Glycosidic bonds are not regarded as true ' crosslinks' . A glycosidic bond has a well-defined directionality, indicated for example by the arrow in the systematic name of isoprimeverose: a-d-Xylp-(1^6)-d-Glc. This is described as Xyl attached to Glc (not Glc to Xyl) because the linkage involves the ano-meric centre of the Xyl, not the Glc. With one postulated exception (Hua; see Section 1.5.2), each sugar residue can form only a single glycosidic bond to another sugar unit, whereas a given sugar unit can in principle have anything from zero up to three (in a C5 sugar), four (in a C6 sugar) or even five (in Kdo) additional sugar residues attached to it - potentially leading to a branched tree-like structure (e.g. RG-I, RG-II or xyloglucan); the reducing terminus is the base of the Tree trunk' (ignore the tree' s roots!). It is chemically feasible for the reducing terminus of one polysaccharide to form a glycosidic bond to a residue within another (identical or non- i dentical) polysaccharide, thus covalently joining the two polysaccharides. However, this is not a true crosslink because the linkage does not differ in its fundamental nature from any glycosidic linkage within either 'individual' polysac-charide. The product is effectively a single larger polysaccharide, albeit maybe with qualitatively dissimilar domains.
Examples of this type of structure are found in pectins, where the reducing terminus of xylogalacturonan or RG' can be glycosidically linked to the non-reducing end of homogalacturonan. It is likely that pectins in muro have structures such as
where ^ denotes a glycosidic bond, HGA is homogalacturonan and XGA is xylogalacturonan (Ishii & Matsunaga 2001; Coenen et al. 2007).
In suspension-cultured angiosperm cells, roughly half the xyloglucan is covalently bonded to acidic pectic domains, probably RG-I (Thompson & Fry 2000; Popper & Fry 2005). The precise chemistry of this xyloglucan-pectin bond is still uncertain; it is stable to prolonged treatment in concentrated
NaOH and urea and a tenable model is a glycosidic bond between the reducing end of the xyloglucan and the Ara/Gal;rich side chains of RG-i . The xyloglucan-RG-I association is most easily demonstrated by ion-exchange chromatography or high-voltage electrophoresis on glass-fibre 'paper': despite angiosperm xyloglucans containing no acidic residues, 44-75% of the 6 M NaOH-extractable xyloglucan (recognized by its conversion to isopri-meverose by Driselase) binds to an anion-exchange column (Popper & Fry 2005) or migrates towards the anode (Thompson & Fry 2000). The formation of these xyloglucan-RG-I bonds has been shown by in-vivo pulse-radiolabelling to occur cosynthetically (probably within the Golgi system), rather than by heterotransglycosylation in the wall (Popper & Fry 2008).
Hrmova et al. (2007) have shown that a barley XTH can catalyse the formation of an MLG^xyloglucan bond by using MLG as donor substrate and a xyloglucan oligosaccharide (e.g. XXXGol) as acceptor substrate in a hetero-transglycosylation reaction:
... GGGGGGGGGGGGGGGGGGGGG... + XXXGol
where ...GGGGGG... is MLG, ^ isa glycosidic bond, and ^ indicates the direction of the enzymic reaction. However, the barley enzyme catalysed this reaction at approximately 0.2% of the rate of the classic XET reaction (where the donor is a xyloglucan). This is slower than with any of various artificial substrates, e.g. hydroxyethylcellulose or cellulose sulphate; the biological significance of the heterotransglycosylation reaction therefore remains unclear.
Recently, an enzyme activity was detected in Equisetum for which the formation of MLG^xyloglucan bonds is the preferred reaction, well exceeding the classic XET reaction rate (Fry et al. 2008a). Thus, at least in Equisetum, the formation of interpolysaccharide glycosidic bonds may be a major in-vivo reaction.
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