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FUNGAL METABOLISM AND FUNGAL PRODUCTS 131

but was then found in the form of mannitol and
trehalose in the fungal sheaths. Similar findings have
been made for plants infected by rust or powdery
mildew fungi (the biotrophic plant pathogens dis-
cussed in Chapter 14). When leaves are exposed to

(^14) CO
2 , the label is found initially in the typical plant
sugars (sucrose, glucose) but spores produced by the
biotrophic fungi are found to contain the label in the
form of fungal carbohydrates. Most plants do not
metabolize these compounds (except in seeds and
fruits), so the conversion of plant sugars to the typical
fungal carbohydrates represents a “metabolic valve”

• a one-way flow of nutrients from the host to the
  fungus. This may help to contribute to the success of
  biotrophic plant pathogens.
  The actual pathways of nutrient translocation in
  fungi are unclear, but nutrients can move both forwards
  and backwards in hyphae, from regions of relative
  abundance to relative shortage. The tubular vacuolar
  system of fungi, described in Chapter 3, may be
  significant in this respect because it can transport
  fluorescent dyes against the general flow of cytoplasm.
  The typical fungal carbohydrates may have several
  other important roles in fungal physiology. For ex-
  ample, mannitol is a common constituent of fungal
  vacuoles, where it has a major role in regulating cellular
  pH (Chapter 8).

Chitin synthesis

Chitin is a characteristic component of fungal walls.
The synthesis of this polysaccharide follows the pat-
tern of synthesis of many polysaccharides and can be
represented by the following general equation:

[Donor-sugar unit] +Acceptor →

Donor + [Acceptor-sugar unit]

For example, in the synthesis of chitin (Fig. 7.11),

fructose-6-phosphate (from the Embden–Myerhof path-

way) is initially converted to N-acetylglucosamine

(GlcNAc) by successive additions of an amino group

(from the amino acid glutamine) and an acetyl group

(from acetyl-CoA). Then GlcNAc reacts with uridine

triphosphate (UTP) to form UDP-GlcNAc plus inorganic

phosphate. The high energy (activated) sugar unit is

then added to the elongating chitin chain, by the

enzyme chitin synthase, discussed in Chapter 4.

UTP +GlcNAc →UDP-GlcNAc +Pi

then:

UDP-GlcNAc +n[GlcNAc] →UDP +n+1[GlcNAc]

A poorly or nonacetylated form of chitin, termed

chitosan, is found in Zygomycota such as Mucorspp.

It is synthesized in the same way as chitin but is then

deacetylated by the enzyme chitin deacetylase.

Lysine biosynthesis

Lysine is an essential amino acid that must be supplied

as a dietary supplement for humans and many farm

animals, because they are unable to synthesize it.

Lysine is produced commercially by large-scale fer-

mentation using the bacterium Brevibacterium flavum.

The interesting feature of this amino acid is that it is

synthesized by two specific pathways that are completely

different from one another. These pathways are

termed DAPand AAA, after their characteristic inter-

mediates, α-diaminopimelic acid and α-aminoadipic

acid (Fig. 7.12). The AAA pathway is found only in the

chitin-containing fungi and some euglenids. All other

organisms that synthesize lysine – the plants, bacteria

and Oomycota – use the DAP pathway. Such a major

Fig. 7.11Structure of chitin and its deacetylated derivative, chitosan.