Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 321: Which of the following aids in the folding of proteins?

A. Proteasomes
B. Chaperones (Correct Answer)
C. Glycoproteins
D. Proteases

Explanation: **Explanation:** **Correct Answer: B. Chaperones** Protein folding is a critical process where a linear polypeptide chain acquires its functional 3D conformation. **Chaperones** (also known as Heat Shock Proteins, e.g., HSP70, HSP60) are specialized proteins that facilitate this process. They prevent the aggregation of unfolded or partially folded polypeptide chains by binding to exposed hydrophobic regions, ensuring the protein reaches its native state efficiently. Some chaperones, like **Chaperonins**, provide a protected "cage" environment for folding to occur in isolation. **Why other options are incorrect:** * **A. Proteasomes:** These are multi-protein complexes involved in protein **degradation**, not folding. They break down ubiquitin-tagged damaged or misfolded proteins into peptides. * **C. Glycoproteins:** These are proteins with carbohydrate chains attached. While glycosylation is a post-translational modification that can affect stability, the primary machinery for folding is the chaperone system. * **D. Proteases:** These are enzymes that catalyze **proteolysis** (the breakdown of proteins into amino acids or smaller peptides) by cleaving peptide bonds. **Clinical Pearls for NEET-PG:** * **Prion Diseases:** Caused by the accumulation of misfolded proteins (PrPSc), leading to neurodegeneration (e.g., Creutzfeldt-Jakob disease). * **Alzheimer’s Disease:** Characterized by the misfolding and aggregation of Amyloid-beta and Tau proteins. * **Cystic Fibrosis:** Often results from a mutation (ΔF508) that causes the CFTR protein to misfold, leading to its premature degradation by the proteasome. * **HSP70** prevents aggregation during synthesis, while **HSP60** (Chaperonin) assists in folding after the protein is fully synthesized.

Question 322: Histones are:

A. Identical to protamine
B. Proteins rich in lysine and arginine (Correct Answer)
C. Proteins with high molecular weight
D. Insoluble in water and very dilute acids

Explanation: **Explanation:** **1. Why the correct answer is right:** Histones are small, highly alkaline proteins found in eukaryotic cell nuclei. Their primary function is to package DNA into structural units called nucleosomes. DNA is negatively charged due to its phosphate backbone. To bind effectively, histones must be positively charged. This positive charge is provided by a high concentration of the basic amino acids **Lysine and Arginine**. The electrostatic attraction between the basic histones and acidic DNA allows for the tight coiling of chromatin. **2. Why the incorrect options are wrong:** * **Option A:** While both histones and protamines bind to DNA, they are not identical. **Protamines** are smaller, even more basic (richer in arginine), and replace histones during spermiogenesis to allow for even denser DNA packing in the sperm head. * **Option C:** Histones are actually **low molecular weight** proteins (typically 11–22 kDa). Their small size is essential for forming the compact octameric core of the nucleosome. * **Option D:** Histones are **soluble in water** and very dilute acids (like 0.2 N HCl). In fact, acid extraction is the standard laboratory method used to isolate histones from chromatin. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Nucleosome Core:** Consists of an octamer of two molecules each of **H2A, H2B, H3, and H4**. * **Linker Histone:** **H1** is the "linker histone" that sits outside the nucleosome core and helps stabilize the 30nm chromatin fiber. * **Epigenetics:** Histone modification (Acetylation/Methylation) occurs at the "tails." **Acetylation** (by HATs) neutralizes the positive charge, relaxing chromatin and increasing transcription (**Euchromatin**). * **Drug Link:** **Hydralazine, Procainamide, and Isoniazid** can induce Drug-Induced Lupus, characterized by the presence of **Anti-histone antibodies**.

Question 323: Which of the following hormones is an example of a peptide hormone?

