Aromatic polyketide synthase

MORITA, H., TAKAHASHI, Y., NOGUCHI, H., ABE, I., Enzymatic formation of unnatural aromatic polyketides by chalcone synthase, Biochem. Biophys. Res. Comm., 2000,279,190-195. 

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, Takahashi Y, Noguchi H (2002) Enzymatic formation of an unnatural C6-C5 aromatic polyketide by plant type III polyketide synthases. Org Lett 4 3623-3626... 

Aromatic polyketides are structurally diverse, often polycyclic molecules that are derived from unreduced polyketone chains. This group of compounds is produced with the help of type II polyketide synthase (PKS), a complex of enzymes that catalyzes the iterative decarboxylative condensation of malonyl-CoA extender units with an acyl starter unit [70], The carbon framework of aromatic polyketides is further decorated with different functionalities, and carbohydrates are often one of them. Their presence has profound effects on physico-chemical and biological properties of aromatic polyketides. For example, anthracycline aglycones are stable and unpolar, while polyglycosylated anthracyclines are quite polar and often... 

A. Actinorhodin, Tetracenomycin, Doxorubicin, and Other Bacterial Aromatic Polyketide Synthases... 

CW Carreras, C Khosla. Purification and in vitro reconstitution of the essential protein components of an aromatic polyketide synthase. Biochemistry 37 2084-2088, 1998. 

J Dreier, AN Shah, C Khosla. Kinetic analysis of the actinorhodin aromatic polyketide synthase. J Biol Chem 274 25108-25112, 1999. 

Y Shen, P Yoon, TW Yu, HG Floss, D Hopwood, BS Moore. Ectopic expression of the minimal whiE polyketide synthase generates a library of aromatic polyketides of diverse sizes and shapes. Proc Natl Acad Sci (USA) 96 3622-3627, 1999. 

Like the related fatty acid synthases (FASs), polyketide synthases (PKSs) are multifunctional enzymes that catalyze the decarboxylative (Claisen) condensation of simple carboxylic acids, activated as their coenzyme A (CoA) thioesters. While FASs typically use acetyl-CoA as the starter unit and malonyl-CoA as the extender unit, PKSs often employ acetyl- or propionyl-CoA to initiate biosynthesis, and malonyl-, methylmalonyl-, and occasionally ethylmalonyl-CoA or pro-pylmalonyl-CoA as a source of chain-extension units. After each condensation, FASs catalyze the full reduction of the P-ketothioester to a methylene by way of ketoreduction, dehydration, and enoyl reduction (Fig. 3). In contrast, PKSs shortcut the FAS pathway in one of two ways (Fig. 4). The aromatic PKSs (Fig. 4a) leave the P-keto groups substantially intact to produce aromatic products, while the modular PKSs (Fig. 4b) catalyze a variable extent of reduction to yield the so-called complex polyketides. In the latter case, reduction may not occur, or there may be formation of a P-hydroxy, double-bond, or fully saturated methylene additionally, the outcome may vary between different cycles of chain extension (Fig. 4b). This inherent variability in keto reduction, the greater variety of... 

Figure 4 (a) The aromatic polyketide biosynthetic cycle (ACP, acyl carrier protein KS, P-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydratase ER, enoyl reductase), (b) The complex polyketide biosynthetic cycle. 

Experiments in vitro with modular PKSs are considerably more straightforward than those with aromatic synthases. Whereas synthesis of aromatic polyketides requires the expression and purification of at minimum three activities, and as many as seven, a single multienzyme from a modular PKS can be competent for synthesis. Additionally, the design rules for the modular PKSs are considerably simpler, as there is a direct correlation between the number and nature of activities present and the resulting structure of the polyketide product. 

CW Carreras, R Pieper, C Khosla. Efficient synthesis of aromatic polyketides in vitro by the actinorhodin polyketide synthase. J Am Chem Soc 118 5158-5159, 1996. 

