Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following is a negative acute-phase protein?

A. Transthyretin (Correct Answer)
B. C-Reactive Protein
C. Ferritin
D. Ceruloplasmin

Explanation: **Explanation:** Acute-phase proteins (APPs) are proteins whose plasma concentrations increase or decrease by at least 25% in response to inflammation, infection, or trauma. This response is primarily mediated by cytokines like IL-6, IL-1, and TNF-α acting on the liver. **1. Why Transthyretin is Correct:** **Transthyretin (Prealbumin)** is a **negative acute-phase protein**. During inflammation, the liver prioritizes the synthesis of "positive" APPs (like CRP) to aid the immune response, leading to a down-regulation in the production of transport proteins. Other major negative APPs include **Albumin** and **Transferrin**. **2. Why the Other Options are Incorrect:** * **C-Reactive Protein (CRP):** A classic positive APP. It is a sensitive marker of systemic inflammation and acts as an opsonin to facilitate phagocytosis. * **Ferritin:** A positive APP. Its levels rise during inflammation to sequester iron, depriving invading pathogens of this essential nutrient (contributing to the "anemia of chronic disease"). * **Ceruloplasmin:** A positive APP. It acts as a ferroxidase and helps in scavenging free radicals during the inflammatory process. **High-Yield Clinical Pearls for NEET-PG:** * **Most Sensitive Marker:** CRP is the most sensitive and rapidly responding positive APP. * **The "Big Three" Negative APPs:** Remember the mnemonic **"PAT"**—**P**realbumin (Transthyretin), **A**lbumin, and **T**ransferrin. * **ESR vs. CRP:** While both indicate inflammation, CRP reflects the acute change more accurately, whereas ESR is influenced by fibrinogen levels and red cell morphology. * **Transthyretin Function:** It transports thyroxine (T4) and Retinol-Binding Protein (RBP). Its short half-life (~2 days) makes it a better marker for acute nutritional status than albumin.

Question 42: What is the most appropriate function of ubiquitination?

A. Electron transport chain
B. Degradation of proteins (Correct Answer)
C. Synthesis of protein
D. Oxidative deamination

Explanation: **Explanation:** Ubiquitin is a small, highly conserved regulatory protein (76 amino acids) found in all eukaryotic cells. Its primary function is to mark unwanted or damaged proteins for destruction via the **Ubiquitin-Proteasome Pathway (UPP)**. 1. **Why Option B is Correct:** Ubiquitination is a three-step enzymatic process (involving enzymes E1, E2, and E3) that attaches ubiquitin molecules to a target protein. Once a protein is "polyubiquitinated," it is recognized and degraded by the **26S proteasome**, a barrel-shaped multi-protein complex. This is the major non-lysosomal pathway for intracellular protein turnover. 2. **Why Other Options are Incorrect:** * **Option A:** The Electron Transport Chain (ETC) involves cytochromes and coenzymes (like NADH and FADH2) for ATP production, not ubiquitin. * **Option C:** Protein synthesis (translation) occurs on ribosomes using mRNA and tRNA; ubiquitin is involved in the *end* of a protein's life cycle, not its beginning. * **Option D:** Oxidative deamination is the process by which amino acids are catabolized to release ammonia (primarily via glutamate dehydrogenase); it does not involve ubiquitin tagging. **High-Yield Clinical Pearls for NEET-PG:** * **ATP-Dependence:** Unlike lysosomal degradation, the ubiquitin-proteasome pathway is **ATP-dependent**. * **Clinical Correlation:** Defects in the ubiquitination system are linked to neurodegenerative diseases like **Parkinson’s** (accumulation of Lewy bodies) and **Alzheimer’s**. * **Bortezomib:** A proteasome inhibitor used clinically in the treatment of **Multiple Myeloma**, highlighting the importance of this pathway in cell cycle regulation.

