Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 251: Which of the following is considered a marker enzyme of mitochondria?

A. Na+ – K+ ATPase
B. Glutamate dehydrogenase (Correct Answer)
C. Lactate dehydrogenase
D. No specific enzyme

Explanation: **Explanation:** Marker enzymes are specific enzymes localized within particular organelles, used to identify or assess the purity of those organelles during cell fractionation. **1. Why Glutamate Dehydrogenase (GDH) is correct:** Glutamate dehydrogenase is a key enzyme involved in amino acid metabolism (oxidative deamination). It is located exclusively within the **mitochondrial matrix**. Because of its specific localization, it serves as a reliable marker for the mitochondrial compartment. Other common mitochondrial markers include **Succinate Dehydrogenase** (Inner Mitochondrial Membrane) and **Cytochrome Oxidase**. **2. Why other options are incorrect:** * **Na+ – K+ ATPase:** This is the classic marker enzyme for the **Plasma Membrane**. It maintains the resting membrane potential by pumping ions against their concentration gradients. * **Lactate Dehydrogenase (LDH):** This is the marker enzyme for the **Cytosol**. It catalyzes the interconversion of pyruvate and lactate during anaerobic glycolysis. * **No specific enzyme:** This is incorrect as almost every organelle has a specific biochemical marker (e.g., Acid phosphatase for Lysosomes, Glucose-6-phosphatase for Endoplasmic Reticulum). **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Matrix Marker:** Glutamate Dehydrogenase. * **Inner Mitochondrial Membrane Marker:** Succinate Dehydrogenase (also part of the TCA cycle and ETC Complex II). * **Outer Mitochondrial Membrane Marker:** Monoamine Oxidase (MAO). * **Lysosome Marker:** Acid Phosphatase. * **Peroxisome Marker:** Catalase. * **Golgi Apparatus Marker:** Galactosyl transferase.

Question 252: Who was awarded the Nobel Prize for determining the complete amino acid sequence of the two polypeptide chains of Insulin?

A. Fredreich Sanger (Correct Answer)
B. Banting and Macleod
C. Paul Muller
D. Alexander Fleming

Explanation: **Explanation:** **Correct Answer: A. Frederick Sanger** Frederick Sanger was awarded his first Nobel Prize in Chemistry (1958) for his work on the structure of proteins, specifically for being the first to determine the complete **amino acid sequence** of the two polypeptide chains of **Insulin**. He used "Sanger’s reagent" (1-fluoro-2,4-dinitrobenzene) to label the N-terminal amino acids. This discovery was monumental as it proved that proteins have a specific, defined chemical sequence rather than being random mixtures. **Analysis of Incorrect Options:** * **B. Banting and Macleod:** They were awarded the Nobel Prize in 1923 for the **discovery of insulin** and its clinical application in treating diabetes, not for its biochemical sequencing. * **C. Paul Müller:** He received the Nobel Prize in 1948 for discovering the insecticidal properties of **DDT**, which was crucial in controlling malaria and typhus. * **D. Alexander Fleming:** He was awarded the Nobel Prize in 1945 for the discovery of **Penicillin**, the first true antibiotic. **High-Yield Clinical Pearls for NEET-PG:** * **Insulin Structure:** It consists of 51 amino acids across two chains: **Chain A (21 residues)** and **Chain B (30 residues)**, linked by two interchain disulfide bonds. There is also one intrachain disulfide bond in Chain A. * **Sanger’s Legacy:** He is one of only two people to win two Nobel Prizes in Chemistry (the second was for **DNA sequencing** using the dideoxy method). * **C-peptide:** In clinical practice, measuring C-peptide levels helps distinguish between Type 1 DM (low/absent) and Type 2 DM (normal/high), as it is released in equimolar amounts with endogenous insulin.

Question 253: Which of the following groups of proteins assist in the folding of other proteins?

