Protein Structure and Function Practice Questions

Q: Mutation in which of the following genes is found in Marfan's syndrome?

Fibrillin I. ### Explanation **Correct Answer: C. Fibrillin I** **Biochemical Basis:** Marfan syndrome is an **autosomal dominant** connective tissue disorder caused by a mutation in the **FBN1 gene** located on **chromosome 15q21**. This gene encodes **Fibrillin-1**, a large glycoprotein that serves as a major structural component of extracellular microfibrils. These microfibrils act as a scaffold for the deposition of elastin. Beyond structural integrity, Fibrillin-1 normally sequesters **Transforming Growth Factor-beta (TGF-β)**; its deficiency leads to excess TGF-β signaling, which causes deleterious effects on vascular smooth muscle and extracellular matrix degradation. **Analysis of Incorrect Options:** * **A. Collagen I:** Mutations here lead to **Osteogenesis Imperfecta**, characterized by brittle bones, blue sclera, and hearing loss. * **B. Collagen IV:** This is the primary collagen of basement membranes. Mutations result in **Alport Syndrome** (hereditary nephritis and sensorineural deafness). * **D. Fibrillin II:** Mutations in the *FBN2* gene (chromosome 5) cause **Congenital Contractural Arachnodactyly (Beals Syndrome)**. While it shares skeletal features with Marfan syndrome (long limbs), it does not typically involve the aorta or eyes. **NEET-PG High-Yield Pearls:** * **Clinical Triad:** Skeletal (arachnodactyly, pectus excavatum), Ocular (**Ectopia lentis**—classically **upward/superolateral** subluxation), and Cardiovascular. * **Most Common Cause of Death:** Aortic root dilation leading to **Aortic Dissection** or rupture. * **Microscopic Finding:** **Cystic Medial Necrosis** of the aorta (fragmentation of elastic fibers). * **Differentiation:** Unlike Homocystinuria (which has downward lens dislocation and intellectual disability), Marfan patients have normal intelligence and upward lens dislocation.

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

Secondary structure. **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).

Q: C-reactive proteins belong to which of the following protein classes?

Beta-1 globulin. **Explanation:** **C-reactive protein (CRP)** is a classic acute-phase reactant synthesized by the liver in response to interleukin-6 (IL-6). Based on serum protein electrophoresis (SPEP), CRP is categorized as a **Beta-1 globulin**. 1. **Why Beta-1 globulin is correct:** On an electrophoresis strip, proteins migrate based on their charge and size. CRP traditionally migrates within the **Beta-fraction**, specifically the Beta-1 region. It is a member of the pentraxin family of proteins and serves as a sensitive marker for systemic inflammation, as it binds to the C-polysaccharide of *Streptococcus pneumoniae*. 2. **Why the other options are incorrect:** * **Alpha-1 globulin:** This fraction primarily contains Alpha-1 antitrypsin, Alpha-fetoprotein (AFP), and Orosomucoid (Alpha-1 acid glycoprotein). * **Alpha-2 globulin:** This fraction includes Haptoglobin, Ceruloplasmin, and Alpha-2 macroglobulin. While these are also acute-phase reactants, CRP does not migrate here. * **Gamma globulin:** This fraction contains immunoglobulins (IgG, IgA, IgM). Note that in some pathological states, CRP may show a slight shift toward the gamma region, but for examination purposes, it is classified as a Beta-globulin. **High-Yield Clinical Pearls for NEET-PG:** * **Most Sensitive Marker:** CRP is one of the most sensitive acute-phase reactants; its levels rise rapidly (within 6 hours) and fall quickly once the inflammatory stimulus is removed. * **hs-CRP:** High-sensitivity CRP is used as a biomarker for predicting **cardiovascular risk** (atherosclerosis is a chronic inflammatory process). * **Negative Acute Phase Reactants:** Remember that **Albumin** and **Transferrin** levels *decrease* during inflammation, unlike CRP. * **ESR vs. CRP:** CRP is a more direct and sensitive measure of the inflammatory response than the Erythrocyte Sedimentation Rate (ESR).

Q: Which amino acid residue is primarily responsible for changing the conformation of the alpha helix in collagen?

