Erythromycin formation

The 11,12-carbonate of erythromycin (32) is an older cycHc ester which had greater stabdity and antibiotic activity by diminishing formation of intramolecular enol ether (27) (136,137). A later analogue, the ll-A/-12-0-cychc carbamate of... 

The sulfuric acid treated aliquot representing the blank forms a cyclic ether anhydroerythromycin.10 The alkaline treatment causes the formation of an unsaturated ketone (9-keto-10-ene) having its absorbance maximum as a shoulder at 236 nm. (e 6000).n i2 Thus, any other UV absorbing species are measured with the blank and subtracted from the absorbance before calculation of the erythromycin concentration. A typical spectrum is shown in Figure 4. 

Deoxyerythronolide B (28), produced by blocked mutants of Streptomyces erythreus, is a common biosynthetic precursor leading to erythromycins. A different route to this compound was developed with aldol methodology.5 In this approach, all the crucial C C bond formations involved in the construction of the carbon framework are exclusively aldol reactions. 

It should be noted that TE-catalyzed cyclization is not Umited to the synthesis ofmacrocycUc peptides by catalyzing the formation of a C—N bond. These enzymes are also responsible for the cyclization of NRP depsipeptide and PK lactone. Indeed, a didomain excised from fengydn synthase was able to catalyze the formation of a macrolactone through the formation of a C—O bond [39]. Several cyclases from PKSs have also been characterized to be functional. For example, when a TE from picromycin synthase was fused to an erythromycin module (DEBS module 3), the resulting hybrid was able to convert a diketide and 2-methylmalonyl-CoA to a triketide ketolactone (Scheme 7.12) [40]. However, their in vitro activity is in... 

Erythromycin inhibits bacterial protein synthesis by reversibly binding with their 50 S ribosomal subunit, thus blocking the formation of new peptide bonds. Erythromycin is classified as a bacteriostatic antibiotic. 

Erythromycin and the other macrolides prevent release of tRNAs from the ribosomal A site after peptide bond formation. 

Macrolides bind to the SOS ribosomal subunit of bacteria but not to the SOS mammalian ribosome this accounts for its selective toxicity. Binding to the ribosome occurs at a site near peptidyltransferase, with a resultant inhibition of translocation, peptide bond formation, and release of oligopeptidyl tRNA. However, unlike chloramphenicol, the macrolides do not inhibit protein synthesis by intact mitochondria, and this suggests that the mitochondrial membrane is not permeable to erythromycin. 

The lincosamide family of antibiotics includes lin-comycin (Lincocin) and clindamycin (Cleocin), both of which inhibit protein synthesis. They bind to the SOS ri-bosomal subunit at a binding site close to or overlapping the binding sites for chloramphenicol and erythromycin. They block peptide bond formation by interference at either the A or P site on the ribosome. Lincomycin is no longer available for human use in the United States. 

Chemistry of the glycoside linkage. Exceptionally fast and efficient formation of glycosides by remote activation, Carbohydr. Res. 80 07 (1980). (e) K. Wiesner, T. Y. R. Tsai, and H. Jiu, On cardioactive steroids. XVI. Stereoselective P-glycosylation of digitoxose the synthesis of digitoxin, Helv. Chim. Acta 60 300 (1985). (f) R. B. Woodward (and 48 collaborators), Asymmetric total synthesis of erythromycin. 3. Total synthesis of erythromycin, J. Am Chem. Soc. 103 3215 (1981). (g) P. G. M. Wuts and S. S. Bigelow, Total synthesis of oleandrose and the avermecin disaccharide, benzyl ot-L-oleandrosyl-ot-L-4-acetOxyoleandroside, J. Org. Chem. 43 3489 (1983). 

A 26-kb gene cluster encoding enzymes for synthesis of the blue antibiotic actinorhodin by Streptomyces coelicolor has been cloned and sequenced.332337 The three large 10-kb genes required for formation of the broad-spectrum antibiotic erythromycin by Saccharopolyspora erythraea have also been cloned and sequenced.337 339 In both cases, the genes... 

