Protein Structure and Function Practice Questions

Q: Which of the following amino acids is most likely to be located in the interior of a soluble protein molecule, not in contact with the water molecules of the solvent?

Valine. ### Explanation **Underlying Concept: The Hydrophobic Effect** The folding of a soluble protein is primarily driven by the **hydrophobic effect**. In an aqueous environment, amino acids with non-polar (hydrophobic) side chains tend to cluster together in the **interior** of the protein to avoid contact with water. Conversely, polar and charged (hydrophilic) amino acids are typically found on the **surface**, where they can form hydrogen bonds or ionic interactions with the solvent. **Why Valine is Correct:** * **Valine** is a branched-chain amino acid (BCAA) with a non-polar, aliphatic side chain. Because it is hydrophobic, it "hides" from the aqueous environment, making it a classic constituent of the protein’s hydrophobic core. **Why the Other Options are Incorrect:** * **Aspartic Acid (A):** This is an acidic amino acid that carries a negative charge at physiological pH. Its high polarity makes it highly soluble in water, positioning it on the protein surface. * **Arginine (B) & Lysine (C):** These are basic amino acids with positively charged side chains. Like aspartic acid, their charge necessitates interaction with water or other polar molecules, placing them on the exterior. **High-Yield NEET-PG Pearls:** 1. **Non-polar (Interior) Amino Acids:** Proline, Glycine, Alanine, **Valine**, Leucine, Isoleucine, Phenylalanine, Methionine, and Tryptophan. 2. **Charged (Exterior) Amino Acids:** Aspartate, Glutamate (Negative); Lysine, Arginine, Histidine (Positive). 3. **Clinical Correlation:** In **Sickle Cell Anemia**, a point mutation replaces a surface-exposed hydrophilic Glutamate with a hydrophobic **Valine** in the $\beta$-globin chain. This creates a "sticky patch" that leads to protein polymerization under deoxygenated conditions. 4. **Membrane Proteins:** Note that in transmembrane proteins, the distribution is reversed: hydrophobic residues face the lipid bilayer, while hydrophilic residues line the internal channel.

Q: Which mucopolysaccharides are found in the eye?

Keratan sulfate I and Hyaluronic acid. **Explanation:** Mucopolysaccharides (Glycosaminoglycans or GAGs) are essential components of the extracellular matrix, providing structural integrity and hydration to ocular tissues. **Why Option A is Correct:** The eye contains two primary GAGs in distinct locations: 1. **Keratan Sulfate I:** This is the specific "corneal" form of Keratan sulfate. It is found in the **corneal stroma**, where it is covalently linked to proteins (lumican and keratocan). Its unique hydration properties are critical for maintaining the precise spacing of collagen fibrils, which ensures **corneal transparency**. 2. **Hyaluronic Acid (Hyaluronan):** This is the predominant GAG in the **vitreous humor**. Unlike other GAGs, it is non-sulfated and not bound to a protein core. It creates a highly hydrated, gel-like matrix that maintains intraocular pressure and supports the retina. **Why Other Options are Incorrect:** * **Option B:** While Heparan sulfate is found in basement membranes (like the lens capsule), it is not the primary functional GAG associated with the bulk of ocular structures compared to Hyaluronic acid. * **Option C:** Keratan sulfate II is the "skeletal" form found in cartilage and bone, not the cornea. * **Option D:** Dermatan sulfate is primarily found in the skin, blood vessels, and heart valves. While Chondroitin sulfate is present in the eye in small amounts, it is not the defining GAG of the vitreous or cornea. **NEET-PG High-Yield Pearls:** * **Keratan Sulfate I vs. II:** Remember "I" for Eye (Cornea) and "II" for "Two" (Skeletal/Cartilage). * **Hyaluronic Acid:** The only GAG that is **not sulfated** and does not form proteoglycans (no protein core). * **Clinical Correlation:** Macular corneal dystrophy is caused by a defect in the sulfation of Keratan sulfate I, leading to corneal opacity and blindness.

Q: Which is the major collagen present in cartilage?

