Subunits coefficients

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... 

D. gigas AOR was the first Mo-pterin-containing protein whose 3D structure was solved. From D. desulfuricans, a homologous AOR (MOD) was purified, characterized, and crystallized. Both proteins are homodimers with-100 kDa subunits and contain one Mo-pterin site (MCD-cofactor) and two [2Fe-2S] clusters. Flavin moieties are not found. The visible absorption spectrum of the proteins (absorption wavelengths, extinction coefficients, and optical ratios at characteristic wavelengths) are similar to those observed for the deflavo-forms of... 

The prevalent receptor model for the excitatory amino acid is a tetrameric complex. As mentioned in the text, there is evidence that the channel conductance depends on the number of subunits that bind a ligand. Estimate the EC50 value and Hill coefficient for a dose-response curve assuming that the occupation at each subunit has a Kd value of 1 pi I, an % of 1, and that activation induces a transition to an active state independent of the state of the other subunits ... 

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. 

Figure 1.7 Structure of a ribosome. It is composed of two subunits the large 60S subunit has a mass of 2800 kDa and is composed of three RNA molecules and about 50 protein molecules. The smaller AOS subunit contains one RNA molecule plus around 30 protein molecules and has an aggregate mass of 1400 kDa. ( S is an abbreviabon for a Svedberg, the unit of the sedimentation coefficient. This is measured in an analybcal ultracentrifuge and is related to, but not simply proporbonal to, molecular mass.)...

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). 

Neurotrophic factors responsible for neuronal survival, dendritic proliferation, and the activation of the different neurotransmission systems are present in the central nervous system [CNS). The most well-known one is the NGF, a peptidergic complex of 140 kd and with a sedimentation coefficient of 7s. NGF has three subunits, a, p, and y. Subunit p is the active part of the molecule. Other neurotrophic factors [F. ffefti 1994) include 1) brain-derived neurotrophic factor [BDNF), 2) neurotrophin 3, 3) neurotrophin 4/5, and 4) ciliary neurotrophic factor. 

Each E. coli cell contains 15,000 or more ribosomes, making up almost a quarter of the dry weight of the cell. Bacterial ribosomes contain about 65% rRNA and 35% protein they have a diameter of about 18 nm and are composed of two unequal subunits with sedimentation coefficients of 30S and 50S and a combined sedimentation coefficient of 70S. Both subunits contain dozens of ribosomal proteins and at least one large rRNA (Table 27-6). 

The ribosomes of eukaryotic cells (other than mitochondrial and chloroplast ribosomes) are larger and more complex than bacterial ribosomes (Fig. 27-9d), with a diameter of about 23 nm and a sedimentation coefficient of about 80S. They also have two subunits, which vary in size among species but on average are 60S... 

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. 

The ribosomes of Escherichia coli (M = 2.3 x 106 dalton) consist of two dissimilar subunits. The larger one is the 50 S unit containing 32 unique proteins (L1-L34) and one molecule each of 5 S and 23 S rRNA. The smaller subunit has a sedimentation coefficient of 30 S and contains one molecule 16 S rRNA and 21 unique proteins (S1-S21). The proteins of both subunits could be separated by RPC on a C 18 column (250 x 4.6 mm do = 30 nm dP = 10 pm) by a gradient water/2-propanol with 0.1 % trifluoroacetic acid in both solvents (Fig. 20). With 2-propanol, 15 proteins of the... 

The apparent molecular weight of asparaginase in 8M urea, 5M guanidinium chloride, or 9Af formamide was first reported as being 19,000-24,000, with an s°o.w of 1.5 to 1.8 S (77), demonstrating the presence of subunits. Arens et al. (65) and Frank and Veros (78) have confirmed the value of the sedimentation coefficient of the subunits obtained in 8 M urea, but the latter authors reported an apparent molecular weight of approximately 32,500 instead of 19,000-24,000, and their value is almost certainly the correct one. 

Equation (8) is an approximation because it ignores intermediate species that have some, but not all, of the binding sites occupied. Even so, the Hill coefficient provides a useful measure of cooperativity. The binding of 02 to hemoglobin is described well by the Hill equation with n 2.8. In the case of phosphofructokinase, which has four subunits, the dependence of the rate on the fructose-6-phos-phate concentration at a fixed, relatively high concentration of ATP is described well with n 3.8. 

The changes in sedimentation coefficient and chemical reactivity caused by PALA fit the model that the enzyme exists in two distinct conformations (T and R) and that the binding of PALA to only one or two of the c subunits causes the entire c6r6 complex to flip from the T to the R state. The dissociation constant for PALA is higher in the T state than in the R state. The equilibrium constant L = [T]/[R] has been calculated to be 250 in the absence of substrates and... 

For the purpose of this calculation it was necessary to know the diffusion coefficient for ligand and whole molecules of hemocyanin (Table I) and the value of the diffusion coefficient for half molecules of hemocyanin. If the level of calcium ion is reduced in pH 9.6 glycine buffer to the point where no whole molecules remain, the half molecules always are contaminated with one-twelfth size (5S) subunits, so that it is impossible to measure directly the diffusion coefficient of half molecules. For a self-associating system, the Gouy method leads to an average diffusion coefficient which is the Z-average, defined by... 

Two of the most important nonlinear optical (NLO) processess, electro-optic switching and second harmonic generation, are second order effects. As such, they occur in materials consisting of noncentrosymmetrically arranged molecular subunits whose polarizability contains a second order dependence on electric fields. Excluding the special cases of noncentrosymmetric but nonpolar crystals, which would be nearly impossible to design from first principles, the rational fabrication of an optimal material would result from the simultaneous maximization of the molecular second order coefficients (first hyperpolarizabilities, p) and the polar order parameters of the assembly of subunits. (1)... 

Whereas a prokaryotic ribosome has a sedimentation coefficient (see Topic G9) of 70S and subunits of 30S and 50S, a eukaryotic ribosome has a sedimentation coefficient of 80S with subunits of 40S and 60S (see Topic G9). The composition of eukaryotic ribosomal subunits is also more complex than prokaryotic subunits (see Topic G9) but the function of each subunit is essentially the same as in prokaryotes. 

We probed the reactivity of the carboxylates using a fluorescent carboxylate-selective chemical dye, N-cyclohexyl-N -(4-(dimethylamino)naphthyl)carbodiimide (NCD4). Using UV-visible spectroscopy, native gel electrophoresis, and denaturing gel electrophoresis, covalent modification with the dye was confirmed (Figure 9.5). The latter showed that the dye was attached to both the S and the L subunit, an expected observation, as the structural data suggested carboxylates on both subunits. A quantification of the number of bound dye molecules could not be achieved owing to the instability of the dye in aqueous solvent a reliable extinction coefficient could not be determined. 

---