Protein Structure and Function Practice Questions

Q: What is the role of selenocysteine?

Antioxidant mechanism. **Explanation:** **Selenocysteine** is often referred to as the **21st amino acid**. It is unique because it contains selenium in place of the sulfur atom found in cysteine. Its primary physiological role is serving as a critical component of **selenoproteins**, which are essential for cellular redox balance and the **antioxidant mechanism**. 1. **Why Option C is Correct:** The most clinically significant selenoprotein is **Glutathione Peroxidase (GPx)**. This enzyme reduces lipid hydroperoxides and free hydrogen peroxide into water, thereby protecting cells from oxidative damage. Other important selenoproteins include **Thioredoxin Reductase** (maintains cellular redox state) and **Iodothyronine Deiodinase** (converts T4 to active T3). 2. **Why Other Options are Incorrect:** * **Option A (Hydroxylation of dopamine):** This process converts dopamine to norepinephrine via the enzyme *Dopamine beta-hydroxylase*, which requires **Vitamin C (Ascorbate)** and Copper as cofactors, not selenocysteine. * **Option B (Oxidation of drugs):** Drug metabolism and oxidation are primarily handled by the **Cytochrome P450** enzyme system in the liver, which utilizes Heme (Iron) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Coding:** Selenocysteine is encoded by the **UGA stop codon**. This "recoding" requires a specific mRNA secondary structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **tRNA:** It has its own unique tRNA, known as **tRNA^Sec**. * **Deficiency:** A deficiency in selenium leads to **Keshan Disease** (an endemic cardiomyopathy). * **Enzyme to Remember:** If a question mentions **Glutathione Peroxidase**, the answer is almost always related to Selenium or Selenocysteine.

Q: Which of the following is a dipeptide?

Carnosine. **Explanation:** **1. Why Carnosine is the Correct Answer:** A dipeptide consists of two amino acids linked by a single peptide bond. **Carnosine** (β-alanyl-L-histidine) is a classic example of a dipeptide. It is composed of **β-alanine** and **L-histidine**. It is found in high concentrations in skeletal muscle and brain tissue, where it acts as a pH buffer and an antioxidant. **2. Why the Other Options are Incorrect:** * **B. Glutathione:** This is a **tripeptide** (γ-glutamyl-cysteinyl-glycine). It is the most abundant intracellular antioxidant. Note the unique γ-glutamyl bond, which protects it from degradation by most peptidases. * **C. Glucagon:** This is a **polypeptide** hormone secreted by the alpha cells of the pancreas. It consists of **29 amino acids** in a single chain. * **D. β-Lipoprotein:** This is a complex **macromolecular assembly** (specifically LDL) composed of lipids and a large apolipoprotein (Apo B-100), which contains over 4,500 amino acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Anserine:** Another important dipeptide (N-methyl carnosine) found in skeletal muscle. * **Aspartame:** An artificial sweetener that is a dipeptide (L-aspartyl-L-phenylalanine methyl ester). * **Glutathione Function:** It serves as a co-factor for the enzyme **Glutathione Peroxidase** to neutralize free radicals (H₂O₂). * **B-alanine:** It is the rate-limiting precursor for carnosine synthesis; its supplementation is often used by athletes to improve buffering capacity during high-intensity exercise.

Q: How many amino acids are present in a single turn of an alpha-helix?

3.6. ### Explanation The **$\alpha$-helix** is the most common secondary structure in proteins, characterized by a tightly packed, right-handed coiled backbone. **Why 3.6 is the Correct Answer:** In an $\alpha$-helix, the polypeptide chain is stabilized by hydrogen bonds between the carbonyl oxygen ($C=O$) of one amino acid and the amide hydrogen ($N-H$) of the amino acid located **four residues ahead** ($i + 4$ hydrogen bonding). This specific bonding pattern results in a repeating unit where: * **Amino acids per turn:** 3.6 residues. * **Pitch (height of one full turn):** 0.54 nm (5.4 Å). * **Rise (distance between consecutive residues):** 0.15 nm (1.5 Å). * Calculation: $3.6 \text{ residues} \times 0.15 \text{ nm} = 0.54 \text{ nm}$. **Analysis of Incorrect Options:** * **Option A (3.3):** This refers to the **$3_{10}$ helix**, which is tighter and thinner than the $\alpha$-helix. It has 3 residues per turn and 10 atoms in the hydrogen-bonded loop. * **Options C & D (10.5 and 11.5):** These values are associated with the **B-DNA double helix**, which has approximately 10.5 base pairs per turn. They are not related to protein secondary structures. **High-Yield Clinical Pearls for NEET-PG:** * **Proline** is known as a **"helix breaker"** because its rigid cyclic structure cannot fit into the $\alpha$-helix and it lacks the necessary N-H group for hydrogen bonding. * **Glycine** also tends to disrupt helices due to its high conformational flexibility. * **Keratin** (hair/nails) and **Hemoglobin** are rich in $\alpha$-helices. * **Amphipathic helices** (one side polar, one side non-polar) are commonly found in transmembrane proteins and serum lipoproteins.

