Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 271: Which class of compounds is characterized by a large amount of carbohydrate and a small amount of protein?

A. Glycoprotein
B. Glycosaminoglycan
C. Proteoglycan (Correct Answer)
D. Glycocalyx

Explanation: **Explanation:** The distinction between these compounds lies in the **ratio of carbohydrate to protein** and the nature of the carbohydrate chains. **1. Why Proteoglycan is Correct:** Proteoglycans are a subclass of glycoproteins where the carbohydrate content is predominant (typically **90-95% carbohydrate** and only 5-10% protein). They consist of a core protein to which long, unbranched polysaccharide chains called **Glycosaminoglycans (GAGs)** are covalently attached. Because of their high sugar content, they function primarily as ground substance in the extracellular matrix, providing hydration and resistance to compression. **2. Why the Other Options are Incorrect:** * **Glycoprotein (A):** These are proteins containing oligosaccharide chains. Unlike proteoglycans, the **protein component is dominant**, and the carbohydrate side chains are usually short and branched. * **Glycosaminoglycan (B):** These are the carbohydrate components themselves (e.g., Heparin, Hyaluronic acid). They are pure polysaccharides and do not contain a protein core unless they are part of a proteoglycan. * **Glycocalyx (D):** This is a functional "sugar coat" on the outer surface of plasma membranes, composed of both glycoproteins and glycolipids. It is a structural feature, not a specific chemical class defined by a carbohydrate-to-protein ratio. **High-Yield Clinical Pearls for NEET-PG:** * **Aggrecan** is the major proteoglycan found in cartilage. * **Hyaluronic acid** is the only GAG that is **non-sulfated** and does not bind to a core protein (it exists as a free GAG). * **Mucopolysaccharidoses (MPS):** These are lysosomal storage disorders (e.g., Hurler and Hunter syndromes) caused by the deficiency of enzymes required to degrade the GAG component of proteoglycans.

Question 272: What is the primary source of nitrogen for the human body?

A. Triacylglycerol
B. Proteins (Correct Answer)
C. Glucose
D. Lipids

Explanation: ### Explanation **1. Why Proteins are the Correct Answer:** Proteins are unique among the major macronutrients because they contain **Nitrogen** in their amino groups ($-NH_2$). While carbohydrates and lipids are composed primarily of carbon, hydrogen, and oxygen, proteins contain approximately **16% nitrogen** by weight. In the human body, dietary proteins are the sole significant source of nitrogen required for the synthesis of essential compounds, including non-essential amino acids, purines, pyrimidines, heme, and neurotransmitters. The "Nitrogen Balance" of the body is a direct reflection of protein metabolism. **2. Why Other Options are Incorrect:** * **A & D (Triacylglycerol/Lipids):** Lipids are primarily composed of hydrocarbon chains. They serve as the body's main energy reserve and structural components of membranes but do not contain nitrogen (with the minor exception of specialized sphingolipids and phospholipids, which are not primary dietary nitrogen sources). * **C (Glucose):** Glucose is a simple carbohydrate ($C_6H_{12}O_6$). It is the primary fuel for the brain and RBCs but lacks nitrogen entirely. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Nitrogen Balance:** Calculated as $N_{in} - N_{out}$. Positive nitrogen balance occurs during growth, pregnancy, and convalescence. Negative nitrogen balance occurs during starvation, burns, and severe infection. * **Kjeldahl Method:** This is the classic laboratory technique used to estimate the protein content of a sample by measuring its nitrogen content (Protein = Nitrogen × 6.25). * **Urea Cycle:** The primary mechanism for disposing of excess nitrogen is the synthesis of urea in the liver, which is then excreted by the kidneys. * **Kwashiorkor:** A clinical state of severe protein deficiency despite adequate calorie intake, leading to edema and "flaky paint" dermatosis due to the inability to maintain nitrogen homeostasis.

Question 273: What will happen to the structure of an alpha-helix if L-alanine is replaced by D-alanine alternatively?

A. Change in optical activity
B. Interruption of structure (Correct Answer)
C. Increased stereoisomerism
D. Increased stability