B Shen, RG Summers, E Wendt-Pienkowski, CR Hutchinson. The Streptomyces glaucescens TcmKL polyketide synthase and TcmN polyketide cyclase genes govern the size and shape of aromatic polyketides. J Am Chem Soc 117 6811-6821, 1995. 

B Shen, CR Hutchinson. Deciphering the mechanism for the assembly of aromatic polyketides by a bacterial polyketide synthase. Proc Natl Acad Sci (USA) 93 6600-6604, 1996. 

C Bisang, PF Long, J Cortes, J Westcott, J Crosby, A-L Matharu, RJ Cox, TJ Simpson, J Staunton, PF Leadlay. A chain initiation factor common to both modular and aromatic polyketide synthases. Nature 401 502-505, 1999. 

Fig. 2a-d. The major architectural paridigms of fatty acid and polyketide synthases. Relationships between genes encoding a bacterial aromatic polyketide synthases, b eukaryotic fatty acid synthases and fungal polyketide synthases, c modular polyketide synthases, and d plant polyketide synthases... 

Chalcone synthase (CHS), the first plant natural product polyketide synthase (PKS) to be characterized at the molecular level (39), catalyzes the condensation of 4-coumaroyl-CoA with three molecules of malonyl-CoA to afford naringenin chalcone, a precursor of the major classes of plant flavonoids. The cloning of a novel type III pentaketide chromone synthase (PCS) from aloe (Aloe arborescens, Liliaceae) rich in aromatic polyketides, especially quinones such as aloe-emodin and emodin, resulted in... 

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. 

Despite their enormous structural diversity, polyketide metabolites are related by their common derivation from highly functionalised carbon chains whose assemblies are controlled by multifunctional enzyme complexes, the polyketide synthases (PKSs) which, like the closely related fatty acid synthases, catalyse repetitious sequences of decarboxylative condensation reactions between simple acyl thioesters and malonate, as shown in Fig. 3 [7]. Each condensation is followed by a cycle of modifying reactions ketoreduction, dehydration and enoyl reduction. In contrast to fatty acid biosynthesis where the full cycle of essentially reductive modifications normally follow each condensation reduction, the PKSs can use this sequence in a highly selective and controlled manner to assemble polyketide intermediates with an enormous number of permutations of functionality along the chain. As shown in Fig. 3, the reduction sequence can be largely or entirely omitted to produce the classical polyketide intermediate which bears a carbonyl on every alternate carbon and which normally cyclises to aromatic polyketide metabolites. On the other hand, the reductive sequence can be used fully or partially after each condensation to produce highly functionalised intermediates such as the Reduced polyketide in Fig. 3. Basic questions to be answered are (i) what is the actual polyketide intermediate... 

Type II polyketide synthases, also referred to as bacterial aromatic polyketide synthases (25) are involved in the biosynthesis of a number of clinically important bacterial aromatic polyketides products exhibiting antitumor or antibiotic activity, such as doxorubicin and oxytetracycline. As mentioned, type II synthases are evolutionarily and structurally related to type II FASs, which occurr as heteromultimeric complexes. In contrast to type I synthases however, where multiple catalytic sites occur within a given subunit, the polypeptides associated with type II synthase complexes are typically monofimctional and dissociable 20,30). 

Expression and Function of Aromatic Polyketide Synthase Genes in Raspberries Rubus idaeus sp.)... 

The starter unit represents an attractive site in the aromatic polyketide scaffold for introducing alternative chemical functionalities. Most aromatic polyketide synthases (PKSs) are primed by acetate through the decarboxylative condensation of malonyl-ACP. Nonacetate-primed PKSs have been sequenced and characterized. Precursor-directed biosynthesis and heterologous recombination of initiation and elongation modules have led to the engineered biosynthesis of completely new aromatic polyketide scaffolds containing novel starter units, as well as the regioselective modification of known aromatic polyketides. 

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