Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 11: All of the following are glycoproteins except?

A. Blood antigen
B. Albumin (Correct Answer)
C. Immunoglobulin
D. hCG

Explanation: **Explanation:** The correct answer is **Albumin**. **1. Why Albumin is the correct answer:** Albumin is a simple, globular protein synthesized by the liver. Unlike glycoproteins, it does not undergo post-translational glycosylation (the covalent attachment of carbohydrate chains). It consists of a single polypeptide chain of 585 amino acids. Its primary functions include maintaining **plasma oncotic pressure** and acting as a non-specific carrier for bilirubin, fatty acids, and various drugs. **2. Analysis of Incorrect Options:** * **Blood Antigens (A):** ABO blood group substances are complex glycoconjugates. The antigenic specificity is determined by the specific carbohydrate moieties (sugars) attached to the protein or lipid backbone on the RBC membrane. * **Immunoglobulins (C):** All antibodies (IgG, IgA, IgM, etc.) are glycoproteins. The carbohydrate content (usually attached to the constant region of the heavy chain) is essential for mediating effector functions like complement activation. * **hCG (D):** Human Chorionic Gonadotropin is a glycoprotein hormone. Like other pituitary hormones (TSH, LH, and FSH), it consists of alpha and beta subunits with significant carbohydrate content (approx. 30% by weight). **3. High-Yield Clinical Pearls for NEET-PG:** * **Glycosylated Hemoglobin (HbA1c):** Note that HbA1c is formed by *non-enzymatic* glycation, not enzymatic glycosylation. * **Acute Phase Reactants:** Most plasma proteins (e.g., Ceruloplasmin, Transferrin, Haptoglobin) are glycoproteins; **Albumin and Transthyretin (Pre-albumin)** are notable exceptions. * **Mucins:** These are high-molecular-weight glycoproteins responsible for the viscosity of mucus. * **Erythropoietin:** Another high-yield example of a glycoprotein hormone.

Question 12: Blood group antigens belong to which class of molecules?

A. Conjugated proteins (Correct Answer)
B. Unconjugated proteins
C. Simple proteins
D. Hemoglobin binding proteins

Explanation: **Explanation:** Blood group antigens (ABO and Rh systems) are primarily **glycoproteins** or **glycolipids** located on the surface of red blood cell membranes. In the context of protein classification, they are categorized as **Conjugated Proteins**. 1. **Why Conjugated Proteins is Correct:** Conjugated proteins consist of a protein molecule joined to a non-protein group (the prosthetic group). Blood group antigens are glycoproteins where the protein backbone is covalently bonded to specific oligosaccharide chains (carbohydrates). The specificity of the ABO blood group is determined by the terminal sugar attached to the H-substance (e.g., N-acetylgalactosamine for Group A and Galactose for Group B). 2. **Why Other Options are Incorrect:** * **Unconjugated/Simple Proteins:** These consist solely of amino acids (e.g., Albumin). Blood group antigens require the carbohydrate moiety for functional and antigenic specificity. * **Hemoglobin binding proteins:** This refers specifically to proteins like **Haptoglobin**, which binds free hemoglobin to prevent oxidative damage and iron loss. While related to RBCs, they do not define blood group antigens. **Clinical Pearls for NEET-PG:** * **H-Substance:** The precursor for A and B antigens. Its absence results in the rare **Bombay Blood Group** (hh phenotype). * **Secretors:** In 80% of the population, these glycoprotein antigens are also found in water-soluble form in body fluids (saliva, semen) due to the *Se* gene. * **Rh Factor:** Unlike ABO antigens (carbohydrates), the Rh antigen is a **transmembrane protein** (polypeptide) without a carbohydrate component, though it still functions within the conjugated membrane complex.

Question 13: Which of the following amino acid substitutions represents a conservative mutation?

A. Glutamic acid to glutamine
B. Histidine to glycine
C. Alanine to leucine (Correct Answer)
D. Arginine to aspartic acid

