Isoleucyl-tRNA synthesis

Bacterial ribosome function Aminoglycosides Tetracyclines Chloramphenicol Macrolides, azalides Fusidic acid Mupirocin Distort SOS ribosomal subunit Block SOS ribosomal subunit Inhibits peptidyl transferase Block translocation Inhibits elongation factor Inhibits isoleucyl-tRNA synthesis No action on 40S subunit Excluded by mammalian cells No action on mammalian equivalent No action on mammalian equivalent Excluded by mammalian cells No action on mammalian equivalent... 

The target of mupirocin is one of a group of enzymes which couple amino acids to their respective tRNAs for delivery to the ribosome and incorporation into protein. The particular enzyme inhibited by mupirocin is involved in producing isoleucyl-tRNA. The basis for the inhibition is a structural similarity between one end of the mupirocin molecule and isoleucine. Protein synthesis is halted when the ribosome encounters the isoleucine codon through depletion of the pool of isoleucyl-tRNA. 

Mupirocin is a topical antibiotic that inhibits isoleucyl tRNA synthetase with the subsequent inhibition of protein synthesis. Mupirocin has become a mainstay in the treatment of Staph, aureus infection and colonization during hospital outbreaks, and it is in this organism that acquired resistance has arisen (Gilbart etal. 1993). 

Many antibiotics, which inhibit protein synthesis, do not bind to ribosomes but block any of a variety of vital chemical processes needed for growth. Among them are pseudomonic acid, which inhibits isoleucyl-tRNA synthetase from many gram-positive bacteria.1111/VV Rapamycin, best known as an immunosuppressant (Box 9-F), inhibits phosphoinositide-3-kinase and also phosphorylation of the cap-binding protein 4G, a component of the eukaryotic initiation factor complex (Fig. 29-11 ).ww The bacterial enzyme peptide deformylase, which is absent from the human body, has been suggested as a target for design of synthetic antibiotics. 01... 

Uracil is removed from the DNA by a uracil glycosidase which excises the base from the sugar ring. This activity is analogous to the hydrolytic activity of the isoleucyl-tRNA synthetase toward Val-tRNAIle. In both cases the hydrolytic site is too small by the size of one methylene group to accommodate the substrate that is to be left intact. In DNA synthesis, the editing is performed by a separate enzyme, since the editing can wait until after polymerization. As this luxury is not permitted in protein synthesis, the hydrolytic function is on the synthetase, so that correction can occur before the misacylated tRNA leaves the enzyme. 

Mupirocin interferes with RNA synthesis and protein svit-thesis in susceptible bacteria. " It specifically and reversibly binds with bacterial isolcucyl transfer-RNA svrr thase to prevent the incorporation of isoleucine into bacletui proteins. High-level, pla-smid-mediated mupirocin nsLv tance in S. aureus has been attributed to the elaboration d a modified isoleucyl tRNA that does not bind mupirocin. Inherent re.sistance in bacilli is likely due to poor cellubi penetration of the antibiotic. " ... 

Isoleucyl-tRNA synthetase (Ile-RS) catalyzes the activation of tRNA with the amino acid isoleucine in an ATP-dependent reaction, prior to its utilization by the ribosome during protein synthesis (Scheme 5). The enzyme uses a ping-pong kinetic mechanism where ATP and isoleucine react to form isoleucine-AMP with the release of pyrophosphate. In the second step of the reaction, the AMP-activated isoleucine is transferred to the appropriate tRNA with release of AMP. Mupirocin was shown to only interfere with the first step of the reaction. Mupirocin was also characterized as an inhibitor with respect to mammalian Ile-tRNA synthetase.Mupirocin acted as a competitive inhibitor versus He, but with a of 20 pmol P about fourfold... 

Mechanisms Mupirocin is a fermentation product of Pseudomonas fluorescens and is unrelated to any other antimicrobial drug. It acts on gram-positive cocci and inhibits protein synthesis by specifically binding to isoleucyl-tRNA synthetase. 

A.R. Fersht, Editing mechanisms in protein synthesis rejection of valine by the isoleucyl-tRNA syndietase. Biochemistry, 16 5 (1977), 1025-1030. 

Isobutyl chloroformate, 202 At-Isobutyrj lphenylalanine, 217 Isocitrate lyase, labeling of, 139 Isodeacetyloolchicine, 568 synthesis of, 569 L-Isoleucine tRNA ligase, 152 Isoleucyl-tRNA synthetase, 90 Isomerase, 132, 158 1-Isoprenaline, 579... 

Maijanov J, Kozmin SA. Spirofungin A. Stereoselective synthesis and inhibition of isoleucyl-tRNA synthetase. Angew. Chem. Int. Ed. 2007 46 8854-8857. 

Singer, P. A., Levinthal, M., Williams, L. S. (1984). Synthesis of the isoleucyl- and valyl-tRNA synthetases and the isoleucine-valine biosynthetic enzymes in a threonine deaminase regulatory mutant of Escherichia coli K-12. J. Mol. Biol. 175, 39-55. 

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