Protein Structure and Function Practice Questions

Q: The amino acid sequence of human chorionic gonadotropin (hCG) is approximately 80% similar to which of the following hormones?

Luteinizing hormone (LH). **Explanation:** Human Chorionic Gonadotropin (hCG) belongs to the family of **glycoprotein hormones**, 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 (α) and Beta (β).** 1. **The Alpha (α) Subunit:** This is identical in all four hormones (hCG, LH, FSH, and TSH). 2. **The Beta (β) Subunit:** This provides biological and immunological specificity. **Why LH is the correct answer:** The β-subunit of hCG shares approximately **80% sequence homology** with the β-subunit of LH. Both hormones bind to the same receptor (LH/hCG receptor). This structural similarity allows hCG to mimic the action of LH, which is crucial for maintaining the corpus luteum and progesterone production during early pregnancy. **Why other options are incorrect:** * **FSH:** While FSH shares the same α-subunit, its β-subunit has significantly lower homology with hCG compared to LH. * **GnRH:** This is a small decapeptide (10 amino acids) produced by the hypothalamus. It is not a glycoprotein and bears no structural resemblance to hCG. **High-Yield NEET-PG Pearls:** * **Biological Half-life:** hCG has a much longer half-life (~24–36 hours) than LH (~20–60 minutes) due to a highly glycosylated C-terminal tail on its β-subunit. * **Clinical Application:** Because of the identical α-subunits, pregnancy tests and immunoassay kits specifically target the **β-subunit of hCG** to avoid cross-reactivity with LH, FSH, or TSH. * **Therapeutic Use:** Due to its LH-like action and longer half-life, hCG is used clinically to trigger ovulation in infertility treatments (acting as an "LH surge").

Q: Which of the following is a negative acute-phase protein?

Transthyretin. **Explanation:** Acute-phase proteins (APPs) are proteins whose plasma concentrations increase or decrease by at least 25% in response to inflammation, infection, or trauma. This response is primarily mediated by cytokines like IL-6, IL-1, and TNF-α acting on the liver. **1. Why Transthyretin is Correct:** **Transthyretin (Prealbumin)** is a **negative acute-phase protein**. During inflammation, the liver prioritizes the synthesis of "positive" APPs (like CRP) to aid the immune response, leading to a down-regulation in the production of transport proteins. Other major negative APPs include **Albumin** and **Transferrin**. **2. Why the Other Options are Incorrect:** * **C-Reactive Protein (CRP):** A classic positive APP. It is a sensitive marker of systemic inflammation and acts as an opsonin to facilitate phagocytosis. * **Ferritin:** A positive APP. Its levels rise during inflammation to sequester iron, depriving invading pathogens of this essential nutrient (contributing to the "anemia of chronic disease"). * **Ceruloplasmin:** A positive APP. It acts as a ferroxidase and helps in scavenging free radicals during the inflammatory process. **High-Yield Clinical Pearls for NEET-PG:** * **Most Sensitive Marker:** CRP is the most sensitive and rapidly responding positive APP. * **The "Big Three" Negative APPs:** Remember the mnemonic **"PAT"**—**P**realbumin (Transthyretin), **A**lbumin, and **T**ransferrin. * **ESR vs. CRP:** While both indicate inflammation, CRP reflects the acute change more accurately, whereas ESR is influenced by fibrinogen levels and red cell morphology. * **Transthyretin Function:** It transports thyroxine (T4) and Retinol-Binding Protein (RBP). Its short half-life (~2 days) makes it a better marker for acute nutritional status than albumin.

Q: Gonadotrophin is a:

Glycoprotein. **Explanation:** **1. Why Glycoprotein is Correct:** Gonadotrophins (FSH, LH, and hCG) are complex **heterodimeric glycoproteins**. They consist of two non-covalently linked subunits: * **Alpha (α) subunit:** Identical in FSH, LH, TSH, and hCG. * **Beta (β) subunit:** Unique to each hormone, conferring biological and immunological specificity. The "glyco-" component refers to the carbohydrate side chains attached to the polypeptide backbone, which are essential for the hormone's stability, half-life, and receptor binding. **2. Why Other Options are Incorrect:** * **Steroid:** Steroid hormones (e.g., Estrogen, Testosterone, Cortisol) are derived from cholesterol and are lipophilic. Gonadotrophins are water-soluble proteins. * **Lipoprotein:** These are complexes of lipids and proteins (e.g., LDL, HDL) used for lipid transport in the blood, not for hormonal signaling. * **Polypeptide:** While gonadotrophins contain polypeptide chains, the term "Polypeptide" usually refers to simple protein hormones without significant carbohydrate modifications (e.g., Insulin, Glucagon, GH). "Glycoprotein" is the more specific and accurate classification. **3. NEET-PG High-Yield Pearls:** * **The "Big Four" Glycoproteins:** Remember the mnemonic **"F-L-A-T"** (FSH, LH, ACTH is NOT one, it's **TSH** and **hCG**) for hormones sharing the same alpha subunit. * **Clinical Correlation:** In pregnancy tests, we measure the **β-hCG** subunit because the α-subunit is identical to LH/FSH/TSH and would cause cross-reactivity. * **Sialic Acid Content:** The half-life of these hormones is determined by their sialic acid content; hCG has the highest sialic acid content and thus the longest half-life (~24 hours).

