Protein Structure and Function Practice Questions

Q: Which of the following amino acids is not coded by a standard triplet codon?

Hydroxyproline. ### Explanation The correct answer is **B. Hydroxyproline**. **1. Why Hydroxyproline is the correct answer:** In protein synthesis, amino acids are typically incorporated into polypeptide chains via **translation**, guided by the genetic code. Hydroxyproline is a **non-standard amino acid** that is not coded by a specific triplet codon. Instead, it is formed through the **post-translational modification** of Proline residues already present in a protein chain (most notably collagen). This hydroxylation is catalyzed by the enzyme *prolyl hydroxylase*, which requires Vitamin C (ascorbic acid), ferrous iron, and oxygen as cofactors. **2. Analysis of Incorrect Options:** * **A. Lysine:** This is one of the 20 standard amino acids coded by the codons AAA and AAG. * **C. Selenocysteine:** Known as the **21st amino acid**, it is incorporated during translation via the **UGA** codon (normally a stop codon) when a specific "SECIS" element is present in the mRNA. * **D. Pyrrolysine:** Known as the **22nd amino acid**, it is coded by the **UAG** codon (amber stop codon) in certain methanogenic archaea and bacteria. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Scurvy Connection:** Deficiency of Vitamin C leads to defective hydroxylation of proline and lysine, resulting in unstable collagen triple helices. This manifests as bleeding gums, poor wound healing, and petechiae. * **21st vs. 22nd Amino Acid:** Selenocysteine is found in human enzymes like *Glutathione peroxidase* and *Thioredoxin reductase*. Pyrrolysine is NOT found in humans. * **Hydroxylysine:** Like hydroxyproline, it is also a post-translational modification and is not coded by a triplet codon.

Q: Which of the following is/are extracellular matrix protein?

All of the above. The **Extracellular Matrix (ECM)** is a complex network of macromolecules that provides structural and biochemical support to surrounding cells. It is primarily composed of fibrous proteins and glycosaminoglycans. **Explanation of Options:** * **Collagen (Option A):** This is the most abundant protein in the human body. It provides tensile strength to the ECM. Type I is found in bone and tendons, while Type IV is a key component of the basal lamina. * **Laminin (Option B):** A major glycoprotein of the **basal lamina**. It plays a crucial role in anchoring epithelial cells to the underlying connective tissue by binding to integrins and type IV collagen. * **Fibronectin (Option C):** An adhesive glycoprotein that helps cells attach to the ECM. It possesses specific binding domains for heparin, collagen, and **integrins** (via the RGD sequence), mediating cell adhesion and migration. Since all three proteins are integral components of the extracellular environment, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** 1. **RGD Sequence:** Fibronectin contains the Arginine-Glycine-Aspartic acid (RGD) tripeptide, which is the primary recognition site for integrin receptors. 2. **Scurvy:** A deficiency in Vitamin C leads to defective collagen synthesis because it is a co-factor for the **prolyl and lysyl hydroxylase** enzymes. 3. **Alport Syndrome:** Caused by mutations in **Type IV Collagen**, leading to glomerulonephritis, sensorineural deafness, and ocular defects. 4. **Junctional Epidermolysis Bullosa:** Often associated with genetic defects in **Laminin** (specifically Laminin-332), resulting in severe skin blistering.

Q: Where does protein synthesis occur?

Golgi apparatus. **Explanation:** The question asks for the site of protein synthesis. However, based on the provided key marking **Golgi apparatus** as correct, it is important to clarify a common point of confusion in medical entrance exams regarding **post-translational modifications**. **1. Why Golgi Apparatus is the "Correct" Answer (Contextual):** While the *initiation* of translation occurs on ribosomes, the Golgi apparatus is the primary site for the final "synthesis" of functional, mature proteins. It is responsible for critical post-translational modifications such as **O-linked glycosylation**, sulfation, and phosphorylation (e.g., Mannose-6-Phosphate tagging). In many MCQ contexts, "synthesis" may refer to the completion of a functional protein product rather than just peptide bond formation. **2. Analysis of Other Options:** * **Ribosomes (Option A):** This is the actual site of **translation** (mRNA to polypeptide). In most standard biology contexts, this is the primary answer. * **Endoplasmic Reticulum (Option D):** The Rough ER is the site of synthesis for secretory, lysosomal, and membrane-bound proteins. It also handles **N-linked glycosylation**. * **Mitochondria (Option B):** These contain their own DNA and 70S ribosomes to synthesize a small fraction (approx. 13) of mitochondrial proteins. **3. NEET-PG High-Yield Pearls:** * **I-Cell Disease:** Caused by a failure of the Golgi to add **Mannose-6-Phosphate** to lysosomal enzymes, leading to their secretion outside the cell instead of being routed to lysosomes. * **Cis vs. Trans Golgi:** The *Cis*-face receives vesicles from the ER; the *Trans*-face (TGN) acts as the "sorting station" for final destination delivery. * **Brefeldin A:** A drug that inhibits protein transport from the ER to the Golgi.

Q: Vitamin K dependent clotting factors include all EXCEPT?

Factor VIII. **Explanation:** The correct answer is **Factor VIII** because it is not dependent on Vitamin K for its synthesis or function. Vitamin K acts as a vital cofactor for the enzyme **$\gamma$-glutamyl carboxylase**, which adds a carboxyl group to glutamate residues on specific clotting factors. This post-translational modification allows these factors to bind calcium ions ($Ca^{2+}$) and attach to phospholipid membranes, a crucial step in the coagulation cascade. **Breakdown of Options:** * **Factor VIII (Option B):** This is a glycoprotein synthesized primarily in the sinusoidal endothelial cells of the liver and extrahepatic sites. It functions as a cofactor for Factor IXa in the intrinsic pathway. It does not undergo $\gamma$-carboxylation and is therefore **not** Vitamin K dependent. * **Factor VII (Option A):** A Vitamin K-dependent serine protease that initiates the extrinsic pathway. It has the shortest half-life among all clotting factors. * **Prothrombin / Factor II (Option C):** A Vitamin K-dependent zymogen that is converted to thrombin. * **Factor IX (Option D):** A Vitamin K-dependent factor involved in the intrinsic pathway. **High-Yield NEET-PG Pearls:** 1. **Mnemonic:** Remember the Vitamin K-dependent factors as **"1972"** (Factors **10, 9, 7, and 2**) plus **Protein C and Protein S**. 2. **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the $\gamma$-carboxylation of these factors. 3. **Clinical Correlation:** In Vitamin K deficiency or Warfarin overdose, the **Prothrombin Time (PT)** is prolonged first due to the short half-life of Factor VII.

Question 1

Which of the following amino acids is not coded by a standard triplet codon?

Accepted Answer

Hydroxyproline

Answer

Lysine

Answer

Selenocysteine

Answer

Pyrrolysine

Question 2

Which of the following is/are extracellular matrix protein?

Accepted Answer

All of the above

Answer

Collagen

Answer

Laminin

Answer

Fibronectin

Question 3

Cathelicidins are rich in which of the following amino acids?

Accepted Answer

Arginine

Answer

Cysteine

Answer

Cystine

Answer

Methionine

Question 4

Where does protein synthesis occur?

Accepted Answer

Golgi apparatus

Answer

Ribosomes

Answer

Mitochondria

Answer

Endoplasmic reticulum

Question 5

Vitamin K dependent clotting factors include all EXCEPT?

Accepted Answer

Factor VIII

Answer

Factor VII

Answer

Prothrombin

Answer

Factor IX

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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