Erythromycin Mode of Action

Erythromycin is a prominent member of the macrolide class of antibiotics.
All macrolides are characterized by a macrocyclic lactone ring structure.
The antibiotic is primarily effective against Gram-positive bacteria, although it also exhibits activity against specific Gram-negative bacteria
Clinically, erythromycin is utilized to treat a wide array of infections, including respiratory tract infections, gastrointestinal (GI) infections, skin infections, and sexually transmitted infections.
It is frequently prescribed as a broad-spectrum alternative for patients with penicillin allergies.

Lactone Ring: Erythromycin possesses a 14-membered macrocyclic lactone ring.
Sugar Substituents: The lactone ring is substituted with two specific sugars:
- D-desosamine: An amino sugar attached via a $\beta$ -glycosidic linkage to the 5-hydroxyl group (C5) of the ring.
- L-cladinose: A neutral sugar attached via an $\alpha$ -glycosidic bond to the 3-hydroxyl group (C3) of the ring.

Erythromycin targets the prokaryotic ribosome to inhibit protein synthesis.
Assembled Ribosome (70S): Composed of two subunits:
- Small Subunit (30S): Contains 16S rRNA.
- Large Subunit (50S): Contains 5S rRNA and 23S rRNA.
Key Sites: The Ribosome features an A site (aminoacyl), P site (peptidyl), and E site (exit), along with the Peptidyl Transferase Center (PTC) and the Nascent Peptide Exit Tunnel (NPET).

Binding Site: Erythromycin binds reversibly to the 50S ribosomal subunit, specifically interacting with the 23S rRNA.
NPET Blockage: It occupies a site within the Nascent Peptide Exit Tunnel, positioned just below the Peptidyl Transferase Center (PTC).
Steric Hindrance: The lactone ring lies against the tunnel wall, while sugar moieties extend toward the PTC. This narrows the tunnel entrance from approximately $18-19\,\text{Å}$ to less than $10\,\text{Å}$ .
Primary Result: The physical blockage prevents the growing nascent peptide chain from passing through the tunnel. Protein synthesis initiates, but once the peptide chain reaches the drug, it cannot proceed.
Peptidyl-tRNA Dissociation: Peptides longer than 8 amino acids are unable to dissociate and get stalled. Smaller peptides can dissociate.
Mode of Action: Primarily bacteriostatic, as it prevents bacterial growth by halting protein production and disrupting cellular metabolism.

Molecular Interactions:
- Hydrogen Bonding: Formed between the desosamine sugar (C5) and residues A2058 and A2059 of the 23S rRNA.
- Hydrophobic Interactions: The lactone ring interacts with residues A2057, C2611, and A2058.
- Sugar Contact: The cladinose sugar (C3) makes close contact with residues C2610 and G2505.
Key Ribosomal Components:
- Central loop of Domain V and Hairpin 35 of Domain II of the 23S rRNA.
- Ribosomal proteins L4 and L22 form part of the tunnel constriction.

Recent research suggests macrolide activity may be sequence-dependent.
Macrolide Arrest Motifs (MAMs): Certain amino acid sequences in the nascent peptide cause the ribosome to stall when a macrolide is present.
Prevalent Motif: The motif $[R/K]X[R/K]$ (where R/K are positively charged residues Arginine or Lysine) is the most common arrest site, designated as $[+]X[+]$ .
Implication: Proteins that do not contain these specific motifs may bypass the steric hindrance of the drug and complete translation.

Erythromycin may also inhibit the assembly of new 50S ribosomal subunits.
Direct Inhibition: The drug binds to precursor ribosomal particles, potentially preventing the addition of necessary ribosomal proteins.
Indirect Inhibition: By stopping the addition of ribosomal proteins, the drug creates an imbalance in components required for ribosome assembly.
Outcome: This leaves rRNA exposed and susceptible to degradation by RNases and prevents required conformational rearrangements.

Phylogenetic Distinction: The identity of the nucleotide at position 2058 is critical.
- Eubacteria: Position 2058 is always an Adenine (A2058), which allows for high-affinity binding.
- Eukaryotes/Archaea: Position 2058 is usually a Guanine (G2058).
Mechanism: Guanine is too bulky to allow erythromycin to bind effectively to the 80S eukaryotic ribosome, protecting human cells.
Side Effects: Despite selective toxicity, side effects occur and include hepatotoxicity, cardiotoxicity, nausea, vomiting, and epigastric distress.

Q: Why does erythromycin not inhibit ribosomes already in the process of translation?
A: Erythromycin must bind at the start of protein synthesis. If a ribosome has already produced a long nascent peptide that occupies the exit tunnel, the drug cannot enter its binding site due to the presence of the existing peptide chain.
Q: What happens to the dissociated peptidyl-tRNAs?
A: They accumulate in the cytoplasm and are scavenged by peptidyl-tRNA hydrolase to recycle free tRNA. If the level of dissociated peptidyl-tRNAs exceeds the capacity of the hydrolases, the depletion of the free tRNA pool can become cytotoxic and disrupt metabolism.