Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 161: All of the following types of collagen are present in cartilage except one. Which is the exception?

A. Type II collagen
B. Type IV collagen (Correct Answer)
C. Type VI collagen
D. Type IX collagen

Explanation: **Explanation:** The correct answer is **Type IV collagen**. **1. Why Type IV is the exception:** Collagen types are broadly categorized into fibrillar and non-fibrillar forms. **Type IV collagen** is a non-fibrillar, network-forming collagen that is a primary constituent of the **basal lamina (basement membrane)**. It is not found in the matrix of cartilage. Its primary role is to provide a structural scaffold for epithelial and endothelial cells, notably in the renal glomerulus and the lens of the eye. **2. Analysis of Incorrect Options:** * **Type II Collagen:** This is the hallmark of cartilage. It constitutes over 50% of all protein in hyaline cartilage and provides tensile strength to the tissue. * **Type IX Collagen:** This is a **FACIT** (Fibril-Associated Collagen with Interrupted Triple helices). It binds to the surface of Type II fibrils, acting as a "bridge" to stabilize the proteoglycan matrix. * **Type VI Collagen:** This type is found in the **pericellular matrix** (chondron) immediately surrounding individual chondrocytes, helping them attach to the wider matrix. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Fibrillar Collagens:** Types **I, II, and III** (1, 2, 3 are "Strong like a Tree"). * **Type I:** Bone, Skin, Tendon (Most abundant). * **Type II:** Cartilage and Vitreous humor (**"Car-two-lage"**). * **Type III:** Reticular fibers, Blood vessels (Granulation tissue). * **Alport Syndrome:** Caused by mutations in **Type IV collagen**, leading to nephritis and sensorineural deafness. * **Goodpasture Syndrome:** Antibodies against the α3 chain of **Type IV collagen**.

Question 162: Which of the following is an octapeptide?

A. Glutathione
B. Angiotensin I
C. Angiotensin II (Correct Answer)
D. Oxytocin

Explanation: **Explanation:** The number of amino acids in a peptide chain determines its classification. **Angiotensin II** is an **octapeptide**, meaning it consists of exactly **8 amino acids**. It is a potent vasoconstrictor produced from Angiotensin I by the action of Angiotensin-Converting Enzyme (ACE) in the lungs. **Analysis of Options:** * **Glutathione (Option A):** It is a **tripeptide** (γ-glutamyl-cysteinyl-glycine). It is the most abundant intracellular antioxidant and plays a crucial role in neutralizing free radicals and maintaining vitamins C and E in their reduced forms. * **Angiotensin I (Option B):** It is a **decapeptide** (10 amino acids). It is physiologically inactive and is formed when Renin acts on Angiotensinogen. * **Oxytocin (Option D):** It is a **nonapeptide** (9 amino acids). Produced in the hypothalamus and stored in the posterior pituitary, it is responsible for uterine contractions and milk ejection. (Note: Vasopressin/ADH is also a nonapeptide). **High-Yield Clinical Pearls for NEET-PG:** * **TRH (Thyrotropin-Releasing Hormone):** A tripeptide. * **Enkephalins:** Pentapeptides (5 amino acids). * **Bradykinin:** A nonapeptide (9 amino acids) that causes vasodilation and pain. * **Glucagon:** A polypeptide consisting of 29 amino acids. * **Insulin:** A 51-amino acid polypeptide with two chains (A chain: 21, B chain: 30) linked by disulfide bonds. * **ACE Inhibitors:** These drugs prevent the conversion of the decapeptide (Angiotensin I) to the octapeptide (Angiotensin II), thereby lowering blood pressure.

Question 163: Which of the following is the 'Shoest peptide'?

A. Angiotensin-III (Correct Answer)
B. Vasopressin
C. Angiotensin-II
D. Angiotensin-I

Explanation: **Explanation:** The term **'Shortest peptide'** (often colloquially referred to or misspelled in some question banks as 'Shoest peptide') refers to the peptide with the fewest amino acid residues among the common components of the Renin-Angiotensin System (RAS). **1. Why Angiotensin-III is correct:** Angiotensin-III is a **heptapeptide** (contains 7 amino acids). It is formed by the action of aminopeptidase on Angiotensin-II. While it has lower pressor activity than Angiotensin-II, it is equally potent in stimulating aldosterone secretion from the adrenal cortex. **2. Why the other options are incorrect:** * **Angiotensin-I:** This is a **decapeptide** (10 amino acids). It is the inactive precursor formed when Renin acts on Angiotensinogen. * **Angiotensin-II:** This is an **octapeptide** (8 amino acids). It is the primary active vasoconstrictor of the RAS, formed from Angiotensin-I by the Angiotensin-Converting Enzyme (ACE). * **Vasopressin (ADH):** This is a **nonapeptide** (9 amino acids). While it is a small peptide, it is longer than Angiotensin-III. **High-Yield Clinical Pearls for NEET-PG:** * **Angiotensinogen:** A large glycoprotein (α2-globulin) synthesized in the liver. * **Sequence of Length:** Angiotensinogen (Large) → Angiotensin I (10) → Angiotensin II (8) → Angiotensin III (7). * **ACE (Kininase II):** A dipeptidyl carboxypeptidase that converts the decapeptide (I) to the octapeptide (II) by removing two amino acids from the C-terminal. * **Site of Action:** Angiotensin-III acts primarily via **AT1 receptors**, similar to Angiotensin-II.