Type II. **Explanation:** Collagen is the most abundant protein in the human body, organized into different types based on its tissue distribution and structural properties. **Correct Answer: Type II (Option B)** Type II collagen is the hallmark of **cartilage** (hyaline, elastic, and fibrocartilage). It consists of three identical alpha-1 (Type II) chains. Its primary function is to provide tensile strength and resist pressure within the cartilaginous matrix, particularly in articulating joints and the vitreous humor of the eye. **Analysis of Incorrect Options:** * **Type I (Option A):** The most abundant collagen in the body. It is found in **"Hard"** structures like **Bone**, skin, tendons, and late scars. (Mnemonic: Type **One** is in **Bone**). * **Type III (Option B):** Also known as **Reticulin**. It is found in extensible tissues like blood vessels, fetal skin, and granulation tissue. It is the first collagen deposited during wound healing. * **Type IV (Option D):** This type does not form fibrils; instead, it forms a meshwork. It is the primary component of the **Basement Membrane** and lens of the eye. (Mnemonic: Type **Four** is in the **Floor**). **High-Yield Clinical Pearls for NEET-PG:** * **Osteogenesis Imperfecta:** Defect in Type I collagen. * **Ehlers-Danlos Syndrome (Vascular Type):** Defect in Type III collagen. * **Alport Syndrome & Goodpasture Syndrome:** Associated with Type IV collagen defects/antibodies. * **Vitamin C** is essential for the hydroxylation of proline and lysine residues during collagen synthesis; deficiency leads to Scurvy.

Q: Which proteins are responsible for preventing the faulty folding of other proteins?

Chaperones. ### Explanation **Correct Option: A. Chaperones** Chaperones (also known as Molecular Chaperones) are a specialized class of proteins that facilitate the correct folding of nascent polypeptide chains. They prevent "faulty folding" by binding to exposed hydrophobic regions of unfolded proteins, preventing inappropriate aggregation and ensuring the protein reaches its native functional conformation. Many chaperones are **Heat Shock Proteins (HSPs)**, such as HSP70, which are upregulated during cellular stress to refold denatured proteins. **Why Incorrect Options are Wrong:** * **B. Histones:** These are highly basic proteins (rich in Lysine and Arginine) that function as "spools" around which DNA winds to form nucleosomes. Their role is structural and regulatory regarding DNA packaging, not protein folding. * **C. Proteases:** These are enzymes that catalyze **proteolysis** (the breakdown of proteins into peptides or amino acids) by cleaving peptide bonds. * **D. Proteasomes:** These are protein complexes responsible for the **degradation** of damaged or unneeded proteins that have been tagged with **Ubiquitin**. While they handle misfolded proteins, they destroy them rather than preventing the faulty folding process itself. **High-Yield Clinical Pearls for NEET-PG:** * **Protein Misfolding Diseases:** Failure of chaperones or accumulation of misfolded proteins leads to proteopathies, such as **Alzheimer’s disease** (Amyloid-β), **Parkinson’s disease** (α-synuclein), and **Prion diseases**. * **Ubiquitin-Proteasome Pathway:** Proteins destined for degradation are tagged with a polyubiquitin chain. This is an ATP-dependent process. * **Cystic Fibrosis:** The most common mutation ($\Delta$F508) leads to the misfolding of the CFTR protein, which is then recognized and degraded by the proteasome before reaching the cell membrane.

Q: Ubiquitin is a small protein present in?

All eukaryotes. **Explanation:** **Ubiquitin** is a highly conserved regulatory protein consisting of 76 amino acids. Its primary function is to mark misfolded or damaged proteins for degradation via the **Ubiquitin-Proteasome Pathway (UPP)**. **1. Why Option A is Correct:** The name "Ubiquitin" is derived from the word "ubiquitous" because it is found in **all eukaryotic cells**, from yeast to humans. It is one of the most evolutionarily conserved proteins known; for example, there is only a 3-residue difference between the ubiquitin found in yeast and that found in humans. This conservation highlights its fundamental role in maintaining cellular proteostasis. **2. Why Options B and C are Incorrect:** Ubiquitin is **absent in prokaryotes** (bacteria and archaea). While some bacteria possess proteins with similar structural folds (like ThiS or MoaD), they do not utilize the classic ATP-dependent ubiquitin-proteasome system for protein degradation. Therefore, it is not found in "all prokaryotes" or "both." **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Kiss of Death":** Ubiquitination is the process of attaching ubiquitin to a lysine residue of a target protein. A chain of at least four ubiquitin molecules (polyubiquitination) signals the **26S Proteasome** to degrade the protein. * **ATP Dependency:** The activation of ubiquitin requires ATP and involves three enzymes: **E1** (Activating), **E2** (Conjugating), and **E3** (Ligase). * **Clinical Correlation:** Defects in the ubiquitin system are linked to neurodegenerative diseases like **Parkinson’s** (Parkin is an E3 ligase) and **Alzheimer’s** (accumulation of ubiquitinated proteins in neurofibrillary tangles), as well as certain cancers (e.g., HPV-induced degradation of p53). * **Angelman Syndrome:** Caused by a mutation in the *UBE3A* gene, which encodes a ubiquitin ligase.