A. Parathormone (Correct Answer)
B. Dopamine
C. Cortisol
D. Thyroxine

Explanation: **Explanation:** Hormones are classified based on their chemical structure into three main categories: **Peptides/Proteins**, **Steroids**, and **Amino acid derivatives**. **1. Why Parathormone (PTH) is Correct:** Parathormone is a classic **peptide hormone** synthesized as a pre-pro-hormone. It consists of a single polypeptide chain of **84 amino acids**. It is water-soluble, circulates freely in the plasma, and acts via cell surface receptors (G-protein coupled receptors) using cAMP as a second messenger to regulate calcium and phosphate homeostasis. **2. Why the other options are incorrect:** * **Dopamine:** This is a **catecholamine**, which is an amino acid derivative synthesized from **Tyrosine**. * **Cortisol:** This is a **steroid hormone** derived from **Cholesterol**. It is lipid-soluble and acts via intracellular receptors to modulate gene transcription. * **Thyroxine (T4):** Although derived from the amino acid **Tyrosine**, it is classified as an **amino acid derivative**. Unlike peptides, it is lipophilic and requires a carrier protein (TBG) for transport. **High-Yield Clinical Pearls for NEET-PG:** * **Peptide Hormones:** Include Insulin, Glucagon, PTH, and all Anterior Pituitary hormones (GH, ACTH, TSH, FSH, LH). * **Amino Acid Derivatives:** Derived from Tyrosine (Epinephrine, Norepinephrine, Dopamine, Thyroid hormones) or Tryptophan (Melatonin, Serotonin). * **Steroid Hormones:** Derived from Cholesterol; includes Glucocorticoids, Mineralocorticoids, and Sex steroids (Estrogen, Progesterone, Testosterone). * **Memory Aid:** If it comes from the Pancreas, Pituitary, or Parathyroid, it is almost always a **Peptide**.

Question 324: Which of the following amino acids will migrate slowest to the anode end at physiological pH?

A. Aspartic acid
B. Glycine
C. Valine
D. Lysine (Correct Answer)

Explanation: ### Explanation The migration of amino acids in an electric field depends on their **net charge** at a specific pH. At physiological pH (~7.4), the direction and speed of migration are determined by the amino acid's side chain (R-group). **1. Why Lysine is the Correct Answer:** * **Net Charge:** Lysine is a **basic amino acid**. At physiological pH, its side chain amino group is protonated, giving the molecule a **net positive charge**. * **Electrophoresis Principle:** Anions (negative) migrate toward the Anode (+), while cations (positive) migrate toward the Cathode (-). * Because Lysine is positively charged, it will migrate toward the cathode. Therefore, it will be the **slowest** to move toward the **anode** (in fact, it moves in the opposite direction). **2. Analysis of Incorrect Options:** * **Aspartic Acid (Option A):** This is an **acidic amino acid**. At pH 7.4, it carries a net negative charge. It will migrate **fastest** toward the anode. * **Glycine (Option B) & Valine (Option C):** These are **neutral, non-polar amino acids**. At physiological pH, they exist primarily as zwitterions (net charge near zero). While they show minimal movement compared to charged residues, they do not carry the positive charge that causes Lysine to move away from the anode. **3. NEET-PG High-Yield Pearls:** * **PI (Isoelectric Point):** If pH > pI, the amino acid is negatively charged (moves to Anode). If pH < pI, it is positively charged (moves to Cathode). * **Basic Amino Acids:** Lysine, Arginine, and Histidine (HAL). Arginine is the most basic. * **Acidic Amino Acids:** Aspartate and Glutamate. * **Clinical Correlation:** This principle is the basis for **Hemoglobin Electrophoresis**. In HbS, a neutral Valine replaces a negatively charged Glutamate, causing the hemoglobin to migrate slower toward the anode than normal HbA.

Question 325: Cytochrome C in bacteria has 50% identity of amino acid sequence with that of a human. Which of the following is the most conserved parameter in these two proteins?