Explanation: **Explanation:** The **alpha-helix** is a rigid, right-handed spiral structure stabilized by intrachain hydrogen bonding. In nature, proteins are composed exclusively of **L-amino acids**. In a right-handed alpha-helix, the R-groups (side chains) of L-amino acids are oriented outward to minimize steric hindrance. If **D-alanine** is introduced alternatively with L-alanine, it creates significant **steric interference**. Because D-amino acids are stereoisomers with a different spatial arrangement, their side chains would point inward or clash with the peptide backbone and the side chains of adjacent L-amino acids. This disrupts the specific $(\phi, \psi)$ dihedral angles required to maintain the helix, leading to the **interruption of the structure**. **Analysis of Options:** * **B (Correct):** The alternating stereochemistry prevents the formation of the regular hydrogen-bonding pattern and causes steric clashes, destabilizing the helix. * **A (Incorrect):** While optical activity would technically change, it is a physical property and not the primary structural consequence of the substitution. * **C (Incorrect):** Stereoisomerism is a property of the individual molecules; replacing one with another does not "increase" the phenomenon itself within the protein's functional context. * **D (Incorrect):** D-amino acids act as "helix breakers" in standard L-proteins, significantly decreasing stability. **High-Yield Clinical Pearls for NEET-PG:** * **Proline** is known as a "helix breaker" because its secondary amino group lacks a hydrogen for bonding and its ring structure creates a kink. * **Glycine** also disrupts helices because its high conformational flexibility makes it entropically unfavorable to stay fixed in a helix. * **D-amino acids** are rarely found in human proteins but are clinically significant as components of **bacterial cell walls** (e.g., D-alanine in peptidoglycan) and certain antibiotics like **Gramicidin**.

Question 274: Which protein is primarily responsible for binding and transporting copper in the blood?

A. Albumin
B. Globulin
C. Transferrin
D. Ceruloplasmin (Correct Answer)

Explanation: **Explanation:** **1. Why Ceruloplasmin is Correct:** Ceruloplasmin is an $\alpha_2$-globulin synthesized in the liver. It is the primary copper-carrying protein in the blood, accounting for approximately **90-95% of circulating copper**. Each molecule of ceruloplasmin can bind 6 to 8 copper atoms tightly. Beyond transport, it functions as a **ferroxidase enzyme**, converting ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$), which is essential for the binding of iron to transferrin. **2. Analysis of Incorrect Options:** * **Albumin:** While albumin binds the remaining 5-10% of plasma copper (loosely bound), its primary role is maintaining oncotic pressure and transporting fatty acids, bilirubin, and various drugs. * **Globulin:** This is a broad category of proteins (Alpha, Beta, Gamma). While ceruloplasmin is a specific type of globulin, "globulin" is too non-specific to be the correct answer. * **Transferrin:** This is the primary transport protein for **iron**, not copper. It carries iron in the $Fe^{3+}$ state to the bone marrow for erythropoiesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency of ceruloplasmin due to a defect in the ATP7B gene. This leads to copper deposition in the liver (cirrhosis), brain (basal ganglia), and eyes (**Kayser-Fleischer rings**). * **Menkes Disease:** A defect in copper absorption (ATP7A gene) leading to "kinky hair" and systemic copper deficiency. * **Acute Phase Reactant:** Ceruloplasmin levels increase during inflammation, infection, or trauma. * **Ferroxidase Activity:** Remember that copper is essential for iron metabolism; thus, copper deficiency can manifest as microcytic anemia.

Question 275: Substitution of which one of the following amino acids in place of alanine would increase the absorbance of a protein at 280 nm?

A. Leucine
B. Arginine
C. Tryptophan (Correct Answer)
D. Proline

Explanation: **Explanation:** The absorbance of proteins at **280 nm** is primarily due to the presence of **aromatic amino acids**. These amino acids contain conjugated double bonds in their side chains (rings) that can absorb ultraviolet light. **1. Why Tryptophan is Correct:** Among the aromatic amino acids (**Tryptophan, Tyrosine, and Phenylalanine**), Tryptophan has the highest molar absorptivity because of its bulky **indole ring**. * **Tryptophan** absorbs the most light at 280 nm. * **Tyrosine** absorbs significantly less than Tryptophan. * **Phenylalanine** absorbs minimally at 280 nm (its peak is closer to 260 nm). Substituting Alanine (a non-aromatic amino acid) with Tryptophan significantly increases the protein's ability to absorb light at this specific wavelength. **2. Why Other Options are Incorrect:** * **Leucine (A):** An aliphatic, branched-chain amino acid. It lacks a conjugated ring system and does not absorb light at 280 nm. * **Arginine (B):** A basic, positively charged amino acid. It does not possess aromatic properties. * **Proline (D):** An imino acid with a cyclic structure, but it is not aromatic and does not contribute to absorbance at 280 nm. **High-Yield Facts for NEET-PG:** * **Beer-Lambert Law:** This principle is used in laboratories to estimate protein concentration based on 280 nm absorbance. * **Order of Absorbance (280 nm):** Tryptophan > Tyrosine > Phenylalanine. * **DNA/RNA Absorbance:** Nucleic acids show maximum absorbance at **260 nm** (due to purine and pyrimidine bases). * **260/280 Ratio:** Used to assess the purity of DNA/RNA samples; a ratio of ~1.8 is considered pure for DNA.

