Deq

FIGURE 8.32 Pyrrolysine and Lysine

Pyrrolysine is (5R,5R)-4-substituted-pyrroline carboxylate. The 4-substituent (shown as X) is thought most likely to be a methyl group, but this is not certain.

Why Selenocysteine?

One plausible hypothesis to explain why UGA is both a stop codon and a selenocysteine codon is evolutionarily based. When oxygen first appeared with the evolution of the photosynthetic process, selenocysteine was replaced by the more stable cysteine in all but a few proteins. This allowed UGA to be reassigned as a stop codon. There are some problems with this hypothesis. Many groups of organisms had already diverged before photosynthesis appeared. That they all independently dropped selenocysteine and reallocated UGA to mean "stop" seems highly unlikely.

Streptomycin and related antibiotics bind to rRNA in the small subunit of the bacterial ribosome.

Tetracycline binds rRNA in the small subunit of both prokaryotic and eukaryotic ribosomes.

Chloramphenicol binds to 23S rRNA and prevents peptide bond formation.

aminoacyl-tRNA synthase, a special tRNA and genes for three accessory proteins are also found in organisms with pyrrolysine. By analogy with selenocysteine, it is believed that the pyrrolysine-tRNA is first charged with lysine, which is then modified to form pyrrolysine. However, the mechanism of pyyrolysine synthesis and insertion remains to be elucidated in detail. In particular, it is unknown how UAG codons for pyrrolysine are distinguished from those still meaning stop. Genome sequence analysis has found genes homologous to those for the pyrrolysine system in occasional eubacteria suggesting that pyrrolysine may be present. However, pyrrolysine itself has not yet been identified directly in these organisms.

Many Antibiotics Work by Inhibiting Protein Synthesis

Many well-known antibiotics work by inhibiting protein synthesis. Most of these are specific for prokaryotic ribosomes. However, very high concentrations of these agents will inhibit the ribosomes of mitochondria and chloroplasts, which are of prokaryotic ancestry.

Aminoglycoside antibiotics bind to the 30S subunit. Streptomycin binds to the 16S rRNA near where the two ribosomal subunits touch. The presence of streptomycin distorts the A-site and hinders binding of incoming charged tRNA. In particular, binding of initiator tRNA-Met is inhibited and so initiation of translation is prevented. Streptomycin-resistant mutants have alterations in nucleotide 523 of 16s rRNA or in ribosomal protein S12 (RpsL), which assists antibiotic binding. Many of the other aminoglycosides, such as gentamycin and kanamycin, bind to multiple sites on the 30S subunit and mainly inhibit the translocation step of protein synthesis. Streptomycin and other aminoglycosides also cause misreading of the mRNA.

Tetracyclines inhibit both bacterial and eukaryotic ribosomes.They bind to the 16S (or 18S) rRNA of the small subunit and block the attachment of charged tRNA. Despite inhibiting both types of ribosome, tetracyclines inhibit bacteria preferentially due to the fact that bacteria actively take them up whereas eukaryotic cells actively export them.

Chloramphenicol binds to the 50S subunit, to the loop of 23S rRNA that interacts with the acceptor stem of the tRNA, and inhibits the peptidyl transferase. Cyclohex-imide binds to the 60S subunit of eukaryotic ribosomes and inhibits the peptidyl trans-ferase. Erythromycin and related macrolide antibiotics bind to the 23S rRNA of bacterial ribosomes and inhibit the translocation step.

Fusidic acid is a steroid derivative that binds to prokaryotic elongation factor EF-G. In the presence of fusidic acid, EF-G, with its bound GDP, is frozen in place aminoglycosides Class of antibiotics that inhibits protein synthesis;includes streptomycin, neomycin, kanamycin, amikacin and gentamycin chloramphenicol An antibiotic that inhibits bacterial protein synthesis cycloheximide An antibiotic that inhibits eukaryotic protein synthesis erythromycin An antibiotic that inhibits bacterial protein synthesis

Fusidic acid An antibiotic that inhibits protein synthesis streptomycin An antibiotic of the aminoglycoside family that inhibits protein synthesis tetracyclines Family of antibiotics that inhibit protein synthesis

Bacterial cell ingested by immune system cell

FIGURE 8.33 Digestive Enzymes Are Activated on Location

A) Proteases destined for export are made as precursors and are cleaved to form the active protease once safely outside the cell. B) Proteases in the membrane-bound lysosome degrade ingested material.

Bacterial cell ingested by immune system cell

FIGURE 8.33 Digestive Enzymes Are Activated on Location

A) Proteases destined for export are made as precursors and are cleaved to form the active protease once safely outside the cell. B) Proteases in the membrane-bound lysosome degrade ingested material.

Active digestive enzyme

on the ribosome. Fusidic acid also inhibits the corresponding eukaryotic elongation factor EF-2; however, in practice, animal cells are unaffected as they do not take up the antibiotic.

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