Biosynthesis of Polyketides

BY T. J. SIMPSON

1 Introduction

Polyketides form a large class of natural products possessing structures of great diversity related by their common formation via the acetate–polymalonate biosynthetic pathway. Acyl units other than acetate, such as propionate, benzoate, and cinnamate, can act as chain-initiating species, and propionate and butyrate as chain elongation units, so that a wide variety of fatty acids, polyacetylenes, single and multiringed phenols, macrolides, flavonoids, and other compounds can be included in this classification.

The literature appearing during 1975 and to mid-1976 is covered in this chapter. An outstanding feature of this period has been the continued growth in 13C n.m.r. methods, in particular the use of doubly labelled 13C-acetate. Analysis of the resultant 13C–13C spin-spin couplings provides a powerful method for determining the manner in which polyketide molecules are assembled on the enzyme surface before the first stable compounds are released, and for probing subsequent molecular rearrangements and cleavage pathways. The scope and methodology of the technique have been discussed in recent reviews.

2 Fatty Acids, Polyacetylenes, and Prostaglandins

Lynen has studied the condensation reaction in fatty acid biosynthesis using dideuteriomalonyl-CoA. No primary iotope effect was observed on the reaction velocity of the yeast-enzyme-catalysed fatty acid synthesis, in which the rate-limiting step is the condensation, or on the condensation itself, studied separately using the β-ketoacyl-(acyl-carrier-protein) synthetase of Escherichia coli. When the condensation was carried out in the presence of tritiated water, no tritium was incorporated into the product. These results exclude condensation mechanisms involving acylation of a malonyl carbanion and indicate a concerted mechanism, (Scheme 1).

2,4,6,8-Tetramethyldecanoic acid (1) is the major fatty acid in the uropygial gland of the goose. Crude cell-free extracts from the gland catalyse the carboxylation of propionyl-CoA but not of acetyl-CoA, whereas more highly purified extracts catalyse both carboxylations. This behaviour was explained by the isolation of a highly specific malonyl-CoA decarboxylase from the crude extracts. Thus acetyl-CoA and methyl-malonyl-CoA are respectively the major chain-primer and elongation agents present in the gland, resulting in the production of the multi-branched fatty acid. Propionate incorporated during chain elongation has been shown to be the branching methyl group donor in biosynthesis of 3- and 13-methylpentacosane (2) and (3), the major cuticular hydrocarbons in the cockroaches Periplaneta americana and P. fulginosa, respectively. In plants, n-alkanes are formed by an elongation of fatty acids followed by decarboxylation, and the 2- and 3-methylalkanes originate from the appropriately branched starter acyl-CoA derived from valine and leucine, whereas in algae the active methyl group from methionine serves as the branching methyl group donor. Cell-free preparations from pea leaves, Pisum sativum, catalysed the decarboxylation of n-dotriacontanoic acid (4), requiring the presence of both ascorbic acid and oxygen, and giving both n-C31 and n-C30 alkanes. Thus decarboxylation and α-oxidation appear to be connected processes; in confirmation, 2-hydroxydotria-contanoic acid, the intermediate in α-oxidation of (4), was converted into the sam two alkanes.

When [1-14C]aleprolic acid (5) was supplied to leaves and seeds of plants belonging to the Flacourtaceae, and also to whole cells of Chlorella vulgaris, cyclopentenyl fatty acids (occurring naturally in seeds and leaves of Flacourtaceae) were synthesized, suggesting that these fatty acids are formed by elongation of aleprolic acid rather than by cyclization of an acyclic fatty acid precursor. Both the availability of aleprolic acid and the ability to use it as a primer for fatty acid synthesis appear as specific characteristics of the Flacourtaceae and thus jointly determine the fatty acid pattern. The main fatty acids in ten strains of acidophilic, thermophilic bacteria isolated from Japanese hot springs are ω-cyclohexyl fatty acids, e.g. (6). Increasing the concentration of glucose in the culture medium increased the production of the cyclohexyl but not the acyclic acids, and from incorporation studies with 14C- and 2H-labelled glucose it was confirmed that the acids are produced by elongation of cyclohexylcarboxylate derived from shikimate, rather than by elongation of cyclohexylpropionate derived from decarboxylation of prephenate. The results are in full agreement with previous studies on cyclohexyl fatty acids from Bacillus acidocaldarius, in which cyclohexylcarboxylate competes with straight- and branched-chain precursors of similar molecular length to determine the fatty acid spectrum. These acids may be the pre- cursors of the n-alkylcyclohexanes present in a number of sediments and crude oils, previously postulated as arising by intramolecular cyclization of unsaturated fatty acids.

Several papers have appeared on the routes to unsaturated fatty acids. Stumpf and co-workers have shown that preparations from saffiower seeds and avocado mesocarp rapidly desaturate stearyl-ACP to oleic acid in the presence of oxygen. Mazliak et al. on the other hand have shown that fractions from a cauliflower homogenate synthesize radioactive oleic acid by an aerobic process from [14C]decanoate, in the presence of ATP, NADPH, Coenzyme A, and oxygen. They proposed a scheme analogous to oleic acid synthesis in anaerobic bacteria, which hardly accounts for the oxygen requirement (Scheme 2): 3-hydroxylauric acid (7) formed from decanoate undergoes β,γ-dehydration to 3-dodecenoic acid which is then elongated to oleic acid (8).

The phleic acids (9) are polyunsaturated acids produced by Mycobacterium phlei, with an unusual distribution of double bonds. When [14C]acetate is incubated with M. phlei, the saturated and unsaturated sections are unequally labelled. [14C]Myristic and [14C]palmitic acids serve as precursors for the phleic acids with m = 12 and 14, respectively. A chain-elongation process involving two acetate units at a time, possibly via crotonate, is postulated; β-hydroxybutyric acid, however, is not incorporated without prior degradation. The biosynthesis of cerulenin (10), an important inhibitor of fatty acid synthetase, has been studied in cultures of Cephalosporium caerulens. The alternate labelling obtained with [l-13C]acetate rules out the possible intermediacy of...