Sedimentation coefficients subunits

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... 

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. 

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... 

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. 

Ribosomes are large macromolecular complexes whose components contain all the information necessary for self-assembly. The E. coli ribosome has a sedimentation coefficient of 70 S and consists of two subunits (50 S and 30 S) with a total mass of 2.8 x 106 Da and with 58 different components. Three of these components are RNA molecules that together comprise 65 percent of the mass and they act as a framework or template for the ordering of the different proteins. When the pure dissociated components are mixed together in the proper order under the correct conditions they spontaneously reassemble to form a fully active ribosome (Fig. 5-1). 

The enzymes from malt sorghum and sweet potato were completely homogeneous by both gel and free-solution electrophoresis, and the three individual wheat components and the soya-bean enzyme were essentially homogeneous a mixture of the three wheat enzymes, however, gave three discernible peaks. Sedimentation studies on the wheat enzymes in the presence of iodine and potassium iodate showed no change in the sedimentation coefficient under these conditions, indicating the absence of intermolecular S — S bonding. There was also, from sedimentation, no evidence of dissociation into subunits in the presence of 8 M urea. 

In the halophile H. halobium, Nakayama et al. [148] have described an aphidicolin-sensitive DNA polymerase that they named DNA polymerase a . The enzyme has a high sedimentation coefficient value, is able to synthesize RNA on a synthetic DNA template, and is associated with a 3 to 5 exonuclease activity. Two major polypeptides of 60 and 70kDa were detected in a purified fraction of H. halobium DNA polymerase a [149]. The authors concluded from these results that this enzyme corresponds to a multi-subunit DNA polymerase able to perform DNA priming like eukaryotic DNA polymerase a. However, the association of the 60 and 70 kDa polypeptides with the polymerase activity has not been demonstrated and the specific activity of the enzyme is very low compared to any other purified DNA polymerases. 

Ribosome s are found in the cytoplasm, on the outer face of the rough ER and in the mitochondrial matrix. Ribosome s are composed of RNA and proteins. The sedimentation coefficient of eukaryotic ribosomes is SOS. The sedimentation coefficient, S, is a unit of measure that describes how fast a macromolecule will sediment when spun in a high speed centrifuge. Larger molecules generally have larger S values. Eukaryotic ribosomes consist of two subunits a 60S large subunit and a 40S small subunit. The 60S subunit is comprised of about 45 proteins and three rRNA s that have S coefficients of 5, 5.8 and 28. The 40S subunit consists of about 33 proteins and 18S rRNA s. 

The ribosomal subunits migrate through the nuclear pores into the cytoplasm where they complex with mRNA, forming 80S ribosomes. Because sedimentation coefficients reflect both shape and particle weight, they are not additive. 

A typical eukaryotic ribosome has a sedimentation coefficient of about 80 and consists of two subunits 408 and 608. These sizes may vary by as much as 10% from one organism to another in contrast with the homogeneity of bacteria ribosomes. The components of the subunits of eukaryotic ribosomes are as follows ... 

The answer is c. (Murray, pp 452—467. Scriver, pp 3—45. Sack, pp 1—40. Wilson, pp 101-120.) Prokaryotic ribosomes have a sedimentation coefficient of 70S and are composed of SOS and 30S subunits. Eukaryotic cytoplasmic ribosomes, either free or bound to the endoplasmic reticulum, are larger—60S and 40S subunits that associate to an SOS ribosome. Nuclear ribosomes are attached to the endoplasmic reticulum of the nuclear membrane. Ribosomes in chloroplasts and mitochondria of eukaryotic cells are more similar to prokaryotic ribosomes than to eukaryotic cytosolic ribosomes. Like bacterial ribosomes, chloroplast and mitochondrial ribosomes use a formylated tRNA. In addition, they are sensitive to many of the inhibitors of protein synthesis in bacteria. 

Fig. 9.— Double Log Plots of (a) Intrinsic Viscosity, (b) the Reciprocal of the Diffiision Coefficient, and (c) Sedimentation Coefficient Data versus Molecular Weight for Human Cervical Mucins. [Key and O, whole mucins and , subunits and A, T-domains. Molecular weights determined from Zimm plots (filled symbols) or the Svedbeig equation using QLS (open symbols). Values for the slopes are in all cases consistent with a random-coil model and not with a rigid sphere or a rod.]...

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