Enzymic degradability of hull-less barley flour alkali-solubilized arabinoxylan fractions by endoxylanases.
The impacts of the arabinose to xylose (A/X) ratio of arabinoxylans (AX) and the endoxylanase substrate specificity on the enzymic degradability of hull-less barley flour AX by endoxylanases were studied by using alkali-solubilized AX (AS-AX) fractions with different A/X ratio, on the one hand, and glycoside hydrolase family 10 and 11 endoxylanases of Aspergillus aculeatus (XAA) and Bacillus subtilis (XBS), respectively, on the other hand. AS-AX were obtained by saturated barium hydroxide treatment of hull-less barley flour water-unextractable AX. Fractionation of AS-AX by stepwise ethanol precipitation resulted in structurally different hull-less barley flour AS-AX fractions. Their A/X ratios increased with increasing ethanol concentration, and this increase in A/X ratio was reflected in their xylose substitution levels. For both XAA and XBS, the enzymic degradability of AX and apparent specific endoxylanase activity decreased with increasing A/X ratio of the AS-AX substrates, implying that both endoxylanases were sterically hindered by arabinose substituents. However, for all AS-AX fractions, hydrolysis end products of lower average degree of polymerization were obtained after incubation with XAA than with XBS, indicating that the former enzyme has a lower substrate specificity toward hull-less barley flour AS-AX than the latter. In addition, apparent specific endoxylanase activities indicated that XBS was approximately 2 times more sensitive to variations in the A/X ratio of AS-AX fractions than XAA. Furthermore, AS-AX with higher A/X ratio were relatively resistant to degradation by XBS.
Trogh
Isabel
I
Laboratory of Food Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium. isabel.trogh@biw.kuleuven.be
Croes
Evi
E
Courtin
Christophe M
CM
Delcour
Jan A
JA
D001230:Aspergillus / Q000201:enzymology; D001412:Bacillus subtilis / Q000201:enzymology; D005591:Chemical Fractionation; D043364:Endo-1,4-beta Xylanases / Q000378:metabolism*; D005433:Flour / Q000032:analysis*; D001467:Hordeum / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D012995:Solubility; D013379:Substrate Specificity; D014990:Xylans / Q000302:isolation & purification / Q000378:metabolism*
D014990:Xylans; C085118:arabinoxylan; D043364:Endo-1,4-beta Xylanases