Q: What is the normal albumin to globulin (A/G) ratio in blood?

2:1. **Explanation:** The **Albumin to Globulin (A/G) ratio** is a vital clinical parameter used to evaluate liver and kidney function, as well as nutritional status. **Why 2:1 is correct:** In a healthy adult, the normal range for total serum protein is approximately 6.0 to 8.3 g/dL [1]. **Albumin**, the most abundant plasma protein (synthesized exclusively by the liver), typically ranges from **3.5 to 5.0 g/dL** [1]. **Globulins** (including alpha, beta, and gamma fractions) range from **2.0 to 3.5 g/dL**. When you divide the average albumin level by the average globulin level, the resulting ratio is approximately **1.5:1 to 2:1**. Therefore, **2:1** is the standard physiological benchmark used in medical examinations. **Analysis of Incorrect Options:** * **A (5:2):** This ratio (2.5:1) overestimates the amount of albumin relative to globulin found in normal physiology. * **C (1:2):** This represents a **reversed A/G ratio**. This is a pathological finding seen in conditions like multiple myeloma (increased globulins) or cirrhosis (decreased albumin). * **D (1:1):** This indicates a relative decrease in albumin or an increase in globulins, often seen in chronic inflammatory states, but it is not the "normal" baseline. **High-Yield Clinical Pearls for NEET-PG:** * **Reversed A/G Ratio (<1.0):** A classic exam favorite. It occurs in **Chronic Liver Disease/Cirrhosis** (decreased synthesis), **Nephrotic Syndrome** (increased loss), and **Multiple Myeloma** (monoclonal gammopathy). * **Albumin Half-life:** Approximately **20 days** [1], making it a marker of chronic rather than acute liver injury. * **Functions:** Albumin is primarily responsible for maintaining **plasma oncotic pressure** (70-80%) and transporting bilirubin, hormones, and drugs [1].

Q: Collagen ------ forms the main part of anchoring fibrils in epithelial tissues?

VII. **Explanation:** The correct answer is **Type VII Collagen**. **1. Why Type VII is Correct:** Type VII collagen is the primary component of **anchoring fibrils**. These fibrils are specialized structures that extend from the basal lamina (specifically the lamina densa) into the underlying papillary dermis. They "anchor" the epidermis to the dermis by looping around Type I and Type III collagen fibers, ensuring dermo-epidermal stability. **2. Analysis of Incorrect Options:** * **Type IV (Option B):** This is a **network-forming collagen** and the major structural component of the **basal lamina** itself. It forms a "mesh-like" scaffold rather than anchoring fibrils. * **Type VIII (Option C):** This is a short-chain collagen found primarily in the **Descemet’s membrane** of the corneal endothelium. * **Type I (Option D):** This is the most abundant collagen in the body, found in **bone, skin, and tendons**. It provides tensile strength but does not form anchoring fibrils. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dystrophic Epidermolysis Bullosa (DEB):** This condition is caused by mutations in the *COL7A1* gene (encoding Type VII collagen). It leads to fragile skin and subepidermal blistering because the epidermis is not properly anchored to the dermis. * **Alport Syndrome:** Associated with mutations in **Type IV collagen**, leading to nephritis and sensorineural deafness. * **Goodpasture Syndrome:** An autoimmune condition where antibodies attack the alpha-3 chain of **Type IV collagen** in glomerular and alveolar basement membranes. * **Mnemonic for Collagen Types:** * Type **I**: **B**one (One/Bone) * Type **II**: **C**artilage (Two/Car-two-lage) * Type **III**: **R**eticular fibers (Three/Reticul-three-um) * Type **IV**: **F**loor (Basement membrane)

Q: Which of the following amino acids, when reacted with diazotized sulfanilic acid under alkaline conditions, forms a red color?

