Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 51: Which amino acid destabilizes the alpha-helix?

A. Proline (Correct Answer)
B. Alanine
C. Glutamine
D. Tryptophan

Explanation: **Explanation:** The **alpha-helix** is a common secondary structure of proteins stabilized by hydrogen bonds between the carbonyl oxygen ($C=O$) of one amino acid and the amide hydrogen ($N-H$) of the amino acid four residues away. **Why Proline is the correct answer:** Proline is known as a **"helix breaker"** for two primary reasons: 1. **Rigid Structure:** Its side chain is cyclized back onto the backbone nitrogen, forming a secondary amino group (imino acid). This rigid ring structure prevents the rotation necessary to fit into the standard alpha-helical geometry. 2. **Lack of Hydrogen Bonding:** Because the nitrogen is part of a ring, it lacks the hydrogen atom required to form the stabilizing hydrogen bonds essential for maintaining the helix. When proline is present, it creates a "kink" or bend, destabilizing the structure. **Analysis of Incorrect Options:** * **B. Alanine:** This is the strongest helix-former. Its small, uncharged side chain fits perfectly into the helical conformation without steric hindrance. * **C. Glutamine:** An uncharged polar amino acid that is generally well-tolerated in alpha-helices. * **D. Tryptophan:** While bulky, it can still be part of a helix, though it is more commonly found in beta-sheets or at the ends of helices. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine** also destabilizes the alpha-helix, but for the opposite reason as proline: it is too flexible (due to having only a hydrogen atom as a side chain), allowing for too many conformational rotations. * **Amino acids that favor Alpha-helix:** MALEK (Methionine, Alanine, Leucine, Glutamate, Lysine). * **Proline’s Role:** While it breaks alpha-helices, it is essential for the **collagen triple helix** and is frequently found in **beta-turns**.

Question 52: Wheat is deficient in which of the following amino acids?

A. Methionine and Lysine
B. Lysine and Threonine (Correct Answer)
C. Threonine and Methionine
D. Arginine and Lysine

Explanation: **Explanation:** The nutritional quality of a protein is determined by its **limiting amino acids**—essential amino acids present in amounts lower than the body's requirements [1]. In nutritional biochemistry, cereals and pulses have distinct deficiency patterns that are frequently tested in NEET-PG. **Why Option B is Correct:** Wheat, like most cereals, is primarily deficient in **Lysine**. However, it is also significantly low in **Threonine**. While Lysine is the "first limiting amino acid" in wheat, Threonine is considered the "second limiting amino acid." Therefore, the combination of Lysine and Threonine represents the most accurate deficiency profile for wheat. **Analysis of Incorrect Options:** * **Option A & C (Methionine):** Methionine is a sulfur-containing amino acid. Cereals (like wheat and rice) are actually **rich in Methionine**. Methionine deficiency is characteristic of **pulses/legumes**. * **Option D (Arginine):** Arginine is a semi-essential amino acid and is generally not the primary limiting factor in staple cereal crops. **Clinical Pearls & High-Yield Facts:** * **Mutual Supplementation:** To achieve a "complete protein" profile, cereals (deficient in Lysine, rich in Methionine) should be consumed with pulses (deficient in Methionine, rich in Lysine) [2]. This is the biochemical basis for traditional diets like *Dal-Chawal* or *Roti-Dal*. * **Limiting Amino Acids Summary:** * **Wheat/Rice:** Lysine (First limiting), Threonine (Second limiting). * **Maize (Corn):** Lysine and **Tryptophan** (Niacin deficiency/Pellagra is common in maize eaters). * **Pulses:** Methionine and Cysteine. * **Reference Protein:** Egg albumin is considered the "standard" or "reference protein" because it has a biological value of 100, containing all essential amino acids in ideal proportions [2].

Question 53: Which of the following proteins possesses a Rossmann fold?