Question 43: Gonadotrophin is a:

A. Steroid
B. Lipoprotein
C. Polypeptide
D. Glycoprotein (Correct Answer)

Explanation: **Explanation:** **1. Why Glycoprotein is Correct:** Gonadotrophins (FSH, LH, and hCG) are complex **heterodimeric glycoproteins**. They consist of two non-covalently linked subunits: * **Alpha (α) subunit:** Identical in FSH, LH, TSH, and hCG. * **Beta (β) subunit:** Unique to each hormone, conferring biological and immunological specificity. The "glyco-" component refers to the carbohydrate side chains attached to the polypeptide backbone, which are essential for the hormone's stability, half-life, and receptor binding. **2. Why Other Options are Incorrect:** * **Steroid:** Steroid hormones (e.g., Estrogen, Testosterone, Cortisol) are derived from cholesterol and are lipophilic. Gonadotrophins are water-soluble proteins. * **Lipoprotein:** These are complexes of lipids and proteins (e.g., LDL, HDL) used for lipid transport in the blood, not for hormonal signaling. * **Polypeptide:** While gonadotrophins contain polypeptide chains, the term "Polypeptide" usually refers to simple protein hormones without significant carbohydrate modifications (e.g., Insulin, Glucagon, GH). "Glycoprotein" is the more specific and accurate classification. **3. NEET-PG High-Yield Pearls:** * **The "Big Four" Glycoproteins:** Remember the mnemonic **"F-L-A-T"** (FSH, LH, ACTH is NOT one, it's **TSH** and **hCG**) for hormones sharing the same alpha subunit. * **Clinical Correlation:** In pregnancy tests, we measure the **β-hCG** subunit because the α-subunit is identical to LH/FSH/TSH and would cause cross-reactivity. * **Sialic Acid Content:** The half-life of these hormones is determined by their sialic acid content; hCG has the highest sialic acid content and thus the longest half-life (~24 hours).

Question 44: Proteins destined for secretion from eukaryotic cells have which of the following in common?

A. A signal peptide located at the carboxy terminus; synthesized on rough ribosomes; embedded within the ER membrane.
B. A signal peptide located at the amino terminus; synthesized on cytoplasmic ribosomes; not embedded within the ER membrane.
C. A signal peptide located at the carboxy terminus; synthesized on rough ribosomes; embedded within the ER membrane.
D. A signal peptide located at the amino terminus; synthesized on rough ribosomes; not embedded within the ER membrane. (Correct Answer)

Explanation: ### Explanation The correct answer is **D**. This question tests the concept of **Protein Targeting (Signal Hypothesis)**, which describes how proteins reach their final destination. **1. Why Option D is Correct:** Proteins destined for secretion (e.g., insulin, digestive enzymes) are synthesized via the **Secretory Pathway**: * **Amino (N) Terminus Signal Peptide:** The "address label" is a sequence of 15–30 hydrophobic amino acids located at the N-terminus. It is the first part of the protein to emerge from the ribosome. * **Rough Endoplasmic Reticulum (RER):** The Signal Recognition Particle (SRP) binds the signal peptide and halts translation until the ribosome docks onto the RER membrane. Synthesis then resumes directly into the ER lumen (**Co-translational translocation**). * **Not Embedded:** Secretory proteins are released into the ER lumen and eventually packaged into vesicles. Unlike transmembrane proteins, they do not remain embedded in the membrane. **2. Why Other Options are Incorrect:** * **Options A & C:** The signal peptide is almost always at the **N-terminus**, not the C-terminus. C-terminal signals (like the KDEL sequence) are typically used for protein *retention* or peroxisomal targeting, not initial ER entry. * **Option B:** While synthesis *starts* on cytoplasmic ribosomes, secretory proteins must finish synthesis on **Rough Ribosomes** to enter the secretory pathway. If they remained in the cytoplasm, they would be destined for the nucleus, mitochondria, or peroxisomes. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **I-Cell Disease:** Caused by a deficiency in *N-acetylglucosamine phosphotransferase*. Proteins destined for lysosomes lack the **Mannose-6-Phosphate** tag and are secreted extracellularly instead, leading to inclusion bodies. * **Signal Peptidase:** This enzyme cleaves the signal peptide once the protein enters the ER lumen. * **Chaperones:** Proteins like **BiP** (Binding immunoglobulin Protein) help in the correct folding of these proteins within the ER.

