Re: what is the meaning of 'S' in 80S, 70S ribosomes?

Hi,

The S in the names of ribosomal subunits, and other macromolecular particles found in the cell (like the 23S proteasome), stands for Svedberg units. This unit is named after Theodor Svedberg, a Swedish chemist who received the 1926 Nobel Prize in Chemistry for his research into disperse systems, (in this case, colloids of macromolecules dispersed in solution). Svedberg pioneered the use of ultracentrifugation to investigate the properties of macromolecules.

Under ultracentrifugation, macromolecules are subjected to centrifugal forces on the order of 1 million gravities. The velocity of a macromolecule’s movement under this force (its sedimentation) is determined by its density, shape, and mass; more massive, more compact (spherical), denser molecules will sediment faster than less massive, less compact, less dense molecules. So, ultracentrifugation has been used to determine these parameters for macromolecules like the ribosomal subunits, or the proteasome.

We can represent the sedimentation of a macromolecule under ultracentrifugation by calculating its sedimentation coefficient (s), as follows:
where m is the macromolecule’s mass, v (pronounced nu-bar) is the reciprocal of the molecule’s density, p is the density of the solution, and f represents the shape of the molecule (the frictional coefficient).

One Svedberg unit (S) is equal to 10^-13 s, so comparing the number of Svedberg units for different macromolecules will tell you about their relative masses, shapes, and densities. The higher the value of S, the faster a molecule will sediment under ultracentrifugation.

You can read more about Theodor Svedberg on the Nobel web site at: http://nobelprize.org/nobel_prizes/chemistry/laureates/1926/svedberg- bio.html, and you can read more about the ultracentrifugation technique in a good college-level Biochemistry textbook, such as Biochemistry by L. Stryer. You can find the pertinent section here: http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.438#472.

Finally, we have an answer in our archives that discusses protein synthesis that may also be of intrest to you: http://www.madsci.org/posts/archives/2000-11/973721404.Cb.r.html.

Keep asking questions!