Ribosomes blocking protein synthesis

Shiga toxin produced by Shigella dysenteriae has similar structural features. The toxin binds to a glycolipid (Gb3), undergoes endocytosis, and the enzymatie Ai fragment, which is a specific N-glycosidase, removes adenine from one particular adenosine residue in the 28S RNA of the 60S ribosomal subunit. Removal of the adenine inactivates the 60S ribosome, blocking protein synthesis. Ricin, abrin, and a number of related plant proteins inhibit eukaryotic protein synthesis in a similar manner (Chapter 25). 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

Pharmacology Telithromycin belongs to the ketolide class of antibacterials and is structurally related to the macrolide family of antibiotics. Telithromycin blocks protein synthesis by binding to domains II and V of 23S rRNA of the 508 ribosomal subunit. Pharmacokinetics ... 

Mechanism of Action Aketolide that blocks protein synthesis by binding to ribosomal receptor sites on the bacterial cell wall. Therapeutic Effect Bactericidal. Pharmacokinetics Protein binding 60%-70%. More of drug is concentrated in WBCs than in plasma, and drug is eliminated more slowly fromWBCs than from plasma. Partially metabolized by the liver. Minimally excreted in feces and urine. Half-life 10 hr. 

The antibacterial action of erythromycin may be inhibitory or bactericidal, particularly at higher concentrations, for susceptible organisms. Activity is enhanced at alkaline pH. Inhibition of protein synthesis occurs via binding to the 50S ribosomal RNA. Protein synthesis is inhibited because aminoacyl translocation reactions and the formation of initiation complexes are blocked (Figure 44-... 

The primary clinical targets of immunotoxins are tumors, based on the principle that the MAb will target the toxin to the tumor cells and the highly toxic moiety will then kill the cancer cells. Examples of toxins are ricin, diphtheria toxin and abrin, which are all glycoproteins. Their toxicity is based on their ability to block protein synthesis at the ribosomal protein assembly site. They are normally extremely toxic and not suitable for therapeutic purposes because they induce liver and vascular toxicity, even at low dose levels. 

C. Tetracyclines bind to the 30S subunit of the bacterial ribosome and block protein synthesis. 

Chloramphenicol blocks translation in bacteria by inhibiting peptidyltransferase of the large ribosomal subunit. It does not interfere with peptidyltransferase in the large subunit of eukaryotic ribosomes. However, the mitochondrion of animal cells contains ribosomes that are similar to bacterial ribosomes, and chloramphenicol can block protein synthesis in this organelle. This could contribute to the side effects of this drug when used in the treatment of animals. 

The mechanisms of toxic action of abrin and ricin are similar. The B-chain attains cell recognition and binding function to facilitate toxin transport across the cell membrane, whereas the A-chain, once internalized by the cell, blocks protein synthesis by catalytically modifying the ribosomes. Both toxins ultimately kill target cells in animal or cell culture models by both necrosis and apoptosis. 

MOA Block protein synthesis at SOS ribosomal subunit. The antibacterial action of the macrolides is bacteriostatic. 

How does toxalbumin relate to ricin Toxalbumins consist of two subunits (A and B) which are joined by a disulfide bond. The A subunit irreversibly binds the 60S ribosomal subunit, which blocks protein synthesis. The B subunit allows for binding and penetration across the gastrointestinal cell wall. Sufficient doses of ricin can cause cell death due to continued inhibition of protein synthesis. 

A. Characteristics. Ricin is a glycoprotein toxin from the seed of the castor bean plant. Altering ribosomal RNA blocks protein synthesis, thereby killing infected cells. Ricin s significance as a potential biological warfare agent relates to its availability worldwide, ease of production, and extreme pulmonary toxicity when inhaled. 

Reflecting the bacterial ancestry of mitochondria, mitochondrial ribosomes resemble bacterial ribosomes and differ from eukaryotic cytosolic ribosomes in their RNA and protein compositions, their size, and their sensitivity to certain antibiotics (see Figure 4-24). For instance, chloramphenicol blocks protein synthesis by bacterial and mitochondrial ribosomes from most organisms, but cycloheximide does not. This sensitivity of mitochondrial ribosomes to the important aminoglycoside class of antibiotics is the main cause of the toxicity that these antibiotics can cause. Conversely, cytosolic ribosomes are sensitive to cycloheximide and resistant to chloramphenicol. In cultured mammalian cells the only proteins synthesized in the presence of cycloheximide are encoded by mtDNA and produced by mitochondrial ribosomes. I... 

