Insulin is a peptide hormone consisting of two polypeptide chains (A and B) linked together by **disulfide bonds**. These bonds are formed between the sulfur atoms of **Cysteine** residues. Specifically, insulin contains three disulfide bridges: two inter-chain (connecting the A and B chains) and one intra-chain (within the A chain). These sulfur-sulfur linkages are critical for the structural stability and biological activity of the hormone. **Analysis of Incorrect Options:** * **Creatine:** This is a nitrogenous organic acid derived from amino acids (Arginine, Glycine, and Methionine). While Methionine (a sulfur-containing amino acid) provides the methyl group for its synthesis, the final structure of Creatine itself does not contain sulfur. * **Inulin:** Often confused with insulin due to the name, inulin is a **polysaccharide** (a polymer of fructose). It is a carbohydrate used clinically to measure the Glomerular Filtration Rate (GFR) and contains no sulfur. * **Creatinine:** This is the anhydride form of creatine, produced by the non-enzymatic breakdown of creatine phosphate in muscles. Like creatine, it is a nitrogenous compound but lacks sulfur. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur-containing amino acids:** Cysteine and Methionine. (Note: Only Cysteine can form disulfide bonds). * **Insulin Structure:** Chain A (21 amino acids) and Chain B (30 amino acids). * **C-peptide:** During the conversion of Proinsulin to Insulin, the C-peptide is cleaved. It is a key marker for endogenous insulin production as it is secreted in a 1:1 ratio with insulin. * **Other Sulfur-containing biomolecules:** Biotin, Thiamine (Vitamin B1), Coenzyme A, and Glutathione.

Question 334: The triple helix structure is characteristic of which biomolecule?

A. Cystine
B. Collagen (Correct Answer)
C. Pectin
D. DNA

Explanation: **Explanation:** The **triple helix** (also known as the tropocollagen unit) is the fundamental structural unit of **Collagen**, the most abundant protein in the human body. It consists of three polypeptide alpha-chains wound around each other in a right-handed rope-like fashion. This structure is stabilized by hydrogen bonds and a repeating primary sequence of **Gly-X-Y**, where Glycine (the smallest amino acid) fits into the restricted central core of the helix, and X and Y are typically Proline and Hydroxyproline. **Analysis of Options:** * **Cystine (A):** This is a dimer formed by two cysteine molecules linked via a disulfide bond. It is crucial for stabilizing protein tertiary structures but does not form a triple helix. * **Pectin (C):** A complex structural polysaccharide (carbohydrate) found in the primary cell walls of terrestrial plants; it is not a protein and lacks a triple helical arrangement. * **DNA (D):** Deoxyribonucleic acid famously exists as a **double helix** (Watson-Crick model), not a triple helix. **High-Yield Clinical Pearls for NEET-PG:** * **Post-translational modifications:** Hydroxylation of Proline and Lysine residues requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy** due to defective triple helix stabilization. * **Osteogenesis Imperfecta:** Often caused by mutations substituting Glycine with bulkier amino acids, preventing proper triple helix formation ("Brittle Bone Disease"). * **Ehlers-Danlos Syndrome:** A group of disorders resulting from defects in the synthesis or processing of collagen fibrils. * **Type I Collagen:** Found in Bone (mnemonic: "B**one**"); **Type II:** Cartilage (mnemonic: "Car**two**lage").

Question 335: Defect in collagen formation is seen in which condition?

A. Scurvy (Correct Answer)
B. Hunter's syndrome
C. Marfan's syndrome
D. All of the above

Explanation: **Explanation:** **Scurvy** is the correct answer because it is caused by a deficiency of **Vitamin C (Ascorbic acid)**, which is a mandatory co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the post-translational hydroxylation of proline and lysine residues in the pro-alpha chains of collagen. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. Without Vitamin C, collagen fibers are under-hydroxylated, leading to poor cross-linking, reduced tensile strength, and fragile connective tissues (manifesting as bleeding gums and poor wound healing). **Why other options are incorrect:** * **Hunter’s Syndrome:** This is a **Mucopolysaccharidosis (Type II)** caused by a deficiency of Iduronate-2-sulfatase. It involves a defect in the degradation of Glycosaminoglycans (GAGs), specifically dermatan and heparan sulfate, not collagen formation. * **Marfan’s Syndrome:** This is an autosomal dominant disorder caused by a mutation in the **FBN1 gene**, which encodes **Fibrillin-1**. Fibrillin-1 is a major component of microfibrils that act as a scaffold for **elastin**, not collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Collagen Synthesis:** Hydroxylation occurs in the **Endoplasmic Reticulum (ER)** and is a post-translational modification. * **Cross-linking:** The final step of collagen stabilization (covalent cross-linking) occurs extracellularly and is catalyzed by the copper-dependent enzyme **Lysyl oxidase**. * **Osteogenesis Imperfecta:** Caused by defects in Type I collagen. * **Ehlers-Danlos Syndrome:** A heterogeneous group of disorders; the vascular type (Type IV) involves defects in Type III collagen.

