Protein Structure and Function Practice Questions

Q: Which amino acid is responsible for forming bends in the alpha-helix structure?

Glycine. **Explanation:** The correct answer is **Glycine (Option A)**. **Why Glycine is Correct:** The alpha-helix is a rigid, right-handed spiral stabilized by intrachain hydrogen bonding. **Glycine** is the smallest amino acid, with only a hydrogen atom as its R-group. This unique structural flexibility allows it to adopt a wide range of conformational angles that are incompatible with the rigid constraints of an alpha-helix. When glycine is incorporated into a helix, its high conformational entropy and lack of a bulky side chain often cause the helix to lose its stability, leading to the formation of **bends or kinks**. *Note:* While **Proline** is the most famous "helix breaker" because its cyclic structure physically prevents hydrogen bonding and creates a fixed bend, **Glycine** is the correct answer here as it is the primary amino acid responsible for providing the flexibility required for tight turns and bends in various protein secondary structures. **Why Other Options are Incorrect:** * **B. Lysine:** A positively charged amino acid that is generally well-tolerated in alpha-helices, though a sequence of many Lysines can cause electrostatic repulsion. * **C. Methionine:** One of the strongest helix-formers. It has a non-polar, unbranched side chain that fits perfectly within the helical geometry. * **D. Glutamine:** An uncharged polar amino acid that is a frequent and stable constituent of alpha-helices. **High-Yield NEET-PG Pearls:** * **Helix Breakers:** Proline (due to its rigid secondary amino group) and Glycine (due to its excessive flexibility). * **Best Helix Formers:** M-A-L-E-K (Methionine, Alanine, Leucine, Glutamate, Lysine). * **Collagen Connection:** Glycine is essential every third residue in collagen (Gly-X-Y) because its small size is the only one that can fit into the crowded center of the triple helix.

Q: Which of the following bonds is preserved during the denaturation of proteins?

Peptide bond. **Explanation:** **Denaturation** is the process by which a protein loses its native three-dimensional conformation (secondary, tertiary, and quaternary structures) due to external stress such as heat, extreme pH, or organic solvents. **Why the Peptide Bond is Preserved:** The peptide bond is a strong **covalent bond** that constitutes the **primary structure** of a protein. Denaturation involves the disruption of non-covalent interactions and disulfide bridges, but it does **not** involve the hydrolysis of the peptide backbone. Therefore, the primary sequence of amino acids remains intact. Only proteolytic enzymes (proteases) can break peptide bonds. **Why Other Options are Incorrect:** * **Hydrogen Bonds (A):** These are weak non-covalent interactions that stabilize alpha-helices and beta-pleated sheets (**secondary structure**). They are easily disrupted by heat or urea. * **Ionic Bonds (C):** Also known as salt bridges, these stabilize the **tertiary structure**. Changes in pH alter the ionization state of amino acid side chains, breaking these bonds. * **All of the above (D):** Since hydrogen and ionic bonds are lost during denaturation, this option is incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Renaturation:** If the denaturing agent is removed, some proteins (like Ribonuclease) can spontaneously refold, 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 aggregation during stress. * **Misfolding Diseases:** Prion diseases (Creutzfeldt-Jakob disease) and Alzheimer’s involve protein misfolding rather than simple denaturation, leading to the formation of insoluble amyloid plaques.

Q: Which of the following amino acids is in order of increasing polarity: Alanine, Valine, Glycine, Isoleucine?

