Protein synthesis inhibitors: Aminoglycosides

Definitions & Key takeaways

Aminoglycosides are a class of antibiotics that inhibit bacterial protein synthesis by binding to the 30S subunit of their ribosomes. This binding disrupts proofreading in bacterial protein synthesis, leading to the production of non-functional or truncated proteins. Examples of the aminoglycoside family include amikacin, gentamicin, neomycin, streptomycin, and tobramycin. Alone, aminoglycosides are effective against Gram-negative aerobic bacteria, but could also treat Gram-positive bacteria if combined with a cell wall synthesis inhibitor, like a beta-lactam antibiotic, or with vancomycin. Notable adverse drug reactions include nephrotoxicity, ototoxicity, neuromuscular blockade, nausea, and allergic reaction.

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Introduction0:00–0:34

Aminoglycosides are antimicrobial antibiotics that inhibit bacterial ribosomes, which are the organelles that make proteins.
Genes are used to synthesize proteins in 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.
Translation is also known as protein synthesis, and it’s when ribosomes use mRNA to assemble proteins from amino acids within the cytoplasm.

Physiology0:34–3:32

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.
In both eukaryotic and prokaryotic cells, 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.
The mRNA serves as a blueprint for the protein that will be synthesized. It’s made up of three nucleotide long sequences, called codons.
Transport RNA, or tRNA, carrying different amino acids can bind to these codons with their matching anticodons. The complete ribosome-mRNA complex 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 in the ribosome, which unlocks the A site for the next tRNA. A process called proofreading occurs here where only tRNAs with the matching anticodon can bind to corresponding mRNA codon.
After the next tRNA binds at the A site, the amino acid detaches from the tRNA in the P site, and gets attached to the amino acid in the A site by the enzyme peptidyl transferase.
This step is called transpeptidation because the peptide chain is transferred from the P site tRNA to the A site tRNA. 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, called translocation, the ribosome slides across the mRNA, and the A site sits above a new codon, the tRNA that was in the A site slides over to the P site, and the tRNA in the P site slides over to the E site.
Now, a new tRNA can bind at the A site, and the process repeats until a long peptide chain, called a protein, is synthesized.
Finally, termination happens when the ribosome comes across a termination codon on the mRNA. tRNA can’t bind to these, so it signals the end of protein synthesis.
To start working, aminoglycosides need to enter the bacteria, and frankly, they aren’t very good at it. Gram positive bacteria have a thicker cell wall compared to Gram negative bacteria, so aminoglycosides can’t even penetrate them.

Mechanism of Action3:32–5:14

They also require an O2-dependent cotransporter on the cell membrane to be transported into the cell. Obviously, these aren’t seen in strictly anaerobic bacteria, so we can rule out aminoglycosides as a treatment for anaerobic infections.
So on their own, aminoglycosides can only target Gram negative aerobic bacteria. Once they enter the bacteria, their main mechanism of action is to irreversibly bind to the 30S units and interfere with the proofreading process, thus causing errors in the protein’s amino acid sequence.
These faulty proteins will eventually lead to the death of the bacteria. When aminoglycosides bind to 30s subunits, they can also prevent the formation of the ribosome-mRNA complex, thus reducing the amount of proteins being synthesized.
Finally, a 30s subunit bound to an aminoglycoside can get stuck to the mRNA. This prevents translocation where the ribosome slides over to the next codon.
Depending on the concentration applied, aminoglycosides can work as bacteriostatic, which stops the bacteria from multiplying, or bactericidal, which outright kills them.
Aminoglycosides are also known for their strong post antibiotic effect because they remain effective hours after the medication is cleared from the body.
Examples of the aminoglycoside family include amikacin, gentamicin, neomycin, streptomycin, and tobramycin. They are used to treat a wide variety of infections in the respiratory tract, urinary tract, blood, bone, and soft tissues.

