Protein Structure and Function Practice Questions

Q: Which amino acid contains an imidazole ring?

Histidine. **Explanation:** The correct answer is **Histidine**. Amino acids are categorized based on the specific chemical structure of their side chains (R-groups). **1. Why Histidine is correct:** Histidine contains an **imidazole ring**, which is a five-membered heterocyclic structure containing two nitrogen atoms. This ring is unique because its pKa is approximately 6.0, close to physiological pH. This allows histidine to function as an effective proton donor or acceptor (buffer) in the active sites of many enzymes, such as carbonic anhydrase. **2. Why the other options are incorrect:** * **Tryptophan:** Contains an **indole ring** (a benzene ring fused to a pyrrole ring). It is the precursor for serotonin and melatonin. * **Arginine:** Contains a **guanidino group**. It is the most basic amino acid and a precursor for Nitric Oxide (NO). * **Tyrosine:** Contains a **phenol ring** (a hydroxyl group attached to a benzene ring). It is a precursor for catecholamines, thyroid hormones, and melanin. **Clinical Pearls for NEET-PG:** * **Buffering Capacity:** Histidine is the most important amino acid for the buffering action of hemoglobin in the blood. * **Pauly’s Test:** This biochemical test is used specifically to detect the presence of Histidine or Tyrosine. * **FIGLU Test:** Formiminoglutamic acid (FIGLU) is an intermediate of histidine metabolism. Increased urinary excretion of FIGLU is a clinical marker for **Folic Acid deficiency**. * **Essentiality:** Histidine is considered a semi-essential amino acid (essential for growth in children and during pregnancy).

Q: Which cellular component is primarily responsible for protein folding?

Chaperone. **Explanation:** **1. Why Chaperones are the Correct Answer:** Protein folding is the process by which a polypeptide chain acquires its functional 3D conformation. **Chaperones** (and chaperonins) are specialized proteins that facilitate this process. They prevent the aggregation of unfolded or partially folded polypeptide chains by binding to exposed hydrophobic regions. While folding can occur spontaneously, chaperones ensure the process is efficient and error-free, especially under cellular stress (Heat Shock Proteins/HSPs). **2. Why Other Options are Incorrect:** * **Endoplasmic Reticulum (ER):** While the Rough ER is the *site* where secretory and membrane proteins are synthesized and folded, it is not the "component" responsible for the folding itself. The folding within the ER is actually performed by resident chaperones (like BiP/GRP78). * **Peroxisome:** These organelles are involved in long-chain fatty acid oxidation (beta-oxidation), hydrogen peroxide metabolism, and bile acid synthesis, not protein folding. * **Lysosome:** These are the "garbage disposals" of the cell. They contain acid hydrolases meant for protein *degradation* (proteolysis), which is the opposite of folding. **3. High-Yield Clinical Pearls for NEET-PG:** * **HSP70:** A major chaperone that prevents aggregation during translation. * **HSP60 (Chaperonins):** Form a cage-like structure (e.g., GroEL-GroES in bacteria) to fold proteins in isolation. * **Clinical Correlation:** Misfolding of proteins is the hallmark of several neurodegenerative diseases, such as **Alzheimer’s** (Amyloid-beta), **Parkinson’s** (alpha-synuclein), and **Prion diseases** (PrPSc). * **Cystic Fibrosis:** Often results from the premature degradation of a slightly misfolded but functional CFTR protein by the ER quality control system.

Q: All of the following hormones are glycoproteins, EXCEPT?

