untitled

Fig. 7.6The structure of mannitol and trehalose, two char-

acteristic “fungal sugars.”

128 CHAPTER 7

acetyl-CoA to produce malate and thence oxaloac-

etate. From this point, oxaloacetate is decarboxylated

to yield phospoenolpyruvate (PEP), shown below, and

sugars are formed from PEP by reversal of the rest of

the Embden–Myerhof pathway:

The important role played by the glyoxylate cycle is

evidenced by the fact that its enzyme levels increase

more than 20-fold when acetate is supplied as the sole

carbon source, and mutant strains of fungi that can-

not grow on acetate often are disrupted in one of the

steps of this pathway. The same processes as described

above could be used to generate sugars from other

substrates like fatty acids, organic acids, amino acids,

ethanol, etc.

Secretion of organic acids as commercial

products

Fungi are important commercial sources of organic

acids (Chapter 1). For example, if Aspergillus nigeris

grown at high glucose levels (15– 20%) and low pH

(about 2.0) it will convert most of the sugar to citric

acidand release this into the culture medium. Large

amounts of oxaloacetate must be generated for this,

by the carboxylation of pyruvic acid discussed earlier.

Similarly Rhizopus nigricans (Zygomycota) produces

large amounts of fumaric acid, another TCA cycle

intermediate. In both cases the growth of the fungus

is severely (and purposefully) restricted by low pH or

some other factor, but the basic metabolic pathways

continue to operate. The fungus behaves like a car with

its engine ticking over: the fuel (substrate) is not used

for growth, so a convenient metabolic intermediate is

released as a kind of exhaust product. We shall see later

that secondary metabolites such as antibiotics are pro-

duced in a similar way. These types of process are termed

energy slippage; the fact that they occur at all indic-

ates that they are necessary, perhaps because fungi need

to keep their normal metabolic processes operating

during periods when growth is temporarily halted.

Mobilizable and energy storage compounds

of fungi

Fungi have a characteristic range of mobilizable and

energy storage compounds, quite different from those

of plants, but strikingly similar to those of insects.

The main energy-storage compounds of fungi include

lipids, glycogen (an α-linked polymer of glucose),

and trehalose(a nonreducing disaccharide, Fig. 7.6).

oxaloacetate + ATP

phosphoenolpyruvate + CO 2 + ADP + Pi

phosphoenolpyruvate

carboxykinase

Fig. 7.5Role of the glyoxylate cycle in generating sugars
for biosynthesis when fungi are grown on nonsugar sub-
strates such as acetate or organic acids.