Svedberg values

Ribosomes are large complexes of protein and rRNA (Figure 31.8). They consist of two subunits—one large and one small—whose relative sizes are generally given in terms of their sedimentation coefficients, or S (Svedberg) values. [Note Because the S values are determined both by shape as well as molecular mass, their numeric values are not strictly additive. For example, the prokaryotic 50S and 30S ribosomal subunits together form a ribosome with an S value of 70. The eukaryotic 60S and 40S subunits form an 80S ribosome.] Prokaryotic and eukaryotic ribosomes are similar in structure, and serve the same function, namely, as the "factories" in which the synthesis of proteins occurs. 

Under the transmission electron microscope, the interior of the bacterial cell appears granular. This is due to the ribosomes, which are essential players in protein synthesis. They ensure, along with the t-RNA, the translation of the genetic code. The ribosomes consist of two parts characterized by their sedimentation speed, expressed in Svedberg values (S). These two sub-units are different in size 30 S and 50 S in procaryotes. The assembled ribosome has a sedimentation constant of 70 S. The 30 S sub-unit contains a 16 S ribosomal RNA molecule (1542 nucleotides) and 21 different protein molecules its molecular mass is 900 KDa. The larger 50 S sub-unit contains two ribosomal... 

Sedimentation coefficients are expressed in Svedbergs (S), after the Swedish biochemist The Svedberg who developed the ultracentrifuge in the 1920s. While S values are indicative of molecular weight, they are not addi-tive-the 70s ribosome is made up of one 50S and one 30S subunit. 

Svedberg obtained the following values for the displacement of gold particles over varying time periods, confirming the anticipated relationship x — K /t. 

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). 

From analysis of a variety of well-characterized proteins, Squire and Himmel192 observed that if proteins are assumed to contain 0.53 g H20 per gram of protein and to have a mean value for v of 0.730 g / cm3 the value of Mr can be predicted by Eq. 3-17 with the standard deviation indicated. Here, S is the sedimentation constant in Svedberg irnits (10 13s). 

With use of the. s 2°0w and D°o,w values cited below (Section II,D,1) an additional estimate of the molecular weight can be made by means of the Svedberg equation (18) the value obtained is 118,000. [Calculations employ as partial specific volume v = 0.745 cm3/g, obtained from the amino acid composition of the protein given in Table III (IP).]... 

Additional size estimates of the 5 M guanidine hydrochloride subunit can be made from the Svedberg equation (18), using the s°0 w and D 0.w values cited in Section II,F, and from the empirical equations of Tan-ford and associates (32, 33), which relate s and [17] values in concentrated guanidine hydrochloride solvents to polypeptide chain length. These calculations yield molecular weights of 21,300, 20,700, and 19,800, respectively, for the E. coli pyrophosphatase subunit (13). 

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. 

Using the sedimentation velocity procedure, Stamm found the average degree of polymerization of purified cotton linters dispersed in cuprammonium hydroxide solution to be 346. It was subsequently shown by Svedberg, however, that reliable values for the diffusion constant could not.be obtained in the ultracentrifuge and consequently the results of Stamm are probably in error. 

The polysaccharide corresponding to the first, immobile fraction was examined by the Svedberg ultracentrifuge technique. The sedimentation constant, S20, was equal to 1.6 X 10 c.g.s. units, and was independent of change of concentration. A 1 % solution of the polysaccharide was calculated from data obtained in this investigation to have a polysaccharide content of 0.91% the polysaccharide thus contained 9% of extraneous matter. The diffusion constant, D20, was found to be 11 X 10 and the partial specific volume, 0.619. A molar frictional ratio of 1.5 was obtained. This indicated that the particles were elongated ellipsoids (see p. 320). A molecular weight of 9,000 was calculated as compared with the value 7,300 obtained by Tennet and Watson. The polysaccharide reacted with immune serum in a dilution of 1 6,000,-... 

Utilizing the buoyant densities listed in Table II and the frictional ratio of 1.11, values for the sedimentation coefficient have been calculated at two different solvent densities the standard S value routinely used to characterize human serum lipoproteins is defined as value of the flotation coefficient in Svedberg units (the negative sedimentation coefficient X 10 sec) in an aqueous NaCl solvent with a density of 1.063 g/ml and a viscosity of 1.021 centipoise (the viscosity of a 1.063 g/ml sodium chloride solution at 26°C). These values are listed in Table II. The S value is very sensitive to small variations in lipoprotein density because the solvent density is close to the lipoprotein density. To compare the particle sizes or molecular weights, values of the sedimentation coefficient (5) in a solvent with a density of 1.20 g/ml and the viscosity of KBr at 25°C are preferred, and the computed values are listed in Table II. 

How well do the sedimentation coefficients and densities predicted by the model match the values actually observed for LDL Excellent agreement with the experimental points is shown by the solid curve of Fig. 2, which is a plot of the values for 525,1.20 given in Table II. However, this agreement was achieved by selecting a value for the partial specific volume of the cholesteryl esters to make the best fit, yielding the value of 1.058 ml/g for this this quantity. [If a value of 1.044 ml/g were employed for the partial specific volume of the cholesteryl esters, as was used by Sata et al. (1972), the values of 525,1.20 listed in Table II would have decreased by about 3,5%. The values of S[ in Table II would have dropped by 1 to 2 Svedbergs.]... 

It is common practice to express measured sedimentation and diffusion constants in terms of the values and D20) that would be found for the given molecules if the measurements were made in water at 20 C., correcting the observed values by the appropriate coefficients, as discussed in detail by Svedberg and Pedersen (122). Under these circumstances, equation (5) becomes... 

If we assume that the protein molecule can be represented by an ellipsoid of revolution, the axial ratio of the ellipsoid determines a value of the frictional ratio. The mathematical relations involved have been derived by Perrin (93) and by Herzog, Illig and Kudar (55). Numerical values giving ///q as a function of the axial ratio of the ellipsoid have been listed by Svedberg and Pedersen [122, p. 41) and by Cohn and Edsall (16, p. 406). However, the derivation of axial ratios from the ///q values is by no means a straightforward process. The measured ///<, is determined not only by the shape, but the hydration of the protein. If the molecule is spherical, but binds w grams of water per g of protein, then the frictional ratio should deviate from unity by the factor... 

Hemocyanin from Brohult (9) horse antibody globulin from Kabat (56) human blood proteins from Cohn, Oncley. Strong, Hughes and Armstrong (17) Oncley, Scatchard and Brown (S6) zein, see Svedberg and Pedersen (122) or Cohn and Edsall (16) present revised value from Foster and Edsall (41). 

Figure 13.12. Sedimentation coefficients (s) for some biological particles, with values expressed in Svedberg units. Nuclear particles have values on the order of 106-107 S, while whole cells have values between 107 and 108 Svedberg units.

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