Question 45: Glutathione consists of cysteine, glutamate, plus which other amino acid?

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

Explanation: **Explanation:** Glutathione (GSH) is a vital **tripeptide** found in high concentrations in most cells. It is composed of three amino acids: **Glutamate, Cysteine, and Glycine**. The structure of glutathione is unique because the glutamate is linked to cysteine via its **gamma-carboxyl group** rather than the standard alpha-carboxyl group, forming a **$\gamma$-glutamyl linkage**. This specific bond protects the molecule from degradation by ordinary intracellular peptidases. The cysteine residue provides a **sulfhydryl (-SH) group**, which is the functional center responsible for its antioxidant properties and its role in redox reactions. **Analysis of Options:** * **Glycine (Correct):** It is the C-terminal amino acid of the tripeptide ($\gamma$-glutamyl-cysteinyl-glycine). * **Leucine, Lysine, and Valine (Incorrect):** These amino acids are not components of the glutathione molecule. While they are essential or common amino acids, they do not participate in the specific enzymatic synthesis of GSH mediated by $\gamma$-glutamylcysteine synthetase and glutathione synthetase. **High-Yield Clinical Pearls for NEET-PG:** * **Antioxidant Function:** Glutathione is the co-factor for **Glutathione Peroxidase**, which neutralizes hydrogen peroxide ($H_2O_2$) into water, protecting RBCs from oxidative stress. * **Redox State:** The ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) is a key indicator of cellular oxidative stress. **NADPH** (from the HMP shunt) is required by **Glutathione Reductase** to regenerate GSH. * **Detoxification:** It plays a crucial role in the liver via the **Meister Cycle** ($\gamma$-glutamyl cycle) for amino acid transport and in neutralizing paracetamol (acetaminophen) metabolites (NAPQI). * **Clinical Correlation:** Deficiency of G6PD leads to decreased NADPH, resulting in low reduced glutathione levels and subsequent **hemolysis** due to Heinz body formation.

Question 46: Albumin binds with which of the following substances?

A. Steroid hormones
B. Calcium
C. Free fatty acids (FFA)
D. Thyroxine (Correct Answer)

Explanation: **Explanation:** Albumin is the most abundant plasma protein, primarily synthesized in the liver. Its physiological importance stems from two main functions: maintaining **plasma oncotic pressure** and acting as a versatile **transport protein** for various ligands. **Why Thyroxine is the Correct Answer:** While albumin is a "generalist" carrier, it plays a significant role in transporting thyroid hormones. Although **Thyroxine-Binding Globulin (TBG)** has the highest affinity for thyroxine (T4), albumin has a much higher capacity. Approximately 10–15% of circulating T4 is bound to albumin. In the context of competitive exams like NEET-PG, when asked about albumin's binding profile, thyroxine is a classic high-yield association alongside bilirubin and drugs. **Analysis of Other Options:** * **Steroid Hormones:** These are primarily transported by specific globulins, such as **Sex Hormone-Binding Globulin (SHBG)** and **Corticosteroid-Binding Globulin (CBG/Transcortin)**. While albumin can bind them non-specifically, it is not their primary carrier. * **Calcium:** Albumin binds to ionized calcium (about 40% of total serum calcium). However, in the hierarchy of biochemical transport "specialties," thyroxine and fatty acids are more frequently tested as primary ligands. * **Free Fatty Acids (FFA):** Albumin is indeed the primary carrier for FFAs (holding up to 7 molecules per albumin monomer). However, in many standardized question banks, thyroxine is prioritized as the "classic" answer for albumin binding specificity. **NEET-PG High-Yield Pearls:** * **Drug Binding:** Albumin primarily binds **acidic drugs** (e.g., Warfarin, Salicylates, Phenytoin), while $\alpha_1$-acid glycoprotein binds basic drugs. * **Bilirubin Transport:** Albumin carries unconjugated bilirubin to the liver. Displacement of bilirubin from albumin by drugs (like Sulfonamides) in neonates can lead to **Kernicterus**. * **Clinical Correlation:** In **Hypoalbuminemia**, the "total" serum calcium level drops, but the "ionized" (active) calcium remains normal. Formula: Corrected Ca = Measured Ca + [0.8 × (4.0 - Albumin)].

