O O

malonyl CoA

1, reduce

2, dehydrate

S-ACP malonyl CoA O

"S-ACP P-ketooctanoyl ACP

hydrolyse H

Q > O decarboxylase O"

2-heptanone

Figure 4.6 A scheme to show the formation of 2-heptanone through a hypothetical polyketide, and the probable scheme by which it is actually biosynthesized formation are probably as indicated in Figure 4.6, although it must be emphasized that this has not yet been proven. Condensation of an acetyl group attached to acyl carrier protein (ACP) with a malonyl group gives first acetoacetyl ACP which is reduced to p-hydroxybutyryl ACP, which is dehydrated and the double bond reduced, in the same kind of step as already met for fatty acid synthesis (Figures 3.3 and 3.5). The resulting butyryl ACP is extended again to hexanoyl ACP, and again to p-keto-octanoyl ACP Decarboxylation of the free p-keto acid gives 2-heptanone (Figure 4.6).

Note that, particularly in micro-organisms and insects, methyl-malonate (equivalent to a propionic acid unit) can replace malonate (resulting in an acetate unit) in the condensation reaction to give a polyketide. Mycolipenic acid contains three propionate units and nine acetate units, erythromycin is made from seven propionates (Figure 4.7).

mycolipenic acid erythromycin

Figure 4.7 Acetogenin structures containing propionate units. Mycolipenic acid is produced in the lipids of Mycobacterium tuberculosis and erythromycin is an antibiotic produced by Streptomyces erythreus. The individual propionyl groups of erythromycin have been indicated by heavy black lines o erythromycin

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