**15. Exoglucanases**

Also known as cellobiohydrolases, these enzymes catalyse the successive hydrolysis of resi‐ dues from the reducing and non-reducing ends of the cellulose, releasing cellobiose molecules as main product, which are hydrolysed by β-glucosidases. They account for 40 to 70% of the to‐ tal component of the cellulase system, and are able to hydrolyse crystalline cellulose.

Exoglucanases have shown specificity on the ends of cellulose, such as*T. reesei* cellobiohy‐ drolase (CBH) I and II that act on the reducing and non-reducing cellulose chain ends, re‐ spectively [112].

These enzymes are monomeric proteins with a molecular weight ranging from 50 to 65 kDa, although there are smaller variants (41.5 kDa) in some fungi, such as *Sclerotium rolfsii*. Low levels of glycosylation (around 12% to none at all) are found in these enzymes; and their op‐ timum pH is 4 to 5, with an optimum temperature from 37 to 60 °C, depending on the spe‐ cific enzyme-substrate combination [137, 140].

Exoglucanases form part of the cellulolytic machinery of the fungi causing white and soft rot and they are found only in some of the basidiomycetes causing the brown rot, such as *Fomi‐ topsis palustris* [141].

Crystalline cellulose (Avicel, bacterial cellulose or filter paper), which is the main form of cellulose in most plant cell walls are good substrates for exoglucanase activity assay, be‐ cause it has a low DP and relatively low accessibility; however, some endoglucanases can release considerable reducing sugars from Avicel [13].
