Peptide chains initiation

Be able to describe the mechanism for peptide chain initiation, elongation, and termination in prokaryotes and eukaryotes on the ribosome. 

Spectinomycin continued to be effective in the treatment of resistant strains of N. gonorrheaesi including the new g-lactamase producing strains.52 In cell-free systems, it inhibited peptide chain initiation.53 It had a very low sensitization rate in guinea pigs.5 t The biosynthesis of spectinomycin from (6-13C)glucose was reported.55... 

Kimball SR, Antonetti DA, Brawley RM, Jefferson LS. 1991. Mechanism of inhibition of peptide chain initiation by amino acid deprivation in perfused rat liver. Regulation involving inhibition of eukaryotic initiation factor 2a phosphatase activity. J Biol Chem 266 1969-1976. 

Mechanisms Interferons are glycoproteins produced in human leukocytes (IFN-a), fibroblasts (IFN-(3), and immune cells (IFN-y)- They exert multiple actions that affect viral RNA and DNA synthesis. Interferons induce the formation of enzymes, including a protein kinase that phosphorylates a factor which blocks peptide chain initiation, a phosphodiesterase that degrades terminal nucleotides of tRNA. and enzymes that activate RNase. 

GILOH, H. and MAGER, J. Inhibition of peptide chain initiation in lysates from ATP-depleted cells. I. Stages in the evolution of the lesion and its reversal by thiol compounds, cyclic AMP or purine derivatives and phosphorylated sxigars. Biochem. Biophys. Acta. (1975)> 414. 295-308. 

Another phase of protein synthesis in which I became interested at Yale is peptide-chain initiation. In 1966, J. Eisenstadt and I proved that the chain initiator fMet-tRNAf is strictly required for the translation of f2 bacteriophage RNA, a natural messenger RNA in a crude extract from E. coli. This requirement was, however, only manifested when the magnesium ion concentration in the extract was low. For this demonstration, we had to deplete the pool, and block the formation, of fMet-tRNAf in the extract. We found two compounds which accomplished this by making formyltetrahydrofolate, the source of the formyl residue in fMet-tRNA(, unavailable trimethoprim, an inhibitor of dihydrofolate reductase, and hydroxylamine which was shown by Bertino to react with methylenetetra-hydrofolate and thereby deplete the pool of formyltetrahydrofolate, produced the result desired. 

This reaction can be utilized for the selective cleavage of peptide chains. Initially, all the disulfide bridges are reduced with dithiothreitol, and then are converted to mixed disulfides through... 

An alternative type of explanation for the specific discrimination against host cell protein synthesis in poliovirus-infected cells stemmed from the observation that initiation of protein synthesis could be selectively inhibited in HeLa cells and in poliovirus-infected HeLa cells by increasing the osmolarity of the growth medium (Sa-borio et al., 1974). The inhibition was independent of the solute used to increase the osmolarity. However, virus-directed protein synthesis was observed to be relatively more resistant to inhibition by hypertonic medium than was cellular protein synthesis, a fact which was interpreted as indicating that initiation of viral RNA translation was intrinsically more efficient than that of cellular mRNA (Nuss et al., 1975). These workers, therefore, proposed that the virus-specific or virus-induced factor involved in suppression of host protein synthesis could function by indiscriminantly lowering the rate of peptide chain initiation. Under such conditions, translation of viral mRNA, when it was synthesized, could occur due to its inherently strong affinity... 

B. Datta, M.K. Ray, D. Chakrabarti, D.E. Wylie and N.K. Gupta, J. Biol Chem., 1989, 264, 20620-20624, Glycosylation of eukaryotic peptide chain initiation factor 2 (eIF-2)-associated 67-kDa polypeptide (p ) and its possible role in the inhibition of elF-2 kinase-catalyzed phosphorylation of the eIF-2 a-subunit. 

The a-amino group of the new aminoacyl-tRNA in the A site carries out a nucleophilic attack on the esterified carboxyl group of the peptidyl-tRNA occupying the P site (peptidyl or polypeptide site). At initiation, this site is occupied by aminoacyl-tRNA mef. This reaction is catalyzed by a peptidyltransferase, a component of the 285 RNA of the 605 ribosomal subunit. This is another example of ribozyme activity and indicates an important—and previously unsuspected—direct role for RNA in protein synthesis (Table 38-3). Because the amino acid on the aminoacyl-tRNA is already activated, no further energy source is required for this reaction. The reaction results in attachment of the growing peptide chain to the tRNA in the A site. 

The peptidyl site (P site) is the site on the ribosome where (f)met-tRNAj initially binds. After formation of the first peptide bond, the P site is a binding site for the growii peptide chain. 

After translation has been initiated (see p. 250), the peptide chain is extended by the addition of further amino acid residues (elongation) until the ribosome reaches a stop codon on the mRNA and the process is interrupted (termination). 

At one point or another during protein synthesis, several other proteins will be associated with the ribosome. These include factors that help in initiating the synthetic process, others that help in elongating the peptide chain, and yet others that play a role in terminating the synthesis of a peptide chain. Beyond this, there is also the mRNA to consider, as well as the aminoacylated tRNA molecules. Finally, since protein biosynthesis consumes energy, there is the hydrolysis of ATP and GTP to AMP and GDP, respectively, by the ribosome. 

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