Hirvonen et al. (1995) evaluated the feasibility of the UV/H202 process for the removal of trichloroethylene (TCE) and erythromycin (perchloroethylene [PCE]) in contaminated groundwater. The formation of chloroacetic acids (CAs) was used as an indication of partial degradation. The dominant byproduct, dichloroacetic acid (DCA), accounted for the major part of the total yield of CAs. The observed concentrations of trichloroacetic acid (TCA) and DCA were relatively low compared with the total amount of TCE and PCE degraded. The effect of initial concentrations of the parent compounds, hydrogen peroxide, and bicarbonate on the yield of by-product was inves-... 

Chloramphenicol. Chloramphenicol (Chloromycetin) is a synthetically produced agent that exerts antibacterial effects similar to those of erythromycin that is, it binds to the 50S subunit of bacterial ribosomes and inhibits peptide bond formation. Chloramphenicol is a broad-spectrum antibiotic that is active against many gram-negative and gram-positive bacteria. This drug is administered systemically to treat serious infections such as typhoid fever, Haemophilus infections such as osteomyelitis, rickettsial infections such as Rocky Mountain spotted fever, and certain forms of meningitis. Chloramphenicol may also be administered topically to treat various skin, eye, and ear infections. 

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-... 

Seno ET, Hutchinson CR (1986) The biosynthesis of tylosin and erythromycin Model systems for the studies of the genetics and biochemistry of antibiotic formation. In Queener SW, Day LE (eds) The Bacteria Volume IX Antibiotic-Producing Streptomyces. Academic Press, Orlando, p 231... 

In the presence of its natural substrates, propionyl-CoA, methylmalonyl-CoA, and NADPH, DEBS 1-TE was initially shown to catalyze the formation of lactone in a cell-free system [36]. Concomitant work on a similar bimodular system called DEBS 1+TE (Fig. 9c) [37] in a cell-free extract and with partially purified protein, demonstrated that it too was competent for biosynthesis of the triketide lactone [33], These experiments set the stage for more rigorous investigation of mechanistic aspects of erythromycin biosynthesis. 

L Tang, H Fu, R McDaniel. Formation of functional heterologous complexes using subunits from the picromycin, erythromycin and oleandomycin polyketide synthases. Chem Biol 7 77-84, 2000. 

Figure 3 Rates of formation of 4-OH-triazolam from triazolam (250 pM) by human liver microsomes in vitro. Each point is the mean ( SE) of four microsomal preparations. Reaction velocities when preparations were preincubated with the macrolide agents are expressed as a percentage of the control velocity with no inhibitor present (inhibitor = 0). Mean IC50 were TAO, 3.3 pM erythromycin, 27.3 pM clarithromycin, 25.2 pM azithromycin, >250 pM. Abbreviations IC50, 50% inhibitory concentrations TAO, troleandomycin. Source Adapted, in part, from Ref. 77.

The enzyme converts erythromycin C into erythromycin A in the presence of AdoMet. Evidence was obtained that the enzyme is associated with the microsomal fraction. The enzyme showed a very high degree of substrate specificity. Aside from erythromycin C, it failed to catalyze the methylation of any other L-mycarosyl moiety tested. Erythromycin A and S-adenosyl-L-homocysteine (AdoHcy) were potent inhibitors of the enzyme, and it was assumed that the Ado-Met AdoHcy ratio could be a major regulatory factor of the final step in the formation of erythromycin A. 

In the course of mechanistic investigations covering these enzymatic reductions, labeling experiments were carried out with biologically produced, selectively deuterated NADPH-mole-cules 4-(/ )-[4- H]NADPH and 4-(5)-[4-2H] NADPH [11], The formation of hydroxy derivatives of opposite stereochemistry is caused by the ketoreductase domains KRl and KR2 from the protein DEBS 1 of the erythromycin poly-ketide-synthase. However, both domains have a preference for the 4-pro-(5)-hydride of the NADPH molecule. Probably the binding of the cofactor in KR domains takes place in an identical manner, whereas the individual y9-keto-acylthioester building blocks in the domains KR 1 and KR 2 of DEBS 1 capture a different orientation relative to the cofactor [11]. 

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