This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Polyketides Biosynthesis|
Expected Taxonomic Range: Magnoliophyta
Polyketide (exclusive of fatty acids) represent a large group of natural products, i.e. secondary metabolites that are widespread in fungi, bacteria and plants [Birch68] [Schroder99a] [Wink03]. Polyketides are often involved as intermediates in the formation of a vast variety of secondary metabolites with mixed biosynthetic origin. Polyketide such as chalcone and 6'-deoxychalcone are precursors in the biosynthesis of thousands of biologically important compounds functioning as pigments, anti-pathogens (phytoalexins), UV protectants, signals for the interaction with microorganisms and mediators of fertility [Schroder99b] [Schroder97a].
The entry enzymes of the biosynthesis of polyketides in plants are the polyketide synthases producing the general backbone of compounds who give rise to a large array of natural products representing an amazing degree of structural diversity. The chalcone synthase (CHS) and stilbene synthase (STS) are the most well-known plant polyketide synthases, however there exists a growing superfamily of related proteins referred to as CHS/STS-type proteins [Schroder99a]. Depending on the specific polyketide synthase involved very diverse compound classes evolve such as chalcones (flavonoid biosynthesis), stilbenes [Schroder99b], stilbenecarboxylates [Eckermann03], benzalacetone [BorejszaWysocki96], pyrone [Eckermann98], C-methylated chalcone [Shen00] and acridone alkaloids [Baumert94] [Junghanns95].
The formation of polyketides resembles the biosynthesis of fatty acids but is different in various aspects. In contrast to fatty acid synthases (FAS) plant polyketide synthases (PKS) are relatively small homodimeric enzymes, which do not use the acyl carrier protein (ACP) as activating agent but utilize acyl-CoA esters directly as substrates [Shen00] [Birch68]. Although the synthesis of chalcone usually does not involve modifications by other proteins the chalcone derivatives reduced at a specific position in the synthesized aromatic ring are frequently found in plants increasing the structural variability of derivatives [Schroder97a].
About This Pathway
The polyketide synthase (PKS) involved in the biosynthesis of aloesone represents a novel type of PKSs catalyzing six successive condensations steps with malonyl-CoA and acetyl-CoA as the starter unit. The product is an aromatic heptaketide, aloesone. The aloesone synthase (EC 2.3.1.-) isolated from rhubarb (Rheum palmatum) belongs to the CHS superfamily (PKS type III). The aloesone synthase is a plant-specific, unique enzyme taking up a central function in the biosynthesis of chromones [Abe04].
Baumert94: Baumert A, Maier W, Groger D, Deutzmann R (1994). "Purification and properties of acridone synthase from cell suspension cultures of Ruta graveolens L." Z Naturforsch [C] 49(1-2);26-32. PMID: 8148006
BorejszaWysocki96: Borejsza-Wysocki W, Hrazdina G (1996). "Aromatic Polyketide Synthases (Purification, Characterization, and Antibody Development to Benzalacetone Synthase from Raspberry Fruits)." Plant Physiol 110(3);791-799. PMID: 12226219
Eckermann03: Eckermann C, Schroder G, Eckermann S, Strack D, Schmidt J, Schneider B, Schroder J (2003). "Stilbenecarboxylate biosynthesis: a new function in the family of chalcone synthase-related proteins." Phytochemistry 62(3);271-86. PMID: 12620338
Eckermann98: Eckermann S, Schroder G, Schmidt J, Strack D, Edrada RA, Helariutta Y, Elomaa P, Kotilainen M, Kilpelaeinen I, Proksch P, Teeri TH, Schroder J (1998). "New pathway to polyketides in plants." Nature 396 (26), 387-390.
Junghanns95: Junghanns KT, Kneusel RE, Baumert A, Maier W, Groger D, Matern U (1995). "Molecular cloning and heterologous expression of acridone synthase from elicited Ruta graveolens L. cell suspension cultures." Plant Mol Biol 27(4);681-92. PMID: 7727746
Schroder99a: Schroder J (1999). "The chalcone/stilbene synthase-type family of condensing enzymes." In: Comprehensive natural products chemistry Vol. 1: Sankawa, U. (editor), Polyketides and other secondary metabolites including fatty acids and their derivatives. Amsterdam, New York: Elsevier, 773-82323.
Rubio06: Rubio S, Larson TR, Gonzalez-Guzman M, Alejandro S, Graham IA, Serrano R, Rodriguez PL (2006). "An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment." Plant Physiol 140(3);830-43. PMID: 16415216
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