Chemistry Reference
In-Depth Information
6.7.2 Endoglycosidases
Type II endoglycosidases are a category of hydrolases which are capable of cleaving spe-
cific internal glycosidic linkages found in glycoproteins. These endoglycosidases cleave
all or part of the carbohydrate moiety from a glycoprotein, depending on the location
of the reactive glycosidic linkage in the glycoprotein. Examples of Type II endoglycosi-
dases include endo-
β
-
N
-acetylglucosaminidases (Endo-D, Endo-H (
EC 3.2.1.96
), Endo-L,
Endo-CI, Endo-CII, Endo-F-Gal and Endo-F), endo-
α
-
N
-acetylgalactosaminidase, endo-
β
-
N
-galactosidases, peptide-
N
-(
N
-acetyl-
-glucosaminyl), aspergine amidase F (PNGaseF
EC
3.5.1.52
) and glycopeptide
N
-glycosidase (Peptide
N
-glycosidase
EC 3.2.2.18
).
Wheat gluten proteins are to a certain extent also glycosylated. Glycans were detected on
both gliadin and glutenin polypeptides. Covalently aggregated LMW glutenins were shown
to contain
N
-glycans with xylose, which demonstrated their sorting in the Golgi apparatus.
203
Tests done with purified endoglycosidases revealed enhanced dough relaxation.
158
This
was explained by assuming better gluten network formation after removing side chains from
gluten proteins which could hinder the building-up of a gluten network.
Since endoglycosidases are an intrinsic side activity of almost all xylanases, cellulases,
glucanases and pectinases, there was limited commercial interest in large-scale production
of such enzymes. The trend seen in the last decade to produce more and more enzymes from
GMOs and even producing protein-engineered enzymes has resulted in the availability of
'mono-component' xylanases; that is xylanases which are purer as a result of the manufactur-
ing process. This trend may lead to increased interest in these endoglycosidase side activities,
since it is clear from the above that observed differences in performance of various xylanases
may very well be the result of the presence or absence of non-xylanase side activities.
β
6.7.3 Cellulases
For cellulose breakdown the combined action of several enzymes is required. Cellulase (endo-
1,4-
-D-glucanase
EC 3.2.1.4)
is the most relevant one. Five general types of cellulases, based
on the type of reaction catalyzed, can be identified.
(1) Endocellulase breaks internal bonds to disrupt the crystalline structure of cellulose and
expose individual cellulose polysaccharide chains; (2) exo-cellulase cleaves 2-4 units from
the ends of the exposed chains produced by endocellulase, resulting in the tetrasaccharides
or disaccharide such as cellobiose. There are two main types of exo-cellulases (cellobio-
hydrolases (CBH);
EC 3.2.1.91
) - one type working progressively from the reducing end,
and one type working progressively from the non-reducing end of cellulose; (3) cellobiase
or
β
β
-glucosidase hydrolyzes the endocellulase product into individual monosaccharides; (4)
oxidative cellulases that depolymerize cellulose by radical reactions, as for instance cel-
lobiose dehydrogenase and (5) cellulose phosphorylases that depolymerize cellulose using
phosphates instead of water. The breakdown of cellulose is schematically shown in Fig. 6.13.
Most fungal cellulases have a two-domain structure with one catalytic domain, and one
cellulose-binding domain, that are connected by a flexible link. This structure is adapted for
working on an insoluble substrate and it allows the enzyme to diffuse two-dimensionally on
a surface in a caterpillar way. However, there are also cellulases (mostly endoglucanases)
that lack a cellulose-binding domain. These enzymes might have a swelling function.
Wholemeal bread formulations differ from those of standard bread as the former contains
a higher level of both water-soluble and insoluble fibre ingredients. Soluble fibres consist
of water extractable (WE) arabinoxylan,
β
-glucans and gums, whereas insoluble fibre is