Q: Which protein is responsible for copper binding?

Ceruloplasmin. **Explanation:** **Ceruloplasmin** is the correct answer because it is the primary copper-carrying protein in the blood, accounting for approximately **90-95% of total plasma copper**. Synthesized in the liver, it is an alpha-2 globulin that functions as a ferroxidase enzyme, oxidizing ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$) to facilitate its binding to transferrin. **Analysis of Incorrect Options:** * **Albumin (A):** While albumin binds many substances (bilirubin, calcium, drugs), it only carries about 5-10% of plasma copper. This "loosely bound" copper is the fraction that is more easily taken up by tissues. * **Globulin (B):** This is a broad category of proteins. While ceruloplasmin is a type of alpha-2 globulin, "globulin" as a general term is non-specific. * **Transferrin (C):** This protein is specifically responsible for the transport of **Iron**, not copper. **Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency in **ATP7B** (a copper-transporting ATPase), leading to decreased incorporation of copper into apoceruloplasmin. This results in **low serum ceruloplasmin levels** and toxic copper accumulation in the liver and basal ganglia (Kayser-Fleischer rings). * **Menkes Disease:** An X-linked recessive disorder caused by a mutation in **ATP7A**, leading to impaired intestinal copper absorption and "kinky hair" syndrome. * **Acute Phase Reactant:** Ceruloplasmin levels rise during inflammation, infection, or pregnancy.

Q: Which of the following represents a covalent bond?

Disulphide bond. **Explanation:** In protein chemistry, bonds are categorized into **covalent** (sharing of electrons) and **non-covalent** (weak electrostatic interactions). **Why the Disulphide bond is correct:** A disulphide bond (S-S bond) is a strong **covalent bond** formed by the oxidation of the sulfhydryl (-SH) groups of two **Cysteine** residues. It is the only covalent bond, other than the peptide bond, that stabilizes the tertiary and quaternary structures of proteins. Because it is covalent, it is significantly stronger than hydrogen or ionic bonds and requires specific enzymes (like protein disulfide isomerase) or strong reducing agents (like β-mercaptoethanol) to break. **Why the other options are incorrect:** * **A. Hydrogen bond:** A non-covalent interaction between a hydrogen atom (covalently bound to an electronegative atom like N or O) and another electronegative atom. It is essential for stabilizing **alpha-helices and beta-sheets**. * **C & D. Electrostatic/Ionic bond:** These are non-covalent interactions between oppositely charged side chains (e.g., the positively charged Lysine and negatively charged Aspartate). They are also known as **salt bridges**. **High-Yield Facts for NEET-PG:** * **Cystine vs. Cysteine:** A "Cystine" molecule is formed when two "Cysteine" residues are linked by a disulphide bond. * **Insulin:** A classic example of a protein stabilized by both inter-chain and intra-chain disulphide bonds. * **Denaturation:** Most denaturing agents (heat, urea) break non-covalent bonds, but **reducing agents** are specifically required to break disulphide bonds. * **Keratin:** The high disulphide bond content in alpha-keratin provides structural rigidity to hair and nails.

Q: Sakaguchi's reaction is specific for which amino acid?

Arginine. **Explanation:** **Sakaguchi’s reaction** is a specific biochemical test used to detect the presence of **Arginine**. The reaction depends on the presence of the **guanidino group** found in the side chain of Arginine. When Arginine reacts with α-naphthol and sodium hypobromite (or hypochlorite) in an alkaline medium, it produces a characteristic **bright red color**. **Analysis of Options:** * **Arginine (Correct):** It is the only proteinogenic amino acid containing a guanidino group, making this test highly specific. * **Tyrosine (Incorrect):** Tyrosine is identified by the **Millon’s test**, which detects the phenolic group, resulting in a white precipitate that turns brick red upon heating. * **Proline (Incorrect):** As an imino acid, Proline reacts with **Ninhydrin** to yield a characteristic **yellow color**, unlike the purple/blue (Ruhemann's purple) seen with other amino acids. * **Cysteine (Incorrect):** Sulfur-containing amino acids like Cysteine are detected using the **Lead Sulfide (Sulfur) test**, where they react with lead acetate in an alkaline medium to form a black precipitate. **High-Yield Clinical Pearls for NEET-PG:** * **Xanthoproteic Test:** Specific for aromatic amino acids (Tyrosine, Phenylalanine, Tryptophan); gives a yellow color with hot concentrated nitric acid. * **Hopkins-Cole Test:** Specific for the indole ring of **Tryptophan**. * **Pauly’s Test:** Specific for **Histidine** and Tyrosine (detects imidazole and phenolic rings). * **Arginine’s Role:** It is a precursor for Nitric Oxide (NO), urea, and creatine, making it a frequent topic in metabolic biochemistry.

