Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 201: Which amino acid is abundant in collagen?

A. Tryptophan
B. Glycine (Correct Answer)
C. Threonine
D. Tyrosine

Explanation: **Explanation:** Collagen is the most abundant protein in the human body and is characterized by a unique triple-helical structure. This structure consists of three polypeptide chains (α-chains) wound around each other. To maintain this tight helix, every third amino acid in the chain must be **Glycine**, resulting in a repeating sequence of **(Gly-X-Y)n**, where X is often Proline and Y is often Hydroxyproline. **Why Glycine is the correct answer:** Glycine is the smallest amino acid, having only a hydrogen atom as its side chain. This minimal size allows it to fit into the restricted space at the center of the triple helix where the three chains come together. Any larger amino acid would cause steric hindrance and destabilize the helix. Glycine accounts for approximately **one-third (33%)** of the total amino acid residues in collagen. **Why other options are incorrect:** * **Tryptophan:** This is a bulky, aromatic amino acid. Collagen is notably deficient in Tryptophan; its presence would disrupt the compact triple helix. * **Threonine:** While present in small amounts in various proteins, it does not play a structural role in the collagen repeat sequence. * **Tyrosine:** Like Tryptophan, this is a large aromatic amino acid and is found in very low quantities in collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Post-translational modification:** Hydroxylation of Proline and Lysine requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy** due to defective collagen cross-linking. * **Osteogenesis Imperfecta:** Often caused by mutations that replace Glycine with a bulkier amino acid, preventing proper triple helix formation. * **Cross-linking:** The stability of collagen fibers depends on copper-dependent **Lysyl oxidase**, which creates covalent cross-links between collagen molecules.

Question 202: What is true about the isopeptide bond?

A. It makes proteins resistant.
B. The bond is formed between the carboxyl terminus of one protein and the amino group of a lysine residue on another.
C. It is involved in post-transcriptional modification of protein, and an enzyme acts as a catalyst for the bond formation.
D. All of the above. (Correct Answer)

Explanation: ### Explanation An **isopeptide bond** is a specialized covalent bond that differs from a standard peptide bond because it forms between the side chain of one amino acid and the backbone or side chain of another, rather than between the alpha-carbon groups. **1. Why Option D (All of the above) is correct:** * **Structural Stability (Option A):** Isopeptide bonds provide significant chemical and mechanical stability. They make proteins resistant to standard proteases (enzymes that break normal peptide bonds). A classic example is the **fibrin clot**, where Factor XIIIa creates isopeptide cross-links to stabilize the clot against fibrinolysis. * **Formation Mechanism (Option B):** By definition, it forms between a carboxyl group (often the C-terminus of a protein like Ubiquitin) and an amino group (typically the **$\epsilon$-amino group of Lysine**) on a target protein. * **Post-translational Modification (Option C):** This bond is not coded by mRNA; it occurs after translation. It requires specific enzymes for formation, such as **Ubiquitin-activating enzymes (E1, E2, E3)** or **Transglutaminases**. **2. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Ubiquitination:** The most high-yield example of an isopeptide bond is the attachment of Ubiquitin to target proteins for proteasomal degradation. * **Blood Clotting:** Factor XIII (Fibrin Stabilizing Factor) is a transglutaminase that catalyzes isopeptide bonds between fibrin monomers. Deficiency leads to delayed bleeding. * **Bacterial Virulence:** Some bacteria use isopeptide bonds to stabilize their pili (surface structures), helping them adhere to host tissues despite mechanical stress. * **Collagen:** While collagen uses different cross-links (lysyl oxidase-mediated), the concept of "cross-linking for strength" is a recurring theme in biochemistry.

Question 203: All of the following hormones are peptide-based except?