A. Proteases
B. Proteosomes
C. Templates
D. Chaperones (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Chaperones** Protein folding is a critical process where a polypeptide chain assumes its functional 3D conformation. **Chaperones** (also known as Heat Shock Proteins, e.g., HSP70) are specialized proteins that facilitate this by preventing the aggregation of unfolded or partially folded polypeptide chains. They do not carry the information for folding themselves; rather, they provide a protected environment or stabilize hydrophobic regions to ensure the protein reaches its native state efficiently. **Analysis of Incorrect Options:** * **A. Proteases:** These are enzymes that catalyze **proteolysis** (the breakdown of proteins into peptides or amino acids) by peptide bond hydrolysis. They are involved in degradation, not folding. * **B. Proteasomes:** These are large multi-protein complexes responsible for the degradation of damaged or unneeded proteins tagged with **ubiquitin**. They act as the cell’s "garbage disposal" unit. * **C. Templates:** In biochemistry, templates usually refer to DNA or RNA strands used during replication or transcription. Protein folding is generally self-assembling (Anfinsen’s dogma) and does not require a physical template. **High-Yield Clinical Pearls for NEET-PG:** * **HSP70 & HSP60:** These are the most common chaperones. HSP70 binds early during translation, while HSP60 (Chaperonins) forms a cage-like structure for folding. * **Prion Diseases:** Result from the **misfolding** of normal PrPᶜ into the β-sheet-rich PrPˢᶜ, leading to neurodegeneration (e.g., Creutzfeldt-Jakob Disease). * **Ubiquitin-Proteasome Pathway:** Deficits in this pathway are linked to Parkinson’s disease (accumulation of α-synuclein in Lewy bodies). * **Cystic Fibrosis:** Often caused by a mutation (ΔF508) that leads to the misfolding and premature degradation of the CFTR protein.

Question 254: Albumin and globulin are classified as:

A. Conjugate proteins
B. Secondary proteins
C. Simple globular proteins (Correct Answer)
D. Derived proteins

Explanation: **Explanation:** Proteins are classified based on their chemical composition and solubility. **Albumin and globulin** are the classic examples of **Simple Globular Proteins**. 1. **Why Option C is Correct:** * **Simple Proteins:** These consist solely of amino acids and do not contain a non-protein (prosthetic) group. Upon hydrolysis, they yield only amino acids. * **Globular Shape:** They possess a spherical or oval shape due to the folding of polypeptide chains. Albumin is highly soluble in water, while globulin is soluble in dilute salt solutions. Both are critical for maintaining oncotic pressure (Albumin) and immune function (Globulins). 2. **Why Other Options are Incorrect:** * **A. Conjugate proteins:** These consist of a simple protein combined with a non-protein component (e.g., Hemoglobin contains heme, Glycoproteins contain carbohydrates). * **B. Secondary proteins:** This term usually refers to the "Secondary Structure" (alpha-helices/beta-sheets) rather than a classification category. If referring to "Secondary Derived Proteins," these are products of protein denaturation or initial hydrolysis (like metaproteins). * **D. Derived proteins:** These are degradation products of natural proteins produced by the action of heat, acids, or enzymes (e.g., proteoses, peptones, and peptides). **High-Yield Clinical Pearls for NEET-PG:** * **Albumin:** The most abundant plasma protein; synthesized in the liver. It is the primary determinant of **Plasma Oncotic Pressure**. * **A/G Ratio:** Normally **1.2 to 2:1**. A reversed A/G ratio (Globulin > Albumin) is a classic finding in **Multiple Myeloma** and **Chronic Liver Disease**. * **Salting Out:** Albumin requires full saturation with ammonium sulfate to precipitate, whereas globulin precipitates at half-saturation.

Question 255: Which protein is the first to be broken down for energy during prolonged starvation?

