untitled

116 CHAPTER 6

present a quite formidable obstacle to enzymatic

degradation, but several types of fungus have evolved

means of degrading them.

The cellulase enzymes

At least three types of enzyme are involved in the com-

plete breakdown of a cellulose chain to its constituent

glucose units, and some of these enzymes exist in

multiple forms of different molecular mass. Collectively

they are termed “cellulase” or, more precisely, the

cellulase enzyme complex. The three main enzymes

in this complex are:

1 An endoglucanase (endo-ββ-1,4-glucanase), which

acts at random points within a cellulose chain,

breaking the molecule into successively smaller

fragments. This enzyme is found in multiple forms,

ranging from about 11,000 to 65,000 Da.

Fig. 6.6Structure and enzymatic breakdown of cellulose. The enzyme cellobiohydrolase(CBH) cleaves disaccharide
residues (cellobiose) from the nonreducing ends of the cellulose chains. The enzyme endo-b-1,4-glucanase(EBG) cleaves
the chains at random to generate shorter chains, thereby providing further ends for the action of CBH. The enzyme
b-glucosidasecleaves cellobiose to two glucose residues for uptake by the fungus.

2 An exo-acting enzyme, termed cellobiohydrolase,

which acts only on the ends of cellulose chains,

cleaving off successive disaccharide units (cel-

lobiose). This enzyme is more uniform than the

endoglucanase, ranging from about 50,000 to

60,000 Da.

3 ββ-glucosidase(or “cellobiase”), which cleaves the

disaccharide cellobiose to glucose, for uptake by

the fungus. This is a wall-bound enzyme, like many

enzymes involved in the final stages of degradation

of polymers.

The combined actions of these three types of enzyme

are shown diagrammatically in Fig. 6.6. All three

enzymes act synergistically and are tightly regulated,

to ensure that a cellulose-degrading fungus does not

release sugars at a faster rate than it can use them. The

endoglucanases, by attacking the cellulose chains at

random, progressively create more ends on which

cellobiohydrolase can act. But the resulting cellobiose