A. Lactate dehydrogenase (Correct Answer)
B. Collagen
C. Insulin
D. Glucagon

Explanation: **Explanation:** **1. Why Lactate Dehydrogenase (LDH) is Correct:** The **Rossmann fold** is a classic structural motif found in proteins that bind nucleotides, particularly the coenzyme **NAD⁺/NADH**. Structurally, it consists of an alternating pattern of beta-strands and alpha-helices ($\beta-\alpha-\beta-\alpha-\beta$ configuration). **Lactate dehydrogenase (LDH)**, an essential enzyme in anaerobic glycolysis, utilizes this fold to bind NAD⁺ during the interconversion of lactate and pyruvate. Other enzymes featuring this fold include Malate dehydrogenase and Alcohol dehydrogenase. **2. Why the Other Options are Incorrect:** * **Collagen:** This is a fibrous protein characterized by a unique **triple helix** structure (tropocollagen) composed of Gly-X-Y repeats. It does not bind nucleotides and lacks the Rossmann fold. * **Insulin:** A peptide hormone consisting of two chains (A and B) linked by disulfide bonds. Its structure is primarily composed of alpha-helices but does not contain the complex Rossmann motif. * **Glucagon:** A linear polypeptide hormone that forms a single alpha-helix in certain environments. It is too small and structurally simple to possess a Rossmann fold. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Definition:** The Rossmann fold is the most common **NAD-binding domain**. * **LDH Isoenzymes:** LDH has five isoenzymes ($LDH_1$ to $LDH_5$). $LDH_1$ (HHHH) is a marker for myocardial infarction, while $LDH_5$ (MMMM) is elevated in liver disease and skeletal muscle injury. * **Supersecondary Structures:** The Rossmann fold is a prime example of a **motif** (supersecondary structure), which is a combination of secondary structures forming a specific functional pattern. * **Key Association:** If a question mentions "Dehydrogenase" and "Nucleotide binding," think Rossmann fold.

Question 54: Albumin is classified as which type of protein?

A. Nucleoprotein
B. Chromoprotein
C. Phosphoprotein
D. Simple protein (Correct Answer)

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Proteins are classified based on their chemical composition into three categories: Simple, Conjugated, and Derived. **Albumin** is a **Simple Protein** because it is composed entirely of amino acids. Upon complete hydrolysis, simple proteins yield only amino acids and no other non-protein components. Albumin is further sub-classified as a globular protein, characterized by its solubility in water and coagulation upon heating. **2. Why the Other Options are Incorrect:** Options A, B, and C are all examples of **Conjugated Proteins**, which consist of a simple protein combined with a non-protein group (prosthetic group): * **A. Nucleoprotein:** These are proteins conjugated with nucleic acids (e.g., Histones, Telomerase). * **B. Chromoprotein:** These contain a colored prosthetic group (e.g., Hemoglobin with heme, Rhodopsin). * **C. Phosphoprotein:** These contain phosphoric acid residues (e.g., Casein in milk, Vitellin in egg yolk). **3. High-Yield Clinical Pearls for NEET-PG:** * **Synthesis:** Albumin is synthesized exclusively in the **liver**. A decrease in serum albumin is a marker of chronic liver disease or nephrotic syndrome. * **Function:** It is the primary contributor (approx. 75-80%) to the **Plasma Colloid Oncotic Pressure**, preventing edema. * **Transport:** It acts as a non-specific carrier for bilirubin, free fatty acids, calcium, and various drugs (e.g., Warfarin, Phenytoin). * **Negative Acute Phase Reactant:** Its levels decrease during acute inflammation. * **Electrophoresis:** Albumin is the fastest-moving protein toward the anode due to its high negative charge.

Question 55: All of the following represent disorders of protein misfolding, except?

A. Alzheimer's disease
B. Tuberculosis (Correct Answer)
C. Cystic fibrosis
D. Creutzfeldt-Jakob disease

Explanation: **Explanation:** The core concept tested here is the pathophysiology of **Proteopathies**—diseases caused by proteins failing to fold into their correct 3D conformations, leading to loss of function or toxic aggregation. [2], [4] **Why Tuberculosis is the correct answer:** Tuberculosis is an **infectious disease** caused by the bacterium *Mycobacterium tuberculosis*. Its pathogenesis involves bacterial invasion, macrophage evasion, and granuloma formation. It is not caused by the endogenous misfolding of human proteins, making it the "except" in this list. **Analysis of incorrect options (Disorders of Misfolding):** * **Alzheimer’s Disease:** Characterized by the misfolding and extracellular accumulation of **Amyloid-beta (Aβ) plaques** and intracellular **Tau protein** neurofibrillary tangles. [3] * **Cystic Fibrosis:** Most commonly caused by the **ΔF508 mutation** in the CFTR gene. This mutation causes the CFTR protein to misfold in the endoplasmic reticulum; the cell’s quality control system recognizes the defect and degrades the protein before it reaches the cell membrane. [5] * **Creutzfeldt-Jakob Disease (CJD):** A classic **Prion disease** where the normal cellular prion protein ($PrP^C$) undergoes a conformational change into the pathological, $eta$-sheet rich form ($PrP^{Sc}$), which is resistant to proteolysis. [1], [2] **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones (Heat Shock Proteins):** These are specialized proteins that assist in correct folding and prevent aggregation. [1] * **Prion Diseases:** These are unique because the misfolded protein itself acts as an infectious agent (e.g., Kuru, Mad Cow Disease). [2] * **Other Misfolding Examples:** Sickle cell anemia (hemoglobin polymerization), $\alpha$1-antitrypsin deficiency (misfolded protein trapped in liver), and Parkinson’s disease ($\alpha$-synuclein). [1]

Question 56: C peptide is a part of which of the following molecules?

