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4TH CONFERENCE OF PHD STUDENTS AT FACULTY OF CHEMICAL AND
BIOENGINEERING
EFFECT OF XYLOGLUCANASE ON THE DEGRADATION OF
HEMICELLULOSE CONTAINING SUBSTRATES
Zsuzsa Benkő
Department of Biochemistry and Food Technology
Budapest University of Technology and Economics
H-1521 Budapest, Hungary
E-mail: [email protected]
Supervisors: Kati Réczey, Matti Siika-aho, Liisa Viikari
In the primary cell wall of higher plants cellulose, hemicelluloses, pectins and
glycoproteins form networks, which can elongate or swell during growth of the cell.
Cellulose and xyloglucan, the major hemicellulosic polysaccharide in various plants, are
associated by hydrogen bonds and this network is regarded as the load-bearing
component while the pectinic polysaccharide network controls the porosity of the cell
wall. Removal of pectin by purified pectolytic enzymes facilitates cellulose degradation
[1] while elimination of xyloglucan from the cellulose surface enhances the hydrolysis
of cellulose by cellulases as well [2].
Figure 1: The xyloglucan molecules in the primary cell wall cover the cellulose
microfibrils and connect adjacent microfibrils.
Xyloglucan (XG) represents about 20-25% of primary cell wall in
dicotyledonous angiosperms, about 2-5 % in grasses and about 10% in soft woods and it
is believed to be a key component in cell enlargement during growth beside the main
load bearing function. It has a cellulose like β-1,4- glucopyranosyl backbone with αxylosyl residues attached to the 6-position, which could be further substituted with β-Dgalactopyranosyl and α-L-fucosyl-galactopyranosyl side chains [3]. Building blocks
(“trains”) of the polymeric XG binds to the cellulose surface, while the so-called
“loops” and “tails” of XG stick into solution [4]. Three distinct XG fractions are part of
the XG network. Approximately 8% of the XG found in the cell wall can be solubilized
by xyloglucanase digestion. This domain may form loops, dead ends or cross-links
between cellulose microfibrils. Another type of XG (~10%) can be extracted using
KOH after the enzymatic treatment. This XG domain is closely associated with the
surface of cellulose. The third domain of xyloglucan is entrapped within or between
cellulose microfibrils and can be solubilized only with cellulases after xyloglucanase
and KOH treatment. This strongly associated structure propose that degrading the
xyloglucan polymer using xyloglucanases could improve the total hydrolysis of
lignocellulosic substrates.
In order to get more information about the role of xyloglucanase and the way of
its enzymatic action eleven different ligncellulosic substrates were hydrolyzed in this
work using different combinations of purified cellobiohydrolase I and II, endoglucanase
II and xyloglucanase of Trichoderma reesei and Aspergillus β-glucosidase.
(Endoglucanase I was not included in the basic cellulase enzyme mixture, since it also
has activity towards xyloglucan.)
Xyloglucanase activity had generally an improving effect on the total hydrolysis
of the lignocellulosic substrates. The results indicate that the application of this enzyme
is required to enhance the total hydrolysis especially in cases when the pretreatment of
the lignocellulosic substrate was not optimal.
References
[1] HAYASHI, T., Xyloglucans in the primary cell wall, Annu. Rev. Plant Physiol.
Plant Mol. Biol., 40 (1989), pp. 139-168.
[2] VINCKEN, J. P. - BELDMAN, G. - VORAGEN, A. G. J., The Effect of
Xyloglucans on the Degradation of Cell-Wall-Embedded Cellulose by the
Combined Action of Cellobiohydrolase and Endolgucanases from Trichoderma
viride, Plant Physiol., 104 (1994), pp. 99-107.
[3] PAULY, M. - ALBERSHEIM, P. - DARVILL, A. - YORK, W. S., Molecular
domains of the cellulose/xyloglucan network in the cell walls of higher plants, The
Plant Journal, 20 (6) (1999), pp. 629-639.
[4] VINCKEN, J. P. – DE KEIZER, A. – BELDMAN, G. – VORAGEN, A. G. J.,
Fractionation of Xyloglucan Fragments and Their Interaction with Cellulose, Plant
Physiol., 108 (1995), pp. 1579-1585.