Q: Which amino acid produces kinks in the beta-pleated structure of proteins?

Proline. **Explanation:** **Why Proline is the Correct Answer:** Proline is unique among the 20 standard amino acids because it is technically an **imino acid**. Its side chain is cyclized and covalently bonded back to the nitrogen of the peptide backbone, forming a five-membered pyrrolidine ring. This rigid, cyclic structure imposes significant steric hindrance and restricts the rotation around the N-Cα bond (the phi angle). In the context of secondary structures: * **Alpha-helices:** Proline acts as a "helix breaker" because it lacks the amide hydrogen necessary for hydrogen bonding. * **Beta-pleated sheets:** The rigid ring structure prevents the polypeptide chain from adopting the flexible, extended conformation required for a regular beta-sheet, thereby introducing **kinks or bends**. **Analysis of Incorrect Options:** * **A. Glycine:** Known as the simplest amino acid (side chain is -H). It provides maximum flexibility to the polypeptide chain. While it is often found in "Beta-turns," it does not cause kinks due to rigidity; rather, it allows for tight packing. * **B. Serine:** A polar, uncharged amino acid with a hydroxyl group. It typically fits well into standard secondary structures without causing structural disruptions. * **D. Alanine:** A small, non-polar amino acid that is considered the "best" former of alpha-helices. It does not disrupt beta-pleated structures. **High-Yield Clinical Pearls for NEET-PG:** * **Hydroxyproline:** A post-translational modification of proline (requiring Vitamin C) essential for collagen stability. Deficiency leads to **Scurvy**. * **Beta-turns:** Proline and Glycine are the two most common amino acids found in Beta-turns (Type I and Type II). * **Ninhydrin Test:** Proline gives a **yellow** color with ninhydrin, unlike other amino acids which give a purple/Ruhemann's purple color.

Q: Which amino acid destabilizes the alpha-helix?

Proline. **Explanation:** The **alpha-helix** is a common secondary structure of proteins stabilized by hydrogen bonds between the carbonyl oxygen ($C=O$) of one amino acid and the amide hydrogen ($N-H$) of the amino acid four residues away. **Why Proline is the correct answer:** Proline is known as a **"helix breaker"** for two primary reasons: 1. **Rigid Structure:** Its side chain is cyclized back onto the backbone nitrogen, forming a secondary amino group (imino acid). This rigid ring structure prevents the rotation necessary to fit into the standard alpha-helical geometry. 2. **Lack of Hydrogen Bonding:** Because the nitrogen is part of a ring, it lacks the hydrogen atom required to form the stabilizing hydrogen bonds essential for maintaining the helix. When proline is present, it creates a "kink" or bend, destabilizing the structure. **Analysis of Incorrect Options:** * **B. Alanine:** This is the strongest helix-former. Its small, uncharged side chain fits perfectly into the helical conformation without steric hindrance. * **C. Glutamine:** An uncharged polar amino acid that is generally well-tolerated in alpha-helices. * **D. Tryptophan:** While bulky, it can still be part of a helix, though it is more commonly found in beta-sheets or at the ends of helices. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine** also destabilizes the alpha-helix, but for the opposite reason as proline: it is too flexible (due to having only a hydrogen atom as a side chain), allowing for too many conformational rotations. * **Amino acids that favor Alpha-helix:** MALEK (Methionine, Alanine, Leucine, Glutamate, Lysine). * **Proline’s Role:** While it breaks alpha-helices, it is essential for the **collagen triple helix** and is frequently found in **beta-turns**.

Q: Which amino acid contains a guanidine group?

