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: The conversion of an optically pure isomer (enantiomer) into a mixture of equal amounts of both dextro and levo forms is called as?

Racemization. ### Explanation **Correct Option: C. Racemization** Racemization is the chemical process by which an optically active substance (a pure enantiomer) is converted into an optically inactive mixture containing equal amounts of both dextrorotatory (+) and levorotatory (–) forms. This mixture is known as a **racemic mixture**. Because the two enantiomers rotate plane-polarized light in opposite directions by equal amounts, the net optical rotation of a racemic mixture is zero. **Why other options are incorrect:** * **A. Polymerization:** This is a process where small molecules (monomers) combine chemically to produce a very large chain-like or network molecule (polymer). It is unrelated to optical activity. * **B. Stereoisomerization:** This is a broad umbrella term for the interconversion of any stereoisomers (including diastereomers or geometric isomers). While racemization is a *type* of stereoisomerization, "Racemization" is the specific and most accurate term for creating a 50:50 mixture of enantiomers. * **D. Fractionation:** This is a separation process (like fractional distillation) used to divide a mixture into its individual components or "fractions" based on physical properties like boiling points. **High-Yield Clinical Pearls for NEET-PG:** * **Thalidomide Tragedy:** A classic example of enantiomer differences. The (R)-enantiomer is a sedative, while the (S)-enantiomer is a potent teratogen causing phocomelia. In the body, these enantiomers can undergo **spontaneous in-vivo racemization**, which is why giving only the "safe" isomer did not prevent toxicity. * **Esmomeprazole vs. Omeprazole:** Omeprazole is a racemic mixture, whereas Esomeprazole is the pure (S)-enantiomer, which offers better bioavailability and more consistent acid suppression. * **Levocetirizine:** The (R)-enantiomer of Cetirizine; it has a higher affinity for H1 receptors and fewer side effects than the racemic mixture.

Question 1

A 5-day-old neonate presents with poor feeding, progressive encephalopathy, and a sweet caramel-like odour to the urine. Plasma amino acid analysis shows markedly elevated leucine, isoleucine, and valine. The newborn screening card is flagged. The relevant catabolic pathway is shown (Image 1), depicting transamination of branched-chain amino acids (leucine, isoleucine, valine) to their corresponding branched-chain α-keto acids, followed by the shared oxidative decarboxylation step (marked with a bold red X) catalysed by the branched-chain α-keto acid dehydrogenase (BCKDH) complex, which converts the α-keto acids to their respective acyl-CoA products. Which coenzyme is directly required by the enzymatic complex blocked at the step indicated in the image?

Accepted Answer

A. Thiamine pyrophosphate (TPP), lipoic acid, CoA, FAD, and NAD+

Answer

B. Pyridoxal phosphate (PLP), FAD, and NAD+

Answer

C. Biotin, ATP, and Mn2+

Answer

D. Tetrahydrofolate (THF), NADPH, and pyridoxal phosphate (PLP)

Question 2

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

Accepted Answer

Hydroxyproline

Answer

Lysine

Answer

Selenocysteine

Answer

Pyrrolysine

Question 3

Cathelicidins are rich in which of the following amino acids?

Accepted Answer

Arginine

Answer

Cysteine

Answer

Cystine

Answer

Methionine

Question 4

The conversion of an optically pure isomer (enantiomer) into a mixture of equal amounts of both dextro and levo forms is called as?

Accepted Answer

Racemization

Answer

Polymerization

Answer

Stereoisomerization

Answer

Fractionation

Question 5

What are amphipathic helices?

Accepted Answer

Alpha helices with predominantly hydrophobic R-groups projecting from one side and predominantly hydrophilic R-groups projecting from the other side of the helix axis.

Answer

Clusters of twisted strands of beta sheet.

Answer

Short segments of amino acids that join two units of secondary structure.

Answer

Supersecondary structures.

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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