Miscellaneous protein synthesis inhibitors

Definitions & Key takeaways

Protein synthesis inhibitors are a class of antibiotics which prevent bacterial ribosomes from synthesizing proteins. They include drugs like chloramphenicol, macrolides, lincosamides, and oxazolidinones.

Most of these drugs act on the 50S subunit of the ribosome, but their mechanisms can be very different. For example, oxazolidinones like linezolid stop the initiation complex from forming. Both the macrolides and lincosamides prevent translocation. Chloramphenicol inhibits peptidyl transferase which is the enzyme that creates the peptide bonds.

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Introduction0:00–0:23

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.

Physiology0:23–3:20

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.

Chloramphenicol3:20–5:30

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.

Macrolides5:30–7:07

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.

Lincosamides7:07–7:58

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.

Oxazolidinones7:58–9:57

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.
Oxazolidinones like linezolid are a new group of drugs. Their method of protein synthesis inhibition involves binding to the 50S subunit of the ribosome, and prevents it from combining with the 30S subunit to form the initiation complex.
Linezolid therefore stops protein synthesis before it even begins. It can be used parenterally as well as perorally.
The oxazolidinones have a unique structure and mechanism so few organisms have developed resistance to it yet. Those that have, typically have mutations in their ribosomal RNA that prevents the medication from binding.
Now, linezolid is effective against a wide variety of gram positive bacteria, but it’s often reserved for bacterial infections that are resistant to other antibiotics.
This includes Methicillin-resistant Staphylococcus aureus, or MRSA, and vancomycin-resistant Enterococcus faecium, or VRE.
When used for short periods, linezolid is a safe drug, with minor side effects, like diarrhea, nausea, vomiting, and rash.

Memory palace9:57–14:18

If used over a long period of time, patients begin to show signs of bone marrow suppression, which often manifests as thrombocytopenia.
Another problem is neurotoxicity and linezolid can cause peripheral neuropathy and optic neuropathy, characterized by washed out perception of colors as well as progressive loss of vision.
Another interesting effect is that linezolid is also a weak inhibitor of monoamine oxidase; an enzyme inside neurons that breaks down the neurotransmitter serotonin.
If a person is also taking antidepressants that increases serotonin, like a monoamine oxidase inhibitor, (MAOi) or a selective serotonin reuptake inhibitor, SSRI, they could develop serotonin syndrome which is characterized by flushing, extremely high body temperature, muscle rigidity, and tremor that may progress to seizures.
So, let’s make a mnemonic to help you memorize and retain all these crazy pharm facts! So let’s set the scene by going to an apartment where a lady is celebrating her 50th birthday, which will help you remember all these drugs act on the 50S ribosome.
Now, she’s putting the candles onto the cake which represents the formation of peptide bonds by peptidyl transferase. The drug that inhibits this step is chloramphenicol which will be represented by a large coral growing out of the cake.
Let’s also put some red eyes all over the coral to help you remember that it’s used topically to treat bacterial conjunctivitis.
Now, to memorize the harmful side effects, let’s put some broken bones sticking out of the cake for bone marrow suppression.
Next to the cake are some red plastic plates that look like red blood cells which represent aplastic anemia. To help you remember this medication is contraindicated for infants and during pregnancy, the lady is pregnant and carries a gray-colored baby doll to help you remember “gray baby syndrome.” A cell that’s chrome-colored, representing cytochrome p450, ate some of the coral and died, which will help you remember chloramphenicol is a cytochrome p450 inhibitor.
Okay, moving on! Translocation will be represented by the door into the apartment, and two groups of drugs will inhibit this process.
On one side of the door, we have four mice carrying large lids for the macrolides. One of them is red for erythromycin; one is wearing an Aztec headdress for azithromycin; one is transparent, or clear, for clarithromycin; and one is wearing a fedora and holding an axe for fidaxomicin.
For indications, let’s put a coughing whooping crane behind the mice to represent whooping cough. Next to it, there’s a very sick person coughing in bed for typical pneumonia and another coughing person with a walking stick, who represents atypical, or “walking” pneumonia.
You can see a slice of pie in the clear mouse’s stomach for H. Pylori and the Aztec mouse is standing on a large clam for chlamydia and other STIs.
The intestines of the mouse with the fedora and axe are showing, for Clostridioides difficile infection that causes pseudomembranous colitis.
For side effects, a long-tailed squirrel who’s quite a cutie wants to join the mice squad and it represents long QT syndrome.
It’s a bit sick however, and is vomiting, which represents GI distress. Okay let’s move on to clindamycin, which also blocks translocation.
So let’s put a mouse dressed as Cinderella on the other side of the door. Now instead of a pumpkin carriage, let’s have her inside a fishbowl, which represents an anaerobic environment.
The fish bowl is sitting on top of a diaphragm to help you remember clindamycin is used to treat anaerobic infections above the diaphragm.
The Cinderella mouse is worried about her acne which is another indication for this medication. For side effects, this medication causes GI distress, so let’s say the stress is giving our poor mouse stomach pains and she’s vomiting outside the fishbowl.
The vomit goes into a closet laying flat on the ground which will help you remember long term use of clindamycin can lead to a superinfection of Clostridioides difficile.
Okay, the last medication is linezolid and we’ll use a pan lid with black and white lines on it. This drug inhibits the formation of the initiation complex where the 30S, and 50S subunits join together along with the mRNA.

Review14:18–14:50

So we can have a pair of newlyweds joining the party. They can represent the 50S and 30S subunits.
The pan lid is the gift they brought and they are carrying it between them which keeps them separated. For indications, let’s have the son of the couple playing with a remote controlled Mars Rover for “MARSA.” He’s using a VR headset to control it which stands for VRE.

Mind map14:50–16:11

Let’s move on to side effects. The kid’s other toy is a dead knight who’s about to be cremated, and he represents serotonin syndrome.
His body is stiff, so there’s muscle rigidity, the hot fire represents hyperthermia, and the armor is glowing red for flushing.
Let’s wrap his arms in bandages to represent peripheral neuropathy, and instead of a sword, he’s holding a broken bone for bone marrow suppression.
All right, as a quick recap. There are numerous classes of antibiotics that inhibit protein synthesis.
Many of these act on the 50S subunit of the ribosome, but their mechanisms can be very different. Oxazolidinones like linezolid stop the initiation complex from forming.
Both the macrolides and lincosamides, like clindamycin, prevent translocation. Chloramphenicol inhibits peptidyl transferase which is the enzyme that creates the peptide bonds.
lincosamides Like clindamycin prevent translocation Chloramphenicol inhibits peptidyl transferase which is the enzyme that creates the peptide bonds But wait there's more here's a mind map with all of the mnemonics from the video Go ahead and pause so you can test yourself to see what you remember Stay