Question 164: Disulfide bonds are formed by pairs of which amino acid?

A. Methionine
B. Homocysteine
C. Serine
D. Cysteine (Correct Answer)

Explanation: **Explanation:** **1. Why Cysteine is Correct:** Disulfide bonds (S-S bonds) are covalent linkages formed by the oxidation of the **sulfhydryl (-SH) groups** of two **Cysteine** residues. When two cysteine molecules are linked via a disulfide bridge, the resulting dimer is known as **Cystine**. These bonds are crucial for stabilizing the tertiary and quaternary structures of proteins (e.g., Insulin, Immunoglobulins). Because they are covalent, they are much stronger than hydrogen bonds or van der Waals forces. **2. Why Other Options are Incorrect:** * **Methionine (A):** Although Methionine is a sulfur-containing amino acid, its sulfur is part of a **thioether group** (C-S-C). It lacks a free sulfhydryl group and therefore cannot form disulfide bonds. * **Homocysteine (B):** While it contains a thiol group, homocysteine is an intermediate in methionine metabolism and is **not a proteinogenic amino acid** (it is not used to build proteins in the body). * **Serine (C):** Serine is structurally similar to cysteine but contains a **hydroxyl group (-OH)** instead of a sulfhydryl group. It forms hydrogen bonds but not disulfide bridges. **3. High-Yield Clinical Pearls for NEET-PG:** * **Insulin Structure:** Insulin consists of two polypeptide chains (A and B) held together by **two interchain** and **one intrachain** disulfide bonds. * **Location of Formation:** Disulfide bonds are typically formed in the **Endoplasmic Reticulum (ER)**, which provides an oxidizing environment, unlike the reducing environment of the cytosol. * **Hair Styling:** Permanent hair waving involves the chemical reduction (breaking) and subsequent oxidation (re-forming) of disulfide bonds in **keratin**. * **Cystinuria:** A clinical condition caused by a defect in the renal transport of **COLA** (Cystine, Ornithine, Lysine, Arginine), leading to hexagonal cystine stones in the urine.

Question 165: Signal peptides are synthesized in which cellular component?

A. Endoplasmic Reticulum (ER)
B. Free ribosomes
C. Membrane-bound ribosomes (Correct Answer)
D. Golgi apparatus

Explanation: **Explanation:** The synthesis of proteins destined for secretion, integration into the plasma membrane, or lysosomal enzymes begins on **free ribosomes** in the cytosol. However, as soon as the **Signal Peptide** (a sequence of 15–30 hydrophobic amino acids at the N-terminus) emerges from the ribosome, it is recognized by the **Signal Recognition Particle (SRP)**. 1. **Why Option C is Correct:** The SRP binds to the signal peptide and temporarily halts translation. This entire complex (ribosome + mRNA + nascent peptide + SRP) then docks onto the **SRP receptor** on the Rough Endoplasmic Reticulum (RER) membrane. Once docked, the ribosome becomes a **membrane-bound ribosome**, and protein synthesis resumes, co-translationally pushing the protein into the ER lumen. Therefore, while synthesis *starts* in the cytosol, the signal peptide is primarily synthesized and processed via membrane-bound ribosomes. 2. **Why Other Options are Incorrect:** * **A. Endoplasmic Reticulum:** The ER is the *destination* or site of entry, but the actual synthesis (translation) is performed by the ribosomes attached to it. * **B. Free Ribosomes:** These synthesize proteins that remain in the cytosol, or go to the nucleus, mitochondria, or peroxisomes. They do not typically complete the synthesis of signal-peptide-bearing proteins. * **D. Golgi Apparatus:** This organelle is involved in post-translational modification (e.g., O-linked glycosylation) and sorting, not protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease:** Caused by a deficiency in *phosphotransferase*, leading to a failure to tag enzymes with **Mannose-6-Phosphate**. Consequently, enzymes are secreted extracellularly rather than being directed to lysosomes. * **Signal Hypothesis:** Proposed by Günter Blobel (Nobel Prize), it explains how proteins are targeted to specific cellular destinations. * **Chaperones:** Proteins like **BiP** (Binding immunoglobulin Protein) help in the correct folding of proteins within the ER lumen.

