Polyketides starter unit

The development of combinatorial biosynthetic strategies, in which components of different polyketide pathways are combined to generate new polyketides, has attracted interest as a new drug discovery tool. Extension of previous approaches focused on combinations of different polyketide synthase components to inclusion of pathways that generate different polyketide starter units provides an additional element of structural diversity. 

By feeding [7- C]3-amino -hydroxybenzoic acid (3,4-AHBA) 242 to liquid cultures of S. nodosus ssp. asukaensis ATCC 29757 and also to S. parvulus Tii 64, intact incorporation of 242 into the manumycins was observed, which showed that 3,4-AHBA indeed serves as the polyketide starter unit for the formation of the lowei chain. 

Long, P.F., Wilkinson, C.J., Bisang, C.P. et al. (2002) Engineering specificity of starter unit selection by the erythromycin-producing polyketide synthase. Molecular Microbiology, 43, 1215. 

The biosynthesis of polyketides (including chain initiation, elongation, and termination processes) is catalyzed by large multi-enzyme complexes called polyketide synthases (PKSs). The polyketides are synthesized from starter units such as acetyl-CoA, propionyl-CoA, and other acyl-CoA units. Extender units such as malonyl-CoA and methylmalonyl-CoA are repetitively added via a decarboxylative process to a growing carbon chain. Ultimately, the polyketide chain is released from the PKS by cleavage of the thioester, usually accompanied by chain cyclization [49]. 

Moore, B.S. and Hertweck, C. (2002) Biosynthesis and attachment of novel bacterial polyketide synthase starter units. Natural Product Reports, 19 (1), 70-99. 

Members of the CHS/STS family of condensing enzymes are relatively modest-sized proteins of 40-47 kDa that function as homodimers. Each enzyme typically reacts with a cinnamoyl-CoA starter unit and catalyzes three successive chain extensions with reactive acetyl groups derived from enzyme catalyzed decarboxylation of malonyl-CoA.11 Release of the resultant tetraketide together with or prior to polyketide chain cyclization and/or decarboxylation yields chalcone or resveratrol (a stilbene). Notably, CHS and STS catalyze identical reactions up to the formation of the intermediate tetraketide. Divergence occurs during the termination step of the biosynthetic cascade as each tetraketide intermediate undergoes a distinct cyclization reaction (Fig. 12.2). 

The polyketide synthases responsible for chain extension of cinnamoyl-CoA starter units leading to flavonoids and stilbenes, and of anthraniloyl-CoA leading to quinoline and acridine alkaloids (see page 377) do not fall into either of the above categories and have now been termed Type TTT PKSs. These enzymes differ from the other examples in that they are homodimeric proteins, they utilize coenzyme A esters rather than acyl carrier proteins, and they employ a single active site to perform a series of decarboxylation, condensation, cyclization, and aromatization reactions. 

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... 

As discussed earlier, the avermectin polyketide backbone is derived from seven acetate and five propionate extender units added to an a branched-chain fatty acid starter, which is either (S( I )-a-mcthylbutyric acid or isobutyric acid. The C25 position of naturally occurring avermectins has two possible substituents a. sec-butyl residue derived from the incorporation of S(+)-a-methy lbutyry 1-CoA ( a avermectins), or an isopropyl residue derived from the incorporation of isobutyiyl-CoA ( b avermectins). These a branched-chain fatty acids, which act as starter units in the biosynthesis of the polyketide ring, are derived from the a branched-chain amino acids isoleucine and valine through a branched-chain amino acid transaminase reaction followed by a branched-chain a-keto acid dehydrogenase (BCDH) reaction (Fig. 5) [23]. 

W Bao, PJ Sheldon, E Wendt-Pienkowski, CR Hutchinson. The Streptomycespeu-cetius dpsC gene determines the choice of starter unit in biosynthesis of the daunoru-bicin polyketide. Biochemistry 38 9752-9757, 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... 

The DEBS 1-TE multienzyme was purified to 90-95% homogeneity and then used in another series of experiments to establish the extent to which alternative starter units could be used by the polyketide synthase [36], Substantial amounts of lactones were obtained in the presence of acetyl-, n-butyryl-, and isobutyryl-CoA, illustrating that the loading didomain exhibits a relaxed specificity for the starter unit (Fig. 10). The utilization of acetyl-CoA and -butyryl-CoA by DEBS 1 + TE was demonstrated in a cell-free system [39], Additionally, in the absence of the reducing cofactor NADPH, cell-free DEBS 1+TE converted... 

A third approach is precursor-directed biosynthesis in which altered starter unit functionality is introduced through the incorporation of a synthetic non-natural diketide intermediate into a polyketide product in vivo. This strategy was first... 

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