have no other role except in secondary metabolism.
It is shown in outline in Fig. 7.13. In this case the
precursor is acetyl-CoA, which is carboxylated to
form malonyl-CoA (a normal event in the synthesis of
fatty acids), then three or more molecules of malonyl-
CoA condense with acetyl-CoA to form a chain. This
chain undergoes cyclization, then the ring systems are
modified to give a wide range of products. These
include the antibiotic griseofulvin(from Penicillium
griseofulvum) used to treat dermatophyte infections
of humans (Chapter 17), the potent aflatoxins(from
Aspergillus flavusand A. parasiticus) discussed later, theochratoxinsfrom various Penicilliumand Aspergillus
spp., and the antibiotic or mycotoxin patulinfrom
Penicillium patulum.
Another important secondary metabolic pathway of
fungi is the isoprenoid pathway for the synthesis of
sterols. Again, acetyl-CoAis the precursor, but three
molecules of this condense to form mevalonic acid (a
6-carbon compound) which is converted to a 5-carbon
isopreneunit (Fig. 7.14). Then the isoprene units
condense head-to-tail to form chains of various
lengths, and the chains undergo cyclization and
further modifications (Fig. 7.15). The products of134 CHAPTER 7
Fig. 7.13Outline of the polyketide pathway. Various intermediates (shown in brackets) can be formed during cycliza-
tion, leading to different end-products. Patulin is derived from 6-methylsalicylic acid. Longer ketide chains than the one
shown here can produce multiple ring systems, leading to products such as the aflatoxins (see Fig. 7.19).