Polyketides avermectin

Ikeda, H., Nonomiya, T., Usami, M. et aL (1999) Organization of the biosynthetic gene cluster for the polyketide anthelmintic macrolide avermectin in Streptomyces avermitilis. Proceedings of the National Academy of Sciences of the United States of America, 96, 9509-9514. 

MacNeil, D.J., Occi, J.L., Gewain, K.M. et al. (1992) Complex organization of the Streptomyces avermetilis genes encoding the avermectin polyketide synthase. Gene, 115, 119-125. 

Another important class of anti-infective natural products introduced in recent years is the avermectins, polyketide-derived macrolides that were originally isolated from several species of Streptomyces. The major drug in this class, ivermectin, was originally developed to treat and control nematodes and parasites in livestock. In recent years, however, the potential of ivermectin for the treatment of human disease has also been realized, and it is now used to treat onchocerciasis (river blindness), a disease that afflicts 40 million people worldwide (De Smet, 1997). 

Eight naturally occurring, structurally related avermectins are produced by Streptomyces avermitilis [7], The avermectin polyketide structure is derived from seven acetate and five propionate residues, together with a single 2-methylbutyric acid or isobutyric acid residue which forms the sec-butyl or isopropyl group attached to the C25 of the spiroketal moiety [8,9] (Fig. 1). The avermectin agly-cone is further modified by glycosylation at C13, with the attachment of two O-methylated oleandrose residues and O-methylation at C5. Thus, S. avermitilis... 

As discussed earlier, the avermectin polyketide backbone is derived from seven acetate and five propionate extender units added to an a branched-chain fatty acid starter, which is either (S( I )-a-mcthylbutyric acid or isobutyric acid. The C25 position of naturally occurring avermectins has two possible substituents a. sec-butyl residue derived from the incorporation of S(+)-a-methy lbutyry 1-CoA ( a avermectins), or an isopropyl residue derived from the incorporation of isobutyiyl-CoA ( b avermectins). These a branched-chain fatty acids, which act as starter units in the biosynthesis of the polyketide ring, are derived from the a branched-chain amino acids isoleucine and valine through a branched-chain amino acid transaminase reaction followed by a branched-chain a-keto acid dehydrogenase (BCDH) reaction (Fig. 5) [23]. 

DJ MacNeil, JL Occi, KM Gewain, T MacNeil. Correlation of the avermectin polyketide synthase genes to the avermectin structure. Implications for designing novel avermectins. Ann NY Acad Sci 123-132, 1994. 

Figure 2 Examples of polyketides. Polyketides demonstrate a broad range of biological activities, including antibiotic (oxytetracycline and erythromycin), antitumor (doxorubicin and dynemicin), antiparasitic (avermectin), and immunosuppressive (FK506). Monensin is used as a bovine feed supplement and an anticoccidial agent.

CJ Dutton, AM Hooper, PF Leadlay, J Staunton. Avermectin biosynthesis. Intact incorporation of a diketide chain-assembly intermediate into the polyketide mac-rocylic ring. Tetrahedron Lett 35 327-330, 1994. 

A final example of metabolic pathway engineering is based on polyketide and nonribosomal peptide biosynthesis. Polyketides and nonribosomal peptides are complex natural products with numerous chiral centers, which are of substantial economic benefit as pharmaceuticals. These natural products function as antibiotics [erythromycin A (65), vancomycin (66)], antifungals (rapamycin, amphotericin B), antiparasitics [avermectin Ala (67)], antitumor agents [epothiolone A (68), calicheamicin yj, and immunosuppressants [FK506 (69), cyclosporin A], Because this exponentially growing and intensely researched field has developed, the reader is directed to review articles for additional details.347-359 Also with the potential economic benefit to develop the next blockbuster pharmaceutical, a number of patents and patent applications have been published.360-366... 

Macrolides and polyethers such as erythromycin A (4), FK 506, rapamycin or avermectin A (5, Scheme 1) are products of modular type I polyketide-synthases. These compounds are distinguished by extraordinary structural diversity and complexity [1,2]. Because of their biological potency, members of this structural class as well as the aromatic polycyclic products of type II polyketide-synthases, tetracyclines and anthara-cyclines, e.g. adriamycin (6), became useful as pharmaceuticals (antibiotics, cytostatics, immunosuppressives) [1,2],... 

Gene clusters (or parts thereof) controlling the biosynthesis of several other complex polyketides, including avermectin [25],rapamycin [26], oleandomycin [164], and soraphen [165], have been isolated and sequenced. In aU cases, the PKSs have been found to be organized into individual modules with each module containing the appropriate sets of active sites. Thus, the modular hypothesis appears to be well-substantiated now in several model systems. Intriguingly, in the case of rapamycin, biosynthesis of the entire macrocycle involves activity of a 12-module PKS as well as a peptide synthetase module (see below). 

There is no major distinction between the avermectins and milbemycins, which are based on the same complex polyketide macrocycle (168) the avermectins are oxygenated at C-13 and bear a disaccharide on this oxygen. They have been isolated from cultures of a... 

The avermectin PKS loading module has been used to generate a hybrid PKS system by replacing the loading module of DEBS 1-TE. The new hybrid PKS produced new hybrid polyketide (triketide lactones) which incorporated the isobutyrate and 2-methylbutyrate starter acids of avermectin biosynthesis, as well as the normal acetate and propionate starter units of erythromycin biosynthesis [51]. 

Figure 12.14. Production of new macrolides after replacing the loading domains of the erythromycin and spiramycin polyketide synthases with domains of the avermectin and tylosin polyketide synthases.

The polyketides are a family of natural products containing many important pharmaceutical agents that are synthesized through the multienzyme complex, polyketide synthase, which can display substantial molecular diversity with respect to chain length, monomer incorporated, reduction of keto groups, and stereochemistry at chiral centers (189). This variability, together with the existence of several discrete forms of polyketide synthase, allows the generation of diverse structures like erythromycin, avermectin, and rapamycin. This biochemical diversity has been considerably expanded by the introduction of new sub strate species that were used by the enzymes to produce new or unnatural polyketides (190). 

Avermectins are a mixture of 16-membered macrocyclic polyketide aglycones with a dimeric o-methyl-a-L-oleandrose attached to C-13, and are widely used for broad spectrum parasite control (Figure 8.2). They vary in three substituents Ri, R2 and R3, where R3 can be either a hydroxyl or hydrogen, in which case there is a double bond... 

A few ER domains are present in polyketide synthases and no ER domains are present in avermectin [28] and pimaricin [30] polyketide synthases. ER domains contain the putative NADP(H)-binding motif LxHxg(a)xGGVG, which was originally proposed for the rat ER [31, 32], or GxGxxAxxxA [33]. 

In avermectin polyketide synthase, the last invariant Ala in the motif sequence is substituted to Thr [28]. KR domain activity is predicted to be required for the formation of avermectin aglycons. In another group of KR domains, the motif sequence corresponding region is not found, which readily accounts for their inactivity. 

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