Protein Structure and Function Practice Questions

Q: An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces which of the following amino acids within the alpha helical structure?

Glycine. **Explanation:** The stability of an **alpha helix** depends on specific bond angles ($\phi$ and $\psi$) and the ability of the polypeptide chain to maintain a rigid, coiled conformation. **Glycine** is the most potent "helix breaker" among the options provided. **Why Glycine is the Correct Answer:** Glycine is the smallest amino acid, with only a hydrogen atom as its R-group. This lack of a bulky side chain grants it **extraordinary conformational flexibility**. In an alpha helix, this high entropy allows the peptide backbone to rotate too freely, preventing the rigid coiling required for helical stability. Consequently, Glycine tends to disrupt the helix and is more commonly found in beta-turns or loops. (Note: **Proline** is the other major helix breaker because its rigid ring structure creates a "kink" and lacks the NH group required for hydrogen bonding). **Analysis of Incorrect Options:** * **A. Alanine:** This is the strongest **helix stabilizer**. Its small, uncharged side chain fits perfectly into the helical structure without steric hindrance. * **B. Aspartic acid:** While charged amino acids can destabilize a helix if clustered (due to electrostatic repulsion), they do not inherently disrupt the structure as fundamentally as Glycine. * **C. Tyrosine:** Though bulky, Tyrosine can be accommodated within an alpha helix, provided it is not part of a dense cluster of large aromatic residues. **High-Yield Clinical Pearls for NEET-PG:** * **Helix Breakers:** Proline (due to rigidity/kinks) and Glycine (due to excessive flexibility). * **Helix Stabilizer:** Alanine (highest helical propensity). * **Collagen Structure:** Glycine is essential for the **collagen triple helix** (Gly-X-Y) because only its small size can fit into the crowded central core of the triple helix. Do not confuse the *alpha helix* (disrupted by Glycine) with the *collagen helix* (requires Glycine).

Q: Where is the glycosyl transferase enzyme responsible for O-glycosylation located?

Golgi apparatus. ### Explanation **1. Why the Golgi Apparatus is Correct:** Post-translational modification of proteins via glycosylation occurs in two distinct patterns: N-linked and O-linked. **O-glycosylation** (the attachment of glycans to the hydroxyl group of **Serine or Threonine** residues) occurs **exclusively in the Golgi apparatus**. Glycosyl transferases in the Golgi cisternae sequentially add sugar residues to the protein backbone. This process is vital for the synthesis of mucins, proteoglycans, and blood group antigens. **2. Why the Other Options are Incorrect:** * **Rough Endoplasmic Reticulum (RER):** The RER is the site for the **initiation of N-glycosylation** (attachment to Asparagine). While the protein backbone is synthesized here, O-glycosylation does not begin until the protein reaches the Golgi. * **Smooth Endoplasmic Reticulum (SER):** The SER is primarily involved in lipid synthesis, steroidogenesis, and detoxification (Cytochrome P450 system), not the glycosylation of secretory proteins. * **Lysosome:** Lysosomes are the site of macromolecule **degradation**, not synthesis. They contain hydrolytic enzymes (like glycosidases) that break down glycoproteins rather than glycosyl transferases that build them. **3. Clinical Pearls & High-Yield Facts:** * **N-glycosylation:** Starts in the **RER** and is completed in the Golgi. It involves a **Dolichol phosphate** intermediate. * **I-Cell Disease:** A high-yield pathology where a defect in phosphotransferase (in the Golgi) fails to phosphorylate mannose residues. This prevents proteins from being targeted to lysosomes, leading to their secretion and subsequent cellular inclusion bodies. * **Amino Acids involved:** * **N-linked:** Asparagine. * **O-linked:** Serine, Threonine (and occasionally Hydroxylysine in collagen).

Q: Human chorionic gonadotropin shares structural similarity with all except?

