Condensation reaction, polyketide biosynthesis

Nature gives us some illustrative examples of iterative methodologies in its biochemical mechanisms. The fatty acid-polyketide biosynthesis is one of them. The assembly of acyl units by sequential Claisen-type condensations to form a polyketide or fatty acid takes place at a multi-enzyme complex, at which the initial molecule is lengthened by one C2-unit per pass of a reaction cycle (Fig. 2). 

C domains can display functions that deviate from typical amide bond formation. Several C domains are postulated to act as ester synthases, catalyzing ester formation instead of amide formation. NRPS modules containing C domains that display this activity are present in the biosynthetic pathways for the kutznerides, cryptophycins, " cereulide, valinomycin, hectochlorin, and beauvericin. Each of these C domains likely utilizes a PCP-bound a-hydroxyl acceptor in the condensation reaction. Another NRPS C domain that catalyzes ester bond formation is involved in the biosynthesis of the polyketide-derived mycotoxins known as the fiimonisins. Du and coworkers have shown that a recombinant PCP-C didomain of an NRPS involved in the biosynthetic pathway of the fnmonisins can catalyze ester bond formation between hydroxyfumonisins and the A-acetylcysteamine thioester of tricarballylic acid, even though PCP-bound tricarballylic acid is not... 

Schroder, J., Plant polyketide synthases a chalcone synthase-type enzyme which performs a condensation reaction with methylmalonyl-CoA in the biosynthesis of C-methylated chalcones. 

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... 

Figure 1. General condensation reaction in polyketide biosynthesis. The starter units are attached to thiol groups of the ketosynthase (KS), and extender units to thiol groups of either acyl carrier protein or acetyl coenzyme A (X).

Fig. 5.3 Key reactions in polyketide biosynthesis. (A) The decarb-oxylative condensation reaction that builds the polyketide chain. (B) Modifications that can occur between condensation steps.

Using 4-courmaroyl-CoA (in most species) and three molecules of malonyl-CoA, chalcone synthase (CHS) carries out a series of sequential decarboxylation and condensation reactions, to produce a polyketide intermediate that then undergoes cyclization and aromatization reactions that form the A-ring and the resultant chalcone structure. The chalcone formed from 4-courmaroyl-CoA is naringenin chalcone. In a few species, caffeoyl-CoA and feruloyl-CoA may also be used as substrates for chalcone formation. Malonyl-CoA is formed from acetyl-CoA by acetyl-CoA carboxylase (ACC). Acetyl-CoA may be produced in mitochondria, plastids, peroxisomes, and the cytosol by a variety of routes. It is the cytosolic acetyl-CoA that is used for flavonoid biosynthesis, and it is produced by the multiple subunit enzyme ATP-citrate lyase that converts citrate, ATP, and CoA to acetyl-CoA, oxaloacetate, ADP, and inorganic phosphate [15]. 

In examining the overall conversion of pyruvate into a fatty acid (Schemes 1.1 and 1.2) it is interesting to note the exploitation of particular chemical properties of sulphur (i), as an easily reduced disulphide (1.6) (ii), as an easily oxidized dithiol and (iii) in reactive thioesters which aid the Claisen-type condensation reactions. Also of crucial importance for the condensation is the use of a malonic acid derivative (1.14) as a source of a stable anion. (Further discussion of fatty acid biosynthesis in relation to polyketide formation is taken up in Chapter 3.)... 

Figure 3. Relationship between polyketide and fatty acid biosynthesis. The simplest ( minimaV) PKSs possess ketosynthase activity and produce linear polyketide products. In contrast, FASs also catalyze successive ketoreduction-dehydration-enoyl reduction reactions following each condensation. Diverse PKSs may perform none, part, or all of this reductive sequence. KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyl reductase.

In nature, the intramolecular condensation of a 1,3-dicarbonyl moiety with a keto group in polyketides is an important step in the biosynthesis of aromatic compounds. Biomimetic transformations of this type have been intensively investigated by Harris. (For a discussion, see Chapter 1.5, this volume.) In the following, the synthesis of some natural products and biologically active compounds using the Knoevenagel reaction will be described. 

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