Polyketides pigments

The main metabohtes produced by Monascus are polyketides formed by the condensation of one acetylcoA with one or more malonylcoAs with a simultaneous decarboxylation as in the case of lipidic synthesis. They consist of the pigments, monacohns, and under certain conditions a mycotoxin. 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

Orphan skeletons are emphasized in boldfece characters. Found both in the Tertiary and the Triassic eras, they have no equivalent in extant organisms and cannot be accounted for by di enetic transformations alone. They comprise xanthenoxanthene quinones, quincyte pigments of likely polyketide origin (Prowse 1991), and three series of isoprenoids polycyclic hydrocarbons from the... 

Most of the pigments of flowers arise from a single polyketide precursor. Phenylalanine is converted to trans-cinnamic acid (Eq. 14-45) and then to cinnamoyl-CoA. The latter acts as the starter piece for chain elongation via malonyl-CoA (step a in the accompanying scheme). The resulting (3-polyketone derivative can cyclize in two ways. The aldol condensation (step b) leads to stilbenecar-boxylic acid and to such compounds as pinosylvin of pine trees. The Claisen condensation (step c) produces chalcones, flavonones, and flavones. These, in turn, can be converted to the yellow fla-vonol pigments and to the red, purple, and blue anthocyanidins.3 c... 

Yu T-W, Shen Y, McDaniel R, Floss HG, Khosla C, Hopwood DA, Moore BS (1998) Engineered biosynthesis of novel polyketides from Streptomyces spore pigment polyketide synthases. J Am Chem Soc 120 7749-7759... 

Figure 10 Examples of unnatural aromatic polyketides. Libraries of aromatic poly-ketides have been generated using combinations from the actinorhodin, tetracenomycin, frenolicin, griseusin, and whiE spore pigment gene clusters.

Streptomycetes also produce a variety of fused ring aromatic polyketides, the carbon skeletons of some of which closely parallel those of their fimgal counterparts. However, with very few exceptions, such as the melanin spore pigment precursor 1,3,6,8-tetrahydroxynaphthalene (10, Scheme 5) of Alternaria alternate and Streptomyces griseus, fimgi and streptomycetes have not been found to produce identical fiised ring structures. 

Yu,T.-W. Shen,Y. McDonald, R. Floss, H. G. Khosla, C. Hopwood, D.A. Moore, B. S., Engineered Biosynthesis of Novel Polyketides from Streptomyces Spore Pigment Polyketide Synthases. J.Amer. Chem. Soc. 1998, 120, 7749. 

Many secondary metabolites with complex chemical structures, including pigments (Figure 2) and monacolins (Figure 2), are synthesized from the polyketide pathway in Monascus spp. (Simpson, 1986). Several effectors controlling the polyketide synthesis of Monascus have been reported by using submerged culture systems (Lin, 1991). Considerable research has been conducted on the industrial production of Monascus in complex liquid media (Shepherd and Carels, 1983). 

Only a few species of Monascus can produce monacolin K (Table 1). Since Monascus pigments and monacolin K are made by the same or similar polyketide-forming enzymes (Turner and Aldridge, 1983), the ability of various species of Monascus to produce monacolin K may be predicted based on its mycelia color. 

Cyclization of the polycarbonyl chain to form aromatic compounds is a very common biosynthetic process. These aromatic compounds can then undergo various further biosynthetic transformations, including ring cleavage reactions. Some polyketides are pigments of fungi and others are serious mycotoxins and these are described in Chapters 7 and 9, respectively. 

A second large group of polyketide metabolites that have been isolated from lichens are heptaketide xanthones. Their structures are mainly based on norli-chexanthone (7.54). An interesting feature of these compounds is the presence of chlorine in several of the metabolites. Lichens also produce some anthraquinone pigments, exemplified by physcion (7.6) from Xanthora species. 

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