A. Skeletal muscle (Correct Answer)
B. Smooth muscle
C. Kidney
D. Liver

Explanation: **Explanation:** In the metabolic response to prolonged starvation, the body shifts from using exogenous glucose to endogenous stores. Once glycogen stores are depleted (within 24 hours), the body initiates **gluconeogenesis** to maintain blood glucose levels for the brain and RBCs. **Skeletal muscle** is the primary source of amino acids (specifically **Alanine and Glutamine**) for this process. It serves as the largest reservoir of protein in the body. Through the **Glucose-Alanine cycle**, skeletal muscle proteins are proteolyzed, and the resulting amino acids are transported to the liver to be converted into glucose. This breakdown is most intense in the early stages of starvation before the brain adapts to using ketone bodies, which eventually exerts a "protein-sparing effect." **Why other options are incorrect:** * **Smooth muscle:** These are vital for the structural integrity of the vasculature and gastrointestinal tract. The body preserves visceral and vascular smooth muscle to maintain basic physiological functions. * **Kidney:** While the kidney becomes a site of gluconeogenesis during late starvation, its structural proteins are not sacrificed for energy; doing so would lead to immediate organ failure. * **Liver:** The liver is the metabolic "factory" that processes nutrients. While it undergoes some size reduction, it does not serve as a primary sacrificial protein store; its role is to convert muscle-derived amino acids into glucose. **Clinical Pearls for NEET-PG:** * **The Glucose-Alanine Cycle (Cahill Cycle):** Transports amino groups from muscle to the liver. * **Protein-Sparing Effect:** Occurs after 3–5 days of starvation when the brain begins using **ketone bodies** (acetoacetate and β-hydroxybutyrate), reducing the demand for gluconeogenesis and slowing skeletal muscle breakdown. * **Negative Nitrogen Balance:** Starvation is a classic state of negative nitrogen balance due to excessive protein catabolism.

Question 256: Histone proteins are rich in which of the following amino acids?

A. Histidine and lysine
B. Lysine and Arginine (Correct Answer)
C. Arginine and Histidine
D. Histidine and Valine

Explanation: **Explanation:** Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. **1. Why Lysine and Arginine are correct:** The fundamental interaction between histones and DNA is based on **electrostatic attraction**. DNA is negatively charged due to its phosphate backbone. To bind tightly to DNA, histone proteins must carry a strong positive charge. **Lysine and Arginine** are basic amino acids that are positively charged at physiological pH. This allows histones to act as a "spool" around which the negatively charged DNA can wrap, facilitating efficient DNA condensation. **2. Analysis of Incorrect Options:** * **Option A & C (Histidine):** While Histidine is technically a basic amino acid, its pKa is close to physiological pH (~6.0). This means it is not consistently or strongly positively charged in the cellular environment compared to Lysine and Arginine. Therefore, it is not a primary constituent of histones. * **Option D (Valine):** Valine is a branched-chain non-polar (hydrophobic) amino acid. It lacks the necessary charge to interact with the DNA backbone. **High-Yield Clinical Pearls for NEET-PG:** * **Nucleosome Core:** Consists of an octamer of histone proteins: two each of **H2A, H2B, H3, and H4**. * **Linker Histone:** **H1** is the "linker" histone that resides outside the nucleosome core and helps stabilize the 30nm chromatin fiber. * **Epigenetics:** Histone tails undergo post-translational modifications (Acetylation, Methylation). **Acetylation** (by HATs) neutralizes the positive charge on Lysine, weakening the histone-DNA bond and leading to **Euchromatin** (transcriptionally active). * **Protamines:** In sperm, histones are replaced by protamines, which are even richer in Arginine for tighter DNA packing.

Question 257: Where is the heme group located within the hemoglobin molecule?

A. A hydrophobic pocket (Correct Answer)
B. A positive region
C. A negative region
D. A polar region

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The heme group is a non-polar, prosthetic group consisting of a protoporphyrin IX ring and a central ferrous iron ($Fe^{2+}$). In both hemoglobin and myoglobin, this heme group is nestled within a **hydrophobic pocket** formed by the folding of the polypeptide globin chains. The primary reason for this environment is to **prevent the oxidation of iron**. For hemoglobin to bind oxygen reversibly, the iron must remain in the ferrous state ($Fe^{2+}$). If heme were exposed to a polar or aqueous environment, the iron would easily oxidize to the ferric state ($Fe^{3+}$), forming **methemoglobin**, which cannot bind oxygen. The hydrophobic residues (like Valine and Phenylalanine) exclude water from the site, ensuring the iron remains reduced and functional. **2. Why the Other Options are Wrong:** * **B & C (Positive/Negative Regions):** Heme is largely non-polar. Placing it in a highly charged (ionic) region would be energetically unfavorable and would not provide the protective "shield" against water molecules. * **D (Polar Region):** A polar environment would facilitate the entry of water and the subsequent oxidation of $Fe^{2+}$ to $Fe^{3+}$, rendering the hemoglobin molecule non-functional for oxygen transport. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Proximal Histidine (F8):** Directly coordinates with the iron atom. * **The Distal Histidine (E7):** Does not touch the heme but helps stabilize the oxygen binding and prevents Carbon Monoxide (CO) from binding too tightly. * **Methemoglobinemia:** Occurs when the iron is oxidized to $Fe^{3+}$. This is treated with **Methylene Blue**. * **Cooperativity:** The movement of iron into the plane of the porphyrin ring upon oxygenation triggers the T-to-R state transition.

