Polyketides tetraketides

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). 

Notably, natural variation in the type III PKS active site cavity, like that observed in Ipomoea and Petunia, does not result in functionally impaired enzymes, but in fact, generates catalytically active enzymes that display both altered substrate and product specificities. Sequential increases in the side chain volume of position 256 in alfalfa CHS2 result in decreases in polyketide chain length and predictable shifts in the ratio of tetraketide to triketide reaction products.32 These results functionally link the volume of the elongation/cyclization lobe in type III PKS to chain length determination. 

FIGURE 16.3 Overview of the biosynthesis of (I) chalcones and (II) 6 -deoxychalcones. The sequential condensation of three molecules of malonyl-CoA (acetate pathway) and p-coumaroyl-CoA (shikimate pathway) is catalyzed by the enzyme chalcone synthase.The production of 6 -deoxychalcones is thought to involve an additional reduction step at the tri- or tetraketide level, catalyzed by polyketide reductase.The origin of the A-ring carbons derived from the acetate pathway is indicated in bold. CoA, coenzyme A. 

CM Kao, M McPherson, RN McDaniel, HFu, DE Cane, C Khosla. Gain of function mutagenesis of a modular polyketide synthase II. Engineered biosynthesis of an eight-membered ring tetraketide lactone. J Am Chem Soc 119 11339-11340,... 

Assign each of these natural products to a general class (such as amino acid metabolite, terpene, polyketide) explaining what makes you choose that class. Then assign them to a more specific part of the general class (for example, tetraketide, sesquiterpene). 

Cell-free systems capable of in vitro synthesis of 6-methylsalicylic acid (6-MS A) and a related tetraketide, orsellinic acid, were developed long before the advent of recombinant DNA technologies in the field of natural product biosynthesis [113-115] (Fig. 5). Since then, the biosynthetic mechanisms and molecular recognition features of 6-methylsalicylic acid synthase (6-MSAS) have been extensively studied. 6-MSAS initiates synthesis with an acetyl group derived from acetyl Co A, extends the polyketide chain to a tetraketide via three decar-boxylative condensations of malonyl CoA-derived extender units, and uses NADPH to specifically reduce one of resulting carbonyls to a hydroxyl group. In its natural producer, Penicillium patulum, the product, 6-MSA is subsequently glycosylated to form the antibiotic patulin [116]. 

Figure 1 Generic polyketide assembly pathway reactions catalyzed by iterative fungal polyketide synthases. The assembly sequence for the squalesatin tetraketide intermediate 37 is shown for illustration.

Squalestatin SI 29 is a potent inhibitor of mammalian squalene synthase. It is produced by Phoma species, and like lovastatin, consists of two polyketide chains a main chain hexaketide and a sidechain tetraketide. Like lovastatin, both chains are methylated, but unusually for a fungal HR polyketide, the main chain is formed from a non-acetate starter unit— benzoate is incorporated at this position. 

While some success has been reported in analogous studies with polyketide assembly intermediates in Streptomyces metabolites, e.g. erythromycin [41] and tylosin [42], similar experiments on fungal polyketides have been more limited. The di- and tetraketide intermediates (44) and (45), variously doubly labelled with and as indicated in Scheme 14, have been incorporated into de-hydro curvular in (46) by cultures of Alternaria cineriae [43]. However, in contrast to the ease of incorporation of assembly intermediates into aspyrone by A. melleus, the experiments in A. cineriae required considerable experimentation to optimise the feeding conditions and the use of the jS-oxidation inhibitors. The initial experiments [43] depended on the use of UV mutants of A. cineriae which had lost the ability to utilise fatty acids and therefore to degrade the fatty... 

Evidence for the initial steps of rifamycin B biosynthesis can be seen in the accumulation of a tetraketide chain elongation intermediate P8/1-0 58 from a mutant of A. mediterranei [92]. This evidence and the demonstration from feeding experiments that the carbons on either side of the ether link in the ansa bridge derive from the same propionate unit [93] indicate that this ether moiety is formed after the initial biosynthesis of a fully extended polyketide chain. 

Evidence for such a modular pathway has been provided from studies into the biosynthesis of the polyketide backbone 77 of (41 )-4-[(E)-2-butenyl]-4-methyl-L-threonine 78 which is incorporated into cyclosporin A in Tolypo-cladium niveum [117]. The proposed biosynthesis of 77 is presented in Scheme 29. In vitro studies using a cell extract have verified unambiguously that the biosynthetic mechanism is processive, that the first PKS free intermediate is the tetraketide 79, and that methylation unequivocally occurs at the stage of the enzyme bound 3-oxo-4-hexenoic acid thioester 80 which is the triketide product from the second elongation cycle. These and other results indicate that the methyl transferase activity is inherent in the second module of the putative PKS. 

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