A. Pro-insulin (Correct Answer)
B. Insulin
C. ACTH
D. Growth hormone

Explanation: **Explanation:** **1. Why Pro-insulin is correct:** Insulin is synthesized in the pancreatic beta cells as a single-chain precursor called **Preproinsulin**. After the signal peptide is removed in the endoplasmic reticulum, it becomes **Pro-insulin**. Pro-insulin consists of three chains: the A-chain, the B-chain, and a connecting segment known as the **C-peptide** (Connecting peptide). During maturation in the Golgi apparatus, endopeptidases cleave the C-peptide, leaving the A and B chains linked by disulfide bonds to form mature, active Insulin. Therefore, C-peptide is an integral structural part of the Pro-insulin molecule. **2. Why other options are incorrect:** * **Insulin:** Mature insulin consists only of the A and B chains. The C-peptide is secreted alongside insulin but is no longer part of the insulin molecule itself. * **ACTH:** Adrenocorticotropic hormone is derived from a different precursor called POMC (Pro-opiomelanocortin). * **Growth Hormone:** This is a single-chain polypeptide hormone produced by the anterior pituitary and does not involve a C-peptide structure. **3. Clinical Pearls for NEET-PG:** * **Equimolar Secretion:** Insulin and C-peptide are secreted into the portal circulation in equal amounts (1:1 ratio). * **Diagnostic Utility:** C-peptide has a longer half-life than insulin and is not cleared by the liver. It is used as a marker of **endogenous insulin production**. * **Factitious Hypoglycemia:** In cases of exogenous insulin overdose, C-peptide levels will be **low** (since commercial insulin lacks C-peptide). In Insulinoma, both insulin and C-peptide levels will be **high**. * **Type 1 vs Type 2 DM:** C-peptide levels are low/absent in Type 1 Diabetes and typically normal or high in early Type 2 Diabetes.

Question 57: What is true about chaperones?

A. Also known as heat shock (HS) proteins (Correct Answer)
B. Possess ATPase activity
C. Always bind to specific areas on unfolded proteins
D. Always act as foldases

Explanation: **Explanation:** **Chaperones** are a specialized group of proteins essential for ensuring the correct folding and assembly of other proteins. 1. **Why Option A is Correct:** Chaperones are also known as **Heat Shock Proteins (HSPs)** because their synthesis is significantly upregulated in response to cellular stress (like high temperatures). This prevents the denaturation and aggregation of proteins under stress. Common examples include HSP70 and HSP60 (Chaperonins). 2. **Analysis of Incorrect Options:** * **Option B:** While many chaperones (like HSP70 and GroEL/ES) utilize ATP hydrolysis to facilitate folding, **not all chaperones possess ATPase activity**. Some act as "holdases" that simply bind to and stabilize proteins without consuming energy. * **Option C:** Chaperones generally recognize and bind to **exposed hydrophobic patches** on unfolded or misfolded proteins. These are non-specific regions that are usually buried in the interior of a natively folded protein, rather than "specific areas" or sequences. * **Option D:** Chaperones can be classified as **Foldases** (which actively assist in folding using ATP) or **Holdases** (which prevent aggregation until folding can occur). Therefore, they do not "always" act as foldases. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** They prevent "illegitimate" interactions between exposed hydrophobic surfaces of nascent polypeptides. * **Prion Diseases:** Misfolding of proteins (PrP to PrPsc) leads to neurodegenerative diseases like Creutzfeldt-Jakob Disease (CJD). * **Alpha-1 Antitrypsin Deficiency:** A classic example of a disease caused by protein misfolding and defective trafficking. * **Ubiquitin-Proteasome Pathway:** Proteins that fail to fold correctly despite chaperone assistance are tagged with ubiquitin and degraded by the 26S proteasome.

Question 58: Which of the following proteins are involved in protein folding?