Isoleucine < Valine < Alanine < Glycine. ### Explanation **Concept: Hydrophobicity and Side-Chain Length** The polarity of non-polar (hydrophobic) amino acids is determined by the size and nature of their R-groups (side chains). In aliphatic amino acids, the longer the hydrocarbon chain, the more **hydrophobic** (less polar) the amino acid becomes. Conversely, the smaller the R-group, the less it interferes with water solubility, making it relatively more polar. * **Isoleucine:** Contains a branched four-carbon aliphatic side chain. It is the most hydrophobic/least polar of the group. * **Valine:** Contains a three-carbon branched side chain. It is less hydrophobic than isoleucine but more so than alanine. * **Alanine:** Contains a simple methyl group (-CH₃). * **Glycine:** The simplest amino acid with only a Hydrogen atom as its R-group. Because it lacks a hydrocarbon chain, it is the least hydrophobic (most polar) among the non-polar amino acids. **Analysis of Options:** * **Option A (Correct):** Correctly ranks them from the longest hydrocarbon chain (Isoleucine) to the shortest/none (Glycine). * **Options B, C, and D:** These are incorrect because they fail to follow the inverse relationship between hydrocarbon chain length and polarity. For example, placing Glycine anywhere but the end of an "increasing polarity" list is incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine:** The only achiral amino acid (no asymmetric carbon). It is essential for the synthesis of Heme, Purines, Creatine, and Glutathione. * **Branched-Chain Amino Acids (BCAA):** Leucine, Isoleucine, and Valine. Their metabolism is impaired in **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. * **Hydropathy Index:** Isoleucine has the highest hydropathy index, making it frequently buried in the hydrophobic core of globular proteins.

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

Proteoglycan. **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.

Q: The Xanthoproteic reaction involves which of the following reagents?

Nitric acid. **Explanation:** The **Xanthoproteic reaction** is a qualitative test used to detect the presence of proteins containing **aromatic amino acids** (Phenylalanine, Tyrosine, and Tryptophan). **1. Why Nitric Acid is Correct:** The reaction involves the addition of **concentrated Nitric acid (HNO₃)** to the protein solution. The nitric acid causes the **nitration of the benzene ring** in aromatic amino acids, resulting in the formation of yellow-colored nitro-derivatives (e.g., nitrotyrosine). Upon the addition of an alkali (like NaOH or NH₄OH), the color deepens to orange due to the formation of a salt of the nitro-compound. **2. Why Other Options are Incorrect:** * **Hydrochloric acid (HCl) & Sulfuric acid (H₂SO₄):** While these are strong mineral acids, they do not possess the specific nitrating properties required to react with the aromatic rings to produce the characteristic yellow color. They may denature or hydrolyze proteins but do not yield a positive Xanthoproteic result. * **Carbolic acid (Phenol):** This is an organic acid. While phenol itself can undergo nitration, it is not a reagent used to test for proteins; rather, it is a compound that would itself react with nitric acid. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Yellow Stain" Fact:** This reaction explains why skin turns yellow when it comes into contact with concentrated nitric acid (the acid reacts with the keratin proteins in the skin). * **Specificity:** Among the aromatic amino acids, **Tyrosine and Tryptophan** give a strongly positive result. Phenylalanine gives a weak or negative result because its benzene ring is highly stable and difficult to nitrate under standard test conditions. * **Key Reagents to Remember:** Concentrated HNO₃ + Heat, followed by an alkali (NaOH) for color intensification.

Q: What is the secondary structure of prion proteins in prion diseases like Creutzfeldt-Jakob disease (CJD)?

Beta sheets. **Explanation:** Prion diseases, such as Creutzfeldt-Jakob disease (CJD), are caused by the conformational change of a normal cellular protein ($PrP^C$) into an abnormal, infectious isoform ($PrP^{Sc}$) [1]. 1. **Why Beta sheets are correct:** The fundamental pathogenesis of prion disease involves a post-translational modification where the **alpha-helical** content of the normal protein is converted into **beta-pleated sheets** [1]. This structural shift makes the protein ($PrP^{Sc}$) resistant to proteolysis (protease-resistant), insoluble, and capable of inducing similar conformational changes in neighboring normal proteins, leading to neurotoxic aggregation [1]. 2. **Why the other options are incorrect:** * **Alpha helix:** This is the predominant secondary structure of the **normal** prion protein ($PrP^C$) [1]. In the disease state, these helices are replaced by beta sheets. * **Beta bend and Beta turn:** These are specific types of secondary structures that involve a change in direction of the polypeptide chain (usually involving 4 amino acids). While they exist in many proteins, they are not the defining structural hallmark of the pathogenic prion transition. **High-Yield Clinical Pearls for NEET-PG:** * **Histology:** Characterized by **spongiform encephalopathy** (vacuolation of neurons and neuropil) without inflammatory infiltrate. * **Diagnosis:** Detection of **14-3-3 protein** in CSF is a sensitive marker for CJD. * **Genetics:** Most cases are sporadic, but familial forms are linked to mutations in the **PRNP gene** on chromosome 20 [1]. * **Staining:** Prion aggregates can be visualized with Congo Red (showing apple-green birefringence) as they behave like amyloid, though they are distinct from classic Alzheimer’s amyloid. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, p. 1284.

