**7. Cellulose degradation mediated by cellulosome**

Selective pressure of evolution is the force driving microorganisms to adapt a new environ‐ ment, in anaerobic conditions is necessary a machinery for the extracellular degradation of substrates, such as the recalcitrant crystalline components of the plant cell wall. Due to this, the anaerobes tend to adopt different strategies for degrading plant components, being the cellulosomes the most remarkable feature.

The occurrence of a cellulosome was first observed in the thermophilic bacterium, *C. thermo‐ cellum* and has now been described in a number of mesophilic anaerobic bacteria and with

Hydrolysis of Biomass Mediated by Cellulases for the Production of Sugars

http://dx.doi.org/10.5772/53719

129

Cellulosomes are large extracellular enzyme complexes capable of degrading cellulose, hem‐ icelluloses, and pectin; they may be the largest extracellular enzyme complexes found in na‐ ture, although the individual cellulosomes size range from 0.65 MDa to 2.5 MDa, some

The cellulosome structure is characterized by two components: (a) the non-enzymatic scaf‐ folding proteins with enzyme binding sites called cohesins, (b) enzymes with dockerins pro‐

Depending of the bacterial species, the scaffolding protein varies in the number of cohesins and cellulose binding modules (CBM) that binds the cellulosome tightly to the substrate and concentrates the enzymes to a particular site of the substrate. Recently, a more complex cel‐ lulosome structure with multiple interacting scaffolding proteins that allows the binding of

The cohesin-dockerin interconnect the different scaffoldin components, whereby the specif‐ icities among the individual cohesin-dockerin complexes dictate the overall supramolecular

In short, the enzymatic cellulosome system may exceed the potential of non-cellulosomal degradative system due to its structural organization, efficient binding to the substrate, the variety of hydrolytic enzymes acting synergistically [52]. Cellulosomes have not been identi‐ fied in bacteria (or eukarya) that grow above 65 ºC, and have not been identified in the

**Figure 5.** Cellulosome structure. A dockerin is appended to catalytic (enzyme) and noncatalytic carbohydrate-binding modules (CBMs). Dockerins bind the cohesins of a noncatalytic scaffoldin, providing a mechanism for cellulosome as‐

some anaerobic fungi particularly *Piromyces* sp.[52, 67].

as much as enzymes has been revealed [52].

Archaea [89].

architecture of the participating components [88].

sembly. Image reproduced with publisher´s permission [90].

polycellulosomes have been reported to be as large as 100 MDa, [87].

teind interacting with cohesins in the scaffolding protein (Figure 5).


\*Termophylic bacteria. \*\*Cel: cellulases; Xyl: xylanases.

**Table 4.** Fungi and bacteria with cellulolytic activity.

The occurrence of a cellulosome was first observed in the thermophilic bacterium, *C. thermo‐ cellum* and has now been described in a number of mesophilic anaerobic bacteria and with some anaerobic fungi particularly *Piromyces* sp.[52, 67].

**Group Fungi Enzymes\*\* Substrate Ref**

128 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

*Schizophyllum commune* Cel, Xyl Microcrystalline cellulose, rice

*P. chrysosporium* Cel, Xyl Red oak, grape seed, barley

*Fomitopsis palustris* Cel Microcrystalline cellulose [75]

*Anaeromyces mucrunatus* Cel, Xyl Orchard grass hay [76]

*Neocallimastix frontalis* Cel, Xyl wheat straw [78] *Orpinomyces sp.* Cel Avicel [79] *N. patriciarum* Cel CMC [80]

*Caecomyces communis* Cel Microcrystalline cellulose,

Cel

Clostridium thermocellum\* Cel Crystalline cellulose [82] *Thermomonospora fusca\** Cel, Xyl wheat straw, oat spelt xylan [83] *Caldicellulosiruptor kristjanssonii\** Cel Microcrystalline cellulose [84]

*Anaerocellum thermophilum\** Cel, Xyl Microcrystalline cellulose,

*T. reesei* Cel, Xyl Wheat straw [68-69] *T. harzianum* Cel Cellulose [70] *A. niger* Cel Sugar cane bagasse [71]

xylan [72]

bran, sorghum [2, 63, 73]

alfalfa hay [77]

xylan [85-86]

[74]

[81]

Oak and cedar sawdust, rice husk, corn stubble, wheat straw and *Jatropha* seed husk

Whatman paper No1, Microcyrtsalline cellulose, Azurine crosslinked hydroxyethylcellulose (AZCL-HEC)

Aerobic fungi

Anaerobic rumen fungi

Aerobic bacteria (cellulosomes)

Anaerobic bacteria (cellulosomes)

(cellulosomes)

(extracellular cellulolytic enzymes)

Ascomycetes

Basidiomycetes

Chytridiomycetes

Actinobacteria

Firmicutes

\*Termophylic bacteria. \*\*Cel: cellulases; Xyl: xylanases.

**Table 4.** Fungi and bacteria with cellulolytic activity.

B. adusta

*Acidothermus cellulolyticus*

*Actinospica robiniae Actinosynnema mirum Catenulispora acidiphila Cellulomonas flavigena Thermobispora bispora Xylanimonas cellulosilytica*

Pycnoporus sanguineus Cel, Xyl

Cellulosomes are large extracellular enzyme complexes capable of degrading cellulose, hem‐ icelluloses, and pectin; they may be the largest extracellular enzyme complexes found in na‐ ture, although the individual cellulosomes size range from 0.65 MDa to 2.5 MDa, some polycellulosomes have been reported to be as large as 100 MDa, [87].

The cellulosome structure is characterized by two components: (a) the non-enzymatic scaf‐ folding proteins with enzyme binding sites called cohesins, (b) enzymes with dockerins pro‐ teind interacting with cohesins in the scaffolding protein (Figure 5).

Depending of the bacterial species, the scaffolding protein varies in the number of cohesins and cellulose binding modules (CBM) that binds the cellulosome tightly to the substrate and concentrates the enzymes to a particular site of the substrate. Recently, a more complex cel‐ lulosome structure with multiple interacting scaffolding proteins that allows the binding of as much as enzymes has been revealed [52].

The cohesin-dockerin interconnect the different scaffoldin components, whereby the specif‐ icities among the individual cohesin-dockerin complexes dictate the overall supramolecular architecture of the participating components [88].

In short, the enzymatic cellulosome system may exceed the potential of non-cellulosomal degradative system due to its structural organization, efficient binding to the substrate, the variety of hydrolytic enzymes acting synergistically [52]. Cellulosomes have not been identi‐ fied in bacteria (or eukarya) that grow above 65 ºC, and have not been identified in the Archaea [89].

**Figure 5.** Cellulosome structure. A dockerin is appended to catalytic (enzyme) and noncatalytic carbohydrate-binding modules (CBMs). Dockerins bind the cohesins of a noncatalytic scaffoldin, providing a mechanism for cellulosome as‐ sembly. Image reproduced with publisher´s permission [90].