Indications5:14–7:21

It is used mainly against aerobic Gram negative bacteria like Proteus species, Escherichia coli, Klebsiella pneumoniae, Enterobacter species, and Serratia species.
Streptomycin is often used against mycobacterium tuberculosis, which thrives in the oxygen-rich lung tissue. Tobramycin is given topically to treat eye infections, or in a nebulized form to treat Pseudomonas aeruginosa infections in people with cystic fibrosis.
Now, remember how the thick cell wall on Gram positive bacteria stops aminoglycosides? If we add a cell wall synthesis inhibitor, such as a beta-lactam antibiotic like penicillin, we can effectively demolish or weaken the cell wall, allowing aminoglycosides to slip through.
This combination is used to treat Gram positive staphylococci and streptococci infections like infective endocarditis. Aminoglycosides are not absorbed from the gut, so neomycin is often given before colorectal surgeries to eliminate the native flora.
For systemic infections, they are given via the intravenous or intramuscular route. They can also be used topically, in the form of creams, gels, or drops.
Bacterial resistance to the aminoglycosides and can be seen in certain strains of P. aeruginosa, S.
aureus, and Enterobacter species. These bacteria developed transferase enzymes that modify aminoglycosides through O-phosphorylation, O-adenylation, or N-acetylation through O-phosphorylation, O-adenylation, or N-acetylation, which inactives the aminoglycosides.
Aminoglycosides are ototoxic and can damage the cochlea and hair cells in the ear. This effect is enhanced if the person is taking loop diuretics, which are also ototoxic.
Another side effect is nephrotoxic which can lead to kidney failure. Aminoglycosides inhibit the release of acetylcholine from motor neurons at the neuromuscular junction, and cause weakness or paralysis.

Side effects & Contraindications7:21–8:16

At high doses, they can even cause potentially fatal respiratory paralysis. This is why they are contraindicated in patients who suffer from myasthenia gravis, a neuromuscular disorder where antibodies bind to and inhibit the acetylcholine receptors on muscle cells.
Other nonspecific side effects include nausea, vomiting, and allergic reactions. So, let’s make a mnemonic to help you memorize and retain all these pharm facts!
All the aminoglycosides are having a wild party. First, we have the medications with “mycin” in their name.
There’s a mouse with wild stripes painted on his body for streptomycin, a mouse with crazy neon glow sticks for neomycin, a particularly brave mouse is riding small cobra for tobramycin, and a mouse dressed as a dapper gentleman for gentamicin.

Memory Palace8:16–10:28

Amikacin is not a mouse, but a human singer holding a giant mic. For the common Gram negative bugs treated by these medications, we have an angry neighbor holding a protest sign for proteus species.
She also brought her friend who’s carrying a large club for klebsiella, and her border collie for E. Coli.
Now, since sausages are made of intestine, let’s use one to represent enterobacter. The border collie is cutting it into smaller pieces with a serrated saw, which stands for serratia.
The cobra has someone’s eye in its mouth to help you remember tobramycine is used to treat eye infections. The eye came from the Mona Lisa, who has cysts and scars on her face to represent pseudomonas in cystic fibrosis patients.
Next, we have the striped mouse, or streptomycin, getting his head stuck in a toilet paper tube for tuberculosis. Finally, we have neomycin, the mouse with the neon glow sticks, who’s actually a surgeon.
At the party, he’s operating on a patient’s colon while using the glow sticks for illumination. Now, someone used a giant pen as a battering ram and smashed a hole in the purple wall.
This is because we use beta lactams, like penicillin, to break down the cell wall of Gram positive bacteria, which stain purple.
This is nice because now the aminoglycoside mice can go through the wall whenever they want. For side effects, some of the mice have been nibbling on a large ear and a kidney which represent ototoxicity and nephrotoxicity.
One mouse refused to eat, so he’s too weak from the hunger to move. Another mouse with a red rash was so grossed out, he started vomiting.
All right, as a quick recap. Aminoglycosides are protein synthesis inhibitors that bind to the 30S ribosomal subunit.
Their main mechanisms is to disrupt proofreading in bacterial protein synthesis. They can be used alone to treat Gram negative aerobic bacteria, but could also treat Gram positive bacteria if combined with a beta lactam antibiotic.
Notable adverse drug reactions include nephrotoxicity, ototoxicity, neuromuscular blockade, nausea, and allergic reaction.

Review10:28–11:03

But wait, there's more: Here's a mind map with all of the mnemonics from the video. Go ahead and pause the video so you can test yourself to see what you remember.
Stay tuned for the answers at the end.

Mind Map11:03–10:31