Protein Structure and Function Practice Questions

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

Type IV collagen. **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**.

Q: Which of the following is an octapeptide?

Angiotensin II. **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.

Q: Which of the following is the 'Shoest peptide'?

Angiotensin-III. **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.

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

Cysteine. **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.

Q: Which enzyme carries out the reverse folding of proteins?

Chaperone. ### 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.

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

None of the above. **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.

Q: Chaperones are used for what primary cellular process?

Protein folding. **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.

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

An amino acid is neutral at its isoelectric pH.. ### 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.

Question 1

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

Accepted Answer

Type IV collagen

Answer

Type II collagen

Answer

Type VI collagen

Answer

Type IX collagen

Question 2

Which of the following is an octapeptide?

Accepted Answer

Angiotensin II

Answer

Glutathione

Answer

Angiotensin I

Answer

Oxytocin

Question 3

Which of the following is the 'Shoest peptide'?

Accepted Answer

Angiotensin-III

Answer

Vasopressin

Answer

Angiotensin-II

Answer

Angiotensin-I

Question 4

Disulfide bonds are formed by pairs of which amino acid?

Accepted Answer

Cysteine

Answer

Methionine

Answer

Homocysteine

Answer

Serine

Question 5

Which enzyme carries out the reverse folding of proteins?

Accepted Answer

Chaperone

Answer

Valine

Answer

Threonine

Answer

Aspartate

Question 6

Signal peptides are synthesized in which cellular component?

Accepted Answer

Membrane-bound ribosomes

Answer

Endoplasmic Reticulum (ER)

Answer

Free ribosomes

Answer

Golgi apparatus

Question 7

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

Accepted Answer

None of the above

Answer

Calmodulin

Answer

Troponin

Answer

Calbindin

Question 8

What does the tertiary structure of a protein describe?

Accepted Answer

The overall three-dimensional shape and folding of a single polypeptide chain

Answer

The sequence of amino acids

Answer

The location of disulfide bonds within the protein

Answer

The formation of loop regions in the protein structure

Question 9

Chaperones are used for what primary cellular process?

Accepted Answer

Protein folding

Answer

Protein targeting

Answer

Protein synthesis

Answer

Protein modification

Question 10

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

Accepted Answer

An amino acid is neutral at its isoelectric pH.

Answer

The carboxyl group can form a positive charge.

Answer

The amino group can form a negative charge.

Answer

All of the above.

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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