Chapter 7
Fungal metabolism and
fungal products
This chapter is divided into the following major
sections:- how fungi obtain energy in different conditions
- coordination of metabolism: how the pathways are
balanced - mobilizable and storage compounds of fungi
- synthesis of chitin and lysine
- the pathways and products of secondary metabolism
In this chapter we discuss the basic metabolic path-
ways of fungi, as a basis for understanding how fungi
grow on different types of substrate and in different
environmental conditions. We also cover some of the
distinctive and unusual aspects of fungal metabolism,
including the production of a wide range of second-
ary metabolites of commercial and environmental
significance, such as the penicillinantibiotics, and
important mycotoxins like the highly carcinogenic
aflatoxinsand the toxic ergot alkaloids. Some of
the material in this chapter will be familiar, basic bio-
chemistry, but it is presented in the specific context
of fungal biology.How do fungi obtain energy in different
conditions?
Figure 7.1 shows an overview of central metabolism
and how this provides the precursors for synthesis
of many other compounds. The spine of the diagram
represents the central energy-yielding pathway, in
which sugars are broken down via the Embden–
Meyerhof (EM) pathway(glycolysis) and the tricar-
boxylic acid (TCA) cycle. Many of the intermediates
can be drawn off to produce other essential meta-
bolites or for synthesis of a wide range of specialized
secondary metabolites.
Although fungi can obtain energy by oxidizing a
wide range of compounds, it is convenient to begin
by considering a fungus growing on a simple sugar
such as glucose. As shown in Fig. 7.2, in the series of
enzymic steps of the Embden–Meyerhof pathway,
glucose is converted to glucose-6-phosphate, then
to fructose-6-phosphate, and finally to fructose-1,6-
biphosphate. These phosphorylation reactions occur
at the expense of energy provided by two molecules
of the energy-rich compound, adenosine triphosphate
(ATP). Then fructose-1,6-biphosphate is split into two
3-carbon compounds, glyceraldehyde-3-phosphate and
dihydroxyacetone phosphate. These two compounds are
interconvertible, and in a further series of enzymic steps
they are converted to pyruvic acid.
Pyruvic acid is one of the key intermediates of cen-
tral metabolism, because it represents a branch-point:
the reaction steps that follow will depend on whether
the fungus is growing in the presence or absence of
oxygen.
In the presence of oxygen, pyruvic acid (a 3-carbon
compound) is transported to the mitochondria,
where it is converted to the 2-carbon compound
acetyl-coenzyme A(acetyl-CoA) by associating with a
molecule of coenzyme-A and releasing a molecule of
CO 2. Then acetyl-CoA combines with oxaloacetate (a
4-carbon compound) to produce citric acid (6-carbon).
Finally, in the reactions of the TCA cycle, citric acid is
converted back to oxaloacate, with the loss of two
molecules of CO 2 (Fig. 7.2).