Question 47: Alpha helix and Beta pleated sheet are examples of which level of protein structure?

A. Primary structure
B. Secondary structure (Correct Answer)
C. Tertiary structure
D. Quaternary structure

Explanation: ### Explanation The correct answer is **Secondary structure**. **1. Why Secondary Structure is Correct:** Secondary structure refers to the local spatial arrangement of the polypeptide backbone, stabilized primarily by **hydrogen bonds** between the carbonyl oxygen (-CO) and the amide nitrogen (-NH) of the peptide bonds. The **Alpha helix** (a right-handed spiral) and **Beta-pleated sheet** (parallel or anti-parallel strands) are the most common repeating patterns. These structures do not involve the side chains (R-groups) of amino acids. **2. Why Other Options are Incorrect:** * **Primary structure:** This is the linear sequence of amino acids held together by **covalent peptide bonds**. It dictates the higher-order folding but does not include helices or sheets. * **Tertiary structure:** This represents the overall 3D conformation of a single polypeptide chain, stabilized by interactions between **R-groups** (disulfide bridges, hydrophobic interactions, ionic bonds). * **Quaternary structure:** This refers to the spatial arrangement and interaction of multiple polypeptide subunits (e.g., the four subunits of Hemoglobin). **3. High-Yield Clinical Pearls for NEET-PG:** * **Proline** is known as an **"Alpha-helix breaker"** because its rigid cyclic structure prevents it from fitting into the helical turn. * **Glycine** is often found in **Beta-turns** because its small size (H-atom side chain) allows for sharp bending. * **Prion diseases** (like Creutzfeldt-Jakob disease) involve a pathological transition where normal alpha-helices are converted into **infectious beta-pleated sheets**, leading to protein aggregation. * **Scurvy:** Defective collagen (triple helix) occurs due to lack of Vitamin C, a cofactor for prolyl hydroxylase.

Question 48: Which of the following represents a covalent bond?

A. Hydrogen bond
B. Disulphide bond (Correct Answer)
C. Electrostatic bond
D. Ionic bond

Explanation: **Explanation:** In protein chemistry, bonds are categorized into **covalent** (sharing of electrons) and **non-covalent** (weak electrostatic interactions). **Why the Disulphide bond is correct:** A disulphide bond (S-S bond) is a strong **covalent bond** formed by the oxidation of the sulfhydryl (-SH) groups of two **Cysteine** residues. It is the only covalent bond, other than the peptide bond, that stabilizes the tertiary and quaternary structures of proteins. Because it is covalent, it is significantly stronger than hydrogen or ionic bonds and requires specific enzymes (like protein disulfide isomerase) or strong reducing agents (like β-mercaptoethanol) to break. **Why the other options are incorrect:** * **A. Hydrogen bond:** A non-covalent interaction between a hydrogen atom (covalently bound to an electronegative atom like N or O) and another electronegative atom. It is essential for stabilizing **alpha-helices and beta-sheets**. * **C & D. Electrostatic/Ionic bond:** These are non-covalent interactions between oppositely charged side chains (e.g., the positively charged Lysine and negatively charged Aspartate). They are also known as **salt bridges**. **High-Yield Facts for NEET-PG:** * **Cystine vs. Cysteine:** A "Cystine" molecule is formed when two "Cysteine" residues are linked by a disulphide bond. * **Insulin:** A classic example of a protein stabilized by both inter-chain and intra-chain disulphide bonds. * **Denaturation:** Most denaturing agents (heat, urea) break non-covalent bonds, but **reducing agents** are specifically required to break disulphide bonds. * **Keratin:** The high disulphide bond content in alpha-keratin provides structural rigidity to hair and nails.