A. ACTH
B. GnRH
C. Thyroxin (Correct Answer)
D. TRH

Explanation: **Explanation:** The classification of hormones based on chemical structure is a high-yield topic for NEET-PG. Hormones are generally divided into three categories: **Peptides/Proteins**, **Steroids**, and **Amino acid derivatives**. **Why Thyroxin is the correct answer:** Thyroxin ($T_4$) is not a peptide; it is an **amino acid derivative**. Specifically, it is derived from the amino acid **Tyrosine**. While it is synthesized from the protein thyroglobulin, the final active hormone consists of an iodinated tyrosine backbone. Unlike peptide hormones, which are water-soluble and act via cell surface receptors, thyroxin is lipophilic and acts via nuclear receptors. **Analysis of incorrect options:** * **ACTH (Adrenocorticotropic Hormone):** This is a polypeptide hormone consisting of 39 amino acids, derived from the precursor molecule POMC (Pro-opiomelanocortin). * **GnRH (Gonadotropin-Releasing Hormone):** This is a **decapeptide** (10 amino acids) produced by the hypothalamus. * **TRH (Thyrotropin-Releasing Hormone):** This is a **tripeptide** (3 amino acids: Glutamate-Histidine-Proline), making it one of the smallest peptide hormones in the body. **NEET-PG Clinical Pearls:** 1. **Tyrosine Derivatives:** Remember the "Three T's and C's" derived from Tyrosine: **T**hyroid hormones ($T_3, T_4$), **T**yramine, and **C**atecholamines (Dopamine, Epinephrine, Norepinephrine). 2. **Tryptophan Derivative:** Melatonin and Serotonin are derived from Tryptophan. 3. **Receptor Location:** All peptide hormones and catecholamines use cell membrane receptors (except Thyroid hormones, which use nuclear receptors despite being amino acid derivatives). 4. **Shortest Peptide:** TRH is frequently tested as the shortest peptide hormone (3 amino acids).

Question 204: In the leucine zipper model, a leucine residue is typically found after how many amino acids?

A. 3 amino acids
B. 6 amino acids
C. 7 amino acids (Correct Answer)
D. 12 amino acids

Explanation: **Explanation:** The **Leucine Zipper** is a common structural motif found in DNA-binding proteins (transcription factors) like **c-Jun and c-Fos**. It consists of two alpha-helices that "zip" together to form a dimer. **Why Option C is correct:** In an alpha-helix, there are approximately **3.6 amino acids per turn**. The leucine zipper motif is characterized by a periodic repetition of a **Leucine residue at every 7th position** (a heptad repeat). This spacing means that Leucine appears every two full turns of the helix on the same side. Because Leucine is a hydrophobic amino acid, these residues create a "hydrophobic strip" that allows two helices to dimerize through hydrophobic interactions, much like the teeth of a zipper. **Analysis of Incorrect Options:** * **Option A (3 amino acids):** This is less than one full turn of an alpha-helix; residues at this interval would not align on the same face. * **Option B (6 amino acids):** While close to two turns, it does not match the precise **heptad (7) repeat** required for the geometric alignment of the hydrophobic interface. * **Option B (12 amino acids):** This spacing is too wide and does not correspond to the structural periodicity of known DNA-binding motifs. **High-Yield NEET-PG Pearls:** * **Function:** Leucine zippers are essential for **protein dimerization**, not direct DNA binding. The actual DNA binding is performed by an adjacent region rich in basic amino acids (Arg, Lys), termed the **bZIP motif**. * **Examples:** Regulatory proteins like **CREB**, **c-Myc**, and the **AP-1** complex utilize this motif. * **Structure:** It is an example of a **coiled-coil** structural framework.

Question 205: Adenosine receptor stability is due to extensive disulfide bonds formed between which amino acid residues?