A. Quaternary structure
B. Tertiary structure (Correct Answer)
C. Amino acid sequence
D. Loop and turn segments

Explanation: ### Explanation The core principle of protein evolution is that **structure is more conserved than sequence.** Even when the primary amino acid sequence diverges significantly over time, the three-dimensional fold (tertiary structure) remains remarkably stable to preserve the protein's biological function. **1. Why Tertiary Structure is Correct:** Cytochrome C is a vital component of the electron transport chain. For it to function correctly (transferring electrons between Complex III and IV), it must maintain a specific 3D shape to hold its heme prosthetic group. Evolution exerts "purifying selection," where mutations that change the amino acid sequence are tolerated as long as they do not disrupt the overall **tertiary fold**. Therefore, even with only 50% sequence identity, the 3D architecture of bacterial and human Cytochrome C remains nearly identical. **2. Why the Other Options are Incorrect:** * **Amino Acid Sequence:** As stated in the question, this has already diverged by 50%. Primary sequences accumulate mutations (neutral or conservative) much faster than structural folds change. * **Quaternary Structure:** Cytochrome C functions as a **monomer**. Quaternary structure refers to the arrangement of multiple polypeptide subunits, which is not applicable here. * **Loop and Turn Segments:** These are the most variable regions of a protein. Unlike alpha-helices or beta-sheets (secondary structure), loops are often located on the surface and can tolerate insertions or deletions without collapsing the protein core. **High-Yield NEET-PG Pearls:** * **Homology:** Proteins with similar folds but low sequence identity are often called "structural homologs." * **Cytochrome C & Apoptosis:** In humans, the release of Cytochrome C from the mitochondria into the cytosol is a key trigger for the **intrinsic pathway of apoptosis** (activating procaspase-9). * **Invariant Residues:** In Cytochrome C, certain residues (like those binding the heme iron) are 100% conserved across all species to maintain redox activity.

Question 326: The collagen triple helix structure is not found in which of the following locations?

A. Cytoplasm (Correct Answer)
B. Golgi apparatus
C. Lumen of endoplasmic reticulum
D. Intracellular vesicles

Explanation: **Explanation:** The synthesis of collagen is a complex process involving both intracellular and extracellular steps. The formation of the **collagen triple helix (procollagen)** occurs exclusively within the membrane-bound organelles of the secretory pathway. **1. Why Cytoplasm is the Correct Answer:** Collagen synthesis begins in the **cytoplasm** with the translation of mRNA into pre-pro-alpha chains. However, these chains are immediately translocated into the **Lumen of the Endoplasmic Reticulum (ER)**. The critical post-translational modifications—specifically the hydroxylation of proline and lysine residues and the subsequent winding of three alpha chains into a **triple helix**—take place inside the ER lumen. Therefore, the organized triple helix structure is never present in the free cytoplasm. **2. Why the Other Options are Incorrect:** * **Lumen of ER:** This is the primary site where the triple helix is assembled. Vitamin C-dependent hydroxylation allows for the hydrogen bonding necessary to stabilize the helix. * **Golgi Apparatus:** Once the triple helix (procollagen) is formed, it is transported to the Golgi for further glycosylation and packaging. * **Intracellular Vesicles:** Procollagen is transported from the Golgi to the plasma membrane via secretory vesicles before being exocytosed into the extracellular space. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin C Deficiency (Scurvy):** Leads to defective hydroxylation of proline/lysine, preventing stable triple helix formation (weak connective tissue). * **Osteogenesis Imperfecta:** Most commonly caused by mutations in Type I collagen genes that interfere with the assembly of the triple helix. * **Amino Acid Composition:** Every third amino acid in the triple helix is **Glycine** (Gly-X-Y) because it is the only amino acid small enough to fit in the crowded central core of the helix. * **Extracellular Steps:** Cleavage of registration peptides (procollagen to tropocollagen) and covalent cross-linking (by lysyl oxidase) occur **outside** the cell.

