Questions: Ribosome Structure and Peptidyl Transferase Activity

Question 1

What is the strongest evidence that rRNA — not ribosomal proteins — catalyzes peptide bond formation?

• A. rRNA is more abundant by mass than ribosomal proteins in the large subunit
• B. High-resolution crystal structures show no ribosomal protein within 18 Å of the peptidyl transferase center
• C. Ribosomal proteins can be completely removed without stopping translation
• D. rRNA sequences are more evolutionarily conserved than ribosomal protein sequences

Answer: (B)

Crystal structures solved by Steitz, Yonath, and Ramakrishnan (Nobel 2009) placed every atom in the ribosome and showed that the active site is entirely surrounded by 23S/28S rRNA — no protein is close enough to contact the reacting groups. This structural evidence directly shows rRNA is the catalyst. The other options are partially true but not the decisive evidence: ribosomal proteins stabilize rRNA folds and assist factor binding, but they cannot be fully stripped without disrupting the ribosome's structure.

Question 2

A researcher treats ribosomes with a drug that specifically cross-links and inactivates all ribosomal proteins, leaving rRNA intact. What would you predict about peptide bond formation?

• A. Peptide bond formation stops entirely, because the proteins are the enzymes
• B. Peptide bond formation continues, because the catalytic activity resides in the rRNA of the peptidyl transferase center
• C. Peptide bond formation slows by about half, because proteins enhance rRNA catalysis
• D. Peptide bond formation accelerates, because the proteins were inhibiting rRNA activity

Answer: (B)

The peptidyl transferase center (PTC) is a ribozyme — RNA does the chemistry. Ribosomal proteins serve structural and regulatory roles (stabilizing rRNA folds, facilitating factor binding, assisting subunit assembly) but are not within catalytic distance of the active site. This is a direct application of the ribosome-as-ribozyme concept. Option C is the most tempting wrong answer, implying a protein-rRNA partnership, but the crystal structure evidence shows proteins are not needed for catalysis.

Question 3

The peptidyl transferase center is located in the small (40S) ribosomal subunit, which is responsible for decoding mRNA and catalyzing peptide bond formation.

• T. True
• F. False

Answer: False

The small subunit (40S in eukaryotes, 30S in prokaryotes) handles decoding — matching mRNA codons to aminoacyl-tRNA anticodons. The peptidyl transferase center resides in the large subunit (60S in eukaryotes, 50S in prokaryotes), which contains the A, P, and E sites for tRNA binding and where the 28S/23S rRNA catalyzes peptide bond formation.

Question 4

The discovery that the ribosome is a ribozyme provides support for the RNA world hypothesis, because it demonstrates that RNA can catalyze the central biochemical reaction of protein synthesis.

• T. True
• F. False

Answer: True

If RNA catalyzes the most fundamental biosynthetic reaction in all of biology — making proteins — then RNA likely preceded proteins as the original catalyst of life. Proteins require ribosomes (RNA) to be made, but ribosomes do not require proteins for their catalytic function. This chicken-and-egg relationship is dissolved if RNA came first: an RNA world could have developed ribozyme-based protein synthesis before protein enzymes took over most other catalytic roles.

Question 5

Describe the mechanism of peptide bond formation at the peptidyl transferase center: what acts as the nucleophile, what is the electrophile, and what is the chemical outcome?

Model answer: The α-amino group of the aminoacyl-tRNA in the A site is the nucleophile (after deprotonation by the rRNA). The electrophile is the carbonyl carbon of the ester bond linking the growing polypeptide chain to the P-site tRNA. Nucleophilic attack forms a tetrahedral intermediate that resolves by breaking the P-site ester bond, transferring the entire polypeptide to the A-site tRNA. The P-site tRNA is left deacylated; the A-site tRNA now carries the elongated peptide chain.

This is a nucleophilic acyl substitution: the ester bond between the peptide and the P-site tRNA is replaced by an amide bond (peptide bond) between the last amino acid of the growing chain and the new amino acid on the A-site tRNA. The ribosome then translocates, moving the peptidyl-tRNA from A to P and the deacylated tRNA from P to E, ready for the next elongation cycle.