Proline. **Explanation:** **Why Proline is Correct:** Collagen is the most abundant protein in the human body, characterized by a unique **triple-helical structure** (tropocollagen). Unlike the standard alpha-helix found in other proteins, the collagen helix is a "left-handed" helix. **Proline** (and its derivative, hydroxyproline) is fundamental to this structure. Due to its cyclic pyrrolidine side chain, Proline is an "alpha-helix breaker" in globular proteins. In collagen, however, its rigid ring structure forces the polypeptide chain to twist into a specific conformation, facilitating the tight packing of the triple helix. Proline provides the necessary **conformational rigidity**, while Glycine (found at every third position) provides the flexibility to fit into the restricted space of the helix core. **Why the Other Options are Incorrect:** * **A. Methionine:** A sulfur-containing amino acid primarily involved in the initiation of translation (AUG codon) and methyl group transfers. It does not play a structural role in collagen. * **C. Alanine:** A small, non-polar amino acid that is a strong "helix former" in standard globular alpha-helices, but it does not dictate the specific triple-helical twist of collagen. * **D. Tyrosine:** A bulky, aromatic amino acid. Large side chains like Tyrosine are generally excluded from the tight interior of the collagen triple helix. **High-Yield Clinical Pearls for NEET-PG:** * **Gly-X-Y Sequence:** Collagen consists of the repeating sequence Glycine-X-Y, where X is usually **Proline** and Y is usually **Hydroxyproline**. * **Post-translational Modification:** Hydroxylation of Proline requires **Vitamin C (Ascorbic acid)** and the enzyme prolyl hydroxylase. Deficiency leads to **Scurvy** due to unstable collagen (decreased melting temperature). * **Cross-linking:** Lysyl oxidase (a copper-dependent enzyme) is responsible for the covalent cross-linking that gives collagen its tensile strength.

Q: All of the following are haemoproteins, EXCEPT:

Adenylate kinase. **Explanation:** The core concept tested here is the identification of **haemoproteins**—proteins that contain a **haem (iron-protoporphyrin IX)** prosthetic group. **Why Adenylate Kinase is the correct answer:** Adenylate kinase (also known as myokinase) is a phosphotransferase enzyme that catalyzes the interconversion of adenine nucleotides ($2 ADP \rightleftharpoons ATP + AMP$). It is **not a haemoprotein**; it does not require a haem group for its catalytic activity. Instead, it typically requires magnesium ($Mg^{2+}$) as a cofactor for the stability of the phosphate bonds. **Analysis of Incorrect Options (Haemoproteins):** * **Catalase:** A vital antioxidant enzyme that converts hydrogen peroxide into water and oxygen. It contains four haem groups. * **Tryptophan pyrrolase (Tryptophan 2,3-dioxygenase):** This is the rate-limiting enzyme in the catabolism of tryptophan to kynurenine. It is a metalloprotein containing haem iron. * **Cytochrome c:** A key component of the Electron Transport Chain (ETC) located in the inner mitochondrial membrane. It is a classic haemoprotein where the iron undergoes reversible oxidation-reduction ($Fe^{2+} \rightleftharpoons Fe^{3+}$). **High-Yield Clinical Pearls for NEET-PG:** * **Common Haemoproteins to remember:** Hemoglobin, Myoglobin, Cytochromes (a, b, c, P450), Catalase, Peroxidase, Tryptophan pyrrolase, and Nitric Oxide Synthase (NOS). * **Lead Poisoning:** Inhibits $\delta$-aminolevulinic acid dehydratase and Ferrochelatase, disrupting haem synthesis. * **Cytochrome P450:** A large family of haemoproteins essential for drug metabolism (Phase I reactions) in the liver. * **Adenylate Kinase** is clinically relevant as a marker of muscle damage and plays a crucial role in maintaining energy homeostasis in cells with high energy demands.

Q: Which of the following best describes the characteristics of a glycoprotein?

Participate in cell surface recognition. **Explanation:** **Glycoproteins** are proteins covalently bonded to carbohydrates. Their primary biological role involves **cell-to-cell interactions**, acting as receptors, ligands, and cell surface markers (e.g., MHC molecules and ABO blood group antigens). The carbohydrate chains project from the cell membrane, forming the glycocalyx, which is essential for **cell surface recognition**, signaling, and immune responses. **Analysis of Options:** * **Option D (Correct):** The oligosaccharide chains of glycoproteins are highly diverse, providing the structural specificity required for cells to recognize and adhere to one another. * **Option A (Incorrect):** While glycoproteins involve sugars, the linkage is specifically **N-glycosidic** (to Asparagine) or **O-glycosidic** (to Serine/Threonine). Simply stating "glycosidic bond" is a general chemical term; the functional hallmark is the branched carbohydrate side chain. * **Option B (Incorrect):** A "core protein" is a characteristic feature of **Proteoglycans**, not glycoproteins. In proteoglycans, numerous GAG chains radiate from a central core protein. * **Option C (Incorrect):** In glycoproteins, the carbohydrate portion consists of **short, branched oligosaccharides** (usually <15 sugar residues). Long, unbranched polysaccharide chains (Glycosaminoglycans) are characteristic of proteoglycans. **High-Yield Clinical Pearls for NEET-PG:** * **Glycoprotein vs. Proteoglycan:** Glycoproteins are mostly protein by weight; Proteoglycans are mostly carbohydrate (up to 95%). * **I-Cell Disease:** A deficiency in the enzyme required to tag glycoproteins with Mannose-6-Phosphate, leading to lysosomal storage issues. * **Erythropoietin:** A clinically significant glycoprotein used to treat anemia. * **Selectins:** Specific glycoproteins that mediate the "rolling" of WBCs during inflammation.

