Svedberg units

Fig. 2. Ultracentrifugal pattern for the water-extractable proteins of defatted soybean meal in pH 7.6, 0.5 ionic strength buffer. Numbers above peaks are approximate sedimentation coefficients in Svedberg units, S. Molecular weight ranges for the fractions are 2S, 8,000—50,000 7S, 100,000—180,000 IIS, 300,000—350,000 and 15S, 600,000—700,000 (9). The 15S fraction is a dimer of the IIS protein (10).

Ribosomes are ancient ribonucleoprotein complexes that are the sites of protein synthesis in living cells. Their core structures and fundamental functional mechanisms have been conserved throughout the three domains of life bacteria, archaea and eukaryotes. All ribosomes are organized into two subunits that are defined by their apparent sedimentation coefficient, measured in Svedberg units (S). There is a general... 

The ribosomal subunits are defined according to their sedimentation velocity in Svedberg units (40S or 60S). This table illustrates the total mass (MW) of each. The number of unique proteins and their total mass (MW) and the RNA components of each subunit in size (Svedberg units), mass, and number of bases are listed. 

S] denotes the Svedberg unit (10 13s). Once such a scaling law is available, it can be used in conjunction with a rapid measurement of the sedimentation coefficient... 

A Svedberg unit is j me asure of the rate of sedimentation of, t nm o-cule due to centnruga-tion. 

It is convenient to classify particles by their rate of sedimentation per unit centrifugal field, a parameter known as the sedimentation coefficient (s), which has units of seconds. A Svedberg unit (S) is defined as a sedimentation coefficient of 1 X 10 13 seconds. 

Ribosomes consist of two subunits of different size, made up of ribosomal RNA (rRNA) and neariy 80 proteins (the number of proteins appiies to rat iiver ribosomes), it is customary to give the sedimentation coef cients (see p. 200) of ribosomes and their components instead of their masses. For exampie, the eukaryotic ribosome has a sedimentation coefficient of 80 Svedberg units (80 S), whiie the sedimentation coef cients of its subunits are 40 S and 60 S (S vaiues are not additive). 

S/ = Svedberg unit, 10 cm/sec/dyn/g. The subscript denotes the solution density at which flotation studies were carried out. [See Scanu and Kruski (S13).]... 

Ribosomal RNAs (rRNAs) are found in association with several proteins as components of the ribosomes—the complex structures that serve as the sites for protein synthesis (see p. 433). There are three distinct size species of rRNA (23S, 16S, and 5S) in prokaryotic cells (Figure 30.2). In the eukaryotic cytosol, there are four rRNA size species (28S, 18S, 5.8S, and 5S). [Note "S" is the Svedberg unit, which is related to the jnolecular weight and shape of the compound.] Together, rRNAs make up eighty percent of the total RNA in the cell. 

At pH 12, the disulfide and noncovalent bonds are both broken, and the monomer with a sedimentation constant of 1.45 Svedberg units is released. From frictional ratios, the monomer appears to exist as a coil with a diameter of 16 A and a length of 150 A. Analysis of the primary structure of K-casein (Loucheux-Lefebvre et al. 1978) suggests considerable secondary structure in the monomer. 23% a-helix, 31% /3-sheets, and 24% 0-turns. In contrast, other investigators, using several different approaches, obtained a-helix contents ranging from 0 to 20.8% (Bloomfield and Mead 1975). Circular dichroism spectra on the monomer indicated 14 and 31% for a-helix and / -sheet, respectively (Loucheux-Lefebvre et al 1978). An earlier study of the optical rotatory dispersion of the K-casein monomer yielded values for the a-helix content ranging from 2 to 16% (Herskovits 1966). 

S refers to the Svedberg unit, the sedimentation coefficient of a particle in the ultracentrifuge, t C, cytosine G, guanosine. 

The bulk of the cellular RNA is ribosomal RNA. Although seven genes exist in E. coli for rRNA, they all lead to essentially the same three ribosomal RNA molecules (see table 28.1) which differ substantially in size. The three rRNAs are always found in a complex with proteins in a functional component known as the ribosome. The ribosome is the site where mRNA and tRNAs meet to engage in protein synthesis. In E. coli, ribosomes are referred to as 70S particles, a measure of their rate of sedimentation and hence their size (S refers to Svedberg units, which are defined in chapter 6). A 70S ribosome consists of two dissociable subunits A 50S subunit and a 30S subunit. Each of these contains both RNA and protein. The 50S subunit contains 23S and 5S rRNAs. The 30S subunit contains a single 16S rRNA (fig. 28.5). Eukaryotic ribosomes are similar in structure, although they are somewhat larger (80S) and con-... 

Svedberg unit (S). The unit used to express the sedimentation constant s ... 

The abbreviation S stands for Svedberg units, and is a measure of how fast a particle sediments in liquid. 

The sedimentation coefficient is often reported in Svedberg units, S, named in honor of The Svedberg, the inventor of the ultracentrifuge 1 S = 10 13 seconds. Sedimentation coefficients of some selected proteins are listed below. 

Sedimentation coefficients are usually expressed in Svedberg units (S), equal to lO g. The smaller the S value, the slower a molecule moves in a centrifugal field. The S values for a number of biomolecules and cellular components are listed in Table 4.2 and Figure 4.14. 

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