Sedimentation coefficient (S value)
In biochemistry, the S value, named after the Swedish physical chemist Theodor Svedberg, is traditionally used to describe the characteristics of ribosomes and their subunits.
This metric is not a simple measure of weight or size. It characterizes the rate of sedimentation of molecular complexes and subunits during ultracentrifugation. It is important to remember the basic rule: the Svedberg value is directly proportional to the molecular mass of the particles under study. The more massive the particle, the faster it settles to the bottom of the tube under centrifugal force, and the higher its S value.
Features of eukaryotic ribosomes (80S)
Eukaryotic cellular ribosomes have a sedimentation coefficient of 80S. Like any other ribosomes, they have a dissociating nature and consist of two unequal parts — the large and small subunits.
- Large subunit (60S): A massive complex that includes three types of ribosomal RNA (rRNA) simultaneously. Their sedimentation coefficients are 5S, 5.8S, and 28S. In addition to nucleic acids, the large subunit contains an extensive protein apparatus — approximately 50 different types of proteins.
- Small subunit (40S): Features a more compact structure. It is based on a single rRNA molecule with a sedimentation coefficient of 18S. Its protein composition is also more modest, numbering about 33 types of proteins.
It is the coordinated action of these two subunits that ensures the accurate reading of genetic information.
Prokaryotic ribosomes (70S) and their differences
Prokaryotic cells are equipped with lighter ribosomes with a sedimentation coefficient of 70S. They are also formed by two parts: a large 50S subunit and a small 30S subunit.
Despite their shared functional purpose, the translation apparatuses of eukaryotes and prokaryotes have several fundamental differences. The main parameters by which they differ include:
- Molecular mass of the subunits: they are significantly lighter in prokaryotes.
- Amount of rRNA: the set of ribosomal RNA molecules varies.
- Mass of rRNA: the sizes of the RNA molecules themselves differ.
- Number and variety of proteins: the eukaryotic apparatus is more complex and protein-rich.
- Ability to bind specific ligands: this fundamental difference is often utilized in pharmacology, as molecules can selectively target one type of ribosome without affecting the other.
Polysomes (Polyribosomes)
To optimize and accelerate protein synthesis, the cell rarely uses single ribosomes. Much more frequently, a polysome (or polyribosome) is formed.
A polysome is a dynamic complex consisting of a single messenger RNA (mRNA) molecule and multiple ribosomes simultaneously participating in translation, moving one after another like cars on a train.
Structural organization of a polysome:
- Each individual ribosome physically occupies and shields an mRNA segment approximately 80 nucleotides long.
- To prevent these molecular machines from interfering with each other during operation, a free distance is maintained between them. The intervals between adjacent ribosomes in the complex are about 100 nucleotides.
Briefly: answers to topical questions
What types of rRNA are found in the prokaryotic ribosome?
The prokaryotic ribosomal subunits (70S) contain the following types of rRNA:
- The small subunit (30S) contains 16S rRNA.
- The large subunit (50S) contains 23S rRNA and 5S rRNA.
Where in the eukaryotic cell does the assembly of ribosomal subunits take place?
The synthesis of rRNA and the formation (assembly) of eukaryotic ribosomal subunits occur in the nucleus, specifically in the nucleolus. The small and large subunits are formed in the nucleolus, after which the finished subunits are transported from the nucleus to the cytoplasm, where, in the presence of mRNA, they assemble into a functioning 80S ribosome.
Which antibiotics (specific ligands) selectively inhibit prokaryotic ribosomes?
Prokaryotic ribosomes are selectively inhibited by the following antibiotics:
- Targeting the 30S subunit:
- Aminoglycosides — irreversibly disrupt tRNA attachment and initiation complex formation.
- Tetracyclines — reversibly block the binding of new aminoacyl-tRNA to the A-site.
- Targeting the 50S subunit:
- Macrolides (e.g., erythromycin) — prevent translocation.
- Chloramphenicol — blocks peptidyl transferase and peptide bond formation.
- Lincosamides.
- Ketolides.
- Linezolid.
- Other groups:
- Oxazolidinones — block subunit binding into the 70S complex.
- Fusidic acid — disrupts peptide chain elongation.
To avoid confusing the subunits: in eukaryotes, all numbers are even (40S, 60S = 80S), whereas in prokaryotes, they are odd tens (30S, 50S = 70S).
Class and exam questions
What is the S value (Svedberg)?
This is a metric that characterizes the rate of sedimentation of molecular particles during ultracentrifugation. It is directly proportional to the molecular mass of the subunits.
What types of rRNA are found in the eukaryotic ribosome?
The small subunit (40S) contains 18S rRNA. The large subunit (60S) contains three types: 5S, 5.8S, and 28S rRNA.
How are ribosomes arranged on mRNA within a polysome?
Each ribosome occupies an mRNA segment about 80 nucleotides long. Meanwhile, the interval between adjacent active ribosomes is approximately 100 nucleotides.
What else to study on this topic
- Functions of rRNA (5S, 5.8S, 18S, 28S) in translation
- Specific ligands binding to prokaryotic ribosomes
- Mechanism of assembly of the 40S and 60S subunits
- Detailed protein composition of the small and large subunits
- Assembly and functioning of the polysome
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