Question 258: Which of the following amino acids frequently induce bends within alpha helices?

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

Explanation: **Explanation:** The stability of an alpha helix depends on the specific R-groups of its constituent amino acids. **Glycine** is unique because its side chain is a single hydrogen atom, making it the smallest and most flexible amino acid. This high degree of conformational flexibility allows it to adopt a wide range of dihedral angles that are incompatible with the rigid, constrained geometry of an alpha helix. Consequently, Glycine acts as a **"helix breaker"** by inducing bends or kinks, often found at the beginning or end of helical segments. **Analysis of Options:** * **Glycine (Correct):** Its small size provides too much entropy/flexibility, destabilizing the organized hydrogen bonding required for a stable alpha helix. * **Threonine & Serine (Incorrect):** These are polar amino acids with hydroxyl groups. While they can sometimes destabilize helices if present in high density (due to bulkiness or competing hydrogen bonds), they do not characteristically "induce bends" like Glycine or Proline. * **Tyrosine (Incorrect):** This is a bulky, aromatic amino acid. While its size can cause steric hindrance, it is generally accommodated within helical structures better than Glycine. **High-Yield NEET-PG Pearls:** * **Proline vs. Glycine:** Both are "helix breakers." Proline induces bends because its rigid cyclic structure creates a **kink** and lacks the NH group for hydrogen bonding. Glycine induces bends because it is **too flexible**. * **Helix Stabilizers:** Alanine and Leucine are the strongest helix formers. * **Collagen Connection:** Glycine is essential in Collagen (Gly-X-Y) because its small size allows the three polypeptide chains to pack tightly into a triple helix.

Question 259: 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 process. 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 **Alpha helix** and **Beta pleated sheet** are the two most common periodic motifs of this level. In an alpha helix, the chain twists into a right-handed spiral, while beta sheets consist of extended strands connected laterally. **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 levels of folding but does not include spatial arrangements like helices. * **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). **High-Yield Clinical Pearls for NEET-PG:** * **Proline** is known as an "alpha-helix breaker" because its rigid structure interferes with 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 conformational change where normal alpha-helices are converted into **infectious beta-pleated sheets**, leading to protein aggregation and neurodegeneration.

Question 260: Which of the following is NOT involved in stabilizing the tertiary structure of a protein?

A. Hydrogen bond
B. Hydrophobic interaction
C. Peptide bond (Correct Answer)
D. Van der Waals forces

Explanation: ### Explanation The **tertiary structure** of a protein refers to its three-dimensional spatial arrangement, where a single polypeptide chain folds into a compact, functional shape. **Why "Peptide Bond" is the Correct Answer:** The **peptide bond** is a strong covalent bond that links amino acids together in a linear sequence. It is the fundamental force responsible for the **primary structure** of a protein. While it provides the backbone, it does not participate in the folding or stabilization of the tertiary structure. Tertiary structure is primarily stabilized by interactions between the **R-groups (side chains)** of amino acids, rather than the peptide backbone itself. **Analysis of Incorrect Options:** * **Hydrogen Bonds:** These occur between polar side chains (e.g., Serine, Threonine) and are vital for stabilizing the folded 3D conformation. * **Hydrophobic Interactions:** This is the **primary driving force** for protein folding. Non-polar side chains (e.g., Valine, Leucine) cluster in the interior of the protein to avoid water, stabilizing the core. * **Van der Waals Forces:** These are weak, short-range attractions between non-polar side chains that contribute to the tight packing of the protein’s interior. **High-Yield Clinical Pearls for NEET-PG:** * **Disulfide Bonds:** These are the only **covalent** bonds involved in stabilizing tertiary and quaternary structures (formed between two Cysteine residues). * **Denaturation:** Agents like heat or urea disrupt tertiary structures by breaking hydrogen and hydrophobic bonds, but they **do not** break peptide bonds (primary structure remains intact). * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in the correct folding of proteins into their tertiary structures.

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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