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

Tryptophan. **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.

Q: Which of the following glycoproteins has both lubricant and protective action?

Mucin. **Explanation:** **Correct Option: A (Mucin)** Mucins are high-molecular-weight, heavily glycosylated proteins (glycoproteins) produced by epithelial tissues. They are the primary constituents of mucus. * **Lubricant Action:** Due to their high carbohydrate content, they trap water, forming a viscous gel that reduces friction during swallowing, digestion, and movement of mucosal surfaces. * **Protective Action:** They form a physical barrier against pathogens, toxins, and mechanical injury. In the stomach, mucin protects the gastric mucosa from acid-pepsin digestion. **Incorrect Options:** * **B. Immunoglobulin:** These are glycoproteins that function as antibodies (immune defense). While they provide protection by neutralizing pathogens, they do not possess lubricating properties. * **C. Collagen:** This is a structural fibrous protein (not primarily a glycoprotein in this functional context) that provides tensile strength to connective tissues like skin, bone, and cartilage. * **D. Albumin:** This is a simple protein (not a glycoprotein) synthesized by the liver. Its primary roles include maintaining oncotic pressure and transporting hormones, drugs, and bilirubin. **High-Yield Clinical Pearls for NEET-PG:** * **Mucin in Pathology:** Overexpression of mucins (like MUC1) is often seen in adenocarcinomas (e.g., breast, pancreatic, and colon cancer) and is used as a tumor marker. * **Vitamin A Connection:** Vitamin A deficiency leads to "Squamous Metaplasia," where mucus-secreting epithelium is replaced by keratinized epithelium, leading to a loss of these lubricant/protective functions (e.g., Xerophthalmia). * **Steatocit:** Mucin is secreted by **Goblet cells** in the intestinal and respiratory tracts.

Question 1

Which amino acid is responsible for forming bends in the alpha-helix structure?

Accepted Answer

Glycine

Answer

Lysine

Answer

Methionine

Answer

Glutamine

Question 2

Which of the following bonds is preserved during the denaturation of proteins?

Accepted Answer

Peptide bond

Answer

Hydrogen bond

Answer

Ionic bond

Answer

All of the above

Question 3

Which of the following amino acids is in order of increasing polarity: Alanine, Valine, Glycine, Isoleucine?

Accepted Answer

Isoleucine < Valine < Alanine < Glycine

Answer

Valine < Glycine < Alanine < Isoleucine

Answer

Alanine < Isoleucine < Glycine < Valine

Answer

Isoleucine < Alanine < Valine < Glycine

Question 4

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

Accepted Answer

Proteoglycan

Answer

Glycoprotein

Answer

Glycosaminoglycan

Answer

Glycocalyx

Question 5

The Xanthoproteic reaction involves which of the following reagents?

Accepted Answer

Nitric acid

Answer

Hydrochloric acid

Answer

Sulfuric acid

Answer

Carbolic acid

Question 6

What is the secondary structure of prion proteins in prion diseases like Creutzfeldt-Jakob disease (CJD)?

Accepted Answer

Beta sheets

Answer

Beta bend

Answer

Beta turn

Answer

Alpha helix

Question 7

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

Accepted Answer

Interruption of structure

Answer

Change in optical activity

Answer

Increased stereoisomerism

Answer

Increased stability

Question 8

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

Accepted Answer

Tryptophan

Answer

Leucine

Answer

Arginine

Answer

Proline

Question 9

Which of the following glycoproteins has both lubricant and protective action?

Accepted Answer

Mucin

Answer

Immunoglobulin

Answer

Collagen

Answer

Albumin

Question 10

The three-dimensional shape of a protein is maintained mainly by which of the following interactions?

Accepted Answer

Strong covalent interactions

Answer

Interactions with other proteins

Answer

Multiple weak interactions

Answer

Interactions with prosthetic groups

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

On this page

Practice by Chapter

Want unlimited practice?