GH. **Explanation:** The correct answer is **GH (Growth Hormone)**. **1. Why GH is the correct answer:** Hormones are chemically classified into proteins/peptides, steroids, or amino acid derivatives. **Glycoproteins** are a specific subset of proteins that contain oligosaccharide chains (carbohydrates) covalently attached to amino acid side-chains. * **Growth Hormone (GH)** and **Prolactin** are pure **polypeptide hormones** (single-chain proteins) secreted by the acidophil cells of the anterior pituitary. They do not contain carbohydrate moieties. **2. Why the other options are incorrect:** Options A, B, and C (**TSH, FSH, and LH**) along with **hCG** (Human Chorionic Gonadotropin) form the family of **Glycoprotein Hormones**. * They are dimeric, consisting of two subunits: **Alpha (α) and Beta (β)**. * The **α-subunit** is identical in all four hormones. * The **β-subunit** is unique to each and confers biological and immunological specificity. * These hormones require glycosylation for proper folding, stability, and biological activity. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Acidophils vs. Basophils:** Remember the mnemonic **"GPA"** (Growth Hormone & Prolactin are produced by Acidophils) and **"B-FLAT"** (Basophils produce FSH, LH, ACTH, and TSH). Note that while ACTH is from basophils, it is a polypeptide, not a glycoprotein. * **Common α-subunit:** Because TSH, FSH, LH, and hCG share the same α-subunit, extremely high levels of hCG (as seen in molar pregnancies) can cross-react with TSH receptors, leading to hyperthyroidism. * **GH Structure:** Human GH is a single polypeptide chain of 191 amino acids with two intramolecular disulfide bridges.

Q: Glutathione is composed of which amino acids?

All of the above. **Explanation:** Glutathione (GSH) is a vital **tripeptide** found in high concentrations in almost all mammalian cells. It is composed of three specific amino acids: **L-Glutamate, L-Cysteine, and Glycine.** The unique structural feature of Glutathione is the linkage between the amino acids. Unlike typical proteins, the amino group of Cysteine is linked to the **gamma-carboxyl group** of Glutamate (rather than the alpha-carboxyl), forming a **$\gamma$-glutamyl linkage**. This makes it resistant to degradation by most intracellular peptidases. * **Why Option D is correct:** Glutathione is chemically defined as **$\gamma$-L-Glutamyl-L-cysteinyl-glycine**. Therefore, all three amino acids (Glycine, Cysteine, and Glutamate) are essential structural components. * **Why individual options (A, B, C) are "wrong":** While Glycine, Cysteine, and Glutamate are indeed part of the molecule, selecting any single one would be incomplete. In the context of a multiple-choice question, "All of the above" is the most accurate description of its composition. **High-Yield Clinical Pearls for NEET-PG:** 1. **The Active Site:** The **thiol (-SH) group of Cysteine** is the functional part of the molecule, responsible for its antioxidant properties and redox reactions. 2. **Function:** It acts as a major intracellular antioxidant, neutralizing free radicals and maintaining Vitamins C and E in their reduced forms. 3. **Enzymatic Role:** It is a cofactor for **Glutathione Peroxidase**, which neutralizes hydrogen peroxide ($H_2O_2$) into water, protecting RBCs from hemolysis. 4. **Clinical Correlation:** In **G6PD deficiency**, a lack of NADPH leads to a failure in regenerating reduced glutathione, resulting in oxidative stress and Heinz body formation.

Q: Which one of the following is a heterocyclic amino acid?

Proline. **Explanation:** The correct answer is **Proline**. **1. Why Proline is the Correct Answer:** A heterocyclic compound is a cyclic structure that contains at least one atom other than carbon (such as Nitrogen, Oxygen, or Sulfur) within the ring. Proline is unique among the 20 standard amino acids because its side chain cyclizes back onto its own amino group. This forms a five-membered **pyrrolidine ring**, making it a heterocyclic amino acid. Technically, proline is an **imino acid** because it contains a secondary amino group (NH) rather than a primary one ($NH_2$). **2. Analysis of Incorrect Options:** * **A. Leucine:** An aliphatic, branched-chain amino acid (BCAA) with a non-cyclic hydrocarbon side chain. * **B. Methionine:** A sulfur-containing amino acid with a linear (non-cyclic) thioether side chain. * **C. Alanine:** The simplest chiral amino acid with a methyl group side chain; it is aliphatic and non-cyclic. **3. High-Yield Clinical Pearls for NEET-PG:** * **Helix Breaker:** Due to its rigid cyclic structure, Proline creates "kinks" in polypeptide chains and is known as a **$\alpha$-helix breaker**. It is rarely found in $\alpha$-helices but is abundant in **$\beta$-turns**. * **Collagen Synthesis:** Proline and its derivative, **Hydroxyproline**, are essential for the stability of the collagen triple helix. Hydroxylation of proline requires **Vitamin C** (Ascorbic acid); deficiency leads to Scurvy. * **Ninhydrin Test:** While most amino acids give a purple/Ruhemann's purple color with ninhydrin, Proline (and Hydroxyproline) gives a characteristic **yellow color** due to its imino group. * **Other Cyclic Amino Acids:** Do not confuse "heterocyclic" with "aromatic." Phenylalanine and Tyrosine are carbocyclic (rings made only of carbon), while **Tryptophan** (Indole ring) and **Histidine** (Imidazole ring) are also heterocyclic.