GH. ### Explanation The correct answer is **GH (Growth Hormone)**. **1. Underlying Medical Concept:** Human Chorionic Gonadotropin (hCG) belongs to the **glycoprotein hormone family**, which also includes Luteinizing Hormone (LH), Follicle-Stimulating Hormone (FSH), and Thyroid-Stimulating Hormone (TSH). All these hormones are heterodimers consisting of two subunits: * **Alpha ($\alpha$) subunit:** Identical in all four hormones (encoded by the same gene). * **Beta ($\beta$) subunit:** Unique to each hormone, providing biological and receptor specificity. **hCG and LH** share the highest degree of structural similarity (approximately 80% of their amino acid sequence in the $\beta$-subunit is identical). This explains why hCG can bind to the LH receptor to maintain the corpus luteum during early pregnancy. **2. Why the other options are incorrect:** * **FSH and TSH:** As members of the glycoprotein family, they share the exact same $\alpha$-subunit as hCG. Therefore, they possess significant structural similarity. * **GH (Growth Hormone):** GH is a **single-chain polypeptide** hormone (not a glycoprotein) and belongs to the Somatotropin family (along with Prolactin and Human Placental Lactogen). It does not share the $\alpha/\beta$ dimer structure, making it structurally distinct from hCG. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cross-reactivity:** Because hCG and TSH share the $\alpha$-subunit, very high levels of hCG (as seen in Hydatidiform mole or Choriocarcinoma) can weakly stimulate TSH receptors, leading to **gestational hyperthyroidism**. * **Biological Half-life:** hCG has a much longer half-life (~24 hours) compared to LH (~20 minutes) due to a highly glycosylated C-terminal tail on its $\beta$-subunit. * **Pregnancy Tests:** Immunological pregnancy tests specifically detect the **$\beta$-subunit of hCG** to avoid cross-reactivity with LH, FSH, or TSH.

Q: Which amino acid is active at neutral pH?

Histidine. **Explanation:** The activity of an amino acid at a specific pH is determined by its **pKa value**, which is the pH at which the side chain is 50% ionized and 50% unionized. **1. Why Histidine is correct:** Histidine is the only amino acid with an imidazole side chain that has a **pKa of approximately 6.0**. Because this value is close to the physiological pH (7.4), histidine can easily function as both a proton donor and a proton acceptor (acid-base catalysis) at neutral pH. This unique property makes it a critical component of the **active sites of many enzymes** (e.g., Carbonic Anhydrase, Chymotrypsin) and allows it to act as an effective **intracellular buffer** (e.g., in Hemoglobin). **2. Why the other options are incorrect:** * **Leucine:** A non-polar, branched-chain amino acid. Its side chain is hydrophobic and does not participate in acid-base chemistry. * **Glycine:** The simplest amino acid with only a hydrogen atom as its R-group. It is chemically inert at neutral pH and is primarily valued for providing flexibility in protein structures (e.g., Collagen). * **Lysine:** A basic amino acid with a high pKa (~10.5). At neutral pH (7.4), it remains almost entirely in its protonated (positively charged) state and cannot readily exchange protons. **Clinical Pearls for NEET-PG:** * **Buffering Capacity:** Histidine is the most important amino acid for the buffering capacity of **Hemoglobin** due to its prevalence and pKa. * **FIGLU Test:** Formiminoglutamic acid (FIGLU) is an intermediate of Histidine metabolism; its excretion in urine is a clinical marker for **Folic Acid deficiency**. * **Precursor:** Histidine undergoes decarboxylation to form **Histamine**, a key mediator of allergic reactions and gastric acid secretion.

Q: Which amino acid has a basic side chain?

Histidine. **Explanation:** Amino acids are categorized based on the chemical properties of their side chains (R-groups). **Histidine** is classified as a **basic amino acid** because its imidazole side chain contains nitrogen atoms that can accept a proton, giving it a positive charge under certain physiological conditions. **Analysis of Options:** * **Histidine (Correct):** Along with **Lysine** and **Arginine**, Histidine belongs to the basic group. It is unique because its pKa is close to physiological pH (~6.0), allowing it to function as an effective buffer in proteins like hemoglobin. * **Methionine (Incorrect):** This is a **sulfur-containing** amino acid. It is non-polar and hydrophobic. * **Leucine (Incorrect):** This is a branched-chain amino acid (BCAA) with a **non-polar, aliphatic** side chain. * **Aspartic acid (Incorrect):** This is an **acidic amino acid** (along with Glutamic acid) because its side chain contains a carboxyl group (-COOH) that donates a proton, becoming negatively charged at physiological pH. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for Basic Amino Acids:** "History Loved Argentina" (**His**tidine, **Lys**ine, **Arg**inine). 2. **Arginine** is the most basic amino acid (highest pI) due to its guanidinium group. 3. **Histidine** is the precursor for **Histamine** (via decarboxylation) and is essential for the "Bohr effect" in hemoglobin due to its buffering capacity. 4. **Ketogenic vs. Glucogenic:** Leucine and Lysine are the only purely ketogenic amino acids.