A. GLUT-1
B. Calnexin
C. Cytochrome 450
D. Protein disulfide isomerase (Correct Answer)

Explanation: **Explanation:** Protein folding is a critical post-translational process occurring primarily in the Rough Endoplasmic Reticulum (RER). It is facilitated by specialized proteins known as **chaperones** and enzymes that catalyze structural modifications. **Why Option D is Correct:** **Protein Disulfide Isomerase (PDI)** is an enzyme located in the lumen of the RER. It catalyzes the formation, breakage, and rearrangement of disulfide bonds between cysteine residues. Since correct disulfide bridging is essential for the stabilization of a protein's tertiary and quaternary structures, PDI plays a direct role in ensuring proteins achieve their functional native conformation. **Analysis of Incorrect Options:** * **A. GLUT-1:** This is a glucose transporter found on the plasma membrane (highly expressed in RBCs and the blood-brain barrier). It facilitates the passive transport of glucose, not protein folding. * **B. Calnexin:** While Calnexin *is* a chaperone involved in protein folding (specifically for glycoproteins), the question asks to identify the protein based on the provided key. In many standard medical examinations, if both are present, PDI is highlighted for its specific enzymatic role in bond formation. *Note: In some contexts, Calnexin is also a correct answer; however, PDI is a classic biochemical marker for RER-mediated folding.* * **C. Cytochrome P450:** These are a superfamily of enzymes primarily involved in the metabolism of drugs, toxins, and endogenous steroids via oxidation-reduction reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones:** Examples include **Hsp70** (prevents premature folding) and **Hsp60** (Chaperonins, provide a folding cavity). * **Misfolding Diseases:** Failure of protein folding leads to proteopathies such as **Alzheimer’s** (Amyloid-beta), **Prion diseases** (PrPsc), and **Cystic Fibrosis** (CFTR degradation). * **Vitamin C:** Essential for the hydroxylation of proline and lysine, a specific type of post-translational modification required for collagen folding.

Question 59: In N-linked glycosylation, a core oligosaccharide is transferred to the polypeptide chain after first being attached to which molecule?

A. Serine
B. Phosphatidyl choline
C. Cholesterol
D. Dolichol (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Dolichol** N-linked glycosylation is a critical post-translational modification that occurs in the **Rough Endoplasmic Reticulum (RER)**. The process begins with the assembly of a core oligosaccharide (GlcNAc, mannose, and glucose) on a specialized long-chain polyisoprenoid lipid called **Dolichol pyrophosphate**, which is embedded in the ER membrane. Once the 14-sugar precursor is fully assembled, it is transferred en bloc to the amide nitrogen of an **asparagine (Asn)** residue within the consensus sequence Asn-X-Ser/Thr. **Analysis of Incorrect Options:** * **A. Serine:** Serine (and Threonine) residues are the attachment sites for **O-linked glycosylation**, which typically occurs in the Golgi apparatus. Unlike N-linked, O-linked sugars are added one by one directly to the protein, not via a lipid carrier. * **B. Phosphatidyl choline:** This is a major structural phospholipid of cell membranes but does not serve as a carrier for oligosaccharide synthesis. * **C. Cholesterol:** While cholesterol is a vital component of animal cell membranes and a precursor for steroid hormones, it plays no role in the glycosylation pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Tunicamycin:** An antibiotic that inhibits the first step of N-linked glycosylation (transfer of GlcNAc-P to dolichol), effectively blocking the entire process. * **I-Cell Disease:** Caused by a deficiency in the enzyme *N-acetylglucosaminyl-1-phosphotransferase*, leading to a failure to tag lysosomal enzymes with **Mannose-6-Phosphate**. This results in enzymes being secreted extracellularly rather than reaching lysosomes. * **Sequence:** N-linked glycosylation occurs on the **Asn-X-Ser/Thr** motif (where X is any amino acid except proline).

Question 60: Strength and rigidity in keratin is due to which amino acid?

A. Leucine
B. Cysteine (Correct Answer)
C. Lithium
D. None of the above

Explanation: **Explanation:** The strength and rigidity of **keratin**, a fibrous structural protein found in hair, nails, and the outer layer of skin, are primarily attributed to the presence of **Cysteine**. Keratin is rich in sulfur-containing cysteine residues. These residues form **disulfide bonds** (S-S bridges) between adjacent polypeptide chains. These covalent cross-links create a stable, rigid, and insoluble matrix. The higher the concentration of cysteine (and thus disulfide bonds), the harder and more rigid the keratin structure (e.g., tortoise shells and nails have more disulfide bonds than hair). **Analysis of Options:** * **Option A (Leucine):** While Leucine is a common hydrophobic amino acid found in the alpha-helical structure of keratin, it contributes to hydrophobic interactions rather than the primary structural rigidity provided by covalent cross-linking. * **Option C (Lithium):** This is an inorganic element used as a mood stabilizer in psychiatry; it is not an amino acid and plays no role in protein structure. * **Option D (None of the above):** Incorrect, as Cysteine is the definitive source of keratin's mechanical strength. **Clinical Pearls for NEET-PG:** * **Permanent Waving (Perming):** This hair-styling process involves chemically reducing (breaking) disulfide bonds, reshaping the hair, and then re-oxidizing them to "set" the new shape. * **Alpha vs. Beta Keratin:** Mammals have **α-keratin** (alpha-helices), while birds and reptiles have **β-keratin** (beta-sheets), which is even tougher. * **Hard vs. Soft Keratin:** "Hard" keratin (nails/hair) has high sulfur content; "Soft" keratin (skin) has lower sulfur content.

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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