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FUNGAL NUTRITION 115

to be secreted exclusively at the tips of Aspergillus
niger. So it seems that enzymes destined for release
through the wall are transported to the hyphal tip,
where they are released by exocytosis and might flow
outwards with the newly formed wall components,
according to the steady-state model of wall growth
described in Chapter 4 (Wessels 1990). Some of these
enzymes would reach the cell surface and be released
into the external environment, whereas others might
become locked in the wall and serve as wall-bound
enzymes – for example, degrading disaccharides and
other small molecules (oligomers) to monosaccharides,
amino acids, etc.

Defense of territory

The release of enzymes to degrade a polymer represents
an investment of resources, so we can expect it to be
coupled with defense of the territory, preventing
other organisms from sharing the breakdown products.
There are a few known examples of noncellulolytic fungi
that grow in close association with cellulose-degraders
(Chapter 12), utilizing some of the enzyme breakdown
products. But these fungi might grow in a mutualistic
association that benefits the cellulolytic fungus.
In general, three factors might help polymer-
degrading fungi to defend their territory.

1 The synthesis of depolymerases is tightly regulated
by feedback mechanisms, discussed later, so the rate
of enzyme production is matched to the rate at
which the breakdown products can be utilized.
2 The final stages of polymer breakdown are achieved
by wall-bound enzymes, so that the most readily uti-
lizable monomers are not available to other organ-
isms; we will see this later in the case of cellulose.
3 A polymer-degrading fungus might produce anti-
biotics or other suppressive metabolites. This is
difficult to demonstrate at the scale of individual
hyphae, but Burton & Coley-Smith (1993) reported
that antibacterial compounds were released by
hyphae of Rhizoctoniaspecies – members of the
Basidiomycota that are known to degrade cellulose.

It may be significant that antibiotics are produced

mainly by polymer-degrading fungi (Chapter 12) and

their production in vitrois associated with nutrient-

limiting growth conditions (Chapter 7). These are the

conditions that polymer-degrading fungi would experi-

ence most of the time, because depolymerases are pro-

duced only when more readily available nutrients are

in short supply. Thus, antibiosis might have evolved

not as an aggressive strategy but for the defense of

territory.

The breakdown of cellulose: a case study of

extracellular enzymes

Cellulose is the most abundant natural polymer on

earth, representing about 40% of all the plant biomass

that is produced and recycled on an annual basis.

Fungi play the principal role in degrading cellulose and

also in utilizing the cellulose breakdown products. So,

we will consider the structure of cellulose and the role

of cellulase enzymes in some detail as a model of how

enzymes function in natural environments.

The chemistry of cellulose

In terms of its chemical structure, cellulose is a relat-

ively simple polymer. It consists mainly of long,

unbranched, chains of between 2000 and 14,000

glucose residues, linked by β-1,4 bonds. The βconfigura-

tion means that each glucose unit is rotated 180

degrees to the next, and the whole chain is made up

of these repeating pairs, shown between the brackets

in Fig. 6.5. Such a molecule should be easily degraded

by the extracellular enzymes of fungi, but the phys-

ical structure of cellulose presents problems of enzyme

access to the substrate. The individual cellulose chains

stack closely together in a near-crystalline manner

to form micelles, which are reinforced by hydrogen-

bonding, and the micelles themselves align to form

microfibrils of about 10 nm diameter, visible in elec-

tron micrographs. These rigid, insoluble microfibrils

H

H HOH

H

H

H H

H

HH

OH

OH

OH

O

HO

HO

HO

β 1

β 1

4

H

n HOH

OH

H

H

H H

H H

H

HH

OH

OH

O OH

O

O

HO

HO

β (^14)

O^4

Fig. 6.5Structure of a single cellulose chain, composed of thousands of repeated disaccharide (cellobiose) units; one
disaccharide unit is shown within the brackets. The β1-4 bonds are also shown.