Q: Flexibility of protein depends on which amino acid?

Glycine. **Explanation:** The flexibility of a protein is primarily determined by the size and nature of the side chains (R-groups) of its constituent amino acids. **Why Glycine is the correct answer:** Glycine is the simplest amino acid, having only a single **hydrogen atom** as its side chain. Because this side chain is so small, it imposes minimal steric hindrance. This allows the polypeptide backbone at the glycine position to rotate more freely and adopt a wider range of dihedral angles (phi and psi) on the **Ramachandran plot**. Consequently, glycine acts as a "hinge" or "flexibility provider," allowing proteins to form tight turns (like Beta-turns) and complex folds that larger amino acids would physically block. **Why the other options are incorrect:** * **Tryptophan & Phenylalanine:** These are large, bulky **aromatic amino acids**. Their massive side chains create significant steric hindrance, restricting the rotation of the peptide bond and decreasing local protein flexibility. * **Histidine:** While smaller than tryptophan, histidine still possesses a relatively rigid imidazole ring side chain, which provides significantly less conformational freedom compared to glycine. **High-Yield Clinical Pearls for NEET-PG:** * **The "Helix Breaker":** While Glycine provides flexibility, **Proline** provides rigidity. Proline is an imino acid that creates a "kink" in the chain and is known as a helix breaker. * **Collagen Structure:** Glycine is essential for the triple helix of collagen. It occurs at every third position (**Gly-X-Y**) because only glycine is small enough to fit into the crowded central core of the triple helix. * **Ramachandran Plot:** Glycine is the only amino acid that can occupy regions of the plot that are "disallowed" for other amino acids due to its lack of a beta-carbon.

Question 1

Which amino acid contains an imidazole ring?

Accepted Answer

Histidine

Answer

Tryptophan

Answer

Arginine

Answer

Tyrosine

Question 2

Which cellular component is primarily responsible for protein folding?

Accepted Answer

Chaperone

Answer

Endoplasmic reticulum

Answer

Peroxisome

Answer

Lysosome

Question 3

Keratin in skin is softer than keratin in nail. What is the reason for this difference?

Accepted Answer

Fewer disulfide bonds

Answer

Fewer salt bridges

Answer

High sodium content

Answer

Different affinity for water

Question 4

All of the following represent disorders of protein misfolding, EXCEPT:

Accepted Answer

Tuberculosis

Answer

Alzheimer's disease

Answer

Cystic fibrosis

Answer

Creutzfeldt-Jakob disease

Question 5

All of the following hormones are glycoproteins, EXCEPT?

Accepted Answer

GH

Answer

TSH

Answer

FSH

Answer

LH

Question 6

Keratin of skin and nail differ because of which type of bond?

Accepted Answer

Disulfide bond

Answer

Covalent bond

Answer

Van der Waals bond

Answer

Hydrogen bond

Question 7

Glutathione is composed of which amino acids?

Accepted Answer

All of the above

Answer

Glycine

Answer

Cysteine

Answer

Glutamate

Question 8

Which one of the following is a heterocyclic amino acid?

Accepted Answer

Proline

Answer

Leucine

Answer

Methionine

Answer

Alanine

Question 9

Flexibility of protein depends on which amino acid?

Accepted Answer

Glycine

Answer

Tryptophan

Answer

Phenylalanine

Answer

Histidine

Question 10

Post-translational modification is seen with which of the following drugs?

Accepted Answer

Streptomycin

Answer

Penicillin

Answer

Sulphonamides

Answer

Cephalosporins

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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