Explanation: ### Explanation **Concept: Conservative vs. Non-conservative Mutations** A **conservative mutation** is a type of missense mutation where one amino acid is replaced by another that possesses **similar physicochemical properties** (e.g., size, charge, or hydrophobicity). Because the new amino acid "conserves" the chemical nature of the original, the overall tertiary structure and function of the protein often remain largely unaffected. **Why Option C is Correct:** * **Alanine to Leucine:** Both alanine and leucine are **non-polar, aliphatic (hydrophobic) amino acids**. Since they share the same chemical property, substituting one for the other in the hydrophobic core of a protein is less likely to cause significant structural disruption. **Analysis of Incorrect Options:** * **A. Glutamic acid to Glutamine:** Glutamic acid is negatively charged (acidic), while Glutamine is uncharged (polar). This change alters the net charge of the protein. * **B. Histidine to Glycine:** Histidine is a bulky, basic (positively charged) amino acid with an imidazole ring. Glycine is the smallest amino acid and lacks a side chain, which would significantly increase backbone flexibility and disrupt folding. * **D. Arginine to Aspartic acid:** This is a radical non-conservative mutation. Arginine is strongly basic (+), while Aspartic acid is acidic (-). This reversal of charge can destroy salt bridges and ionic interactions. **High-Yield Clinical Pearls for NEET-PG:** * **Sickle Cell Anemia:** A classic example of a **non-conservative** mutation where Glutamic acid (polar/charged) is replaced by Valine (non-polar) at the 6th position of the $\beta$-globin chain. * **HbC Disease:** Glutamic acid is replaced by Lysine at the same 6th position. * **Glycine** is unique because it is achiral and acts as a "helix breaker." * **Proline** is an imino acid that causes "kinks" or bends in $\alpha$-helices.

Question 14: What is Serotonin?

A. 5-Hydroxy tryptophan
B. 5-Hydroxy tryptamine (Correct Answer)
C. 5-Carboxy tryptamine
D. 5-Carboxy tryptophan

Explanation: **Explanation:** **1. Why Option B is Correct:** Serotonin is a biogenic amine synthesized from the essential amino acid **L-Tryptophan**. The synthesis occurs in a two-step process: * **Step 1:** Tryptophan is hydroxylated by *Tryptophan hydroxylase* (the rate-limiting enzyme) to form **5-Hydroxy tryptophan**. * **Step 2:** 5-Hydroxy tryptophan undergoes decarboxylation by *Aromatic L-amino acid decarboxylase* (using Vitamin B6 as a cofactor) to form **5-Hydroxy tryptamine (5-HT)**, which is the chemical name for Serotonin. **2. Why Other Options are Incorrect:** * **Option A (5-Hydroxy tryptophan):** This is the immediate **precursor** of serotonin, not serotonin itself. * **Options C & D (5-Carboxy variants):** These are chemically incorrect terms. The synthesis involves the removal of a carboxyl group (decarboxylation), not the addition of one. **3. High-Yield Clinical Pearls for NEET-PG:** * **Metabolism:** Serotonin is degraded by **MAO-A** (Monoamine oxidase) into **5-HIAA** (5-Hydroxyindoleacetic acid). * **Clinical Marker:** Elevated urinary levels of **5-HIAA** are a diagnostic marker for **Carcinoid Syndrome**. * **Localization:** While known as a neurotransmitter, the majority (~90%) of the body's serotonin is found in the **Enterochromaffin cells** of the GI tract. * **Melatonin Connection:** Serotonin is the direct precursor to Melatonin in the pineal gland (via acetylation and methylation). * **Cofactor:** Like other biogenic amine syntheses, the hydroxylation step requires **Tetrahydrobiopterin (BH4)**.

Question 15: Aminoacyl tRNA is required for all except:

A. Hydroxyproline (Correct Answer)
B. Methionine
C. Cystine
D. Lysine

Explanation: **Explanation:** The core concept tested here is the difference between **standard amino acids** (incorporated during translation) and **post-translational modifications**. **Why Hydroxyproline is the correct answer:** Protein synthesis occurs via translation, where **Aminoacyl tRNA** (tRNA charged with an amino acid) carries specific amino acids to the ribosome based on mRNA codons. **Hydroxyproline** is a non-standard amino acid. It does not have a specific codon or a corresponding tRNA. Instead, proline is first incorporated into the polypeptide chain (specifically collagen) via prolyl-tRNA. Once the chain is formed, proline residues are hydroxylated by the enzyme **prolyl hydroxylase**. Since hydroxyproline is formed *after* the protein is synthesized, it never exists as an aminoacyl-tRNA complex. **Analysis of Incorrect Options:** * **B. Methionine:** An essential amino acid and the universal "start" codon (AUG) initiator. It requires Met-tRNA for translation. * **C. Cystine:** While cystine is a dimer of two cysteines, it is formed by the oxidation of **Cysteine** residues already present in the polypeptide chain. However, in the context of this question, Cysteine itself requires a tRNA. (Note: Some examiners use Cystine/Cysteine interchangeably in this context, but both rely on the initial translation of Cysteine via tRNA). * **D. Lysine:** A standard basic amino acid that is coded by AAA/AAG and requires Lysyl-tRNA for incorporation into proteins. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vitamin C Connection:** Prolyl hydroxylase requires **Vitamin C (Ascorbic acid)** and **Ferrous iron (Fe²⁺)** as cofactors. Deficiency leads to **Scurvy** due to defective collagen cross-linking. 2. **Selenocysteine:** Known as the "21st amino acid," it is unique because it *does* have its own tRNA (Sec-tRNA), unlike hydroxyproline. 3. **Collagen Markers:** Urinary hydroxyproline levels are a clinical marker for bone resorption/collagen breakdown.

