Sedimentation Coefficient (S Value)
In biochemistry, the characteristics of ribosomes and their subunits are traditionally described using the S value (Svedberg unit), named after the Swedish physical chemist Theodor Svedberg.
This parameter is not a simple measure of weight or size. It characterizes the rate of sedimentation of molecular complexes and subunits during ultracentrifugation. The fundamental rule to remember is that the Svedberg value is directly proportional to the molecular mass of the particles being analyzed. The more massive the particle, the faster it sediments to the bottom of the tube under centrifugal force, and the higher its S value.
Eukaryotic Ribosomes (80S)
Eukaryotic cellular ribosomes have a sedimentation coefficient of 80S. Like all ribosomes, they dissociate into two unequal parts: a large and a small subunit.
- Large Subunit (60S): A massive complex containing three types of ribosomal RNA (rRNA): 5S, 5.8S, and 28S. In addition to nucleic acids, the large subunit includes an extensive protein framework of approximately 50 distinct proteins.
- Small Subunit (40S): Has a more compact structure based on a single 18S rRNA molecule and around 33 distinct proteins.
The coordinated action of these two subunits ensures the precise reading of genetic information.
Prokaryotic Ribosomes (70S) and Key Differences
Prokaryotic cells contain smaller, lighter ribosomes with a sedimentation coefficient of 70S, formed by a 50S large subunit and a 30S small subunit.
Despite their shared fundamental function, eukaryotic and prokaryotic translational machinery differ significantly in several key aspects:
- Subunit Molecular Mass: Prokaryotic subunits are considerably lighter.
- rRNA Count: The set of ribosomal RNA molecules varies.
- rRNA Mass: The physical size and mass of the RNA molecules differ.
- Protein Composition: The eukaryotic apparatus is structurally more complex and protein-rich.
- Ligand Binding Specificity: This fundamental difference is widely exploited in pharmacology, allowing drugs to selectively target one type of ribosome while sparing the other.
Polysomes (Polyribosomes)
To optimize and accelerate protein synthesis, cells rarely utilize isolated, single ribosomes. Instead, they form polysomes (polyribosomes).
A polysome is a dynamic complex consisting of a single mRNA molecule being translated simultaneously by multiple ribosomes moving along it like cars on a train track.
Structural Organization of a Polysome:
- Each individual ribosome physically covers and protects an mRNA segment approximately 80 nucleotides long.
- To prevent steric hindrance and collisions during elongation, a spacer distance of about 100 nucleotides is maintained between adjacent ribosomes.