Question 49: Sakaguchi's reaction is specific for which amino acid?

A. Tyrosine
B. Proline
C. Arginine (Correct Answer)
D. Cysteine

Explanation: **Explanation:** **Sakaguchi’s reaction** is a specific biochemical test used to detect the presence of **Arginine**. The reaction depends on the presence of the **guanidino group** found in the side chain of Arginine. When Arginine reacts with α-naphthol and sodium hypobromite (or hypochlorite) in an alkaline medium, it produces a characteristic **bright red color**. **Analysis of Options:** * **Arginine (Correct):** It is the only proteinogenic amino acid containing a guanidino group, making this test highly specific. * **Tyrosine (Incorrect):** Tyrosine is identified by the **Millon’s test**, which detects the phenolic group, resulting in a white precipitate that turns brick red upon heating. * **Proline (Incorrect):** As an imino acid, Proline reacts with **Ninhydrin** to yield a characteristic **yellow color**, unlike the purple/blue (Ruhemann's purple) seen with other amino acids. * **Cysteine (Incorrect):** Sulfur-containing amino acids like Cysteine are detected using the **Lead Sulfide (Sulfur) test**, where they react with lead acetate in an alkaline medium to form a black precipitate. **High-Yield Clinical Pearls for NEET-PG:** * **Xanthoproteic Test:** Specific for aromatic amino acids (Tyrosine, Phenylalanine, Tryptophan); gives a yellow color with hot concentrated nitric acid. * **Hopkins-Cole Test:** Specific for the indole ring of **Tryptophan**. * **Pauly’s Test:** Specific for **Histidine** and Tyrosine (detects imidazole and phenolic rings). * **Arginine’s Role:** It is a precursor for Nitric Oxide (NO), urea, and creatine, making it a frequent topic in metabolic biochemistry.

Question 50: Which amino acid produces kinks in the beta-pleated structure of proteins?

A. Glycine
B. Serine
C. Proline (Correct Answer)
D. Alanine

Explanation: **Explanation:** **Why Proline is the Correct Answer:** Proline is unique among the 20 standard amino acids because it is technically an **imino acid**. Its side chain is cyclized and covalently bonded back to the nitrogen of the peptide backbone, forming a five-membered pyrrolidine ring. This rigid, cyclic structure imposes significant steric hindrance and restricts the rotation around the N-Cα bond (the phi angle). In the context of secondary structures: * **Alpha-helices:** Proline acts as a "helix breaker" because it lacks the amide hydrogen necessary for hydrogen bonding. * **Beta-pleated sheets:** The rigid ring structure prevents the polypeptide chain from adopting the flexible, extended conformation required for a regular beta-sheet, thereby introducing **kinks or bends**. **Analysis of Incorrect Options:** * **A. Glycine:** Known as the simplest amino acid (side chain is -H). It provides maximum flexibility to the polypeptide chain. While it is often found in "Beta-turns," it does not cause kinks due to rigidity; rather, it allows for tight packing. * **B. Serine:** A polar, uncharged amino acid with a hydroxyl group. It typically fits well into standard secondary structures without causing structural disruptions. * **D. Alanine:** A small, non-polar amino acid that is considered the "best" former of alpha-helices. It does not disrupt beta-pleated structures. **High-Yield Clinical Pearls for NEET-PG:** * **Hydroxyproline:** A post-translational modification of proline (requiring Vitamin C) essential for collagen stability. Deficiency leads to **Scurvy**. * **Beta-turns:** Proline and Glycine are the two most common amino acids found in Beta-turns (Type I and Type II). * **Ninhydrin Test:** Proline gives a **yellow** color with ninhydrin, unlike other amino acids which give a purple/Ruhemann's purple color.

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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