Q: If cellular proteins do not fold into a specific conformation, their functions are affected. Certain disorders arise if specific proteins are misfolded. Which of the following disorders arises due to conformational isomerization?

Familial fatal insomnia. **Explanation:** The correct answer is **Familial Fatal Insomnia (FFI)**. This disorder belongs to a group of conditions known as **Prion Diseases** (Transmissible Spongiform Encephalopathies). **1. Why Familial Fatal Insomnia is correct:** Prion diseases are caused by **conformational isomerization**, where a normal cellular protein ($PrP^C$, primarily alpha-helical) undergoes a structural transition into a pathological, misfolded isoform ($PrP^{Sc}$, rich in beta-pleated sheets). This misfolded protein is resistant to proteolysis and acts as a template to induce further misfolding of normal proteins. In FFI, this process specifically targets the thalamus, leading to profound sleep disturbances and autonomic dysfunction. **2. Why the other options are incorrect:** * **Hepatitis Delta:** This is an infectious disease caused by the Hepatitis D virus (HDV), a subviral satellite that requires the presence of Hepatitis B virus for replication. It is not a protein misfolding disorder. * **Pernicious Anemia:** This is an autoimmune megaloblastic anemia caused by a deficiency of **Intrinsic Factor**, leading to Vitamin B12 malabsorption. * **Lesch-Nyhan Syndrome:** This is an X-linked recessive metabolic disorder caused by a deficiency of the enzyme **HGPRT** in the purine salvage pathway, leading to hyperuricemia and self-mutilation. **Clinical Pearls for NEET-PG:** * **Prion diseases** include Creutzfeldt-Jakob Disease (CJD), Kuru, and Bovine Spongiform Encephalopathy (Mad Cow Disease). * The hallmark of prion pathology is the **$\alpha$-helix to $\beta$-sheet transition**. * Other high-yield protein misfolding diseases: **Alzheimer’s** (Amyloid-$\beta$), **Parkinson’s** ($\alpha$-synuclein), and **Transthyretin Amyloidosis**.

Q: Proteins synthesized in the rough endoplasmic reticulum will first travel to which cellular organelle?

Golgi body. ### Explanation The correct answer is **C. Golgi body**. **Underlying Concept:** This question tests the understanding of the **Secretory Pathway** (Anterograde transport). Proteins destined for secretion, membrane integration, or lysosomal enzymes are synthesized by ribosomes on the **Rough Endoplasmic Reticulum (RER)**. Once synthesized, these proteins are packaged into **COPII-coated transport vesicles**, which bud off from the ER and travel directly to the **cis-Golgi network**. The Golgi body acts as the "post office" of the cell, where proteins undergo post-translational modifications (like glycosylation and sulfation) and sorting. **Analysis of Incorrect Options:** * **A. Mitochondria:** Most mitochondrial proteins are synthesized by free ribosomes in the cytosol or within the mitochondria themselves. They do not pass through the RER-Golgi pathway. * **B. Cytosol:** Proteins synthesized on the RER are sequestered within the ER lumen or membrane; they do not return to the cytosol. Proteins found in the cytosol are synthesized by **free ribosomes**. * **D. Lysosome:** While many RER-synthesized proteins are destined for the lysosome, they must **first** pass through the Golgi body to receive the **Mannose-6-Phosphate (M6P)** tag, which targets them specifically to the lysosome. **High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease:** Caused by a deficiency in N-acetylglucosaminyl-1-phosphotransferase. This leads to a failure of the Golgi to add the M6P tag, causing lysosomal enzymes to be secreted extracellularly rather than delivered to lysosomes. * **Vesicular Transport:** * **COPII:** Anterograde transport (ER → Golgi). * **COPI:** Retrograde transport (Golgi → ER). * **Clathrin:** Trans-Golgi → Lysosomes; Plasma membrane → Endosomes (Receptor-mediated endocytosis).

