Convergence in the biosynthesis of acetogenic natural products from plants, fungi, and bacteria

The biosynthesis of chrysophanol and the naphthylisoquinoline alkaloids were investigated. These polyketidic compounds are produced convergently in a organism-specific manner. In prokaryotes, chrysophanol is formed via folding mode S or S′ and in eukaryotes via mode F. Furthermore, the naphthylisoquinoline alkaloids are the only tetrahydroisoquinoline alkaloids not derived from amino acids. This review deals with polyketides to which nature has developed different biosynthetic pathways in the course of evolution. The anthraquinone chrysophanol is the first example of an acetogenic natural product that is, in an organism-specific manner, formed via more than one polyketide folding mode: In eukaryotes, like e.g., in fungi, in higher plants, and in insects, it is synthesized via folding mode F, while in prokaryotes it originates through mode S. It has, more recently, even been found to be synthesized by a third pathway, named mode S′. Thus, chrysophanol is the first polyketide synthase product that originates through a divergent–convergent biosynthesis (depending on the respective producing organisms). A second example of a striking biosynthetic convergence is the isoquinoline alkaloids. While all as yet investigated representatives of this large family of plant-derived metabolites (more than 2500 known representatives!) are formed from aromatic amino acids, the biosynthetic origin of naphthylisoquinoline alkaloids like dioncophylline A is unprecedented in following a route to isoquinolines in plants: we have shown that such naphthylisoquinolines represent the as yet only known polyketidic di- and tetrahydroisoquinolines, originating from acetate and malonate units, exclusively. Both molecular halves, the isoquinoline part and the naphthalene portion, are even synthesized from a joint polyketide precursor, the first proven case of the F-type folding mode in higher plants. The biosynthetic origins of the natural products presented in this paper were elucidated by feeding 13C 2-labeled acetate (or advanced precursors) to the respective producing organisms, with subsequent NMR analysis of their 13C 2 incorporation patterns using the potent cryoprobe methodology, thus making the full polyketide folding pattern visible.