Miscellaneous protein synthesis inhibitors

Protein synthesis inhibitors include many different classes of medications that prevent bacterial ribosomes from synthesizing proteins. The ones that target the 50S subunit of the ribosome include chloramphenicol, macrolides, lincosamides, and oxazolidinones.

Okay, first, let’s look at how genes become proteins. There’s two steps: transcription and translation. During transcription, a specific gene on the DNA is “read” and a copy is made called a messenger RNA, or mRNA, which is like a blueprint with instructions on what protein to build. Translation is also known as protein synthesis, and it’s when organelles called ribosomes assemble the protein from amino acids within the cytoplasm.

Now, prokaryotic cells, like bacteria, have smaller ribosomes than eukaryotic cells, like those found in humans. Bacterial ribosomes are made up of a 50S subunit and a 30S subunit which combine to form a 70S ribosome. Eukaryotic ribosomes are made up of a 60S and a 40S subunit that form a 80S ribosome. Since these proteins are different, we can create medications that selectively interfere with the bacterial ones.

Protein synthesis involves initiation, elongation, and termination. In bacteria, initiation occurs when the 50S and 30S subunits bind to the mRNA sequence to form a ribosome-mRNA complex, also called the initiation complex. The mRNA serves as a blueprint for the protein that will be synthesized. It’s made up of three nucleotide-long sequences, called codons, on top of which transport RNA, or tRNA, carrying amino acids can bind with their matching anticodon. The complete ribosome has 3 sites where tRNA can enter and bind. These are called the A, or aminoacyl site, the P, or peptidyl site, and the E, or exit site.

Elongation starts when the first tRNA, carrying a formylmethionine amino acid, enters the P site and binds to the start codon. This causes a conformational change which unlocks the A site for the next tRNA. The next tRNA binds at the A site, the amino acid detaches from the tRNA in the P site, and a peptide bond is formed by an enzyme called peptidyl transferase between the amino acids in the P and A sites, a process known as transpeptidation. Now, the A site has the newly formed peptide chain dangling from it, while the P site has an empty tRNA with no amino acids. In the final stage of elongation, the tRNA in the P site slides over to the E site, and the tRNA in the A site slides over to the P site. The ribosome slides over so the free A site is over the next codon on the mRNA and this process is called translocation. Next, a new tRNA with the matching anticodon binds, and the process repeats until a long peptide chain called a protein is synthesized.

Okay, so let’s look at some protein synthesis inhibitors. Chloramphenicol is a bit of a loner, and is the single representative in its group. It works during elongation by inhibiting peptidyl transferase, thus preventing peptide bond formation between new amino acids and the polypeptide chain. It’s a broad spectrum bacteriostatic medication, meaning it limits the growth of bacteria, rather than eradicating them. It’s usually administered parenterally, and it readily crosses the blood-brain barrier so it could be used to treat bacterial meningitis. Resistance to chloramphenicol occurs through acetyltransferase enzymes, which add an acetyl group to chloramphenicol to inactivate it. Chloramphenicol is effective against some strains of Haemophilus influenzae, Neisseria meningitidis, Bacteroides species, and Rickettsia species. However, due to its severe toxicity, it’s no longer used systemically in most countries with access to safer alternatives. It is also commonly used in the form of eye ointments to treat bacterial conjunctivitis. For the side effects, it’s very toxic to the bone marrow and could kill off the hematopoietic cells, leading to bone marrow suppression. This will usually lead to aplastic anemia first, followed by a decrease in platelets and leukocytes. It’s also notoriously teratogenic, as it readily crosses the placenta, so it should not be given to pregnant people or newborns. This is because infants lack the hepatic enzyme glucuronosyl transferase which normally metabolizes chloramphenicol. When this medication accumulates, it causes “gray baby syndrome”, where the infant is anemic and cyanotic, where the skin is a pale, or grayish color, and it can lead to cardiovascular collapse. Chloramphenicol also inhibits the hepatic enzymes in the cytochrome p450 family. These enzymes break down many other drugs, like warfarin, so when they are inhibited, it increases the action of those medications.

Next, let’s look at the macrolide antibiotics which include erythromycin, azithromycin, and clarithromycin. There’s also a novel macrolide called fidaxomicin. They work by binding to the 50S subunit of the ribosome and blocking translocation, so the ribosome can’t slide to the next codon on the mRNA. Most macrolides can be administered perorally as well as parenterally. Fidaxomicin is administered only perorally and it’s not absorbed in the systemic circulation. It is only used for Clostridioides difficile infection that causes pseudomembranous colitis. Now, in general, the rest of the macrolides are broad spectrum bacteriostatic medications effective against Gram positive and Gram negative bacteria alike. Specifically, they are used as the first line therapy for Bordetella pertussis, which causes whooping cough. They can also be used to treat typical pneumonia as well as atypical pneumonia caused by Mycoplasma pneumoniae or Legionella species. bacteria. Clarithromycin is used in combination with amoxicillin and omeprazole to treat Helicobacter pylori which causes peptic ulcers. Azithromycin is often used to treat sexually transmitted infections like chlamydia. Finally, resistance against macrolides typically involves developing efflux proteins that actively pump macrolides out of the cell. In addition, methylase enzymes can alter the ribosomal target site of the drug, and thus, decrease the drug binding. Side effects of macrolides are rare and the most common is gastrointestinal problems like diarrhea, nausea, and vomiting. The more serious side effects include a prolonged QT interval, so they should be avoided in people with arrythmias, and hepatotoxicity, so they are contraindicated in people with liver disease. Both erythromycin and clarithromycin can inhibit cytochrome p450.

Lincosamides are represented by clindamycin, which is the most commonly used member of the family. They function similarly to macrolides by binding to the 50S subunit of the ribosome and they inhibit translocation. Clindamycin can be given perorally, parenterally, or as a topical cream. It’s used to treat anaerobic bacterial infections of the lungs and mouth, in other words above the diaphragm. It is also used for Gram positive bacteria, such as group A streptococcus, especially in patients with penicillin allergy. Lincosamides are also used in toxin-mediated conditions, such as toxic shock syndrome, because they can reduce toxin production by inhibiting protein synthesis. Topical formulations of clindamycin can also be given to treat acne. Clindamycin tastes extremely bitter so it's not commonly prescribed to children. Common side effects include GI distress like diarrhea, nausea, vomiting, and cramps. Long term use can cause bacterial superinfection by Clostridioides difficile, formerly known as Clostridium difficile, which can cause pseudomembranous colitis.