Q: What is the most abundant glycoprotein present in the basement membrane?

Laminin. **Explanation:** The basement membrane is a specialized form of extracellular matrix (ECM) that provides structural support and influences cell behavior. **Why Laminin is the correct answer:** Laminin is a large, cross-shaped heterotrimeric glycoprotein (composed of $\alpha$, $\beta$, and $\gamma$ chains). It is the **most abundant non-collagenous glycoprotein** in the basement membrane. Its primary role is to serve as the "glue" that anchors epithelial cells to the basal lamina by binding to cell surface receptors (integrins) and other ECM components like Type IV collagen and nidogen. **Analysis of Incorrect Options:** * **B. Collagen type IV:** While this is the most abundant **protein** overall in the basement membrane (forming the structural scaffold), it is classified as a structural protein rather than a primary adhesive glycoprotein in this context. * **C. Fibronectin:** This is a major glycoprotein of the **interstitial** extracellular matrix and connective tissue, rather than the basement membrane. It plays a key role in cell adhesion and wound healing. * **D. Heparan sulfate:** This is a **proteoglycan** (specifically Perlecan in the basement membrane), not a glycoprotein. Its primary function is to provide a negative charge barrier for selective filtration (e.g., in the renal glomerulus). **High-Yield Clinical Pearls for NEET-PG:** * **Goodpasture Syndrome:** Autoantibodies against the non-collagenous (NC1) domain of **Type IV Collagen**. * **Alport Syndrome:** Genetic defect in the synthesis of **Type IV Collagen** (presents with nephritis and deafness). * **Junctional Epidermolysis Bullosa:** Often associated with mutations in **Laminin-332**. * **Nidogen (Entactin):** A glycoprotein that functions as a molecular bridge, linking laminin and type IV collagen networks.

Question 1

In quaternary structure, subunits are linked by:

Accepted Answer

Non-covalent bonds

Answer

Peptide bonds

Answer

Disulphide bonds

Answer

Covalent bonds

Question 2

An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces which of the following amino acids within the alpha helical structure?

Accepted Answer

Glycine

Answer

Alanine

Answer

Aspartic acid

Answer

Tyrosine

Question 3

Where is the glycosyl transferase enzyme responsible for O-glycosylation located?

Accepted Answer

Golgi apparatus

Answer

Rough Endoplasmic Reticulum (RER)

Answer

Smooth Endoplasmic Reticulum (SER)

Answer

Lysosome

Question 4

Human chorionic gonadotropin shares structural similarity with all except?

Accepted Answer

GH

Answer

FSH

Answer

LH

Answer

TSH

Question 5

Which amino acid is active at neutral pH?

Accepted Answer

Histidine

Answer

Leucine

Answer

Glycine

Answer

Lysine

Question 6

Which amino acid has a basic side chain?

Accepted Answer

Histidine

Answer

Methionine

Answer

Leucine

Answer

Aspartic acid

Question 7

If cellular proteins do not fold into a specific conformation, their functions are affected. Certain disorders arise if specific proteins are misfolded. Which of the following disorders arises due to conformational isomerization?

Accepted Answer

Familial fatal insomnia

Answer

Hepatitis delta

Answer

Pernicious anemia

Answer

Lesch-Nyhan syndrome

Question 8

Proteins synthesized in the rough endoplasmic reticulum will first travel to which cellular organelle?

Accepted Answer

Golgi body

Answer

Mitochondria

Answer

Cytosol

Answer

Lysosome

Question 9

An immunoglobulin molecule represents which level of organized protein structure?

Accepted Answer

Quaternary structure

Answer

Primary structure

Answer

Secondary structure

Answer

Tertiary structure

Question 10

What is the most abundant glycoprotein present in the basement membrane?

Accepted Answer

Laminin

Answer

Collagen type IV

Answer

Fibronectin

Answer

Heparan sulfate

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

On this page

Practice by Chapter

Want unlimited practice?