Arabinoxylan is a form of the
hemicellulose found in both the primary and secondary
cell walls of plants which in addition to
xylose contains substantial amounts of another
pentose sugar,
arabinose.
[Scheller, H. V., & Ulvskov, P. (2010). Hemicelluloses. Annual Review of Plant Biology, 61, 263-289. ] The term arabinoxylan usually refers to feruloyl-arabinoxylan from
grasses and other
commelinids containing moieties of the phenolic
ferulic acid that can undergo oxidative coupling (in the same way as
lignin units) forming
Cross-link between arabinoxylan chains and with lignin. Whilst arabinose has been found linked to xylan in non-commelinid plants, ferulic acid has not been reported on these and unlike feruloyl-arabinoxylan these arabinoxylans are not
monophyletic. The remainder of this article refers to feruloyl-arabinoxylan from cell walls of
grasses and other
Commelinids species.
Structure
As with all xylan, the backbone of arabinoxylan chains is composed of a large number of 1,4-linked β-D-xylopyranosyl units. In arabinoxylan many of these xylose units are 3-linked with single α-L-arabinofuranosyl units and some of these arabinose in turn have ester-linked ferulic acid residues.
These feruloyl units can undergo radical oxidative coupling forming ferulic acid dehydrodimers and possibly higher oligomers that covalently crosslink arabinoxylan chains. This mode of cross-linking is a key feature of both primary and secondary cell walls in
grasses and other
Commelinids species.
Due to its importance in food, the structure of arabinoxylan from wheat grain and other cereals has been intensively studied. In particular, the arabinoxylan from wheat endosperm that gives rise to white flour has a simpler structure than that from most tissues. Apart from the structural features described above, it has xylose units di-substituted with 2 and 3-linked arabinose residues.
Arabinoxylans from tissues other than endosperm are structurally more complex. The xylan backbone is often heavily substituted with glucuronic acid resides and other sugars and these arabinoxylan polymers are sometimes termed glucuronarabinoxylans or heteroxylans.
Functions
Arabinoxylans chiefly serve a structural role in the
.
[Wakabayashi K, et al (2005). Physiologia Plantarum. 125:127–134] They are also the reservoirs of large amounts of
ferulic acid and other
which are
covalent bond to them. Phenolic acids may also be involved in defense including protection against
.
Arabinoxylans are one of the main components of soluble and insoluble which are shown to exert various health benefits.
