Polyketides from Fungi 2 Polyketide Biosynthesis

The second largest class of compounds reported from macroalgae is the polyketides, which comprise approximately a quarter of known algal compounds (Blunt et al. 2007). Polyketides are polymers of acetate (C2) and occasionally propionate (C3) and are very similar to fatty acids in their biosynthetic origin. Polyketides can be found in plants, animals, bacteria, and fungi. With a range of activities as broad as their structures, the polyketides are a diverse family of natural products classified based upon the polyketide synthases (PKSs) responsible for their biosynthesis, primarily type I and type II. 

Fungi like Pyricularia contain a melanin built from 1,8-dihydroxynaphthalene (DHN), called DHN melanin [Butler Day, 1998]. The biosynthetic pathway is short and simple (Fig. 20.1). The origin of the building blocks is not amino acids, but a polyketide originating from acetyl coenzyme A and malonyl coenzyme A. Besides DHN this fungal melanin may contain other poorly characterized constituents. All the enzymes of the DHN melanin biosynthesis pathway are known, have been cloned, except the last one, and are more or less well described [Butler Day, 1998). 

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