Within every living cell, molecular machines known as ribosomes build the proteins that perform a vast array of tasks. These protein factories are composed of two parts: a small subunit and a large subunit. The subunits remain separate until a cell needs to create a new protein, at which point they come together. They then lock onto a genetic blueprint to translate its code into a functional protein.
The Building Blocks of Ribosomes
Each ribosomal subunit is a complex assembly of ribosomal RNA (rRNA) and proteins. The rRNA forms the structural and functional core of the ribosome, while proteins on the surface help stabilize the structure. A key distinction exists in the ribosomes of different life forms. Prokaryotic organisms, such as bacteria, have 70S ribosomes, made of a small 30S subunit and a large 50S subunit.
In contrast, eukaryotic cells, found in plants and animals, possess larger 80S ribosomes in their cytoplasm, composed of a small 40S subunit and a large 60S subunit. The “S” refers to the Svedberg unit, a measure of a particle’s size determined by how quickly it settles in a centrifuge. This unit is not additive, which is why the masses of the individual subunits (e.g., 50S and 30S) do not sum to the mass of the complete ribosome (70S).
Assembling for Protein Synthesis
The two ribosomal subunits float freely in the cytoplasm when inactive and must come together to initiate protein synthesis. Assembly begins when the small ribosomal subunit binds to a messenger RNA (mRNA) molecule. The mRNA carries genetic instructions, copied from DNA, for building a protein. The small subunit then scans the mRNA until it locates the start codon, a specific three-letter sequence.
This recognition is the cue for the large ribosomal subunit to join the complex. It positions itself so the mRNA is held between the two subunits, creating a complete ribosome. This docking encloses the mRNA template within a groove at their interface. The assembled ribosome is now ready to begin the main phase of protein production, known as elongation.
The Role of Each Subunit in Translation
During the elongation phase of translation, each subunit performs a specialized job. The small subunit’s primary function is to decode the genetic message. It reads the codons on the mRNA template and ensures accuracy by matching them with the correct transfer RNA (tRNA) molecules, each carrying a specific amino acid. This codon-anticodon pairing is necessary for correct translation.
The large subunit’s main role is to catalyze the formation of peptide bonds, the chemical links that join one amino acid to the next. This activity occurs within specific regions known as the A (aminoacyl), P (peptidyl), and E (exit) sites. The A site accepts the incoming tRNA, the P site holds the tRNA with the elongating protein chain, and the E site is where the empty tRNA is discharged.
Targeting Subunits for Medical Treatments
The structural differences between prokaryotic 70S and eukaryotic 80S ribosomes are an advantage in medicine. These distinctions allow antibiotics to selectively target and disrupt bacterial protein synthesis without harming human cells. By binding to the 30S or 50S bacterial subunits, these drugs halt the production of proteins required for bacterial survival.
For instance, tetracycline antibiotics bind to the bacterial 30S small subunit. This action blocks the A site, preventing tRNA molecules from delivering their amino acids and stopping protein elongation. In contrast, macrolide antibiotics like erythromycin target the bacterial 50S large subunit. They bind within the polypeptide exit tunnel, creating a blockage that prevents the growing amino acid chain from leaving the ribosome.
When Subunit Function Fails
Errors in the function or assembly of our own ribosomes can lead to diseases known as “ribosomopathies.” These conditions arise from genetic mutations that affect ribosomal components like proteins or rRNA. Although ribosomes are in every cell, these diseases often manifest with specific symptoms affecting particular tissues.
An example is Diamond-Blackfan anemia (DBA). This condition is typically caused by a mutation in a gene that codes for a ribosomal protein, leading to a deficiency in functional ribosomes. This ribosomal haploinsufficiency, or having only one functional copy of the gene, impairs the maturation of red blood cells in the bone marrow, resulting in severe anemia.