Q: Misfolded proteins are likely produced due to a defect in which cellular component?

Rough endoplasmic reticulum. **Explanation:** The **Rough Endoplasmic Reticulum (RER)** is the primary site for the synthesis, folding, and quality control of proteins destined for secretion or membrane insertion [1]. It is studded with ribosomes that translate mRNA into polypeptide chains [1]. Within the RER lumen, specialized proteins called **chaperones** (e.g., BiP, calnexin) assist in the correct folding of these proteins [2]. If the RER environment is disrupted or if the quality control mechanism fails, proteins remain **misfolded**, leading to "ER stress" and the Unfolded Protein Response (UPR). **Analysis of Options:** * **A. Cholesterol:** This is a structural lipid found in cell membranes and a precursor for steroid hormones; it is not involved in protein synthesis or folding. * **B. Mitochondria:** These are the "powerhouses" of the cell, primarily responsible for ATP production via oxidative phosphorylation and apoptosis regulation. * **D. Smooth Endoplasmic Reticulum (SER):** The SER lacks ribosomes and is primarily involved in **lipid synthesis**, steroid hormone production, and detoxification (especially in hepatocytes) [1]. It does not play a role in protein folding. **Clinical Pearls for NEET-PG:** * **Nissl Bodies:** In neurons, the RER is visualized as Nissl bodies. A decrease in Nissl substance (chromatolysis) occurs after axonal injury. * **Protein Aggregation Diseases:** Failure of protein folding is the hallmark of neurodegenerative diseases like **Alzheimer’s** (Amyloid-beta/Tau), **Parkinson’s** (alpha-synuclein), and **Prion diseases**. * **Golgi Apparatus:** Once correctly folded in the RER, proteins move to the Golgi for post-translational modification (e.g., glycosylation) and sorting.

Question 1

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

Accepted Answer

Long stretches of left-handed alpha-helices occur in proteins.

Answer

It is one of the most important secondary structures.

Answer

It has a net dipole moment.

Answer

All hydrogen bonds are aligned in the same direction.

Question 2

Mutation in which of the following genes is found in Marfan's syndrome?

Accepted Answer

Fibrillin I

Answer

Collagen I

Answer

Collagen IV

Answer

Fibrillin II

Question 3

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

Accepted Answer

Secondary structure

Answer

Primary structure

Answer

Tertiary structure

Answer

Quaternary structure

Question 4

Protein disulfide isomerase is involved in?

Accepted Answer

Protein folding

Answer

Protein synthesis

Answer

Protein degradation

Answer

Protein quaternary structure formation

Question 5

C-reactive proteins belong to which of the following protein classes?

Accepted Answer

Beta-1 globulin

Answer

Alpha-1 globulin

Answer

Alpha-2 globulin

Answer

All of the above

Question 6

Which of the following statements regarding insulin is true?

Accepted Answer

Two polypeptide chains are bound by disulfide linkages

Answer

Produced by alpha cells of the pancreas

Answer

Shifts potassium into the cell

Answer

Subcutaneous insulin has a half-life of 60 minutes

Question 7

Which amino acid residue is primarily responsible for changing the conformation of the alpha helix in collagen?

Accepted Answer

Proline

Answer

Methionine

Answer

Alanine

Answer

Tyrosine

Question 8

All of the following are haemoproteins, EXCEPT:

Accepted Answer

Adenylate kinase

Answer

Catalase

Answer

Tryptophan pyrrolase

Answer

Cytochrome c

Question 9

Which of the following best describes the characteristics of a glycoprotein?

Accepted Answer

Participate in cell surface recognition

Answer

Protein linked with a glycosidic bond

Answer

Core protein

Answer

Sugar residues are long in the carbohydrate portion of glycoproteins

Question 10

Misfolded proteins are likely produced due to a defect in which cellular component?

Accepted Answer

Rough endoplasmic reticulum

Answer

Cholesterol

Answer

Mitochondria

Answer

Smooth endoplasmic reticulum

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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