Question 327: The heme prosthetic group is found in which of the following?

A. Myoglobin (Correct Answer)
B. Cytochrome oxidase
C. Xanthine oxidase
D. Tyrosine

Explanation: **Explanation:** **1. Why Myoglobin is Correct:** Myoglobin is a monomeric heme protein found primarily in skeletal and cardiac muscle. Its primary function is the storage of oxygen and facilitating its diffusion to the mitochondria. The **heme prosthetic group** consists of a protoporphyrin IX ring with a central ferrous iron ($Fe^{2+}$) atom. This iron atom forms six coordination bonds: four with the nitrogen atoms of the porphyrin ring, one with the proximal histidine (F8) of the globin chain, and one available for reversible oxygen binding. **2. Analysis of Incorrect Options:** * **Cytochrome oxidase (Complex IV):** While this enzyme does contain heme ($a$ and $a_3$), it also critically contains **copper centers** ($Cu_A$ and $Cu_B$). In many NEET-PG contexts, if "Heme protein" is the primary identifier, Myoglobin or Hemoglobin are the classic prototypes. However, technically, Cytochrome oxidase is a hemeprotein; in single-choice questions, Myoglobin is often the preferred "textbook" example for basic heme structure. * **Xanthine oxidase:** This is a metalloenzyme that requires **Molybdenum (Mo)**, Iron-sulfur clusters, and FAD as cofactors, but it does **not** contain a heme group. * **Tyrosine:** This is a non-essential amino acid, not a complex protein or enzyme containing a prosthetic group. **3. Clinical Pearls & High-Yield Facts:** * **Heme-containing enzymes:** Remember the mnemonic **"CCC PHT"** — Cytochromes, Catalase, Cytochrome P450, Peroxidase, Hemoglobin, Tyrosine hydroxylase (some isoforms), and Tryptophan pyrrolase. * **Non-heme iron:** Ferritin, Hemosiderin, and Transferrin contain iron but **lack** the heme (porphyrin) ring. * **Clinical Marker:** Myoglobin is the **earliest cardiac marker** to rise following a Myocardial Infarction (within 1–3 hours), though it lacks specificity compared to Troponins. * **Toxicology:** Carbon monoxide (CO) has a 200x higher affinity for the heme in hemoglobin/myoglobin than oxygen, leading to tissue hypoxia.

Question 328: Aminoacyl t-RNA is not required for which of the following?

A. Proline
B. Lysine
C. Hydroxylysine (Correct Answer)
D. Methionine

Explanation: **Explanation:** The correct answer is **Hydroxylysine**. The fundamental concept here is the difference between **primary protein synthesis** (translation) and **post-translational modification**. Aminoacyl-tRNA is required only for the 20 standard amino acids that are coded by mRNA codons and incorporated into a polypeptide chain during translation on the ribosome. **Why Hydroxylysine is the correct answer:** Hydroxylysine is a **non-standard amino acid** found primarily in collagen. It does not have a specific codon in the genetic code and, therefore, does not have a corresponding aminoacyl-tRNA. Instead, it is formed by the **post-translational hydroxylation** of Lysine residues already incorporated into the polypeptide chain. This reaction is catalyzed by the enzyme *Lysyl hydroxylase*. **Why the other options are incorrect:** * **Proline, Lysine, and Methionine** are all standard (proteogenic) amino acids. * They are encoded by specific mRNA codons (e.g., AUG for Methionine). * Each requires a specific aminoacyl-tRNA synthetase to charge them onto their respective tRNAs for delivery to the ribosome during protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vitamin C Dependency:** The hydroxylation of Proline and Lysine requires **Vitamin C (Ascorbic acid)** as a co-factor. Deficiency leads to **Scurvy**, characterized by defective collagen synthesis and capillary fragility. 2. **Enzymes:** *Prolyl hydroxylase* and *Lysyl hydroxylase* require molecular oxygen and $\alpha$-ketoglutarate, in addition to Vitamin C and $Fe^{2+}$. 3. **Function:** Hydroxyproline provides thermal stability to the collagen triple helix via hydrogen bonding, while Hydroxylysine is essential for the **cross-linking** of collagen fibers and the attachment of carbohydrate moieties (glycosylation).

