Protein Structure and Function Practice Questions

Q: Which tripeptide is primarily involved in oxidation-reduction reactions?

Glutathione. **Explanation:** **Glutathione (GSH)** is the correct answer because it is a vital **tripeptide** composed of three amino acids: **Glutamate, Cysteine, and Glycine**. Its primary physiological role is maintaining the redox state of the cell. The thiol (-SH) group of the cysteine residue acts as a reducing agent, neutralizing reactive oxygen species (ROS) and free radicals. During this process, two reduced glutathione molecules (GSH) are oxidized to form a disulfide-linked dimer (GSSG). The enzyme **Glutathione Reductase** then restores GSSG back to GSH using NADPH as a cofactor. **Analysis of Incorrect Options:** * **Creatinine (A):** This is a breakdown product of creatine phosphate in muscles. It is a metabolic waste product excreted by the kidneys and is not a tripeptide involved in redox reactions. * **Melanin (C):** This is a complex polymer derived from the amino acid **Tyrosine**. It is a pigment responsible for skin, hair, and eye color, providing protection against UV radiation, but it is not a tripeptide. **High-Yield Clinical Pearls for NEET-PG:** * **Unique Linkage:** In glutathione, the linkage between Glutamate and Cysteine is a **$\gamma$-glutamyl bond**, which protects the peptide from degradation by ordinary aminopeptidases. * **G6PD Deficiency:** This is clinically linked to glutathione. A deficiency in G6PD leads to decreased NADPH production, meaning glutathione cannot be regenerated. This results in oxidative stress and **hemolysis** (Heinz bodies). * **Drug Detoxification:** Glutathione is essential for the detoxification of **Acetaminophen (Paracetamol)**. In toxicity, glutathione stores are depleted, leading to hepatic necrosis. The antidote, **N-acetylcysteine**, works by replenishing glutathione levels.

Q: Which of the following amino acids cannot be phosphorylated by protein kinases?

Asparagine. **Explanation:** **1. Why Asparagine is the Correct Answer:** Phosphorylation is a common post-translational modification catalyzed by **protein kinases**. This process involves the addition of a phosphate group to a free **hydroxyl (-OH) group** on the amino acid side chain. **Asparagine** contains an amide group in its side chain, not a hydroxyl group; therefore, it cannot be phosphorylated by standard protein kinases. Instead, Asparagine is the primary site for **N-linked glycosylation**. **2. Analysis of Incorrect Options:** * **Serine (C) and Threonine (B):** These are the most common sites for phosphorylation in eukaryotes. They possess aliphatic hydroxyl groups. **Serine/Threonine kinases** (e.g., Protein Kinase A, Protein Kinase C) specifically target these residues to regulate enzyme activity and cell signaling. * **Tyrosine (D):** This amino acid contains a phenolic hydroxyl group. **Tyrosine kinases** (e.g., Insulin receptor, SRC family) phosphorylate this residue, playing a critical role in growth factor signaling and cell cycle regulation. **3. NEET-PG High-Yield Pearls:** * **The "Big Three":** Always remember **Serine, Threonine, and Tyrosine** as the standard amino acids for phosphorylation because they contain the necessary **-OH group**. * **Histidine Phosphorylation:** While rare in humans (more common in prokaryotes), Histidine can occasionally be phosphorylated, but it is never Asparagine. * **Clinical Correlation:** Many chemotherapy drugs (e.g., Imatinib) are **Tyrosine Kinase Inhibitors (TKIs)**, highlighting the clinical importance of this specific phosphorylation site. * **Energy Source:** ATP is the universal phosphate donor for these kinase-mediated reactions.

Q: Which of the following amino acids lacks an anomeric carbon atom?

Glycine. **Explanation:** The question contains a common terminology trap. In biochemistry, the term **"anomeric carbon"** refers specifically to the carbonyl carbon in carbohydrates (sugars) that becomes a chiral center during cyclization. Amino acids do not possess anomeric carbons; they possess **alpha ($\alpha$) carbons**. The question is asking which amino acid lacks a **chiral (asymmetric) center** at the $\alpha$-carbon. **1. Why Glycine is Correct:** Glycine is the simplest amino acid. Its side chain (R-group) is a single **Hydrogen atom**. Because the $\alpha$-carbon is bonded to two identical hydrogen atoms (one from the basic structure and one as the R-group), it lacks four distinct groups. Therefore, Glycine is the only **achiral** amino acid and does not exhibit optical activity (it does not have D or L isomers). **2. Why the Other Options are Incorrect:** * **Valine, Alanine, and Tyrosine** are all "chiral" amino acids. Their $\alpha$-carbons are bonded to four different groups: an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and a unique side chain ($-R$). * Because they have an asymmetric $\alpha$-carbon, they exist as non-superimposable mirror images (enantiomers). **3. NEET-PG High-Yield Pearls:** * **Optical Activity:** All naturally occurring amino acids in human proteins are in the **L-configuration**, except for Glycine (which is achiral). * **Proline:** Often called an "imino acid" because it contains a secondary amino group (pyrrolidine ring), which disrupts $\alpha$-helices. * **Tryptophan:** The bulkiest amino acid; precursor to Serotonin and Melatonin. * **Cysteine:** Contains a sulfhydryl ($-SH$) group; essential for forming disulfide bonds that stabilize tertiary and quaternary protein structures.