Fig. 38.24. General mechanism of action of drugs that block protein synthesis by binding to ribosomal units.

Penicillins, cephalosporins, tetracyclines, acrinomvdns These are examples of classes of antibiotics each from a microorganism producing c) otoxic agents. The first three classes are selectively c) otoxic to bacteria, and the fourth cytotoxic to mammalian cancer cells, unfortunately with poor selective toxicity. The penicillins and cephalosporins inhibit bacterial cell wall synthesis, and tetracyclines selectively block protein synthesis at the bacterial ribosome. The actinomycins intercalate in a relative nonselective manner the... 

Tetracycline Inhibits the growth of bacteria by blocking protein synthesis at the ribosomes. We have performed a study of the thermodynamics of cation binding by tetracycline In aqueous... 

The question remains whether CAP under certain circumstances blocks protein synthesis at the 80 S ribosome. In a cell-free system of reticulocyte microsomes (Weisberger et al., 1964 Beard et oL, 1969) an inhibition of the association of mRNA and the 40 S ribosomal subunit was reported. In the reticulocyte system used 70-100% inhibition of [ Cjleucine incorporation was reported at 0.1 pmole/ml of CAP with either endogenous template RNA or small amounts of poly(U). These results could not be confirmed by Zelkowitz et al. (1968). We also found no striking inhibition of poly (U)-directed amino acid incorporation by CAP in messenger-depleted cell-free systems from rat liver or rat embryos (Uehlin et al, 1974). From all this it seems clear that generally CAP has no striking effect on protein synthesis at the 80 S ribosome level. But inhibition of mammalian microsomal protein synthesis under certain conditions cannot be ruled out. 

The presence of ribonuclear protein particles active in amino acid uptake has been demonstrated in nuclear preparations. The so-called nuclear ribosomes require ATP and GTP for amino acid uptake, exactly as cytoplasmic ribosomes require the same triphosphonucleo-tides for activity. There seems to be one major difference between nuclear and cytoplasmic ribosomes. RNase, which effectively blocks protein synthesis in the cytoplasm, is without effect on protein synthesis in the nucleus. The fact that DNA is required for amino acid incorporation into nuclear protein cannot be invoked as an argument in favor of the existence of such a metabolic pathway in the nucleus, because the effect of DNA is indirect and DNase interferes with the generation of ATP in the nucleus and therefore, abolishes the only source of energy in the system. 

Previous reports that, in eukaryotic cells chloramphenicol specifically inhibits protein synthesis by mitochondrial ribosomes, whereas cy-cloheximide blocks protein synthesis by cytoplasmic ribosomes were confirmed by a number of groups98a-c. 

A natural antibacterial compound diat blocks protein synthesis at the 70S ribosomes of Gram-positive bacteria. 

The experimental approach in most of the systems studied so far was to follow the greening process at different levels (ultrastructure, composition of membranes, development of activity) and try to establish a correlation between the different parameters observed which will lead to the formulation of explanatory hypotheses. The validity of the hypotheses can then be assessed by inducing alteration in the process at one level and studying the effect at the other levels. The use of protein and nucleic acid synthesis inhibitors has so far been the method of choice. Cycloheximide was extensively used to block protein synthesis by cytoplasmic 80 S type ribosomes, while chloramphenicol, spectinomycin, and lyncomycin were used to inhibit protein synthesis by the chloroplast ribosomes. Actinomycin D and, more recently, rifampicin have been used as inhibitors of RNA synthesis. Initially, rifampicin was considered to be specific for the DNA-dependent RNA polymerase of the chloroplast. Although it appears that in some algae, such as Chlamydomonas and Acetabularia, rifampicin has a reasonable degree of specificity, its action against RNA polymerase of the chloroplast partially purified from plants was not found to be specific. ... 

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