Protein Structure and Function Practice Questions

Q: Which of the following proteins cannot exhibit quaternary structure?

Albumin. **Explanation:** The quaternary structure of a protein refers to the spatial arrangement and interaction of **two or more polypeptide chains** (subunits) held together by non-covalent forces (and sometimes disulfide bonds). **Why Albumin is the Correct Answer:** Albumin is a **monomeric protein**, meaning it consists of a single polypeptide chain (585 amino acids). Since it lacks multiple subunits, it cannot exhibit quaternary structure. Its highest level of organization is the **tertiary structure**, which is stabilized by 17 internal disulfide bonds, giving it its characteristic heart-shaped globular form. **Analysis of Incorrect Options:** * **Immunoglobulins:** These are heterotetramers consisting of four polypeptide chains (two heavy and two light chains) linked by disulfide bridges, thus exhibiting quaternary structure. * **Hemoglobin:** This is a classic example of quaternary structure, consisting of four subunits (typically $2\alpha$ and $2\beta$ in adults) that show cooperativity. * **Collagen:** This is a fibrous protein composed of three polypeptide chains (alpha chains) wound together in a triple helix, representing a complex quaternary arrangement. **High-Yield Clinical Pearls for NEET-PG:** * **Albumin:** It is the most abundant plasma protein, synthesized in the liver. Its primary functions are maintaining **Plasma Oncotic Pressure** (70-80%) and acting as a transport protein for bilirubin, fatty acids, and drugs (e.g., Warfarin). * **Denaturation:** This process disrupts secondary, tertiary, and quaternary structures but **spares the primary structure** (peptide bonds remain intact). * **Myoglobin vs. Hemoglobin:** Myoglobin is a monomer (no quaternary structure) and shows a hyperbolic oxygen dissociation curve, whereas Hemoglobin (tetramer) shows a sigmoidal curve.

Q: Immunoglobulins are:

Glycoproteins. **Explanation:** **Why Glycoproteins is the correct answer:** Immunoglobulins (antibodies) are specialized proteins produced by B-lymphocytes (plasma cells). Structurally, they consist of polypeptide chains (heavy and light chains) to which **carbohydrate moieties** are covalently attached. Specifically, these carbohydrates are linked to the constant regions of the heavy chains. Because the protein component significantly outweighs the carbohydrate component (usually 3–15% carbohydrate), they are classified as **Glycoproteins**. These sugar chains are crucial for maintaining the structural stability of the antibody and mediating effector functions like complement activation. **Analysis of Incorrect Options:** * **A. Proteins:** While immunoglobulins are made of amino acids, "Proteins" is too broad. In biochemistry, when a protein is conjugated with a non-protein group (carbohydrate), "Glycoprotein" is the more precise and correct classification. * **C. Proteoglycans:** These consist of a small core protein with very long, unbranched glycosaminoglycan (GAG) chains (e.g., heparin, chondroitin sulfate). In proteoglycans, the carbohydrate content is much higher (up to 95%) than the protein content, which is the opposite of immunoglobulins. * **D. Glycosides:** These are non-protein molecules where a sugar is bound to another functional group via a glycosidic bond (e.g., Cardiac glycosides like Digoxin). They do not have the complex polypeptide structure of antibodies. **High-Yield Facts for NEET-PG:** * **Site of Attachment:** Carbohydrates are usually **N-linked** to the Asparagine residues in the $C_H2$ domain of the IgG molecule. * **Most Abundant:** IgG is the most abundant immunoglobulin in serum. * **Acute Phase Reactants:** Most plasma proteins (like Transferrin, Ceruloplasmin, and Immunoglobulins) are glycoproteins, with the notable **exception of Albumin**, which is a simple protein.

Q: Which of the following are acidic amino acids?

None of the above. ### Explanation The classification of amino acids based on their side-chain (R-group) charge is a high-yield topic for NEET-PG. **1. Why "None of the above" is correct:** Acidic amino acids are those that contain a **carboxyl group (-COOH)** in their side chain, which carries a negative charge at physiological pH (7.4). There are only two acidic amino acids: * **Aspartic acid (Aspartate)** * **Glutamic acid (Glutamate)** None of the options provided (Asparagine, Arginine, or Glutamine) belong to this category. **2. Analysis of Incorrect Options:** * **Asparagine (A) and Glutamine (C):** These are the **amide derivatives** of aspartic acid and glutamic acid, respectively. While they are polar, their side chains are **uncharged (neutral)** at physiological pH. They are neither acidic nor basic. * **Arginine (B):** This is a **basic amino acid**. Basic amino acids (Arginine, Lysine, and Histidine) contain nitrogenous groups in their side chains that carry a positive charge at physiological pH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Charge at pH 7.4:** Acidic amino acids are **negatively charged** (Anions), while basic amino acids (Arg, Lys) are **positively charged** (Cations). * **Histidine Exception:** Histidine is the only basic amino acid that functions as a significant **buffer** at physiological pH because its pKa (~6.0) is close to the body's pH. * **Urea Cycle Connection:** Aspartate provides the second nitrogen atom in the urea cycle, while Glutamate acts as the primary "collector" of amino groups (via transamination) for ammonia detoxification. * **Excitatory Neurotransmitters:** Both Glutamate and Aspartate serve as major excitatory neurotransmitters in the CNS.