Question 336: Which of the following amino acids can be phosphorylated?

A. Cysteine
B. Leucine
C. Methionine
D. Serine (Correct Answer)

Explanation: **Explanation:** **1. Why Serine is Correct:** Phosphorylation is a critical post-translational modification (PTM) where a phosphate group is added to an amino acid residue by enzymes called **kinases**. For this reaction to occur, the amino acid must possess a free **hydroxyl (-OH) group** in its side chain. **Serine**, along with **Threonine** and **Tyrosine**, contains this essential hydroxyl group, making them the primary targets for phosphorylation in eukaryotic cells. This process acts as a molecular "on/off" switch, regulating enzyme activity and signal transduction pathways. **2. Why the Other Options are Incorrect:** * **Cysteine (A):** Contains a **sulfhydryl (-SH) group**. While it is involved in forming disulfide bonds and can undergo palmitoylation, it is not a standard target for phosphorylation. * **Leucine (B):** An aliphatic, branched-chain amino acid with a non-polar hydrocarbon side chain. It lacks the functional group required for phosphate attachment. * **Methionine (C):** A sulfur-containing amino acid (thioether). Like Leucine, it lacks a hydroxyl group and cannot be phosphorylated. **3. Clinical Pearls & High-Yield Facts:** * **The "Big Three":** Always remember **Serine, Threonine, and Tyrosine** as the amino acids that undergo phosphorylation. * **Enzymes:** Phosphorylation is catalyzed by **Kinases** (using ATP), while the removal of phosphate is catalyzed by **Phosphatases**. * **Clinical Relevance:** Dysregulation of protein phosphorylation (especially by Tyrosine Kinases) is a hallmark of many cancers. Drugs like **Imatinib** (a tyrosine kinase inhibitor) are used in treating Chronic Myeloid Leukemia (CML). * **Other PTMs:** * **N-glycosylation** occurs on Asparagine. * **O-glycosylation** occurs on Serine or Threonine. * **Hydroxylation** (important for collagen) occurs on Proline and Lysine (requires Vitamin C).

Question 337: What is the number of amino acids in the A chain and B chain of insulin?

A. 30, 21
B. 28, 32
C. 32, 28
D. 21, 30 (Correct Answer)

Explanation: **Explanation:** Insulin is a peptide hormone synthesized in the beta cells of the islets of Langerhans in the pancreas. It is initially produced as a single-chain precursor called **Preproinsulin**, which undergoes post-translational modifications to become active insulin. **1. Why Option D is Correct:** Mature human insulin consists of **51 amino acids** arranged in two polypeptide chains: * **A chain (Acidic):** Contains **21 amino acids**. * **B chain (Basic):** Contains **30 amino acids**. These two chains are linked together by two interchain disulfide bridges (A7-B7 and A20-B19). There is also one intrachain disulfide bridge within the A chain (A6-A11). **2. Why Other Options are Incorrect:** * **Option A (30, 21):** This reverses the order. In biochemistry nomenclature, the A chain is always the shorter chain (21) and the B chain is the longer one (30). * **Options B & C:** These numbers do not correspond to the structural components of human insulin. While different species (like bovine or porcine insulin) have variations in specific amino acid sequences, the total count of 21 and 30 remains highly conserved across mammals. **3. High-Yield Clinical Pearls for NEET-PG:** * **Proinsulin:** Consists of A chain, B chain, and a **C-peptide** (Connecting peptide). * **C-peptide:** It contains **31 amino acids**. It is cleaved from proinsulin to form active insulin and is secreted in equimolar amounts with insulin. * **Clinical Utility:** Measuring C-peptide levels helps distinguish between Type 1 Diabetes (low C-peptide) and Type 2 Diabetes (normal/high C-peptide), and identifies factitious hypoglycemia (exogenous insulin lacks C-peptide). * **Zinc:** Insulin is stored in pancreatic granules as a **hexamer** coordinated with zinc ions.

Question 338: Who was the first to determine the sequence of a polypeptide?

