Chrysophanol, biosynthesis

Remarkably, the same compound can be produced in different ways by different organisms. The best known example is probably the polyketide anthraquinone, chrysophanol 9, which occurs in both eukaryotes (higher plants, lichens, fungi and insects) and prokaryotes, but is produced through different folding modes of polyketide chains.10 Similarly, it has also been demonstrated that the biosynthesis of gibberellins involves different metabolic sequences in fungi and plants.11... 

If identical plant and microbial polyketides are indeed derived by common or closely related pathvwiys, it would appear that higher plant species can produce both types of synthases required for the formation of mode F and mode S-cyclised fused ring polyketides. The biosynthesis of aloesaponarin II by steptomycetes is known to involve an iterative type IIPKS and, by analogy with other fungal aromatic polyketide synthases, it is likely that a type I PKS is responsible for the formation of chrysophanol. However, at present little is known of the nature of PKSs responsible for the formation of fused ring polyketides in plants. 

Leistner in Germany and Burnett in Great-Britain have performed much work on the biosynthesis of anthraquinones [5,23-28]. Two different pathways for the biosynthesis of anthraquinones in plants exist either the polyketide pathway or the shikimate pathway [5], Anthraquinones synthesised according to the former pathway are substituted in both ring A and C. Examples are emodin and chrysophanol. These polyketide-derived anthraquinones are found in Rhamnaceae, Polygonaceae and Leguminosae... 

Chrysophanol (l,8-Dihydroxy-3-methylanthraquinone) Biosynthesis in Higher Plants. J. Chem. Soc. Chem. Commun. 1969, 210. 

SCHEME 2.16 Different folding patterns in the biosynthesis of chrysophanol (80) in eukaryotes and prokaryotes. 

It is remarkable that besides the main precursor chrysophanol (75), emodin (71) and islandicin (73) are also incorporated into ergochromes. The incorporations are, respectively, 3.6 and 11.8 times less than for chrysophanol (75), but still significant. As emodin (71) and islandicin (73) arise from the same polyketide precursor (15), their incorporation represents an offshoot of ergochrome biosynthesis, the participation of which was precisely established (see Table VI). Only few comparative incorporations with differently labeled precursors have been carried out in other natural product areas. Therefore, it is still uncertain to what extent such branching of biosynthetic pathways occurs. 

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