A pharmaceutical company has modified one of its existing antibiotics to have an improved toxicity profile. The new antibiotic blocks protein synthesis by first entering the cell and then binding to active ribosomes. The antibiotic mimics the structure of aminoacyl-tRNA. The drug is covalently bonded to the existing growing peptide chain via peptidyl transferase, thereby impairing the rest of protein synthesis and leading to early polypeptide truncation. Where is the most likely site that this process occurs?
Q42
A 4-year-old boy with beta thalassemia requires regular blood transfusions a few times per month because of persistent anemia. He is scheduled for a splenectomy in the next several months. Samples obtained from the boy’s red blood cells show a malformed protein with a length of 160 amino acids (in normal, healthy red blood cells, the functional protein has a length of 146 amino acids). Which of the following best accounts for these findings?
Q43
An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?
Transcription/translation US Medical PG Practice Questions and MCQs
Question 41: A pharmaceutical company has modified one of its existing antibiotics to have an improved toxicity profile. The new antibiotic blocks protein synthesis by first entering the cell and then binding to active ribosomes. The antibiotic mimics the structure of aminoacyl-tRNA. The drug is covalently bonded to the existing growing peptide chain via peptidyl transferase, thereby impairing the rest of protein synthesis and leading to early polypeptide truncation. Where is the most likely site that this process occurs?
A. E site
B. 30S small subunit
C. A site (Correct Answer)
D. 40S small subunit
E. P site
Explanation: ***A site***
- The **A (aminoacyl) site** is where incoming aminoacyl-tRNAs bind during translation, bringing new amino acids to the ribosome. Since the antibiotic mimics **aminoacyl-tRNA** and is covalently bonded to the peptide chain by **peptidyl transferase**, its action must occur at the A site.
- Binding at the A site and subsequent peptide bond formation with the antibiotic would lead to premature polypeptide truncation, as no further amino acids can be added.
*E site*
- The **E (exit) site** is where deacylated tRNAs are released from the ribosome after having delivered their amino acid to the growing peptide chain in the P site.
- The antibiotic's mechanism of action, involving binding and covalent incorporation into the peptide, does not align with the function of the E site.
*30S small subunit*
- The **30S small ribosomal subunit** in prokaryotes is primarily involved in mRNA binding and decoding, ensuring the correct aminoacyl-tRNA binds to the mRNA codon.
- While the antibiotic binds to active ribosomes, its key action described as mimicking aminoacyl-tRNA and being incorporated by peptidyl transferase points to a specific binding site within the ribosome rather than the entire subunit's general function.
*40S small subunit*
- The **40S small ribosomal subunit** is found in **eukaryotic ribosomes**, not prokaryotic ones, and is involved in mRNA binding during initiation.
- The question implies an antibiotic targeting bacterial protein synthesis (given its discussion of modifying an existing antibiotic), making eukaryotic ribosomal subunits an unlikely target.
*P site*
- The **P (peptidyl) site** holds the tRNA carrying the growing polypeptide chain. Peptidyl transferase activity forms a peptide bond between the amino acid in the A site and the peptide in the P site.
- While peptidyl transferase is involved, the antibiotic *mimics* aminoacyl-tRNA, which is delivered to the A site for peptide bond formation, rather than the P site which already holds the growing chain.
Question 42: A 4-year-old boy with beta thalassemia requires regular blood transfusions a few times per month because of persistent anemia. He is scheduled for a splenectomy in the next several months. Samples obtained from the boy’s red blood cells show a malformed protein with a length of 160 amino acids (in normal, healthy red blood cells, the functional protein has a length of 146 amino acids). Which of the following best accounts for these findings?
A. Nonsense mutation
B. Silent mutation
C. Missense mutation
D. Splice site mutation (Correct Answer)
E. Frameshift mutation
Explanation: ***Splice site mutation***
- A **splice site mutation** can lead to the retention of an **intron** or the **skipping of an exon**, resulting in an abnormal mRNA sequence.
- If a cryptic splice site is used or an intron is retained, it can lead to the inclusion of additional amino acids in the final protein, thus increasing its length from 146 to 160 amino acids.
*Nonsense mutation*
- A **nonsense mutation** results in a **premature stop codon**, which would produce a **truncated protein** shorter than 146 amino acids.
- This type of mutation does not explain the observed increase in protein length.
*Silent mutation*
- A **silent mutation** changes a single nucleotide but does **not alter the amino acid sequence** of the protein due to the redundancy of the genetic code.
- This would result in a normal protein length of 146 amino acids and no observed malformation.
*Missense mutation*
- A **missense mutation** changes a single nucleotide leading to a **different amino acid**, but it typically **does not alter the total length** of the protein.
- While it can lead to a *malformed protein*, it wouldn't explain the increased length from 146 to 160 amino acids.
*Frameshift mutation*
- A **frameshift mutation** is caused by the **insertion or deletion of nucleotides** not divisible by three, leading to a shift in the reading frame downstream.
- This often results in a **premature stop codon** and a **shorter, non-functional protein**, or a completely altered sequence that is usually unstable, rather than a longer protein with 160 amino acids.
Question 43: An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?
A. Glycosylation
B. Acylation
C. Carboxylation
D. Phosphorylation
E. Ubiquitination (Correct Answer)
Explanation: ***Ubiquitination***
- The description of a **chemically-tagged protein** attaching to **lysine residues** on a newly formed polypeptide strongly suggests **ubiquitin**, a small protein that marks other proteins for degradation.
- The subsequent lysis by a **barrel-shaped complex** (the **proteasome**) into amino acids and the chemically-tagged proteins is the hallmark of the **ubiquitin-proteasome pathway**, a major mechanism for targeted protein degradation.
*Glycosylation*
- Involves the **covalent attachment of carbohydrate moieties** to proteins, typically at asparagine, serine, or threonine residues.
- While it is a common post-translational modification, it does not involve a "chemically-tagged protein" marking for proteasomal degradation.
*Acylation*
- Refers to the addition of an **acyl group** (e.g., fatty acids like myristate or palmitate) to a protein, often impacting membrane association.
- This process is distinct from the described mechanism of protein tagging and subsequent degradation by a barrel-shaped complex.
*Carboxylation*
- Involves the **addition of a carboxyl group** to a protein, most notably to glutamate residues in clotting factors, requiring vitamin K.
- This modification is not involved in marking proteins for degradation and does not utilize a specific "chemically-tagged protein" for this purpose.
*Phosphorylation*
- Refers to the **addition of a phosphate group** to a protein, typically at serine, threonine, or tyrosine residues, to regulate protein activity, signaling, and interactions.
- While it is a common regulatory mechanism, it does not involve a "chemically-tagged protein" targeting the protein for complete degradation into amino acids by a proteasome.