Question 166: Which enzyme carries out the reverse folding of proteins?

A. Valine
B. Threonine
C. Chaperone (Correct Answer)
D. Aspartate

Explanation: ### Explanation **Correct Option: C. Chaperone** Protein folding is a complex 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 process. Their primary functions include: * **Preventing Misfolding:** They bind to exposed hydrophobic regions of nascent polypeptides to prevent inappropriate aggregation. * **Reverse Folding/Refolding:** They can unfold misfolded proteins and provide a protected environment (like the "Anfinsen cage" in Chaperonins) to allow them to refold into their correct native state. * **Stabilization:** They stabilize proteins during cellular stress (like high temperatures). **Why Incorrect Options are Wrong:** * **A, B, and D (Valine, Threonine, Aspartate):** These are individual **amino acids**. While they are the building blocks of proteins, they do not possess enzymatic or catalytic activity to facilitate the folding or refolding of other protein chains. Valine is non-polar, Threonine is polar uncharged, and Aspartate is acidic. **High-Yield Clinical Pearls for NEET-PG:** * **Prion Diseases:** Result from the accumulation of misfolded proteins (PrPSc) which are rich in $\beta$-sheets and resistant to proteolysis. * **Alzheimer’s Disease:** Characterized by the deposition of amyloid-beta plaques due to protein misfolding. * **Cystic Fibrosis:** Often caused by a mutation ($\Delta$F508) that leads to the misfolding and subsequent degradation of the CFTR protein in the endoplasmic reticulum. * **HSP70 & HSP60:** These are the most common families of chaperones; HSP60 (Chaperonins) specifically forms a barrel-shaped structure for protein folding.

Question 167: All of the following are examples of calcium binding proteins, EXCEPT?

A. Calmodulin
B. Troponin
C. Calbindin
D. None of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **None of the above** because all three listed options (Calmodulin, Troponin, and Calbindin) are well-characterized **calcium-binding proteins**. Calcium acts as a vital second messenger in the body, but to exert its effects, it must bind to specific sensor proteins that undergo conformational changes. 1. **Calmodulin (Option A):** This is the most ubiquitous calcium-binding protein found in almost all eukaryotic cells. It contains four **EF-hand motifs** (a common structural domain for calcium binding). Once bound to $Ca^{2+}$, it activates various enzymes like Phosphodiesterase and Myosin Light Chain Kinase (MLCK). 2. **Troponin (Option B):** Specifically, **Troponin C** is the subunit responsible for binding calcium in cardiac and skeletal muscle. This binding shifts the tropomyosin complex, exposing myosin-binding sites on actin filaments to initiate muscle contraction. 3. **Calbindin (Option C):** This protein is primarily found in the intestines and kidneys. Its synthesis is dependent on **Vitamin D**. It facilitates the transport of calcium across epithelial cells, ensuring efficient absorption and reabsorption. **High-Yield Clinical Pearls for NEET-PG:** * **EF-Hand Motif:** This is the signature "helix-loop-helix" structural domain found in Calmodulin, Troponin C, and Parvalbumin. * **Vitamin D Link:** Calbindin is a key marker of Vitamin D action; its deficiency leads to impaired calcium absorption. * **Annexins:** Another group of calcium-binding proteins that bind to phospholipids in a calcium-dependent manner. * **C-reactive protein (CRP):** Though an acute-phase reactant, it is also technically a calcium-binding protein.

Question 168: What does the tertiary structure of a protein describe?

A. The sequence of amino acids
B. The location of disulfide bonds within the protein
C. The formation of loop regions in the protein structure
D. The overall three-dimensional shape and folding of a single polypeptide chain (Correct Answer)