Question 329: Which substance is commonly found in skin, hair, and nails?

A. Vimentin
B. Keratin (Correct Answer)
C. Laminin
D. Nectin

Explanation: **Explanation:** **Keratin** is the correct answer because it is the primary structural protein found in the epidermis, hair, and nails. It belongs to the family of **Intermediate Filaments (IFs)**. Specifically, epithelial cells contain Type I (acidic) and Type II (basic) keratins, which heterodimerize to provide mechanical strength and water-proofing to the skin and its appendages. **Analysis of Incorrect Options:** * **Vimentin:** This is also an intermediate filament, but it is characteristic of cells of **mesenchymal origin** (e.g., fibroblasts, endothelial cells, and smooth muscle). In pathology, vimentin staining is used as a marker for sarcomas. * **Laminin:** This is a major glycoprotein component of the **basal lamina** (basement membrane). It plays a crucial role in cell adhesion and signaling, rather than forming the bulk structure of hair or nails. * **Nectin:** These are Ca²⁺-independent **cell adhesion molecules** found in adherens junctions. They help in cell-to-cell docking but do not serve as structural bulk proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Hard vs. Soft Keratin:** Hair and nails contain "hard" keratin (high sulfur content due to **cysteine cross-links**), while the skin contains "soft" keratin. * **Diagnostic Marker:** Keratin (Cytokeratin) is a key immunohistochemical marker used to identify **Carcinomas** (epithelial origin tumors). * **Clinical Correlation:** Mutations in keratin genes lead to blistering diseases like **Epidermolysis Bullosa Simplex** (K5/K14) and **Epidermolytic Hyperkeratosis** (K1/K10).

Question 330: What is the initial amino acid in prokaryotic protein synthesis?

A. Arginine
B. Methionine
C. Formyl-methionine (Correct Answer)
D. Alanine

Explanation: ### Explanation **Correct Option: C. Formyl-methionine (fMet)** In prokaryotes (such as bacteria), protein synthesis is initiated by a specific modified amino acid called **N-formyl-methionine (fMet)**. The initiation codon **AUG** codes for methionine; however, in prokaryotes, the initiator tRNA (tRNA$_f^{Met}$) carries a methionine that has been formylated by the enzyme *transformylase*. This formyl group mimics a peptide bond, helping the initiator tRNA bind directly to the P-site of the ribosome. **Analysis of Incorrect Options:** * **B. Methionine:** While Methionine is the universal starting amino acid for **eukaryotic** translation, it is not the *initial* form in prokaryotes. Prokaryotes use the formylated version to distinguish initiation from internal methionine residues. * **A. Arginine & D. Alanine:** These are common amino acids found within polypeptide chains but do not serve as initiator amino acids in any known biological system. **NEET-PG High-Yield Pearls:** 1. **Clinical Relevance:** fMet is recognized by the human immune system as a **PAMP** (Pathogen-Associated Molecular Pattern). Since human mitochondrial and cytosolic proteins do not start with fMet (except in mitochondria, which share prokaryotic ancestry), its presence triggers neutrophil chemotaxis via formyl peptide receptors. 2. **Mitochondrial Exception:** Human mitochondria, following the endosymbiotic theory, also use **fMet** for protein synthesis initiation. 3. **Deformylation:** In many bacteria, the formyl group (and sometimes the methionine itself) is removed post-translationally by *peptide deformylase* and *methionine aminopeptidase*. 4. **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA of the 30S subunit binds to this purine-rich sequence to correctly position the fMet-tRNA at the AUG start codon.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free