Q: Which of the following is an aromatic amino acid?

Tryptophan. ### Explanation **Correct Answer: D. Tryptophan** **1. Why Tryptophan is Correct:** Aromatic amino acids are characterized by the presence of a benzene or indole ring in their side chains. There are three primary aromatic amino acids: **Phenylalanine, Tyrosine, and Tryptophan**. Tryptophan contains an **indole ring** system, making it the most complex aromatic amino acid. These molecules are relatively non-polar and can participate in hydrophobic interactions. Notably, they are responsible for the ultraviolet light absorption of proteins at **280 nm**. **2. Why the Other Options are Incorrect:** * **A. Serine:** This is a **hydroxy-containing** (polar, uncharged) amino acid. It plays a crucial role in the catalytic sites of enzymes (serine proteases) and is a site for O-linked glycosylation. * **B. Histidine:** While it contains an **imidazole ring**, it is traditionally classified as a **basic (positively charged)** amino acid. Although it has aromatic properties, in the context of standard NEET-PG classification, it is grouped with Lysine and Arginine. * **C. Leucine:** This is a **branched-chain aliphatic** amino acid. It is purely ketogenic and plays a vital role in muscle protein synthesis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Precursor Functions:** Tryptophan is the precursor for **Serotonin, Melatonin, and Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (primarily Tryptophan) leads to pellagra-like symptoms due to Niacin deficiency. * **Essentiality:** Phenylalanine and Tryptophan are **Essential**, while Tyrosine is **Non-essential** (derived from Phenylalanine). * **Metabolic Fate:** All three aromatic amino acids are **both glucogenic and ketogenic**.

Q: The peptide bond has all the following characteristics, EXCEPT?

Free to rotate. ### Explanation The peptide bond is the fundamental linkage in protein structure, formed between the $\alpha$-carboxyl group of one amino acid and the $\alpha$-amino group of another. **Why "Free to rotate" is the correct answer:** The peptide bond is **rigid and restricted**, meaning it **cannot rotate** freely. This lack of rotation is due to the **partial double bond character** (resonance) between the Carbon and Nitrogen atoms. While the peptide bond itself is fixed, rotation is permitted only around the single bonds involving the $\alpha$-carbon: the **Phi ($\phi$)** bond (N-C$\alpha$) and the **Psi ($\psi$)** bond (C$\alpha$-C). **Analysis of other options:** * **A. Covalent in nature:** Correct. The peptide bond is a strong covalent amide linkage formed via a dehydration (condensation) reaction. * **B. Planar in nature:** Correct. Due to the double bond character, the six atoms involved in the peptide group (C$\alpha$, C, O, N, H, and the next C$\alpha$) lie in a single plane. * **C. Partial double bond character:** Correct. Resonance allows electrons to be shared between the C-O and C-N bonds. This makes the C-N bond shorter than a typical single bond and prevents rotation. **High-Yield Clinical Pearls for NEET-PG:** * **Configuration:** Most peptide bonds in proteins are in the **trans** configuration to minimize steric hindrance between R-groups. * **Biuret Test:** This clinical chemistry test detects peptide bonds (requires at least two) to quantify protein levels. * **Ramachandran Plot:** Used to visualize the allowed values of $\phi$ and $\psi$ angles, which determine the secondary structure of proteins (e.g., $\alpha$-helix, $\beta$-sheets). * **Cleavage:** Peptide bonds are highly stable and require proteolytic enzymes (proteases) or strong acids/bases at high temperatures to be hydrolyzed.

Q: Which of the following is not a transport protein?