Question 276: Which amino acid is primarily responsible for the formation of histone-nucleic acid complexes?

A. Alanine
B. Threonine
C. Leucine
D. Lysine (Correct Answer)

Explanation: ### Explanation **1. Why Lysine is Correct:** The interaction between histones and DNA is primarily driven by **electrostatic attraction**. DNA is a highly negatively charged molecule due to its phosphate backbone. For histones to bind tightly to DNA and facilitate the packaging of chromatin, they must possess a strong positive charge. **Lysine** (along with Arginine) is a basic amino acid that carries a positive charge at physiological pH. These positively charged side chains form ionic bonds with the negative phosphate groups of the DNA, stabilizing the nucleosome structure. **2. Why the Other Options are Incorrect:** * **A. Alanine:** This is a small, non-polar, hydrophobic amino acid. It lacks the charge necessary to interact with the DNA backbone. * **B. Threonine:** This is a polar, uncharged amino acid containing a hydroxyl group. While it can participate in hydrogen bonding, it cannot provide the strong electrostatic attraction required for histone-DNA binding. * **C. Leucine:** This is a branched-chain, hydrophobic amino acid. It is typically found in the interior of proteins or involved in protein-protein interactions (like leucine zippers), not in binding to negatively charged nucleic acids. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Histone Acetylation:** This is a key epigenetic modification. When the positive charge of **Lysine** is neutralized by acetylation (via Histone Acetyltransferases - HATs), the affinity between histones and DNA decreases. This results in "relaxed" chromatin (**Euchromatin**), which is transcriptionally active. * **Histone Deacetylation:** Removal of acetyl groups by HDACs restores the positive charge, leading to tightly packed **Heterochromatin**, which is transcriptionally silent. * **Amino Acid Composition:** Histones are exceptionally rich in **Arginine and Lysine** (approx. 20-30%). * **Linker Histone:** H1 is the linker histone that binds to the entry/exit sites of DNA on the nucleosome core.

Question 277: Why is ubiquitin called 'ubiquitin'?

A. It is present in all eukaryotic cells. (Correct Answer)
B. It takes part in important cellular reactions.
C. It is highly conserved across species.
D. None of the above.

Explanation: **Explanation:** The term **Ubiquitin** is derived from the word **"ubiquitous,"** which means "found everywhere." It was given this name because it is expressed in **all eukaryotic cells**, from simple yeast to complex mammals. It is a small (76 amino acids) regulatory protein that plays a critical role in the **Ubiquitin-Proteasome Pathway**, marking damaged or unneeded proteins for degradation. **Analysis of Options:** * **Option A (Correct):** The name specifically refers to its universal distribution across all eukaryotic cell types and tissues. * **Option B (Incorrect):** While ubiquitin is vital for cellular reactions (like protein turnover, DNA repair, and cell cycle regulation), its name is a descriptor of its *location/presence*, not its *function*. * **Option C (Incorrect):** Ubiquitin is indeed one of the most highly conserved proteins known (only 3 amino acids differ between yeast and humans). However, "ubiquitin" refers to its widespread presence, not the evolutionary stability of its sequence. **High-Yield NEET-PG Pearls:** 1. **Mechanism:** Ubiquitin attaches to the **Lysine** residue of target proteins (Ubiquitination). 2. **Degradation:** Polyubiquitinated proteins are recognized and degraded by the **26S Proteasome** in an ATP-dependent process. 3. **Clinical Correlation:** Defects in the ubiquitin system are linked to neurodegenerative diseases like **Parkinson’s** (Lewy bodies contain ubiquitin) and **Alzheimer’s disease**. 4. **Bortezomib:** A proteasome inhibitor used in the treatment of **Multiple Myeloma**, highlighting the clinical importance of this pathway.

Question 278: Which level of protein structure is not affected by protein denaturation?