Question 16: In which of the following is the highest concentration of cystine found?

A. Melanin
B. Chondroitin sulfate
C. Myosin
D. Keratin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Keratin**. **1. Why Keratin is correct:** Keratin is a fibrous structural protein found in hair, nails, and the outer layer of the skin. Its high mechanical strength and rigidity are primarily due to a high concentration of the sulfur-containing amino acid **Cysteine**. When two cysteine residues are oxidized, they form a **disulfide bond**, creating a **Cystine** molecule. These disulfide cross-links stabilize the polypeptide chains, making keratin insoluble and resistant to stretching or enzymatic digestion. In human hair, cystine accounts for approximately 14% of the total protein content. **2. Why the other options are incorrect:** * **Melanin:** This is a pigment derived from the amino acid **Tyrosine**, not a protein rich in sulfur-containing amino acids. * **Chondroitin sulfate:** This is a **glycosaminoglycan (GAG)**, a complex carbohydrate found in cartilage and connective tissue. It is not a protein and does not contain cystine. * **Myosin:** This is a contractile protein found in muscle. While it contains various amino acids, it lacks the dense disulfide cross-linking characteristic of keratin. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hard vs. Soft Keratin:** "Hard" keratin (hair/nails) has a higher cystine content than "soft" keratin (skin). * **Homocystinuria:** A deficiency in cystathionine beta-synthase leads to high levels of homocysteine; patients often have "ectopia lentis" and skeletal abnormalities. * **Cystinuria:** A defect in the renal transport of COAL (Cystine, Ornithine, Arginine, Lysine), leading to **hexagonal cystine stones** in the urine. * **Permanent Waving (Perms):** This process involves chemically breaking and reforming the disulfide bonds (cystine) in hair keratin to change its shape.

Question 17: Which amino acid can protonate and deprotonate at neutral pH?

A. Histidine (Correct Answer)
B. Leucine
C. Glycine
D. Arginine

Explanation: **Explanation:** The ability of an amino acid to protonate and deprotonate at a specific pH depends on its **pKa value** (the pH at which the protonated and deprotonated forms are in equilibrium). **Why Histidine is correct:** Histidine contains an **imidazole side chain** with 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. This unique property allows it to participate in acid-base catalysis, making it a frequent resident at the active sites of enzymes (e.g., Carbonic Anhydrase). **Why the other options are incorrect:** * **Leucine:** An aliphatic, non-polar amino acid. Its side chain cannot gain or lose protons. * **Glycine:** The simplest amino acid with a hydrogen atom as its R-group; it lacks an ionizable side chain. * **Arginine:** A basic amino acid with a very high pKa (~12.5). At neutral pH, it remains almost entirely in its protonated (positively charged) state and does not readily deprotonate. **High-Yield Clinical Pearls for NEET-PG:** * **Buffering Capacity:** Due to its pKa being near physiological pH, Histidine is the most effective amino acid buffer in the body. This explains why **Hemoglobin** (rich in Histidine) is an excellent blood buffer. * **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.

Question 18: Chymotrypsin cleaves the peptide bond at the carboxyl-terminal side of which amino acid residues?

A. Phenylalanine
B. Tryptophan
C. Tyrosine
D. All of these (Correct Answer)

Explanation: **Explanation:** Chymotrypsin is a serine protease secreted by the pancreas that plays a vital role in protein digestion. Its specificity is determined by its **hydrophobic pocket** (S1 pocket), which is large enough to accommodate bulky, non-polar side chains. **1. Why "All of these" is correct:** Chymotrypsin preferentially cleaves peptide bonds on the **carboxyl-terminal side** of large hydrophobic amino acids, specifically the **aromatic amino acids**: * **Phenylalanine (Option A)** * **Tryptophan (Option B)** * **Tyrosine (Option C)** Because all three amino acids possess the bulky aromatic rings required to fit into the enzyme's active site pocket, chymotrypsin acts on all of them, making Option D the correct choice. It can also occasionally cleave at Leucine and Methionine, though with much lower affinity. **2. Comparison with other Proteases (Distinguishing incorrect options):** In the NEET-PG context, it is crucial to differentiate Chymotrypsin from other digestive enzymes: * **Trypsin:** Cleaves at the carboxyl side of basic amino acids (**Lysine and Arginine**). * **Elastase:** Cleaves at the carboxyl side of small neutral amino acids (**Alanine, Glycine, and Serine**). * **Pepsin:** An acidic protease that also favors aromatic residues but acts primarily in the stomach. **3. Clinical Pearls & High-Yield Facts:** * **Zymogen Activation:** Chymotrypsin is secreted as the inactive proenzyme **Chymotrypsinogen**, which is activated by **Trypsin** in the duodenum. * **Catalytic Triad:** The mechanism of action involves a specific "catalytic triad" of three amino acids: **Aspartate, Histidine, and Serine**. * **Diagnostic Use:** Fecal chymotrypsin levels can be measured to assess pancreatic exocrine function (e.g., in Chronic Pancreatitis or Cystic Fibrosis).

