A protein synthesis inhibitor is a substance that stops or slows the growth or proliferation of cells by disrupting the processes that lead directly to the generation of new proteins. 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.