SUBSTRATE SUBSITE RECOGNITION OF THE XYLOGLUCAN ENDO-TRANSGLYCOSYLASEOR XYLOGLUCAN-SPECIFIC ENDO-(1-]4)-BETA-D-GLUCANASE FROM THE COTYLEDONS OF GERMINATED NASTURTIUM (TROPAEOLUM-MAJUS L) SEEDS
C. Fanutti et al., SUBSTRATE SUBSITE RECOGNITION OF THE XYLOGLUCAN ENDO-TRANSGLYCOSYLASEOR XYLOGLUCAN-SPECIFIC ENDO-(1-]4)-BETA-D-GLUCANASE FROM THE COTYLEDONS OF GERMINATED NASTURTIUM (TROPAEOLUM-MAJUS L) SEEDS, Planta, 200(2), 1996, pp. 221-228
We have investigated recognition requirement transglycosylase/xylogluc
an-specific endo-(1 --> 4)-beta-D-glucanase (NXET) from the cotyledons
of nasturtium seedlings. Seed xyloglucans are composed almost entirel
y of the Glc, subunits XXXG, XLXG, XXLG and XLLG, where G represents a
n unsubstituted glucose residue, X a xylose-substituted glucose residu
e and L a galactosyl-xylose-substituted glucose residue. Thus in the x
yloglucan sequence shown below, the xylose (Xyl) residues at the backb
one glucose (Glc) residues numbered - 3, - 2, + 2 and + 3 may be galac
tose-substituted, and NXET cleaves between the unsubstituted glucose a
t - 1 and the xylose-substituted glucose at + 1, which never carries a
galactosyl substituent. [GRAPHICS] We have isolated the xyloglucan ol
igosaccharides XXXGXXXG and XLLGXLLG from NXET digests of tamarind see
d xyloglucan, have modified them enzymatically using a pure xyloglucan
oligosaccharide-specific alpha-xylosidase from nasturtium seeds to gi
ve GXXGXXXG and GLLGXLLG, and have identified and compared the product
s of NXET action on XXXGXXXG, GXXGXXXG, XLLGXLLG and GLLGXLLG. We have
also compared the molar proportions of XXXG, XLXG, XXLG and XLLG in n
ative tamarind and nasturtium seed xyloglucans with those in NXET dige
sts of these polysaccharides. Using these and existing data we have de
monstrated that NXET action does not require xylose-substitution at gl
ucose residues - 4, - 2, + 1 and + 3 and that xylose substitution at 2, is a requirement. There may also be a requirement for xylose subst
itution at - 3. We have demonstrated also that galactosyl substitution
of a xylose residue at + 1 prevents, and at - 2 modifies, chain-cleav
age. A partial model for the minimum substrate binding requirement of
NXET is proposed.