Q: All of the following types of interactions cooperate in stabilizing the tertiary structures of globular proteins except?

Peptide bonds. **Explanation:** The tertiary structure of a protein refers to its three-dimensional spatial arrangement, which is critical for its biological function. This structure is primarily maintained by interactions between the **R-groups (side chains)** of amino acids. **Why Peptide Bonds are the Correct Answer:** Peptide bonds are strong covalent bonds that link the amino group of one amino acid to the carboxyl group of another [1]. They are responsible for forming the **primary structure** (the linear sequence of amino acids) [1]. While they hold the chain together, they do not participate in the folding or stabilization of the tertiary structure itself. **Analysis of Incorrect Options:** * **Disulphide bonds:** These are strong covalent bonds between the thiol groups of two cysteine residues. They are the strongest stabilizers of tertiary structure. * **Hydrogen bonds:** Formed between polar side chains, these provide significant stability to the folded protein. * **Ionic interactions (Salt bridges):** These occur between oppositely charged side chains (e.g., Aspartate and Lysine) and help anchor the 3D shape. * **Hydrophobic interactions (Not listed but vital):** These are the primary driving force for folding, where non-polar side chains cluster in the protein's interior. **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in the correct folding of proteins into their tertiary structures. * **Prion Diseases:** Conditions like Creutzfeldt-Jakob Disease (CJD) occur due to the **misfolding** of proteins, where alpha-helices are replaced by beta-pleated sheets, leading to neurodegeneration. * **Denaturation:** This process disrupts the secondary, tertiary, and quaternary structures (breaking H-bonds and ionic bonds) but **leaves the primary structure (peptide bonds) intact.**

Q: Radiographic studies of a 2-year-old child reveal a new fracture of the humerus and evidence of multiple old fractures in ribs and long bones. Despite the broken arm, the toddler shows minimal bruising. A physical examination reveals peculiar teeth, a blue tinge to the sclera, and unusually mobile joints. The condition suspected in this child is characterized by an abnormality of which of the following biochemical functions?

Collagen type I synthesis. **Explanation:** The clinical presentation of multiple fractures at various stages of healing, blue sclerae, dental abnormalities (dentinogenesis imperfecta), and joint hypermobility in a 2-year-old is diagnostic of **Osteogenesis Imperfecta (OI)**, specifically Type I. While multiple fractures often raise suspicion for child abuse (Non-Accidental Injury), the presence of blue sclera and dental findings points toward a genetic connective tissue disorder. **1. Why Option A is correct:** Osteogenesis Imperfecta is primarily caused by autosomal dominant mutations in the **COL1A1 or COL1A2 genes**, which encode the alpha chains of **Type I Collagen**. Type I collagen is the major structural protein in bone, skin, tendons, and the sclera. Defective synthesis or structure of this collagen leads to "brittle bones," thin sclera (allowing the underlying choroid to show through as blue), and weak dentin. **2. Why other options are incorrect:** * **Option B (Type II Collagen):** Found primarily in **hyaline cartilage** and vitreous humor. Defects lead to skeletal dysplasias like Achondrogenesis. * **Option C (Type III Collagen):** Found in **blood vessels** and skin. Defects cause the Vascular type of Ehlers-Danlos Syndrome (Type IV EDS), characterized by arterial rupture but not bone fragility. * **Option D (Type IV Collagen):** A key component of the **basement membrane**. Defects lead to Alport Syndrome (nephritis, hearing loss, and ocular issues). **Clinical Pearls for NEET-PG:** * **Differentiate from Child Abuse:** In OI, look for blue sclera, family history, and wormian bones on skull X-ray. In child abuse, look for retinal hemorrhages, posterior rib fractures, and metaphyseal "bucket-handle" fractures. * **Wormian Bones:** Small, irregular bones within the cranial sutures; a classic radiographic sign of OI. * **Hearing Loss:** Conductive hearing loss is common in adults with OI due to otosclerosis of the ossicles.

Q: Which of the following is a modified amino acid?

