What Antibiotics Inhibit Protein Synthesis?

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll first review the basic steps of building new proteins through protein synthesis. Then we'll look at two main ways that antibiotics inhibit protein synthesis.

What Are Antibiotics?

It might start with a tingle in the back of the throat, or maybe a sniffle. You have a cough drop and go to bed early, hoping to ward of the sickness. However, when you wake up the illness seems to have taken over. Your nose is running, you're coughing up something thick and green, and your body feels like you've been hit by a truck.

Sometimes when we are sick like this, bacteria are the culprit. Bacteria are single celled living organisms that can be pathogenic, or cause disease to humans. If your immune system can't fight off the bacteria on its own, your doctor might prescribe an antibiotic, which is a type of medication that specifically kills bacteria. They won't work on other illnesses like a virus, which is why it is important to have a doctor test you for a bacterial infection before taking them.

But how do antibiotics work? For many, the answer is that they shut down the bacteria's manufacture of important molecules called proteins. Proteins are tiny molecules that are used for structure and function inside all living things. Both humans and bacteria have to make protein during a process called protein synthesis in order to stay alive. For example, these tiny molecules are responsible for making energy for our cells and for making new cells when we need them.

However, the way bacteria and humans make proteins is a little bit different. So antibiotics can specifically stop bacteria from making proteins while leaving the human cells alone. But before we delve into how this happens, let's refresh about how protein synthesis happens.

Process of Protein Synthesis

Proteins are small molecules needed for all structure and function. Cells make proteins using protein synthesis. The instructions for making proteins are found in a molecule called DNA, which contains the master blueprints for the cell. Cells make a copy of this important document to use during the manufacturing process; this copy is called messenger RNA (mRNA). After the mRNA is copied it is taken to a small compartment called a ribosome. In bacteria, the ribosome is made of two parts, a 30S subunit (S here stands for svedberg, a unit of measurement that indicates how fast this particle moves in a centrifuge) and a 50S subunit. The 30S subunit is smaller, hence the smaller number. Each subunit is made of some of its own RNA, called ribosomal RNA (rRNA) and protein.

Structure of the bacterial ribosome

During protein synthesis, the two parts come together and sandwich the mRNA in the middle. The mRNA is pushed through three sites: A (acceptor site) where it enters, P (peptidyl) in the middle, and E (exit) where the mRNA exits. When the RNA enters the A site, the ribosome reads the instructions and tells another molecule called tRNA to go get the building blocks of the protein, which are called amino acids. Elongation happens at the A site; this is where the amino acids in the A and P site are connected together to make a protein. Once all the instructions in the mRNA are read, the amino acid chain is released and folds together to make a functional protein.

Protein synthesis at the ribosome

Antibiotics inhibit this process at different steps to prevent protein synthesis in bacteria. Let's look at the different types of inhibition of protein synthesis next.

Inhibition of 30S Ribosome

A patient comes into the emergency room coughing up blood and complaining of a long term chest cold. X-rays and chest samples are taken and the doctor finds the patient has tuberculosis, a very serious infection of the lungs. He prescribes an aminoglycoside, a class of antibiotics that work on the 30S subunit.

The doctor sees a lung with tuberculosis shown by the arrows in the right picture when the patient arrives at the ER
tuberculosis lung

Aminoglycosides like streptomycin and neomycin attach to a smaller piece of ribosomal RNA (rRNA) within the 30S subunit. When they are attached, the ribosome doesn't read the mRNA correctly and makes mistakes creating the protein. If the protein is not made correctly, it won't be able to do its job. However, some strains of tuberculosis have become resistant, or immune, to streptomycin. In response, many doctors now prescribe antibiotics that work on other parts of the bacteria than protein synthesis.

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