Histidine. ### Explanation The question refers to the **Pauly’s Test**, a specific biochemical reaction used to detect amino acids containing a phenolic group or an imidazole ring. **1. Why Histidine is Correct:** Histidine contains an **imidazole ring**. When histidine reacts with diazotized sulfanilic acid (the Pauly reagent) in an alkaline environment (usually sodium carbonate), it undergoes a coupling reaction to form a **cherry-red colored** azo dye. This test is highly specific for histidine and tyrosine. **2. Analysis of Incorrect Options:** * **Tyrosine (Option A):** While Tyrosine also gives a positive Pauly’s test (forming a red-orange color) because of its **phenolic group**, Histidine is the classic textbook answer for the "red color" reaction in this specific context. However, in most clinical biochemistry exams, if both are present, Histidine is prioritized for the "deep red" result. * **Arginine (Option B):** Arginine contains a **guanidino group**. It is identified by the **Sakaguchi test**, which produces a bright red color when reacted with alpha-naphthol and sodium hypobromite. * **Cysteine (Option C):** Cysteine contains a **sulfhydryl (-SH) group**. It is identified by the **Nitroprusside test**, which yields a red color, or the Sullivan test. **3. High-Yield Clinical Pearls for NEET-PG:** * **Xanthoproteic Test:** Detects aromatic amino acids (Tyrosine, Tryptophan, Phenylalanine) using concentrated nitric acid (yellow color). * **Millon’s Test:** Specific for **Tyrosine** (due to the phenol group), yielding a brick-red precipitate. * **Hopkins-Cole Test:** Specific for **Tryptophan** (indole ring), showing a violet ring. * **Ninhydrin Test:** General test for all alpha-amino acids (purple/Ruhemann's purple), except Proline (yellow).

Q: Carnosine is a major constituent of which of the following?

Muscle. **Explanation:** **Carnosine** (beta-alanyl-L-histidine) is a dipeptide composed of the amino acids **beta-alanine** and **histidine**. It is found in high concentrations in skeletal muscle and the brain, but it is considered a major constituent specifically of **skeletal muscle**. **Why Muscle is Correct:** In skeletal muscle, carnosine acts as an essential **intracellular buffer**. During high-intensity exercise, muscles produce lactic acid, leading to a drop in pH (acidosis). Carnosine helps maintain the acid-base balance by buffering hydrogen ions, thereby delaying muscle fatigue and improving performance. It also possesses antioxidant properties, protecting muscle cells from oxidative stress. **Why Other Options are Incorrect:** * **Kidney:** While the kidneys are involved in the metabolism of various amino acids, carnosine is not a major structural or functional constituent of renal tissue. * **Brain:** Although carnosine and its methylated derivative, **Anserine**, are present in the brain (acting as neuroprotective agents), the concentration is significantly lower compared to the bulk found in skeletal muscle. * **Adipose Tissue:** This tissue primarily stores triglycerides and does not require the specific buffering capacity provided by carnosine. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Carnosine = $\beta$-alanine + L-histidine (Note: $\beta$-alanine is the rate-limiting precursor). * **Anserine:** A related dipeptide found in muscle (especially in birds) is $N$-methyl carnosine. * **Enzyme:** It is synthesized by *carnosine synthetase* and degraded by *carnosinase*. * **Homocarnosine:** A dipeptide found exclusively in the **brain**, composed of GABA and Histidine. * **Function:** Primary role is pH buffering and acting as a free radical scavenger.

Question 1

Which of the following amino acids is most likely to be located in the interior of a soluble protein molecule, not in contact with the water molecules of the solvent?

Accepted Answer

Valine

Answer

Aspartic acid

Answer

Arginine

Answer

Lysine

Question 2

Which mucopolysaccharides are found in the eye?

Accepted Answer

Keratan sulfate I and Hyaluronic acid

Answer

Keratan sulfate I and Heparan sulfate

Answer

Chondroitin sulfate and Keratan sulfate II

Answer

Chondroitin sulfate and Dermatan sulfate

Question 3

Which is the major collagen present in cartilage?

Accepted Answer

Type II

Answer

Type I

Answer

Type III

Answer

Type IV

Question 4

Which proteins are responsible for preventing the faulty folding of other proteins?

Accepted Answer

Chaperones

Answer

Histones

Answer

Proteases

Answer

Proteasomes

Question 5

Ubiquitin is a small protein present in?

Accepted Answer

All eukaryotes

Answer

All prokaryotes

Answer

Both eukaryotes and prokaryotes

Answer

None of the above

Question 6

What is the normal albumin to globulin (A/G) ratio in blood?

Accepted Answer

2:1

Answer

5:2

Answer

1:2

Answer

1:1

Question 7

Collagen ------ forms the main part of anchoring fibrils in epithelial tissues?

Accepted Answer

VII

Answer

IV

Answer

VIII

Answer

I

Question 8

Which of the following is NOT a protein misfolding disease?

Accepted Answer

Ehlers-Danlos syndrome

Answer

Prion disease

Answer

Alzheimer's disease

Answer

Beta thalassemia

Question 9

Which of the following amino acids, when reacted with diazotized sulfanilic acid under alkaline conditions, forms a red color?

Accepted Answer

Histidine

Answer

Tyrosine

Answer

Arginine

Answer

Cysteine

Question 10

Carnosine is a major constituent of which of the following?

Accepted Answer

Muscle

Answer

Kidney

Answer

Brain

Answer

Adipose tissue

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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