Collagen. **Explanation:** The correct answer is **Collagen** because it is a **structural protein**, not a transport protein. In biochemistry, proteins are classified by their functional roles. Transport proteins bind and move specific molecules or ions across cell membranes or through the bloodstream, whereas structural proteins provide mechanical support and shape to tissues. * **Collagen (Correct Option):** It is the most abundant protein in the human body. It provides tensile strength to connective tissues such as bone, cartilage, tendons, and skin. It does not possess binding sites for the systemic transport of ligands. * **Transferrin (Incorrect):** This is a classic transport protein (beta-globulin) synthesized in the liver. Its primary function is to bind and transport **iron (Fe³⁺)** in the plasma to the bone marrow and other tissues. * **Ceruloplasmin (Incorrect):** This is an alpha-2 globulin that acts as the primary **copper-transporting** protein in the blood. It also functions as a ferroxidase, converting Fe²⁺ to Fe³⁺ so it can be loaded onto transferrin. * **Hemoglobin (Incorrect):** This is a specialized transport protein found in erythrocytes. It is responsible for the transport of **oxygen** from the lungs to the peripheral tissues and assists in the transport of carbon dioxide and protons. **High-Yield Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency in **Ceruloplasmin**, leading to copper deposition in the liver and brain (Kayser-Fleischer rings). * **Collagen Structure:** It is a fibrous protein characterized by a **triple helix** (Gly-X-Y sequence), where X and Y are often Proline and Hydroxyproline. * **Albumin:** The most versatile transport protein in the plasma, carrying free fatty acids, bilirubin, calcium, and various drugs.

Q: All of the following are storage proteins, except?

Ricin. **Explanation:** The classification of proteins is based on their biological functions. Storage proteins serve as a reservoir of essential nutrients (like amino acids or metal ions) for the development of embryos or for metabolic needs. **Why Ricin is the Correct Answer:** **Ricin** is not a storage protein; it is a potent **Phytotoxin** (toxic plant protein) derived from the castor bean (*Ricinus communis*). It functions as a **Type II Ribosome-Inactivating Protein (RIP)**. Mechanistically, it inhibits protein synthesis by removing an adenine residue from the 28S rRNA of the 60S ribosomal subunit, leading to cell death. This makes it a biological toxin rather than a nutritional reserve. **Analysis of Incorrect Options:** * **Ovalbumin:** The primary protein found in egg white. It serves as a source of amino acids for the developing bird embryo. * **Ferritin:** A critical intracellular protein that **stores Iron** in a non-toxic form and releases it in a controlled manner. It is the primary diagnostic marker for iron deficiency anemia. * **Glutelin:** A major storage protein found in the endosperm of cereal grains (e.g., Glutenin in wheat). It provides nitrogen and amino acids during seed germination. **High-Yield Clinical Pearls for NEET-PG:** * **Other Storage Proteins:** Casein (Milk), Myoglobin (stores Oxygen in muscles), and Zein (Maize). * **Ricin Mechanism:** It acts as an N-glycosidase. Because it irreversibly halts protein synthesis, it is studied in targeted cancer immunotherapy (immunotoxins). * **Ferritin Levels:** Decreased in Iron Deficiency Anemia; increased in Hemochromatosis and as an **Acute Phase Reactant** during inflammation/infection.

Q: Which of the following is a property of amino acids at their isoelectric pH?

Maximum precipitability. The **isoelectric point (pI)** is the specific pH at which an amino acid or protein carries no net electrical charge, existing primarily as a **zwitterion** (dipolar ion). **Why Maximum Precipitability is Correct:** At the isoelectric pH, the net charge on the molecule is zero. This leads to a loss of electrostatic repulsion between neighboring protein molecules. Without these repulsive forces, the molecules tend to aggregate and clump together due to hydrophobic interactions, resulting in **minimum solubility** and **maximum precipitability**. This principle is utilized in laboratory techniques like isoelectric focusing and the precipitation of caseins in milk (curdling). **Analysis of Incorrect Options:** * **A. Maximum buffering action:** Buffering capacity is maximal at a pH equal to the **pKa** of the ionizing group (where [Acid] = [Conjugate Base]). At the pI, the buffering capacity is actually at its minimum. * **C. Maximum solubility:** Solubility is at its **minimum** at the pI because the lack of net charge reduces the interaction between the protein and the water solvent. * **D. Mobility in an electric field:** Electrophoretic mobility depends on a net charge. Since the net charge is **zero** at the pI, the molecule will not migrate toward either the anode or the cathode. **High-Yield Clinical Pearls for NEET-PG:** * **Zwitterion:** An amino acid at its pI that has both a positive (—NH₃⁺) and a negative (—COO⁻) charge, making it electrically neutral. * **Electrophoresis:** Proteins are separated based on their charge; if the buffer pH is greater than the pI, the protein becomes negatively charged (anion) and moves toward the **Anode**. * **Clinical Application:** The solubility of insulin is lowest at its pI (pH 5.4). This property is exploited in the formulation of long-acting insulin analogs (like Glargine) which precipitate at physiological pH (7.4) to slow absorption.