A. Cysteine (Correct Answer)
B. Methionine
C. Arginine
D. Alanine

Explanation: **Explanation:** **1. Why Cysteine is Correct:** Disulfide bonds (also known as S-S bridges or cystine links) are covalent bonds formed by the oxidation of the sulfhydryl (-SH) groups of two **Cysteine** residues. In G-protein coupled receptors (GPCRs) like the **Adenosine receptor**, these bonds are critical for structural integrity. Specifically, a highly conserved disulfide bond between the second and third extracellular loops (ECL2 and transmembrane helix 3) stabilizes the receptor's active conformation, allowing it to bind ligands effectively. **2. Why the Other Options are Incorrect:** * **Methionine (B):** Although Methionine contains sulfur, it is a thioether. The sulfur atom is bonded to a methyl group, meaning it lacks the free sulfhydryl (-SH) group required to form disulfide bridges. * **Arginine (C):** This is a basic, positively charged amino acid. It is involved in ionic interactions (salt bridges) and hydrogen bonding, but cannot form covalent disulfide bonds. * **Alanine (D):** This is a simple, non-polar hydrophobic amino acid with a methyl side chain. It lacks the functional groups necessary for complex stabilizing bonds. **3. High-Yield Clinical Pearls for NEET-PG:** * **Post-translational Modification:** Disulfide bond formation occurs in the **Endoplasmic Reticulum (ER)**. * **Reducing vs. Oxidizing Environments:** Disulfide bonds are typically found in **extracellular proteins** (like insulin and immunoglobulins) because the extracellular environment is oxidizing, whereas the cytosol is reducing. * **Keratin:** The strength of hair and nails is primarily due to extensive disulfide cross-linking between cysteine residues in keratin. * **Cystinuria:** A clinical condition caused by a defect in the renal transport of Cystine (two cysteines linked by a disulfide bond), leading to hexagonal renal stones.

Question 206: Misfolding of protein is associated with all of the following diseases in humans, except?

A. Alzheimer's disease
B. Bovine spongiform encephalopathy
C. Scrapie disease (Correct Answer)
D. Parkinson's disease

Explanation: ### Explanation The core concept tested here is the association between **protein misfolding (proteopathy)** and human diseases. Protein misfolding occurs when a protein fails to achieve its functional 3D conformation, leading to the formation of insoluble aggregates (like amyloid) that are toxic to cells. **Why "Scrapie disease" is the correct answer:** While Scrapie is indeed a protein misfolding disease (a prion disease), it occurs exclusively in **sheep and goats**. The question specifically asks for diseases associated with protein misfolding **in humans**. Therefore, Scrapie is the exception. **Analysis of Incorrect Options:** * **A. Alzheimer’s Disease:** Characterized by the misfolding and accumulation of **Amyloid-beta (Aβ)** plaques extracellularly and **Tau protein** (neurofibrillary tangles) intracellularly. * **B. Bovine Spongiform Encephalopathy (BSE):** Also known as "Mad Cow Disease." While it primarily affects cattle, it is zoonotic. The human variant caused by consuming BSE-infected meat is **variant Creutzfeldt-Jakob Disease (vCJD)**. In the context of medical exams, BSE is often grouped with human prion pathologies. * **D. Parkinson’s Disease:** Caused by the misfolding and aggregation of the protein **alpha-synuclein**, which forms **Lewy bodies** in the dopaminergic neurons of the substantia nigra. **High-Yield Clinical Pearls for NEET-PG:** * **Prion Diseases (Human):** Creutzfeldt-Jakob Disease (CJD), Kuru, Fatal Familial Insomnia (FFI), and Gerstmann-Sträussler-Scheinker syndrome (GSS). * **Prion Mechanism:** Normal prion protein ($PrP^C$, alpha-helical) transforms into the pathological isoform ($PrP^{Sc}$, beta-pleated sheet), which is resistant to proteases. * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that prevent misfolding by assisting in the correct folding of nascent polypeptides. * **Transthyretin (TTR):** Misfolding of this protein leads to Familial Amyloid Polyneuropathy.