A. Pehr Edman
B. Frederick Sanger (Correct Answer)
C. John Kendrew
D. Oakley Fulthrop

Explanation: **Explanation:** The correct answer is **Frederick Sanger**. In 1953, Sanger successfully determined the complete amino acid sequence of **Insulin**, a peptide hormone. This was a landmark achievement in biochemistry because it proved that proteins are distinct chemical entities with a specific, defined sequence, rather than random mixtures of amino acids. For this work, Sanger was awarded his first Nobel Prize in Chemistry (1958). **Analysis of Options:** * **Frederick Sanger (Correct):** He used "Sanger’s Reagent" (1-fluoro-2,4-dinitrobenzene) to label the N-terminal amino acid, allowing him to sequence the A and B chains of bovine insulin. * **Pehr Edman:** While he did not sequence the first protein, he developed the **Edman Degradation** method. This technique uses Phenylisothiocyanate (PITC) to sequence amino acids one by one from the N-terminus without destroying the rest of the peptide bond, which became the standard for automated sequencing. * **John Kendrew:** He was the first to determine the **3D structure** of a protein (Myoglobin) using X-ray crystallography, not the primary sequence. * **Oakley Fulthrop:** This refers to the Oakley-Fulthorpe method, which is a double-diffusion technique used in immunology to detect antigens/antibodies, unrelated to protein sequencing. **High-Yield Facts for NEET-PG:** * **Sanger’s Reagent:** 1-fluoro-2,4-dinitrobenzene (DNFB). * **Edman’s Reagent:** Phenylisothiocyanate (PITC). * **Double Nobel Laureate:** Frederick Sanger is the only person to win two Nobel Prizes in Chemistry (the second was for DNA sequencing). * **Insulin Structure:** Consists of 51 amino acids (A chain: 21, B chain: 30) linked by two interchain disulfide bridges.

Question 339: Which of the following statements is NOT true for an alpha-helix?

A. It is one of the most important secondary structures.
B. It has a net dipole moment.
C. All hydrogen bonds are aligned in the same direction.
D. Long stretches of left-handed alpha-helices occur in proteins. (Correct Answer)

Explanation: **Explanation:** The alpha-helix is a fundamental secondary structure of proteins, characterized by a right-handed coiled conformation. **Why Option D is the Correct Answer (The False Statement):** While left-handed alpha-helices are theoretically possible, they are energetically unfavorable due to steric hindrance between the side chains (R-groups) and the polypeptide backbone. In nature, proteins are composed of **L-amino acids**, which preferentially form **right-handed alpha-helices**. Long stretches of left-handed helices do not occur in stable proteins; they are only found as very short segments or in rare, non-natural synthetic peptides. **Analysis of Other Options:** * **Option A:** True. The alpha-helix and beta-pleated sheet are the two most common and stable secondary structures, stabilized by hydrogen bonding. * **Option B:** True. Each peptide bond has a small dipole. In an alpha-helix, these dipoles point in the same direction along the helix axis, resulting in a **net dipole moment** (positive at the N-terminus, negative at the C-terminus). * **Option C:** True. Hydrogen bonds form between the carbonyl oxygen ($C=O$) of the $n^{th}$ residue and the amide hydrogen ($NH$) of the $(n+4)^{th}$ residue. These bonds are oriented parallel to the helix axis, all pointing in the same direction. **High-Yield Facts for NEET-PG:** * **Pitch:** 0.54 nm (5.4 Å) per turn. * **Residues per turn:** 3.6 amino acids. * **Helix Breakers:** **Proline** (introduces a kink and lacks an NH group for H-bonding) and **Glycine** (too flexible). * **Stabilization:** Intrachain hydrogen bonding is the primary stabilizing force. * **Clinical Correlation:** Mutations affecting alpha-helical structures in collagen or keratin can lead to diseases like Osteogenesis Imperfecta or Epidermolysis Bullosa.

Question 340: The alpha-helix is a common structural motif in proteins. What level of protein structure does the alpha-helix represent?

A. Primary structure
B. Secondary structure (Correct Answer)
C. Tertiary structure
D. Quaternary structure

Explanation: **Explanation:** The **alpha-helix** is a fundamental component of the **Secondary structure** of proteins. This level of structure refers to the local spatial arrangement of the polypeptide backbone, stabilized primarily by **hydrogen bonds** between the carbonyl oxygen ($C=O$) and the amide nitrogen ($N-H$) of peptide bonds located four residues apart ($i+4$ rule). **Why the other options are incorrect:** * **Primary structure:** Refers solely to the linear sequence of amino acids held together by covalent peptide bonds. It dictates the higher levels of folding but does not describe spatial arrangement. * **Tertiary structure:** Represents the overall three-dimensional folding of a single polypeptide chain, stabilized by side-chain interactions (disulfide bridges, hydrophobic effects, ionic bonds). * **Quaternary structure:** Refers to the spatial arrangement and interaction of multiple polypeptide subunits (e.g., Hemoglobin). **High-Yield NEET-PG Pearls:** * **Proline** is known as an "alpha-helix breaker" because its rigid cyclic structure lacks the necessary $N-H$ group for hydrogen bonding and creates steric hindrance. * **Right-handed vs. Left-handed:** Naturally occurring proteins almost exclusively contain right-handed alpha-helices. * **Pitch and Rise:** Each turn of the helix contains **3.6 amino acids** and has a pitch of **0.54 nm**. * **Clinical Correlation:** Misfolding of secondary structures (e.g., alpha-helix to beta-sheet transition) is a hallmark of **Prion diseases** and **Alzheimer’s disease** (amyloid plaque formation).

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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