Arginine. **Explanation:** The correct answer is **Arginine**. Arginine is a basic, semi-essential amino acid characterized by the presence of a **guanidino (guanidine) group** in its side chain. This group consists of a central carbon atom bonded to three nitrogen atoms, making Arginine the most basic amino acid (pKa ≈ 12.5) as it remains protonated and positively charged at physiological pH. **Analysis of Options:** * **Histidine (A):** Contains an **imidazole** ring. It is a basic amino acid often involved in enzyme active sites due to its pKa being near physiological pH. * **Proline (B):** Contains a secondary amino group (pyrrolidine ring) and is technically an **imino acid**. It causes "kinks" in alpha-helices. * **Tryptophan (C):** Contains an **indole** ring. It is the precursor for serotonin, melatonin, and niacin. * **Arginine (D):** Contains the **guanidine** group. It is a key intermediate in the Urea Cycle and the precursor for Nitric Oxide (NO). **High-Yield Clinical Pearls for NEET-PG:** * **Urea Cycle:** Arginine is cleaved by the enzyme *Arginase* to produce Urea and Ornithine. * **Nitric Oxide Synthesis:** Arginine is the sole precursor of Nitric Oxide (NO), a potent vasodilator, via the enzyme *Nitric Oxide Synthase (NOS)*. * **Creatine Synthesis:** Arginine, along with Glycine and Methionine (SAM), is required for creatine synthesis. * **Glucogenic nature:** Arginine is a purely glucogenic amino acid as it enters the TCA cycle via α-ketoglutarate.

Q: C peptide is a part of which of the following molecules?

Pro-insulin. **Explanation:** **1. Why Pro-insulin is correct:** Insulin is synthesized in the pancreatic beta cells as a single-chain precursor called **Preproinsulin**. After the signal peptide is removed in the endoplasmic reticulum, it becomes **Pro-insulin**. Pro-insulin consists of three chains: the A-chain, the B-chain, and a connecting segment known as the **C-peptide** (Connecting peptide). During maturation in the Golgi apparatus, endopeptidases cleave the C-peptide, leaving the A and B chains linked by disulfide bonds to form mature, active Insulin. Therefore, C-peptide is an integral structural part of the Pro-insulin molecule. **2. Why other options are incorrect:** * **Insulin:** Mature insulin consists only of the A and B chains. The C-peptide is secreted alongside insulin but is no longer part of the insulin molecule itself. * **ACTH:** Adrenocorticotropic hormone is derived from a different precursor called POMC (Pro-opiomelanocortin). * **Growth Hormone:** This is a single-chain polypeptide hormone produced by the anterior pituitary and does not involve a C-peptide structure. **3. Clinical Pearls for NEET-PG:** * **Equimolar Secretion:** Insulin and C-peptide are secreted into the portal circulation in equal amounts (1:1 ratio). * **Diagnostic Utility:** C-peptide has a longer half-life than insulin and is not cleared by the liver. It is used as a marker of **endogenous insulin production**. * **Factitious Hypoglycemia:** In cases of exogenous insulin overdose, C-peptide levels will be **low** (since commercial insulin lacks C-peptide). In Insulinoma, both insulin and C-peptide levels will be **high**. * **Type 1 vs Type 2 DM:** C-peptide levels are low/absent in Type 1 Diabetes and typically normal or high in early Type 2 Diabetes.

Question 1

The amino acid sequence of human chorionic gonadotropin (hCG) is approximately 80% similar to which of the following hormones?

Accepted Answer

Luteinizing hormone (LH)

Answer

Follicle-stimulating hormone (FSH)

Answer

Gonadotropin-releasing hormone (GnRH)

Answer

None of the above

Question 2

Which of the following is a negative acute-phase protein?

Accepted Answer

Transthyretin

Answer

C-Reactive Protein

Answer

Ferritin

Answer

Ceruloplasmin

Question 3

Gonadotrophin is a:

Accepted Answer

Glycoprotein

Answer

Steroid

Answer

Lipoprotein

Answer

Polypeptide

Question 4

Which protein is responsible for copper binding?

Accepted Answer

Ceruloplasmin

Answer

Albumin

Answer

Globulin

Answer

Transferrin

Question 5

Which of the following represents a covalent bond?

Accepted Answer

Disulphide bond

Answer

Hydrogen bond

Answer

Electrostatic bond

Answer

Ionic bond

Question 6

Sakaguchi's reaction is specific for which amino acid?

Accepted Answer

Arginine

Answer

Tyrosine

Answer

Proline

Answer

Cysteine

Question 7

Which amino acid produces kinks in the beta-pleated structure of proteins?

Accepted Answer

Proline

Answer

Glycine

Answer

Serine

Answer

Alanine

Question 8

Which amino acid destabilizes the alpha-helix?

Accepted Answer

Proline

Answer

Alanine

Answer

Glutamine

Answer

Tryptophan

Question 9

Which amino acid contains a guanidine group?

Accepted Answer

Arginine

Answer

Histidine

Answer

Proline

Answer

Tryptophan

Question 10

C peptide is a part of which of the following molecules?

Accepted Answer

Pro-insulin

Answer

Insulin

Answer

ACTH

Answer

Growth hormone

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

On this page

Practice by Chapter

Want unlimited practice?