Question 207: Proteins are linear polymers of amino acids that fold into compact structures. Sometimes, these folded structures associate to form homo- or hetero-dimers. Which one of the following refers to this associated form?

A. Denatured state
B. Molecular aggregation
C. Precipitation
D. Quaternary structure (Correct Answer)

Explanation: ### Explanation **1. Why Quaternary Structure is Correct:** Proteins are organized into four levels of structure. While the **primary** (sequence), **secondary** (local folding like $\alpha$-helices), and **tertiary** (3D folding of a single polypeptide) levels describe individual chains, the **Quaternary structure** refers specifically to the spatial arrangement and association of multiple polypeptide subunits. These subunits can be identical (**homo-dimers/multimers**) or different (**hetero-dimers/multimers**), held together by non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) and sometimes disulfide bridges. **2. Why Other Options are Incorrect:** * **A. Denatured state:** This refers to the loss of the native 3D conformation (secondary, tertiary, and quaternary) due to heat, pH changes, or chemicals, usually leading to loss of function. * **B. Molecular aggregation:** This is a pathological process where misfolded proteins clump together (e.g., Amyloid fibrils in Alzheimer's). Unlike quaternary structure, aggregation is usually non-functional and irreversible. * **C. Precipitation:** This is a physical phenomenon where proteins become insoluble and settle out of a solution (e.g., at their isoelectric point), rather than a specific level of structural organization. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hemoglobin (Hb):** The classic example of quaternary structure (a heterotetramer: $\alpha_2\beta_2$). * **Cooperativity:** Quaternary structure allows for "allosteric regulation." In Hb, the binding of $O_2$ to one subunit increases the affinity of other subunits (Sigmoid curve). * **Isoenzymes:** Many enzymes exist as quaternary complexes (e.g., **LDH** is a tetramer of H and M subunits; **CK** is a dimer of B and M subunits). * **Chaperones:** These are specialized proteins (Heat Shock Proteins) that assist in the correct folding and assembly of quaternary structures, preventing "aggregation."

Question 208: Which of the following proteins is synthesized by free ribosomes?

A. Cytosolic proteins (Correct Answer)
B. Secretory proteins
C. Membrane proteins
D. None of the above

Explanation: In eukaryotic cells, protein synthesis occurs in two distinct locations based on the ultimate destination of the protein. This concept is central to the **Signal Hypothesis**. ### 1. Why Cytosolic Proteins are Correct **Free ribosomes** (ribosomes suspended in the cytosol) are responsible for synthesizing proteins that function within the intracellular environment. These include: * **Cytosolic proteins** (e.g., enzymes of glycolysis like Hexokinase). * **Nuclear proteins** (e.g., Histones, DNA polymerase). * **Mitochondrial and Peroxisomal proteins.** These proteins lack a signal peptide for the Endoplasmic Reticulum (ER) and are released directly into the cytosol after translation. ### 2. Why Other Options are Incorrect * **Secretory Proteins (B) & Membrane Proteins (C):** These are synthesized by **membrane-bound ribosomes** on the **Rough Endoplasmic Reticulum (RER)**. These proteins possess an N-terminal **Signal Peptide** that is recognized by the **Signal Recognition Particle (SRP)**, which docks the ribosome-mRNA complex to the RER membrane. * *Secretory proteins* (e.g., Insulin, Albumin) are sequestered into the ER lumen. * *Membrane proteins* (e.g., Receptors, Ion channels) are integrated into the ER membrane during synthesis. ### High-Yield Clinical Pearls for NEET-PG * **Signal Recognition Particle (SRP):** A cytosolic ribonucleoprotein that inhibits premature folding and facilitates docking to the RER. * **I-Cell Disease:** A defect in the post-translational modification (mannose-6-phosphate tagging) of lysosomal enzymes, which are synthesized on the RER but fail to reach the lysosomes. * **Chaperones:** Proteins like **Hsp70** help in the correct folding of proteins synthesized by both free and bound ribosomes.

