Octaketide

Aloe (Aloe arborescens) is a medicinal plant rich in aromatic polyketides such as pharmaceutically important aloenin (a hexaketide pyrone), aloesin (a heptaketide chromone), and barbaloin (an octaketide anthrone) (Fig. 4a). Pentaketide chromone synthase (PCS) and octaketide synthase (OKS) are novel plant-specific type III PKSs, which were obtained from the aloe plant by RT-PCR cloning using degenerate oligonucleotide primers based on the conserved sequences of known CHS enzymes [30-33]. The deduced amino acid sequences of PCS and OKS are 91% identical (368/403), and show 50-60% identity to those of other CHS superfamily type III PKSs of plant origin OKS shares 60% identity (240/403) with CHS from... 

Abe I, Oguro S, Utsumi Y, Sano Y, Noguchi H (2005) Engineered biosynthesis of plant polyketides chain length control in an octaketide-producing plant Type III polyketide synthase. J Am Chem Soc 127 12709-12716... 

Karppinen K, Hokkanen J, Manila S, Neubauer P, Hohtola A (2008) Octaketide-producing type III polyketide synthase from Hypericum perforatum is expressed in dark glands accumulating hypericins. FEBS J 275 4329-4342... 

Morita H, Kondo S, Kato R, Wanibuchi K, Noguchi H, Sugio S, Abe I, Kohno T (2007) Crystallization and preliminary crystallographic analysis of an octaketide-producing plant type III polyketide synthase. Acta Crystallograph Sect F Struct Biol Cryst Commun 63 947-949... 

MAT has stringent specificity toward malonyl-CoA, which is reflected in the exclusive utilization of malonyl extender units by Type n PKS 7 Together, these four enzymes consist of the smallest set of enzymes required for the synthesis of a complete polyketide chain. For example, the minimal PKS from the actinorhodin (act) biosynthetic pathway synthesizes an octaketide (Cjg) backbone from eight malonyl-CoA equivalents, the tetracenomycin (tan) minimal PKS synthesizes a decaketide (C20) backbone from ten equivalents of malonyl-CoA (Figure 8A), and the pradimycin (pms) minimal PKS synthesizes a dodecaketide (C24) backbone from twelve equivalents of malonyl-CoA. ... 

The Cj symmetrical macrodiolide elaiophylin (41) consists of two identical polyketide chains which are linked to a deoxyfucose at C-13 [83]. Here, it was assumed that macrodilactone formation is the last biosynthetic step, and only one glycosyl transfer step is necessary prior to lactonization. However, incorporation experiments with the complete (glycosylated) octaketide half 42 failed. Later, after changing the fermentation parameters, the asymmetrical mono-glycosyl derivative 43 was isolated, indicating that the two glycosyl transfer steps take place after generation of the macrodiolide 44 [83-85] (Scheme 16). 

Fig. 4. Polyketide biosynthesis by gene products of the act PKS cluster. Presence of the KS/AT, CLF, and ACP is sufficient for the production of two 16-carbon polyketides, SEK4 and SEK4b both in vivo [ 103] and in vitro [107]. In the presence of the act ketoreductase (KR), aromatase (ARO) and cyclase (CYC), the octaketide intermediate is converted into DMAC. DMAC can be converted into 8-methoxy DMAC both in vivo and in vitro through the S-adenosylmethionine (Adomet)-dependent action of the tcmO methyltransferase [207]...

PCS-catalyzed condensation of five molecules of malonyl-CoA to produce 5,7-dihydroxy-2-methyl chromone new to this plant (40). Another novel Aloe arborescens type III PKS that produces two hitherto unknown aromatic octaketides, SEK4 and SEK4b, has recently been reported (41). The application of plant cell cultures for the production of the polyketide hypericin from St. John s wort (Hypericum performatum, Hypericaceae) has been investigated (42). 

Figure 10 Conversion of malonyl-ACP to acetyl-ACP and polyketide production by KS/CLF mutants, (a) ESMS analysis of initial sample of malonyl-ACP before decarboxylation and after incubation with either (middle) KS(ala)/CLF(gln) (60 min), or (right) KS(gln)/CLF(ala) (1 min), (b) Time course of decarboxylation of malonyl CoA by KS(ala)/CLF(gln), and production of octaketides (SEK-4 + SEK-4b) from malonyl ACP by KS(cys)/CLF(ala), both in the presence (triangles) and absence (squares) of acetyl-ACP.

Another example of the possible simultaneous operation of both F and S cyclisation modes in the same plant species, is the formation of the naphthalene octaketides eleutherin (65) and eleutherinol (66) in Eleutherine bulbosa The structure of eleutherin corresponds to that of a mode F folded polyketide, whereas the cyclisation pathway originally proposed by Birch and Donovan for the biosynthesis of eleutherinol would require mode S folding, although its formation by rearrangement of an advanced eleutherin precursor is an alternative possibility. 

Figure 7 Apparent mode F and mode Sfoldedfused ring octaketides of higher plants...

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