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

Explanation: **Explanation:** **Denaturation** is the process by which a protein loses its native three-dimensional conformation due to external stress (such as heat, extreme pH, or organic solvents). 1. **Why Primary Structure is the Correct Answer:** The primary structure consists of the linear sequence of amino acids held together by **covalent peptide bonds**. Denaturation involves the disruption of non-covalent interactions (hydrogen bonds, hydrophobic interactions, and ionic bonds). Since peptide bonds are strong covalent bonds, they are not broken during standard denaturation; they can only be cleaved by proteolytic enzymes or strong acids/bases via hydrolysis. Therefore, the primary structure remains intact. 2. **Why Other Options are Incorrect:** * **Secondary Structure:** Involves $\alpha$-helices and $\beta$-pleated sheets stabilized by **hydrogen bonds**. These bonds are weak and easily disrupted by heat or pH changes. * **Tertiary Structure:** Represents the overall 3D folding of a single polypeptide chain. It is stabilized by disulfide bridges, salt bridges, and hydrophobic interactions, all of which are disrupted during denaturation. * **Quaternary Structure:** Refers to the spatial arrangement of multiple polypeptide subunits. The non-covalent forces holding these subunits together are the first to be disrupted during denaturation. **High-Yield Clinical Pearls for NEET-PG:** * **Renaturation:** If the denaturing agent is removed, some proteins can spontaneously refold into their native state (e.g., Ribonuclease), proving that the primary structure contains all the information necessary for folding. * **Chaperones:** These are specialized proteins (Heat Shock Proteins) that assist in the correct folding of proteins and prevent misfolding during cellular stress. * **Prion Diseases:** These occur when a normal protein ($\text{PrP}^c$) undergoes a conformational change (not denaturation) into a pathological $\beta$-sheet rich form ($\text{PrP}^{sc}$), which is resistant to standard denaturation.

Question 279: Which enzymatic activity is characteristic of ribosomes?

A. Peptidyl transferase (Correct Answer)
B. Peptidase
C. Carboxylase
D. Dehydratase

Explanation: **Explanation:** The correct answer is **Peptidyl transferase**. **1. Why Peptidyl Transferase is Correct:** Ribosomes are the cellular machinery responsible for protein synthesis (translation). The core enzymatic activity of the ribosome is **peptidyl transferase**, which catalyzes the formation of a peptide bond between the carboxyl group of the growing polypeptide chain (at the P-site) and the amino group of the incoming amino acid (at the A-site). Crucially, in both prokaryotes (23S rRNA) and eukaryotes (28S rRNA), this activity is mediated by the **ribosomal RNA (rRNA)** itself rather than a protein. Therefore, the ribosome is classified as a **ribozyme** (an RNA molecule with catalytic activity). **2. Why Other Options are Incorrect:** * **B. Peptidase:** These enzymes (also called proteases) break peptide bonds to degrade proteins into smaller peptides or amino acids. This is the opposite of the ribosome's synthetic function. * **C. Carboxylase:** These enzymes catalyze the addition of a carboxyl group (CO₂), often requiring Biotin as a cofactor (e.g., Acetyl-CoA carboxylase). * **D. Dehydratase:** These enzymes catalyze the removal of a water molecule to form a double bond (e.g., δ-aminolevulinic acid dehydratase in heme synthesis). **3. High-Yield Clinical Pearls for NEET-PG:** * **Ribozyme Concept:** The discovery that the 23S/28S rRNA is the catalyst proves that "not all enzymes are proteins." * **Antibiotic Target:** Several antibiotics inhibit this specific activity. For example, **Chloramphenicol** binds to the 50S subunit and inhibits peptidyl transferase in bacteria. * **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA of the small subunit recognizes this mRNA sequence to initiate translation. * **Energy Source:** While peptidyl transferase itself doesn't require ATP/GTP, the *loading* of tRNA (aminoacyl-tRNA synthetase) requires ATP, and *translocation* requires GTP.

Question 280: 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:** Protein structure is organized into four distinct levels based on the complexity of the folding. The **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 most common examples are the **$\alpha$-helix** and **$\beta$-pleated sheet**. * **Alpha ($\alpha$) helix:** A spiral structure stabilized by intrachain hydrogen bonds (parallel to the axis). * **Beta ($\beta$) pleated sheet:** Formed by adjacent segments of polypeptide chains (parallel or anti-parallel) stabilized by interchain hydrogen bonds. **Why other options are incorrect:** * **Primary structure:** Refers only to the linear sequence of amino acids held together by covalent **peptide bonds**. It determines the higher levels of folding but does not include helices or sheets. * **Tertiary structure:** Represents the overall 3D folding of a single polypeptide chain, stabilized by disulfide bridges, hydrophobic interactions, and ionic bonds (e.g., Myoglobin). * **Quaternary structure:** Refers to the spatial arrangement and interaction of multiple polypeptide subunits (e.g., Hemoglobin). **High-Yield Clinical Pearls for NEET-PG:** * **Proline** is known as an **"alpha-helix breaker"** because its rigid ring structure disrupts the helical turn. * **Prion diseases** (e.g., Creutzfeldt-Jakob disease) involve a conformational change where normal $\alpha$-helices are replaced by pathological **$\beta$-sheets**, leading to protein aggregation. * **Scurvy** involves defective hydroxylation of proline/lysine, affecting the stability of the collagen triple helix.

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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