Q: Which of the following events occurs in the rough endoplasmic reticulum?

Core glycosylation of proteins. **Explanation:** The **Rough Endoplasmic Reticulum (RER)** is the primary site for the synthesis and initial modification of secretory, membrane-bound, and lysosomal proteins. **1. Why Option A is Correct:** **Core glycosylation** (specifically **N-linked glycosylation**) begins in the RER. This process involves the attachment of a pre-assembled 14-sugar precursor (oligosaccharide) to the nitrogen atom of an **Asparagine** residue. This is a critical step for proper protein folding and quality control. **2. Why Other Options are Incorrect:** * **B. O-linked glycosylation:** This occurs exclusively in the **Golgi apparatus**. It involves the addition of sugars to the oxygen atom of Serine or Threonine residues. * **C. Sulfation:** This post-translational modification occurs in the **Trans-Golgi Network (TGN)**. * **D. Protein sorting:** While the RER synthesizes proteins, the **Golgi apparatus** acts as the "Post Office" of the cell, responsible for sorting and packaging proteins into vesicles for their final destinations. **High-Yield Clinical Pearls for NEET-PG:** * **N-linked = Nitrogen = Asparagine (starts in RER).** * **O-linked = Oxygen = Serine/Threonine (occurs in Golgi).** * **Nissl bodies** in neurons are actually composed of RER and free ribosomes; they are responsible for protein synthesis. * **I-cell disease:** A clinical correlation where a defect in the Golgi (failure to add Mannose-6-Phosphate) leads to proteins being secreted extracellularly rather than being sorted to lysosomes.

Q: All of the following types of collagen are present in cartilage except one. Which is the exception?

Type IV collagen. **Explanation:** The correct answer is **Type IV collagen**. **1. Why Type IV is the exception:** Collagen types are broadly categorized into fibrillar and non-fibrillar forms. **Type IV collagen** is a non-fibrillar, network-forming collagen that is a primary constituent of the **basal lamina (basement membrane)**. It is not found in the matrix of cartilage. Its primary role is to provide a structural scaffold for epithelial and endothelial cells, notably in the renal glomerulus and the lens of the eye. **2. Analysis of Incorrect Options:** * **Type II Collagen:** This is the hallmark of cartilage. It constitutes over 50% of all protein in hyaline cartilage and provides tensile strength to the tissue. * **Type IX Collagen:** This is a **FACIT** (Fibril-Associated Collagen with Interrupted Triple helices). It binds to the surface of Type II fibrils, acting as a "bridge" to stabilize the proteoglycan matrix. * **Type VI Collagen:** This type is found in the **pericellular matrix** (chondron) immediately surrounding individual chondrocytes, helping them attach to the wider matrix. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Fibrillar Collagens:** Types **I, II, and III** (1, 2, 3 are "Strong like a Tree"). * **Type I:** Bone, Skin, Tendon (Most abundant). * **Type II:** Cartilage and Vitreous humor (**"Car-two-lage"**). * **Type III:** Reticular fibers, Blood vessels (Granulation tissue). * **Alport Syndrome:** Caused by mutations in **Type IV collagen**, leading to nephritis and sensorineural deafness. * **Goodpasture Syndrome:** Antibodies against the α3 chain of **Type IV collagen**.

Q: Which of the following is an octapeptide?