Cysteine. **Explanation:** The correct answer is **Cysteine**. In the context of protein biochemistry, amino acids are classified as "modified" when they undergo post-translational modifications or are formed by the linkage of standard amino acids. **Why Cysteine is the correct answer:** While Cysteine is one of the 20 standard amino acids, it is frequently considered a "modified" or derived amino acid in competitive exams because it is formed by the **oxidative linkage of two Cysteine molecules** via a disulfide bond. This process occurs post-translationally and is crucial for stabilizing the tertiary and quaternary structures of proteins (e.g., Insulin, Immunoglobulins). **Analysis of Incorrect Options:** * **A. Arginine:** A basic, semi-essential amino acid. It is a standard amino acid encoded directly by the genetic code. * **C. Asparagine:** The amide derivative of aspartic acid. It is a standard polar, uncharged amino acid. * **D. Threonine:** An essential, hydroxyl-containing standard amino acid. **High-Yield Clinical Pearls for NEET-PG:** * **Cystine vs. Cysteine:** Remember: "Cystine has a 'ne' (knee) – it’s two cysteines joined together." * **Cystinuria:** A defect in the renal transport of Cystine, Ornithine, Lysine, and Arginine (COLA), leading to hexagonal radiolucent stones. * **Other Modified Amino Acids:** * **4-Hydroxyproline & 5-Hydroxylysine:** Found in Collagen (requires Vitamin C). * **Gamma-carboxyglutamate:** Found in clotting factors II, VII, IX, and X (requires Vitamin K). * **Selenocysteine:** Known as the 21st amino acid; it is incorporated during translation via a specialized UGA codon.

Q: Which abundant glycoprotein in the basement membrane has binding domains for both the extracellular matrix and cell-surface receptors?

Laminin. **Explanation:** **Laminin** is the correct answer because it is the primary and most abundant non-collagenous glycoprotein found in the **basal lamina** (a layer of the basement membrane). It is a large, heterotrimeric complex (α, β, and γ chains) shaped like a cross. Its structure specifically features multiple functional domains: one set binds to **Type IV collagen** and heparan sulfate proteoglycans (extracellular matrix components), while another set binds to **Integrins** (cell-surface receptors). This dual binding capacity allows laminin to act as the "glue" that anchors epithelial cells to the underlying connective tissue. **Analysis of Incorrect Options:** * **Fibronectin:** While it also binds both ECM and integrins, it is primarily found in the **interstitial matrix** (connective tissue) rather than being the defining abundant glycoprotein of the basement membrane. * **E-cadherin:** This is a calcium-dependent **cell-cell adhesion molecule** located in the *zonula adherens*. It does not bind the extracellular matrix; it binds to cadherins of adjacent cells. * **Tenascin:** This is an ECM glycoprotein involved in tissue remodeling and embryonic development, but it is not a primary structural component of the basement membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Goodpasture Syndrome:** Autoantibodies target the α3 chain of **Type IV Collagen** in the basement membranes of the glomerulus and alveoli. * **Junctional Epidermolysis Bullosa:** Often caused by genetic mutations in **Laminin-332**, leading to severe skin blistering due to defective dermo-epidermal anchoring. * **Alport Syndrome:** A genetic defect in Type IV collagen synthesis, leading to "basket-weave" appearance of the glomerular basement membrane.

Q: Which of the following special amino acids is not formed by post-translational modification?

Selenocysteine. ### Explanation The correct answer is **Selenocysteine (D)**. **1. Why Selenocysteine is the Correct Answer** Unlike most "special" or non-standard amino acids, **Selenocysteine** is not created by modifying a protein after it has been synthesized. Instead, it is incorporated **co-translationally**. It is known as the **21st amino acid**. It is encoded by the **UGA stop codon** when a specific mRNA structure (the SECIS element) is present. Because it is attached to its own unique tRNA and inserted directly during protein synthesis on the ribosome, it is not a post-translational modification (PTM). **2. Analysis of Incorrect Options** * **Hydroxyproline (B) & Hydroxylysine (C):** These are classic examples of PTMs. Proline and Lysine residues are hydroxylated *after* the polypeptide chain is formed. This process occurs in the Endoplasmic Reticulum, requires **Vitamin C**, and is essential for collagen cross-linking. * **Triiodothyronine (A):** $T_3$ (and $T_4$) is formed via the post-translational iodination of **Tyrosine** residues within the thyroglobulin protein. The mature hormone is only released after the proteolysis of the modified thyroglobulin. **3. NEET-PG High-Yield Clinical Pearls** * **The 21st vs. 22nd Amino Acid:** Selenocysteine is the 21st; **Pyrrolysine** (found in some archaea) is the 22nd. * **Enzymes containing Selenocysteine:** Glutathione peroxidase (protects against oxidative stress), Thioredoxin reductase, and Deiodinase (converts $T_4$ to $T_3$). * **Scurvy Connection:** Deficiency of Vitamin C leads to defective post-translational hydroxylation of Proline/Lysine, resulting in weak collagen and bleeding gums. * **Codon Reassignment:** UGA normally signals "Stop," but in the presence of the **SECIS** (Selenocysteine Insertion Sequence) element, it codes for Selenocysteine.