Question 19: Which amino acid is typically not found in an alpha-helix?

A. Alanine
B. Leucine
C. Proline (Correct Answer)
D. Isoleucine

Explanation: **Explanation:** The **alpha-helix** is a common secondary structure of proteins stabilized by hydrogen bonds between the carbonyl oxygen (C=O) and the amide hydrogen (N-H) of amino acids four residues apart. **Why Proline is the correct answer:** Proline is known as a **"helix breaker"** for two primary structural reasons: 1. **Rigid Cyclic Structure:** Proline’s side chain is covalently bonded to its amino group, forming a pyrrolidine ring. This creates significant steric hindrance and prevents the peptide backbone from rotating into the specific dihedral angles ($\phi$ and $\psi$) required for an alpha-helix. 2. **Lack of Hydrogen Bonding:** Proline is an **imino acid**. When it forms a peptide bond, it lacks a free hydrogen atom on its nitrogen. Consequently, it cannot participate as a hydrogen bond donor to stabilize the helix, leading to a "kink" or termination of the helical structure. **Why the other options are incorrect:** * **Alanine (A):** This is the most "helix-friendly" amino acid. Its small, uncharged side chain provides optimal stability and fits perfectly within the helical geometry. * **Leucine (B) & Isoleucine (D):** These are hydrophobic, aliphatic amino acids that are frequently found in alpha-helices, particularly in the transmembrane segments of proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Glycine** is also often excluded from alpha-helices, but for the opposite reason: it is too flexible (due to having only a hydrogen atom as a side chain), making the helix entropically unstable. * **Hydroxyproline:** Found in **Collagen**, it requires Vitamin C for synthesis. Deficiency leads to **Scurvy** due to unstable triple helices. * **Alpha-keratin** (hair/nails) is almost entirely alpha-helical, while **Silk Fibroin** is a classic example of beta-pleated sheets.

Question 20: Which among the following is the shortest peptide?

A. Angiotensin-III (Correct Answer)
B. Vasopressin
C. Angiotensin-II
D. Angiotensin-I

Explanation: To answer this question, one must know the specific amino acid counts of common biologically active peptides, particularly those in the Renin-Angiotensin-Aldosterone System (RAAS). ### **Explanation of the Correct Answer** **Angiotensin-III** is a **heptapeptide**, meaning it consists of **7 amino acids**. It is formed by the action of aminopeptidase on Angiotensin-II (which removes one amino acid from the N-terminus). While less potent than Angiotensin-II in terms of vasoconstriction, it is equally potent in stimulating aldosterone secretion. Among the given options, it has the fewest number of amino acids. ### **Analysis of Incorrect Options** * **Angiotensin-I (Option D):** This is a **decapeptide (10 amino acids)**. It is the inactive precursor formed when Renin acts on Angiotensinogen. * **Angiotensin-II (Option C):** This is an **octapeptide (8 amino acids)**. It is formed from Angiotensin-I by the action of Angiotensin-Converting Enzyme (ACE), which removes two C-terminal amino acids. * **Vasopressin (Option B):** Also known as Antidiuretic Hormone (ADH), it is a **nonapeptide (9 amino acids)** produced in the hypothalamus and stored in the posterior pituitary. ### **High-Yield Clinical Pearls for NEET-PG** * **RAAS Cascade Summary:** Angiotensinogen (452 aa) → Angiotensin I (10 aa) → Angiotensin II (8 aa) → Angiotensin III (7 aa). * **Glutathione:** A very common high-yield **tripeptide** (3 aa: Glutamate, Cysteine, Glycine). * **TRH (Thyrotropin-Releasing Hormone):** One of the shortest peptide hormones (3 aa). * **Oxytocin:** Like Vasopressin, it is also a **nonapeptide** (9 aa). * **Insulin:** A 51-amino acid polypeptide consisting of two chains (A chain: 21, B chain: 30) linked by disulfide bridges.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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