XXII. BIOCHEMICKÝ ZJAZD - Jesseniova lekárska fakulta

Lectures
DIffERENCIES BETWEEN TWO ENDOXYLANASES frOM GH5
Mária Vršanská, Katarína Šuchová, Vladimír Puchart and Peter Biely
Department of enzymology of Carbohydrates, Institute of Chemistry,
Center for Glycomics, SAS, Bratislava
Endoxylanase A(XynA) produced by plant pathogen Erwinia chrysanthemi and endoxylanase
IV(XynIV) isolated from Trichoderma reesei RUT C-30 are classified into glycoside
hydrolase family 5 (GH5). A detailed study of both endoxylanases showed on differencies
in their mode of action on variety of plant glucuronoxylans and linear xylooligosaccharides.
The hydrolytic action of XynA was found to be absolutely dependend on the presence of
4-O-methyl-D-glucuronosyl (MeGlcA) side residues in both oligosaccharides and polysaccharides.
Neutral linear xylooligosaccharides are resistant towards enzymatic action of
XynA. As a rule, the enzyme attacked the second glycosidic linkage following the MeGlcA
branch towards the reducing end. The productive complex of XynA with glucuronoxylan
is formed in such a way that the MeGlcA residue is bound to the xylopyranosyl residue
accommodated at the hypothetical subsite -2. Depending on the distribution of MeGlcA
residues on glucuronoxylan main chain, XynA generated series of shorter and longer
aldouronic acids in which the MeGlcA is linked exclusively to the second xylopyranosyl
residues from the reducing end. Endoxylanase Trichoderma reesei XynIV also from GH5
do not recognize uronic acid chains as substrate determinants. XynIV showed lover afinity
toward deacetylated glucuronoxylan than to acetylated beechwood glucuronoxylan
or to rhodymenan, a sea alga β-1,3-β-1,4-xylan. The behaviour on polymeric substrates
suggested that XynIV prefers xylan with irregularities in the main chain such as the replacement
of β-1,4-linkages by β-1,3-linkages as it is in rhodymenan, or substitution of
xylopyranosyl residues as it is in acetylglucuronoxylan. [1- 3 H]-Linear xylooligosaccharides
are exclusively cleaved by XynIV at the first glycosidic linkage from the reducing end, to
give [1- 3 H]-xylose as the only radioactive product. All experimental evidence suggests
that the substrate-binding site of XynIV is composed of three subsites -II, -I and +I with
a serious steric barrier at the position of the hypothetical subsite +II. Our data point to
a great diversity among endoxylanases belonging to the GH5 family.
XXII. Biochemistry Congress, Martin
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