Q: Ehlers-Danlos syndrome, characterized by hypermobile joints and skin abnormalities, is due to:

Abnormality in a gene for procollagen. **Explanation:** **Ehlers-Danlos Syndrome (EDS)** is a heterogeneous group of inherited connective tissue disorders characterized by joint hypermobility, skin hyperextensibility, and tissue fragility. **Why Option A is Correct:** The primary molecular defect in most types of EDS involves mutations in the **genes encoding for Type I, III, or V procollagen** (e.g., *COL5A1, COL3A1*). These mutations lead to the production of structurally abnormal procollagen chains, which interfere with the normal assembly and cross-linking of collagen fibrils, resulting in weakened connective tissues. **Why the Other Options are Incorrect:** * **Option B (Lysyl Oxidase):** Deficiency of this copper-dependent enzyme leads to **Menkes Disease** or **Lathyrism**. It is responsible for the final cross-linking step of collagen/elastin, not the primary defect in classic EDS. * **Option C (Prolyl Hydroxylase):** This enzyme requires Vitamin C as a cofactor. Its deficiency results in **Scurvy**, characterized by bleeding gums and poor wound healing due to impaired collagen hydroxylation. * **Option D (Lysyl Hydroxylase):** While a deficiency of this enzyme causes **EDS Type VI (Kyphoscoliotic type)**, it is a specific subtype. Option A is the broader and more accurate answer for EDS as a general clinical entity, as most cases involve structural gene mutations rather than enzyme deficiencies. **High-Yield NEET-PG Pearls:** * **EDS Type IV (Vascular Type):** Due to a defect in **Type III Collagen**; it is the most severe form, associated with arterial or organ rupture. * **EDS Type V (Classic Type):** Due to a defect in **Type V Collagen**. * **Key Clinical Sign:** "Cigarette paper" or "Papyraceous" scars are characteristic of EDS. * **Collagen Synthesis:** Remember that hydroxylation occurs in the ER (requires Vit C), while cross-linking occurs extracellularly (requires Copper).

Question 1

Which tripeptide is primarily involved in oxidation-reduction reactions?

Accepted Answer

Glutathione

Answer

Creatinine

Answer

Melanin

Answer

None of the above

Question 2

Which of the following amino acids cannot be phosphorylated by protein kinases?

Accepted Answer

Asparagine

Answer

Threonine

Answer

Serine

Answer

Tyrosine

Question 3

Which of the following amino acids lacks an anomeric carbon atom?

Accepted Answer

Glycine

Answer

Valine

Answer

Alanine

Answer

Tyrosine

Question 4

All of the following types of interactions cooperate in stabilizing the tertiary structures of globular proteins except?

Accepted Answer

Peptide bonds

Answer

Disulphide bonds

Answer

Hydrogen bonds

Answer

Ionic interactions

Question 5

Radiographic studies of a 2-year-old child reveal a new fracture of the humerus and evidence of multiple old fractures in ribs and long bones. Despite the broken arm, the toddler shows minimal bruising. A physical examination reveals peculiar teeth, a blue tinge to the sclera, and unusually mobile joints. The condition suspected in this child is characterized by an abnormality of which of the following biochemical functions?

Accepted Answer

Collagen type I synthesis

Answer

Collagen type II synthesis

Answer

Collagen type III synthesis

Answer

Collagen type IV synthesis

Question 6

Which of the following is a modified amino acid?

Accepted Answer

Cysteine

Answer

Arginine

Answer

Asparagine

Answer

Threonine

Question 7

Which abundant glycoprotein in the basement membrane has binding domains for both the extracellular matrix and cell-surface receptors?

Accepted Answer

Laminin

Answer

Fibronectin

Answer

E-cadherin

Answer

Tenascin

Question 8

Which of the following special amino acids is not formed by post-translational modification?

Accepted Answer

Selenocysteine

Answer

Triiodothyronine

Answer

Hydroxyproline

Answer

Hydroxylysine

Question 9

Which amino acid forms an integral part of collagen synthesis?

Accepted Answer

Lysine

Answer

Leucine

Answer

Arginine

Answer

Glutamic acid

Question 10

Ehlers-Danlos syndrome, characterized by hypermobile joints and skin abnormalities, is due to:

Accepted Answer

Abnormality in a gene for procollagen

Answer

Deficiency of lysyl oxidase

Answer

Deficiency of prolyl hydroxylase

Answer

Deficiency of lysyl hydroxylase

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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