8
1.2.1.4 Beta Glucosidases
Beta glucosidases or cellobiases (EC 3.2.1.21) are enzymes that catalyze the hydrolysis of terminal, nonreducing beta-D-glucosyl residues with release of beta-Dglucose (Leah et al. 1995). Beta glucosidases (BGLs) catalyze the final reaction in
cellulose hydrolysis, namely, the hydrolysis of cellobiose to two molecules of glucose, and are responsible for the regulation of the cellulolytic cascade through their
own feedback inhibition by their reaction product glucose. Most of the microbial
BGLs employed in biomass hydrolysis belong to GH family 3, while they can be
found in families 1, 3, 9, 30, and 116 (Teugjas and Väljamäe 2013). BGL action is
considered as a critical step in cellulose hydrolysis since the substrate of BGL – cellobiose – is a strong inhibitor of CBHs and its hydrolysis is essential to overcome
product inhibition of the exoglucanases. Since glucose accumulation can lead to
BGL inhibition which in turn leads to CBH inhibition through accumulation of cellobiose, the regulation of cellulase production in response to the hydrolysis of cellulose is of critical importance in most of the organisms producing these enzymes.
In several cases the BGLs are also inhibited by their substrate, believed to be caused
by the transglycosylation reaction capable of being performed by these enzymes
(Bohlin et al. 2013).
1.2.1.5 Other Cellulolytic Enzymes and Accessory Proteins
It has long been recognized that the hydrolysis of the dense crystalline lattices of
cellulose has to be mechanically disrupted for access of the hydrolytic enzymes and
the role of a “swelling factor” which was nonhydrolytic and was proposed as early
as 1950 (Reese et al. 1950). “Swollenin”, a protein with sequence similarity to plant
expansions, was described in T. reesei by Saloheimo et al. (Saloheimo et al. 2002).
It was believed that swollenin and similar nonhydrolytic swollenin-like proteins act
like a zipper opening up the cross-linking of cellulose microfibrils just like plant
expansins (Arantes and Saddler 2010). It was also proposed originally that these
proteins lack hydrolytic activity since only negligible quantities of sugar release
were observed with their independent action, while they enhanced hydrolysis of
cellulosic substrates (Gourlay et al. 2012). The mechanism of promoting cellulose
breakdown was speculated to be through a nonhydrolytic weakening of hydrogen
bonding (Jäger et al. 2011, Gourlay et al. 2012). However, the most recent works
have indicated that the protein does have hydrolytic activity and shows a unique
mode of action with similarities to the action of both endoglucanases and exoglucanases (Andberg et al. 2015). Apart from swollenin, the “disrupting” or “amorphogenesis inducing” class of biomass-degrading proteins includes expansins, bacterial
expansin-like proteins, fungal expansin-like proteins, loosenin, etc. (Arantes and
Saddler 2010; Gourlay et al. 2013).
Revolutionary changes in the conventional cellulose deconstruction paradigm
have emerged with the discovery of a class of enzymes that share conserved structural features binding a metal ion and following a hitherto undescribed oxidative
mechanism (Vaaje-Kolstad et al. 2010). These types of enzymes which are now
considered ubiquitous have been termed as Lytic Polysaccharide Mono Oxygenases
(LPMOs). The most important feature of these enzymes is their ability to attack the
R.K. Sukumaran et al.
1.2.1.4 Beta Glucosidases
Beta glucosidases or cellobiases (EC 3.2.1.21) are enzymes that catalyze the hydrolysis of terminal, nonreducing beta-D-glucosyl residues with release of beta-Dglucose (Leah et al. 1995). Beta glucosidases (BGLs) catalyze the final reaction in
cellulose hydrolysis, namely, the hydrolysis of cellobiose to two molecules of glucose, and are responsible for the regulation of the cellulolytic cascade through their
own feedback inhibition by their reaction product glucose. Most of the microbial
BGLs employed in biomass hydrolysis belong to GH family 3, while they can be
found in families 1, 3, 9, 30, and 116 (Teugjas and Väljamäe 2013). BGL action is
considered as a critical step in cellulose hydrolysis since the substrate of BGL – cellobiose – is a strong inhibitor of CBHs and its hydrolysis is essential to overcome
product inhibition of the exoglucanases. Since glucose accumulation can lead to
BGL inhibition which in turn leads to CBH inhibition through accumulation of cellobiose, the regulation of cellulase production in response to the hydrolysis of cellulose is of critical importance in most of the organisms producing these enzymes.
In several cases the BGLs are also inhibited by their substrate, believed to be caused
by the transglycosylation reaction capable of being performed by these enzymes
(Bohlin et al. 2013).
1.2.1.5 Other Cellulolytic Enzymes and Accessory Proteins
It has long been recognized that the hydrolysis of the dense crystalline lattices of
cellulose has to be mechanically disrupted for access of the hydrolytic enzymes and
the role of a “swelling factor” which was nonhydrolytic and was proposed as early
as 1950 (Reese et al. 1950). “Swollenin”, a protein with sequence similarity to plant
expansions, was described in T. reesei by Saloheimo et al. (Saloheimo et al. 2002).
It was believed that swollenin and similar nonhydrolytic swollenin-like proteins act
like a zipper opening up the cross-linking of cellulose microfibrils just like plant
expansins (Arantes and Saddler 2010). It was also proposed originally that these
proteins lack hydrolytic activity since only negligible quantities of sugar release
were observed with their independent action, while they enhanced hydrolysis of
cellulosic substrates (Gourlay et al. 2012). The mechanism of promoting cellulose
breakdown was speculated to be through a nonhydrolytic weakening of hydrogen
bonding (Jäger et al. 2011, Gourlay et al. 2012). However, the most recent works
have indicated that the protein does have hydrolytic activity and shows a unique
mode of action with similarities to the action of both endoglucanases and exoglucanases (Andberg et al. 2015). Apart from swollenin, the “disrupting” or “amorphogenesis inducing” class of biomass-degrading proteins includes expansins, bacterial
expansin-like proteins, fungal expansin-like proteins, loosenin, etc. (Arantes and
Saddler 2010; Gourlay et al. 2013).
Revolutionary changes in the conventional cellulose deconstruction paradigm
have emerged with the discovery of a class of enzymes that share conserved structural features binding a metal ion and following a hitherto undescribed oxidative
mechanism (Vaaje-Kolstad et al. 2010). These types of enzymes which are now
considered ubiquitous have been termed as Lytic Polysaccharide Mono Oxygenases
(LPMOs). The most important feature of these enzymes is their ability to attack the
R.K. Sukumaran et al.