Q: Which of the following best describes the typical structure of proteins like collagen and keratin?

Fibrous. **Explanation:** Proteins are broadly classified based on their overall shape and solubility into **fibrous** and **globular** types. **Why Fibrous is correct:** Fibrous proteins, such as **collagen, keratin, and elastin**, consist of polypeptide chains arranged in long strands or sheets. They are characterized by their high tensile strength, structural stability, and insolubility in water. * **Collagen** provides structural integrity to connective tissues (skin, bone, tendons). * **Keratin** provides mechanical protection (hair, nails, outer layer of skin). Their structure is dominated by repetitive secondary motifs (like the triple helix in collagen) rather than complex tertiary folding. **Why other options are incorrect:** * **B. Globular:** These proteins (e.g., Hemoglobin, Myoglobin, Enzymes) are folded into compact, spherical shapes. They are generally water-soluble and perform dynamic functions like catalysis and transport. * **C. Branched:** Proteins are linear polymers of amino acids linked by peptide bonds. While glycans (carbohydrates) can be branched, protein backbones themselves are **never branched**. * **D. Compound:** Also known as conjugated proteins, these contain a protein molecule bound to a non-protein prosthetic group (e.g., Glycoproteins, Lipoproteins). While collagen is a glycoprotein, "Fibrous" is the more fundamental structural classification requested. **NEET-PG High-Yield Pearls:** * **Collagen:** The most abundant protein in the human body. It features a repeating sequence of **Gly-X-Y** (where X is often Proline and Y is Hydroxyproline). * **Vitamin C:** Essential for the hydroxylation of proline and lysine residues in collagen; deficiency leads to **Scurvy** (defective cross-linking). * **Keratin:** Rich in **Cysteine** residues; the degree of disulfide bonding determines the hardness of the tissue.

Question 1

What is the role of selenocysteine?

Accepted Answer

Antioxidant mechanism

Answer

Hydroxylation of dopamine

Answer

Oxidation of drugs

Answer

None of the above

Question 2

Which of the following is a dipeptide?

Accepted Answer

Carnosine

Answer

Glutathione

Answer

Glucagon

Answer

β-Lipoprotein

Question 3

How many amino acids are present in a single turn of an alpha-helix?

Accepted Answer

3.6

Answer

3.3

Answer

10.5

Answer

11.5

Question 4

Which of the following is an aromatic amino acid?

Accepted Answer

Tryptophan

Answer

Serine

Answer

Histidine

Answer

Leucine

Question 5

The peptide bond has all the following characteristics, EXCEPT?

Accepted Answer

Free to rotate

Answer

Covalent in nature

Answer

Planar in nature

Answer

Partial double bond character

Question 6

The stability of a toxoid derived from snake venom is primarily attributed to which type of chemical bond?

Accepted Answer

Disulfide bonds

Answer

Hydrogen bonds

Answer

Ionic bonds

Answer

Van der Waals forces

Question 7

Which of the following is not a transport protein?

Accepted Answer

Collagen

Answer

Transferrin

Answer

Ceruloplasmin

Answer

Hemoglobin

Question 8

All of the following are storage proteins, except?

Accepted Answer

Ricin

Answer

Ovalbumin

Answer

Ferritin

Answer

Glutelin

Question 9

Which of the following is a property of amino acids at their isoelectric pH?

Accepted Answer

Maximum precipitability

Answer

Maximum buffering action

Answer

Maximum solubility

Answer

Mobility in an electric field

Question 10

Which of the following best describes the typical structure of proteins like collagen and keratin?

Accepted Answer

Fibrous

Answer

Globular

Answer

Branched

Answer

Compound

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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