Inhibitors of Protein Synthesis: How Antibiotics Target the Bacterial Ribosome

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  • 0:05 Inhibiting Protein Synthesis
  • 0:45 Target: Bacterial 70S Ribosome
  • 2:45 Antibiotics That…
  • 5:13 Shared Antibiotic…
  • 5:57 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Proteins carry out tons of incredibly important functions within cells. In this lesson, we will learn how antibiotics can fight bacteria by shutting down the ribosome, the protein synthesis factory of the cell.

Inhibiting Protein Synthesis

When you hear the word 'protein,' you might think of a juicy steak or the protein powder that bodybuilders put into their milkshakes. But, anyone who's learned a little bit about cell biology knows that proteins are the workhorses of cells, carrying out tons of essential functions like catalyzing enzymatic reactions, sensing and passing on signals and making important physical structures. Without making proteins, most cells wouldn't be able to carry out their day-to-day functions at all. Many types of antibiotics make use of this fact of life by attempting to prevent bacteria from making proteins. In this lesson, we will take a closer look at how these antibiotics work on a molecular level.

Target: Bacterial 70S Ribosome

All of the antibiotics that target bacterial protein synthesis do so by interacting with the bacterial ribosome and inhibiting its function. The ribosome might not seem like a very good target for selective toxicity, because all cells, including our own, use ribosomes for protein synthesis.

The good thing is that bacteria and eukaryotes have ribosomes that are structurally different. Bacteria have so-called 70S ribosomes and eukaryotes have 80S ribosomes. No, not '70s and '80s ribosomes, although that would be pretty entertaining. The S stands for 'Svedberg unit,' and it refers to the rate at which particles sediment down into the tube during high-speed ultracentrifugation. Basically, it tells us about the ribosome's molecular weight and shape.

70S and 80S ribosomes are different enough that antibiotics can specifically target one and not the other. Let's take a closer look at the bacterial 70S ribosome and see where some different kinds of antibiotics act on it. Remember that ribosomes are made of RNA and protein and that they have two subunits, one large and one small.

The bacterial 70S ribosome's subunits are the 50S subunit and the 30S subunit. Yes, I know, 50 + 30 = 80, not 70, but this is not a math mistake. Using the Svedberg unit to measure ribosomes means that things don't always add up perfectly because rates of sedimentation are not additive like molecular weights are.

Before we get into the specifics of how antibiotics inhibit bacterial ribosomes, let's briefly review how ribosomes work. First, a tRNA loaded with a particular amino acid enters the ribosome at the A site. The tRNA's anticodon has to match the codon, or group of three nucleotides on the mRNA. Then, at the P site of the ribosome, a peptide bond forms between the previous amino acid and the new amino acid. Finally, the empty tRNA exits at the E site. This process repeats for the whole length of the mRNA, and the polypeptide chain continues to grow.

Antibiotics that Inhibit Ribosomes

Okay, let's talk about antibiotics. There are many different antibiotics that all inhibit protein synthesis by interacting with the bacterial 70S ribosome in different ways. We'll go through several examples briefly so that you can get an idea of how these antibiotics really work.

Let's start with the tetracyclines. These antibiotics bind to the 30S subunit at the A site and prevent the attachment of tRNAs carrying amino acids. This means that the next bead on the polypeptide string can't be brought into the ribosome.

Another antibiotic, chloramphenicol, interacts with the 50S subunit of the ribosome and prevents the formation of peptide bonds. When chloramphenicol is around, the amino acid beads can't be linked together into a polypeptide string.

The next class of protein synthesis inhibitors is the aminoglycosides. These seem to have a variety of different mechanisms, but one well-known member of this family is streptomycin, which binds to the 30S subunit and causes the ribosome to misread the genetic code. This could make a well-ordered polypeptide string into a jumbled-up mess that can't carry out its function.

The macrolides, such as erythromycin, are thought to inhibit protein synthesis by binding to the 50S subunit and blocking the tunnel where the polypeptide string is supposed to exit. This clogs up the ribosome and stops translation.

That's already a lot of antibiotics, but there are still two classes of protein synthesis inhibitors left. These last two are newer antibiotics that have been really important in combating multi-drug-resistant pathogens, especially those that are resistant to vancomycin. You can learn more about vancomycin in another lesson, but basically, it used to be used as a last resort to kill bacteria that are resistant to almost every other type of antibiotic. But now, there are more and more bugs that are resistant even to vancomycin. Let's see how these last two very important protein synthesis inhibitors work.

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