Explanation: **Explanation:** The **tertiary structure** of a protein refers to the comprehensive **three-dimensional arrangement** of all atoms in a single polypeptide chain. It is formed when the secondary structural elements (alpha-helices and beta-pleated sheets) fold into a compact, functional shape. This folding is stabilized by various interactions between the R-groups (side chains) of amino acids, including hydrophobic interactions, hydrogen bonds, ionic bonds (salt bridges), and disulfide bridges. **Analysis of Options:** * **Option A:** Describes the **Primary structure**, which is the linear sequence of amino acids linked by peptide bonds. * **Option B:** While disulfide bonds stabilize the tertiary structure, they are a *component* of it, not the definition. The tertiary structure encompasses the entire spatial arrangement, not just one type of bond. * **Option C:** Loop regions and turns are elements of **Secondary structure** (or supersecondary motifs) that connect more organized patterns like helices and sheets. **High-Yield NEET-PG Pearls:** 1. **Driving Force:** The primary driving force for tertiary folding is the **hydrophobic effect**, where non-polar side chains are buried in the protein interior to avoid water. 2. **Denaturation:** When a protein is denatured (by heat or pH), the tertiary structure is lost, but the primary structure (peptide bonds) remains intact. 3. **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in the correct folding of polypeptide chains into their functional tertiary states. 4. **Clinical Relevance:** Misfolding of proteins can lead to proteopathies, such as **Amyloidosis** or **Alzheimer’s disease**, where proteins aggregate into insoluble fibrils.

Question 169: Chaperones are used for what primary cellular process?

A. Protein folding (Correct Answer)
B. Protein targeting
C. Protein synthesis
D. Protein modification

Explanation: **Explanation:** **1. Why Protein Folding is Correct:** Chaperones (also known as **Molecular Chaperones**) are specialized proteins that assist in the correct non-covalent assembly and folding of newly synthesized polypeptide chains. They prevent the "misfolding" of proteins and inhibit the formation of non-functional protein aggregates. Many chaperones are **Heat Shock Proteins (HSPs)**, such as HSP70, which are upregulated during cellular stress to stabilize denatured proteins and facilitate their refolding. **2. Why Other Options are Incorrect:** * **Protein Targeting (B):** This refers to the transport of proteins to specific organelles (e.g., mitochondria, nucleus). While some chaperones assist in keeping proteins unfolded for translocation, the *primary* process of targeting is governed by **Signal Sequences** and **Signal Recognition Particles (SRP)**. * **Protein Synthesis (C):** This is the process of translation occurring on ribosomes. Chaperones act *after* or *during* the synthesis of the polypeptide chain, but they do not catalyze the formation of peptide bonds. * **Protein Modification (D):** This refers to post-translational modifications (PTM) like glycosylation, phosphorylation, or hydroxylation. These are enzymatic processes (e.g., by kinases or transferases) distinct from the folding activity of chaperones. **Clinical Pearls for NEET-PG:** * **Protein Misfolding Diseases:** Defective chaperone activity or protein aggregation leads to "Conformational Diseases" such as **Alzheimer’s** (Amyloid-β), **Parkinson’s** (α-synuclein), and **Prion diseases**. * **HSP60 & HSP70:** These are the most common chaperones. HSP60 forms a barrel-shaped structure (Chaperonin) that provides a "safe cage" for folding. * **ATP Dependency:** Most chaperone-mediated folding is an **energy-dependent process** requiring ATP hydrolysis.

Question 170: Which of the following is true about the charge on an amino acid?

A. The carboxyl group can form a positive charge.
B. The amino group can form a negative charge.
C. An amino acid is neutral at its isoelectric pH. (Correct Answer)
D. All of the above.

Explanation: ### Explanation **1. Why Option C is Correct:** The **isoelectric point (pI)** is the specific pH at which an amino acid exists as a **zwitterion** (a dipolar ion). In this state, the molecule carries both a positive charge (on the amino group, $-NH_3^+$) and a negative charge (on the carboxyl group, $-COO^-$). These charges cancel each other out, resulting in a **net electrical charge of zero**. Because it is electrically neutral, the amino acid does not migrate toward either the anode or the cathode in an electric field. **2. Why Other Options are Incorrect:** * **Option A:** The carboxyl group ($-COOH$) is acidic. It can either be neutral (protonated) or carry a **negative charge** (deprotonated as $-COO^-$). It cannot form a positive charge. * **Option B:** The amino group ($-NH_2$) is basic. It can either be neutral or carry a **positive charge** (protonated as $-NH_3^+$). Under physiological conditions, it does not form a negative charge. * **Option D:** Since A and B are chemically incorrect, "All of the above" is invalid. **3. High-Yield Clinical Pearls for NEET-PG:** * **Solubility:** Amino acids have **minimum solubility** at their pI because the lack of a net charge decreases the electrostatic interaction with water molecules. * **Electrophoresis:** This principle is used to separate proteins. If $pH > pI$, the protein becomes negatively charged (anion) and moves toward the **Anode**. If $pH < pI$, it becomes positively charged (cation) and moves toward the **Cathode**. * **Calculation:** For simple amino acids, $pI = (pK_1 + pK_2) / 2$. * **Buffering:** Amino acids and proteins act as the best buffers at a pH near their $pK_a$ values, not necessarily at their pI.

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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