Protein synthesis ribosome cycle

Nilsson L and Nygard O (1986). The mechanism of the protein-synthesis elongation cycle in eukaryotes. Effect of ricin on the ribosomal interaction with elongation factors. Eur J Biochem, 161, 111-117. 

RF-1, RF-2, and RF-3 in E. coli) help to dissociate the newly synthesized protein chain, the mRNA and the ribosomal subunits from one another. The mRNA is free to be used in another cycle of protein synthesis, as are the ribosomal subunits. 

At each stage in protein synthesis on the ribosome—-initiation, elongation, and termination—a different set of protein factors is engaged by the ribosome. Why do such protein factors, which are crucial to the translation, exist separate from the ribosome Why must they cycle on and... 

The antibiotic fusidic acid inhibits protein synthesis by preventing EF-G from cycling off of the ribosome. Fusidic-acid-resistant mutants of EF-G have been isolated. Fusidic acid resistance is recessive to sensitivity. In other words, an E. coli cell containing two EF-G genes, one resistant and one sensitive, is still sensitive to the antibiotic. Why Hint Look at fig. 29.2.)... 

Mitochondria can have different shapes, depending on the kind of cell they are in. The number of mitochondria present in a cell also varies with cell type and may range from a single large mitochondrion to thousands. The region inside the inner membrane is called the matrix. This is where the Krebs cycle that converts pyruvate into C02 and energy takes place, so it contains a lot of enzymes. Mitochondria contain ribosomes, small particles composed of RNA and protein that are the sites of protein synthesis. Mitochondria also contain their own special DNA. 

In eucaryotic cells, glycolysis, gluconeogenesis and fatty acid synthesis takes place in the cytosol, while the Krebs cycle is isolated within mitochondria glycogen is made in glycogen granules, lipid is synthesized in the endoplasmic reticulum and lysosomes carry on a variety of hydrolytic activities. As in procaryotic cells, ribosomes in the cytosol are the site of protein synthesis. 

A number of these enzymes are expressed in other tissues as well but cause a notable deficiency predominantly in red blood cells because of the life span of the erythrocyte after the loss of protein synthesis. Once an enzyme is degraded or otherwise becomes nonfunctional, it cannot be replaced by new or other compensating proteins because of the lack of nucleus, mitochondria, ribosomes, and other cell organelles in mature red cells. Disorders have been described in the EMP, HMP, Rapoport-Luebering cycle, the glutathione pathway (Figure 21-9), purine-pyrimidine metabolism and methemoglobin reduction. 

The second phase of protein synthesis is the elongation cycle. This phase begins with the insertion of an aminoacyl-lRKA into the empty A site on the ribosome. The particular species inserted depends on the mRNA codon in the A site. The cognate ami noacyl-tKN A does not simply leave the synthetase and diffuse to the A site. Rather, it is delivered to the A site in association with a 4. Tkd protein called elongation factor Tu (EF-Tu). Elongation factor Tu, another member of the G-protein family, requires GTP to bind aminoacyl-tRNA (Figure 30.23) and to bind the ribosome. The binding of... 

Figure 30.31 The SRP targeting cycle, (1) Protein synthesis begins on free ribosomes. (2) After the signal sequence has exited the ribosome, it is bound by the SRP, and protein synthesis halts. (3) The SRP-ribosome complex docks with the SRP receptor in the ER membrane. (4) The SRP and SRP receptor simultaneously hydrolyie bound GlPs. Protein synthesis resumes and the SRP is free to bind another signal sequence, (5) The signal peptidase may remove the signal sequence as it enters the lumen of the ER, (6) Protein synthesis continues as the protein is synthesized directly into the ER. (7) On completion of protein synthesis, the ribosome is released and the protein tunnel in the translocon closes. [After H. Lodish et al. Molecular Cell Biology, 5th ed, (W. H. Freeman and Company. 2004), Fig. 16.6,]...

The cycle of peptide-chain elongation continues until one of the three stop codons (UAA, UAG, UGA) is reached. There is no aminoacyl-tRNA complementary to these codons, and instead a termination factor or a release factor (RF) with bound GTP binds to the ribosome and induces hydrolysis of both the aminoacyl-linkage and GTP, thereby releasing the completed polypeptide chain from the ribosome. The 475 amino acid-long sequence of rabbit liver RF has been deduced from its cDNA sequence, and it shows 90% homology with mammalian trypto-phanyl-tRNA synthetase (Lee et al., 1990). It has also been reported that for efficient and accurate termination, an additional fourth nucleotide (most commonly an A or a G) after the stop codon is required (Tate and Brown, 1992). The exact role of the fourth nucleotide in the termination of protein synthesis is not fully understood at present. 

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