Polyketide intermediates, biosynthesis

A characteristic of legumes is the biosynthesis of 6 -deoxychalcones (chalcones lacking a hydroxyl at the C-6 position), which are the substrates for the production of 5-deoxyflavo-noids. The formation of 6 -deoxychalcones requires the activity of polyketide reductase (PKR) (also known as chalcone reductase or chalcone ketide reductase) in conjunction with CHS. It is thought that CoA-linked polyketide intermediates diffuse in and out of the CHS active site, and while unbound are reduced to alcohols by PKR. The resultant hydroxyl groups are then removed from the PKR products in the final cyclization and aromatization steps catalyzed by CHS. 

Our initial studies focussed on the ACP component, as we believed that it could have a central role in stabilisation and cyclisation of the inherently unstable polyketide intermediates necessarily involved in chain assembly. The proposed pathway shown in Scheme 3 differs from the generally accepted pathway in a number of important aspects. First, we believe that the assembly pathway is a processive pathway, as in Type I polyketide biosynthesis, and that the necessary modifications occur during the... 

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... 

Stable isotope labelling is also proving to have an important role in studies on the enzymology of polyketide biosynthesis. The acyl carrier protein (AGP) components of polyketide synthases (PKSs) are believed to play a central role in the control of the assembly and stabilisation of polyketide intermediates, especially of the highly oxygenated intermediates necessarily involved in biosynthesis of... 

In animals, the breakdown of lipids involves conversion of propionyl-CoA to succinyl-CoA. Methylmalonyl-CoA is a metabolic intermediate in this process. In vivo, it is necessary to convert the 2-(S)-form of methylmalonyl-CoA to the 2-(R)-form, for reaction with methylmalonyl-CoA mutase. This reaction is catalyzed by methylmalonyl-CoA epimerase (MMCE) [4, 66-68]. Methylmalonate is also employed in polyketide antibiotic biosynthesis, in the form of methylmalonate units, although less is known about the stereochemical requirements of these processes [69, 70]. 

The 2-naphthonate moiety is most likely of polyketide origin, but the exact nature of the nascent linear polyketide intermediate and its subsequent folding pattern to afford the 2-naphthonate backbone cannot be predicted in the absence of isotope labeling experiments. Since the biosynthesis of 1-naphthoic acid moiety in 301 is catalyzed by the iterative type I PKSs, and a close examination of the oifs within the 302 cluster identified, in addition to kedE that encodes the enediyne PKS, kedU38 that resides in the middle of the 15-gene type I PKS locus and encodes a type I PKS with a similar domain organization as in 301, it was proposed that KedL138 catalyzes the formation of a nascent intermediate, which is further modified by the other activities within the type I PKS locus to yield 3,6,8-trihydroxy-2-naphthoic acid as a key intermediate. 

This and other information show that nine Cg units from malonyl-coenzyme A and one C3 unit from propionyl-coenzyme A condense to form the linear polyketide intermediate shown below. These units are joined by acylation reactions that are the biosynthetic equivalent of the malonic ester synthesis we studied in Section 18.7. These reactions are also similar to the acylation steps we saw in fatty acid biosynthesis (Special Topic E in WileyPLUS). Once formed, the linear polyketide cyclizes by enzymatic reactions akin to intramolecular aldol additions and dehydrations (Section 19.6). These steps form the tetracyclic core of akiavinone. Phenolic hydroxyl groups in akiavinone arise by enolization of ketone carbonyl groups present after the aldol condensation steps. Several other transformations ultimately lead to daunomycin ... 

The mechanisms by which aromatic PKSs control structmal diversity are distinct from those for noniterative type I PKSs for reduced polyketide biosynthesis. Aromatic PKSs catalyze the biosynthesis of various polycyclic, mostly aromatic, polyketides (Fig. 2), involving a linear poly- -ketone intermediate such as 5 or 6, and using malonyl coenzyme A (CoA) as an extender unit exclusively (pathway B in Fig. 1). The principal challenges faced by aromatic PKSs are to choose a starter unit, to determine the munber of extensions, to control the folding of the linear poly-j0-ketone intermediates, and to carry out regiospecific reduction, aromatization, and cyclizations of the correctly folded polyketide intermediates into polycyclic metabolites. 

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]. 

Figure 4.9 The biosynthesis of 4-methyl-3-heptanone can be supposed to proceed through either of two routes. The imaginary polyketide intermediates are shown, although the former route is more likely because it allows decarboxylation of a p-keto-acid at the final stage The final structure shown in this figure is (S)-(+)-4-methyl-3-heptanone, an ant trail pheromone...

The acetate labeling results clearly demonstrated a polyketide origin for the naphthoate fragment. This resulted in the hypothesis that the first enzyme-free intermediate in azinomycin biosynthesis would be naphthoate 102, with condensation to fonn a polyketone chain, reduction, cyclization, and dehydration/aromati-... 

---