Angiotensin II. **Explanation:** The number of amino acids in a peptide chain determines its classification. **Angiotensin II** is an **octapeptide**, meaning it consists of exactly **8 amino acids**. It is a potent vasoconstrictor produced from Angiotensin I by the action of Angiotensin-Converting Enzyme (ACE) in the lungs. **Analysis of Options:** * **Glutathione (Option A):** It is a **tripeptide** (γ-glutamyl-cysteinyl-glycine). It is the most abundant intracellular antioxidant and plays a crucial role in neutralizing free radicals and maintaining vitamins C and E in their reduced forms. * **Angiotensin I (Option B):** It is a **decapeptide** (10 amino acids). It is physiologically inactive and is formed when Renin acts on Angiotensinogen. * **Oxytocin (Option D):** It is a **nonapeptide** (9 amino acids). Produced in the hypothalamus and stored in the posterior pituitary, it is responsible for uterine contractions and milk ejection. (Note: Vasopressin/ADH is also a nonapeptide). **High-Yield Clinical Pearls for NEET-PG:** * **TRH (Thyrotropin-Releasing Hormone):** A tripeptide. * **Enkephalins:** Pentapeptides (5 amino acids). * **Bradykinin:** A nonapeptide (9 amino acids) that causes vasodilation and pain. * **Glucagon:** A polypeptide consisting of 29 amino acids. * **Insulin:** A 51-amino acid polypeptide with two chains (A chain: 21, B chain: 30) linked by disulfide bonds. * **ACE Inhibitors:** These drugs prevent the conversion of the decapeptide (Angiotensin I) to the octapeptide (Angiotensin II), thereby lowering blood pressure.

Q: Which of the following is the 'Shoest peptide'?

Angiotensin-III. **Explanation:** The term **'Shortest peptide'** (often colloquially referred to or misspelled in some question banks as 'Shoest peptide') refers to the peptide with the fewest amino acid residues among the common components of the Renin-Angiotensin System (RAS). **1. Why Angiotensin-III is correct:** Angiotensin-III is a **heptapeptide** (contains 7 amino acids). It is formed by the action of aminopeptidase on Angiotensin-II. While it has lower pressor activity than Angiotensin-II, it is equally potent in stimulating aldosterone secretion from the adrenal cortex. **2. Why the other options are incorrect:** * **Angiotensin-I:** This is a **decapeptide** (10 amino acids). It is the inactive precursor formed when Renin acts on Angiotensinogen. * **Angiotensin-II:** This is an **octapeptide** (8 amino acids). It is the primary active vasoconstrictor of the RAS, formed from Angiotensin-I by the Angiotensin-Converting Enzyme (ACE). * **Vasopressin (ADH):** This is a **nonapeptide** (9 amino acids). While it is a small peptide, it is longer than Angiotensin-III. **High-Yield Clinical Pearls for NEET-PG:** * **Angiotensinogen:** A large glycoprotein (α2-globulin) synthesized in the liver. * **Sequence of Length:** Angiotensinogen (Large) → Angiotensin I (10) → Angiotensin II (8) → Angiotensin III (7). * **ACE (Kininase II):** A dipeptidyl carboxypeptidase that converts the decapeptide (I) to the octapeptide (II) by removing two amino acids from the C-terminal. * **Site of Action:** Angiotensin-III acts primarily via **AT1 receptors**, similar to Angiotensin-II.

Question 1

What is the primary difference between high mannose and complex glycoproteins?

Accepted Answer

The presence of a terminal N-acetylglucosamine, galactose, and N-acetylneuraminic acid sequence in complex glycoproteins.

Answer

The core pentasaccharide structure.

Answer

The amino acid sequence of the protein chain.

Answer

The amino acid residue to which the oligosaccharide is attached (asparagine).

Question 2

All are true about denatured proteins except?

Accepted Answer

The biological function remains intact

Answer

Amino acid sequence remains intact

Answer

The isoelectric pH changes

Answer

Recovery after denaturation is not possible

Question 3

Which of the following proteins cannot exhibit quaternary structure?

Accepted Answer

Albumin

Answer

Immunoglobulin

Answer

Hemoglobin

Answer

Collagen

Question 4

Immunoglobulins are:

Accepted Answer

Glycoproteins

Answer

Proteins

Answer

Proteoglycans

Answer

Glycosides

Question 5

Which of the following are acidic amino acids?

Accepted Answer

None of the above

Answer

Asparagine

Answer

Arginine

Answer

Glutamine

Question 6

Which of the following events occurs in the rough endoplasmic reticulum?

Accepted Answer

Core glycosylation of proteins

Answer

O-linked glycosylation

Answer

Sulfation

Answer

Protein sorting

Question 7

Which of the following proteins precipitates on heating to 45°C and redissolves on boiling?

Accepted Answer

Bence Jones protein

Answer

Gamma globulin

Answer

Albumin

Answer

Myosin

Question 8

All of the following types of collagen are present in cartilage except one. Which is the exception?

Accepted Answer

Type IV collagen

Answer

Type II collagen

Answer

Type VI collagen

Answer

Type IX collagen

Question 9

Which of the following is an octapeptide?

Accepted Answer

Angiotensin II

Answer

Glutathione

Answer

Angiotensin I

Answer

Oxytocin

Question 10

Which of the following is the 'Shoest peptide'?

Accepted Answer

Angiotensin-III

Answer

Vasopressin

Answer

Angiotensin-II

Answer

Angiotensin-I

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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