Question 209: Aspartate is similar to glutamate in the same way that:

A. Valine is similar to threonine
B. Asparagine is similar to glutamine (Correct Answer)
C. Phenylalanine is similar to tryptophan
D. Phenylalanine is similar to histidine

Explanation: **Explanation:** The core concept behind this question is the **homologous relationship** between amino acids based on their side-chain length. **Why the correct answer is right:** Aspartate (Asp) and Glutamate (Glu) are both acidic, negatively charged amino acids. The only structural difference between them is a single **methylene (-CH₂-) group** in the side chain: Aspartate has one methylene group, while Glutamate has two. Similarly, **Asparagine (Asn)** and **Glutamine (Gln)** are the amide derivatives of aspartate and glutamate, respectively. They follow the exact same structural logic: Asparagine has one methylene group in its side chain, while Glutamine has two. Therefore, the relationship between Asp and Glu is identical to the relationship between Asn and Gln. **Analysis of incorrect options:** * **A. Valine vs. Threonine:** Valine is a branched-chain non-polar amino acid, whereas Threonine is a polar uncharged amino acid containing a hydroxyl group. They are not structural homologs. * **C. Phenylalanine vs. Tryptophan:** While both are aromatic, their structures differ significantly (phenyl ring vs. indole ring). * **D. Phenylalanine vs. Histidine:** Phenylalanine is purely non-polar/hydrophobic, whereas Histidine is a basic, polar amino acid with an imidazole ring. **High-Yield NEET-PG Pearls:** * **Acidic Amino Acids:** Aspartate and Glutamate (carry a negative charge at physiological pH). * **Amide Amino Acids:** Asparagine and Glutamine (polar but uncharged). * **Methylene Bridge Rule:** Many amino acid pairs differ by a single carbon (CH₂). For example, **Serine** is to **Threonine** what **Glycine** is to **Alanine** (addition of a methyl group). * **Clinical Correlation:** Glutamine is the most abundant amino acid in the blood and serves as the primary non-toxic carrier of ammonia to the liver and kidneys.

Question 210: Cystine is formed by?

A. Hydroxylation of cysteine molecule
B. Carboxylation of cysteine molecule
C. Peptide bond between two cysteine molecules
D. Disulfide bond between two cysteine molecules (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is correct:** Cystine is a dimeric amino acid formed by the **oxidation** of two cysteine molecules. The sulfhydryl (-SH) groups of two cysteine residues react to form a covalent **disulfide bond (S-S)**. This reaction is crucial for stabilizing the tertiary and quaternary structures of proteins (e.g., insulin, immunoglobulins). While cysteine is a standard proteinogenic amino acid, cystine is considered a post-translational modification product. **2. Why other options are incorrect:** * **Option A (Hydroxylation):** Hydroxylation involves adding an -OH group (e.g., proline to hydroxyproline). It does not result in cystine formation. * **Option B (Carboxylation):** Carboxylation involves adding a -COOH group (e.g., glutamate to γ-carboxyglutamate via Vitamin K). This is not the mechanism for cystine synthesis. * **Option C (Peptide bond):** A peptide bond forms between the α-amino group of one amino acid and the α-carboxyl group of another. While two cysteines can be linked by a peptide bond in a protein chain, the specific molecule "Cystine" refers specifically to the disulfide linkage of their side chains. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cystinuria:** A defect in the renal transport of COAL (Cystine, Ornithine, Arginine, Lysine), leading to **hexagonal cystine crystals** in urine and renal stones. * **Cystinosis:** A lysosomal storage disorder where cystine accumulates due to a defective transporter (cystinosin). * **Reducing Agents:** Disulfide bonds in cystine can be broken by reducing agents like β-mercaptoethanol or dithiothreitol (DTT). * **Keratin:** Hair and nails are rich in cystine; the "perm" process in hair styling involves breaking and reforming these disulfide bonds.

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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