Enzyme Systems

Cellulolytic bacteria and fungi produce a variety of different cellulases and related glycoside hydrolases, which together convert plant cell wall polysaccharides to simple fermentable sugars. Cellulolytic bacteria and fungi employ different strategies for the degradation of the

Table 5.1. Major glycoside hydrolase families and their enzymatic activities. The glycoside hydrolase families (GHn) in which some members exhibit standard cellulase activities are shown in bold. GH families that include cellulases exclusively are followed by an asterisk (*). See CAZy website for more details: http://www. cazy. org/.

GH Family Enzymes

GH1 Numerous activities, including p-glucosidase, p-galactosidase, p-mannosidase, and

P-glucuronidase; but not p-xylosidase activity

GH2 Numerous activities, including p-galactosidase, p-mannosidase, and p-glucuronidase;

but neither p-glucosidase nor p-xylosidase activities GH3 Numerous activities, notably not only p-glucosidase and p-xylosidase activities, but also

glucan 1,3-p-glucosidase, glucan 1,4-p-glucosidase, and exo-1,3(4)-glucanase activities

GH5 Broad spectrum of cellulase and hemicellulase activities, including cellulase, xylanase,

1,3-p-mannanase; p-mannosidase, glucan 1,3-p-glucosidase, licheninase, glucan endo-1,6-p-glucosidase, mannan endo-1,4-p-mannosidase, endo-1,6-p-galactanase, and xyloglucan-specific endo-1,4-p-glucanase activities GH6* Cellulase activities in both aerobic bacteria and fungi (not found in archaea): both endo — and exo-glucanase (cellobiohydrolase) activities GH7* Cellulase activities exclusive to the fungi: both endo — and exo-glucanase (cellobiohydrolase) activities

GH8 Cellulase, lichenanase, xylanase activities; exclusive to bacteria

GH9* Endo-, processive endo-, and exo-glucanase (cellobiohydrolase) activities in bacteria, plants, and fungi (but not in archaea)

GH10 Endo-1,4-p-xylanase and endo-1,3-p-xylanase activities in bacteria and fungi

GH11 Xylanase activities in bacteria and fungi

GH12 Endoglucanase, xyloglucanase, and 1,3(4)-p-glucanase in the three domains of life GH16 Endo-1,3-p-glucanase, endo-1,3(4)-p-glucanase, lichenanase, and xyloglucanase activities

GH17 Glucan 1,3-p-glucosidase and lichenanase activities GH18 Chitinases

GH19 Chitinases

GH26 p-Mannanase and 1,3-p-xylanase activities

GH30 1,6-p-Glucanase and p-xylosidase activities

GH39 p-Xylosidase activity

GH42 p-Galactosidase activity

GH43 Broad spectrum of hemicellulase activities, including xylanase, arabinanase, p-

arabinofuranosidase, p-xylosidase, and galactan 1,3-p-galactosidase activities in bacteria and fungi

GH44 Endoglucanase and xyloglucanase activities, mainly in bacteria GH45* Endoglucanase activity, mainly in fungi (some bacteria)

GH47 a-Mannosidase activity, mainly in fungi

GH48* Cellobiohydrolases and endo-processive cellulases; mainly in bacteria; an important enzyme in all cellulosomes and in some noncellulosomal systems GH51 a-L-Arabinofuranosidase and endoglucanase activities

GH52 p-Xylosidase activity

GH53 Endo-1,4-p-galactanase activity

GH54 a-L-Arabinofuranosidase and p-xylosidase activities, mainly in fungi

GH55 Exo — and endo-1,3-glucanase activities, mainly in fungi

GH61 Exclusive to fungi. In some cases, annotated as endoglucanases, but probably disrupt cellulose structure rather than cleaving glucoside bonds.

GH62 a-L-Arabinofuranosidase activity

GH64 1,3-p-Glucanase activities; mainly in bacteria

GH67 a-Glucuronidase and xylan a-1,2-glucuronosidase activities

GH74 Xyloglucanase and endoglucanase activities

GH81 1,3-p-Glucanase activity

plant cell wall polysaccharide substrates, which reflect the complement and type(s) of enzymes produced by a given microbe. The resultant “cellulase system” may be characterized by free enzymes, cell-bound enzymes, multifunctional enzymes, cellulosomes, or any combination of the latter (Bayer et al. 2006; Wilson 2008).