16
cellulolyticum, C. acetobutylicum, Acetivibrio celluloyticus, Bacteriodes cellulosolvans, Ruminococcus albus, R. flavifaciens, etc. (Fontes and Gilbert 2010). There
are also anaerobic fungi like Neocalimastix, Pyromices, and Orpinomyces which
employ the cellulosomes for degradation of celluloses (Haitjema et al. 2014). These
systems employed by the anaerobic microorganisms are called “complexed systems” as the cellulosomes are multiprotein complexes anchored to the microbial cell
wall. Cellulosomes are the largest extracellular enzyme complexes found in nature,
and there are polycellulosomes as large as 100 MDa (Doi and Kosugi 2004).
Cellulosome contain high-molecular weight noncatalytic proteins called scaffoldin
onto which the enzymes are attached. The modular cellulases and hemicellulases
produced by anaerobic microbes contain a dockerin appended to the catalytic module (the enzyme) and a noncatalytic carbohydrate binding module (CBM) (Fontes
and Gilbert 2010). Dockerins are proteins of ~70 aminoacids usually present in
single copy at the C terminal end of cellulolytic enzymes. They serve the purpose of
anchoring the enzyme to the large scaffoldin protein which bears modules called
cohesins that directly bind the dockerins. Cohesins are modules that are ~150 residues in length and are present as internal repeats in the scaffoldin. Typically about
1–11 cohesin modules are found in a scaffoldin, and it is recognized that the interaction of cohesions with dockerins may not be highly specific allowing different dockerins (bearing different enzymes) to be assembled on the cellulosome complex.
Also the scaffoldin molecules contain a cellulose-specific family 3 CBM and a C
terminal divergent dockerin which serve respectively the functions of targeting the
cellulosome to the cellulose and to the bacterial cell wall (Fontes and Gilbert 2010).
A typical cellulosome assembly is represented in Fig. 1.5.
The co-localization of different enzymes and CBMs on the cellulosome allows
them to act in close proximity on the cellulose surface, which in turn is proposed to
enhance the hydrolytic ability (Resch et al. 2013).
In addition to the anaerobic cellulolytic bacteria inhabiting the rumen or gut
microbiomes and aquatic environments, there are anaerobic fungi that are capable
of efficient cellulose degradation. It is now known that an early branch of fungi
belonging to the order Neocallimastigomycota inhabit the digestive tracts of mammalian and reptilian herbivores that consume highly fibrous diets (Haitjema et al.
2014). They are suggested to be responsible for 40–70% of plant biomass digestion
in the ruminant and nonruminant herbivores (Akin et al. 1990). These fungi possess
both the complexed and the free enzymes and are believed to act by developing a
highly branched rhizoidal network of rhizomycelia that penetrates the substrate and
exposes it for attack by the secreted cellulases (Haitjema et al. 2014). While the
studies on anaerobic fungi have confirmed that the enzymes of these fungi can form
large complexes and they encode fungal dockerin domains, more is yet to be known
about the cohesins or scaffoldins in them. While most of the studies have identified
fungal dockerin domains in the cellulolytic enzymes elaborated by the anaerobic
fungi, the type and structure of scaffoldins have largely remained elusive. Recently,
scaffoldins have been described in Neocallimastix (Wang et al. 2014). It is also
known that the dockerins displayed by one enzyme can bind another cellulase from
the same organism (Nagy et al. 2007), implying that the mechanism of cellulose
R.K. Sukumaran et al.
cellulolyticum, C. acetobutylicum, Acetivibrio celluloyticus, Bacteriodes cellulosolvans, Ruminococcus albus, R. flavifaciens, etc. (Fontes and Gilbert 2010). There
are also anaerobic fungi like Neocalimastix, Pyromices, and Orpinomyces which
employ the cellulosomes for degradation of celluloses (Haitjema et al. 2014). These
systems employed by the anaerobic microorganisms are called “complexed systems” as the cellulosomes are multiprotein complexes anchored to the microbial cell
wall. Cellulosomes are the largest extracellular enzyme complexes found in nature,
and there are polycellulosomes as large as 100 MDa (Doi and Kosugi 2004).
Cellulosome contain high-molecular weight noncatalytic proteins called scaffoldin
onto which the enzymes are attached. The modular cellulases and hemicellulases
produced by anaerobic microbes contain a dockerin appended to the catalytic module (the enzyme) and a noncatalytic carbohydrate binding module (CBM) (Fontes
and Gilbert 2010). Dockerins are proteins of ~70 aminoacids usually present in
single copy at the C terminal end of cellulolytic enzymes. They serve the purpose of
anchoring the enzyme to the large scaffoldin protein which bears modules called
cohesins that directly bind the dockerins. Cohesins are modules that are ~150 residues in length and are present as internal repeats in the scaffoldin. Typically about
1–11 cohesin modules are found in a scaffoldin, and it is recognized that the interaction of cohesions with dockerins may not be highly specific allowing different dockerins (bearing different enzymes) to be assembled on the cellulosome complex.
Also the scaffoldin molecules contain a cellulose-specific family 3 CBM and a C
terminal divergent dockerin which serve respectively the functions of targeting the
cellulosome to the cellulose and to the bacterial cell wall (Fontes and Gilbert 2010).
A typical cellulosome assembly is represented in Fig. 1.5.
The co-localization of different enzymes and CBMs on the cellulosome allows
them to act in close proximity on the cellulose surface, which in turn is proposed to
enhance the hydrolytic ability (Resch et al. 2013).
In addition to the anaerobic cellulolytic bacteria inhabiting the rumen or gut
microbiomes and aquatic environments, there are anaerobic fungi that are capable
of efficient cellulose degradation. It is now known that an early branch of fungi
belonging to the order Neocallimastigomycota inhabit the digestive tracts of mammalian and reptilian herbivores that consume highly fibrous diets (Haitjema et al.
2014). They are suggested to be responsible for 40–70% of plant biomass digestion
in the ruminant and nonruminant herbivores (Akin et al. 1990). These fungi possess
both the complexed and the free enzymes and are believed to act by developing a
highly branched rhizoidal network of rhizomycelia that penetrates the substrate and
exposes it for attack by the secreted cellulases (Haitjema et al. 2014). While the
studies on anaerobic fungi have confirmed that the enzymes of these fungi can form
large complexes and they encode fungal dockerin domains, more is yet to be known
about the cohesins or scaffoldins in them. While most of the studies have identified
fungal dockerin domains in the cellulolytic enzymes elaborated by the anaerobic
fungi, the type and structure of scaffoldins have largely remained elusive. Recently,
scaffoldins have been described in Neocallimastix (Wang et al. 2014). It is also
known that the dockerins displayed by one enzyme can bind another cellulase from
the same organism (Nagy et al. 2007), implying that the mechanism of cellulose
R.K. Sukumaran et al.