RhamnogalacturonanII domains

RG-II is a small but exceedingly complex pectic domain. It has a backbone of at least eight a-GalA residues, to which are attached five different types of side chain: the first four are designated A-D, each with a precisely (not stochastically) defined primary structure (O'Neill et al. 2004). The side chains typically have the following composition:

A (octasaccharide), a-l-Gal, P-GlcA, a-MeXyl, a-Fuc, P-Rha, a-GalA, p-GalA, p-Api(®)

B (octa- or nonasaccharide), P-Ara, a-Rha (x1 or x2), a-Arap, P-Gal, a-MeFuc, a-AceA, P-Rha, P-Api(®) C (disaccharide), a-Rha, a(?)-Kdo(®) D (disaccharide), P-Ara, P-Dha(®).

where (®) indicates the residue attached to the oligo-GalA backbone.

All four of these side chains are themselves acidic (unlike those of RG-I) and contain very unusual residues, including some known only from RG-II. The fifth side chain is a single a-Ara residue (Melton et al. 1986). Side chain B has acetyl groups on the AceA and MeFuc residues (O'Neill et al. 2004). There are no reports of methyl esters in RG-II. If RG-II has one copy of each side chain, it is DP 30 (~5 kDa). The dimerization of RG-II by borate crosslinks is discussed later.

The linkages between the side-chains and RG-II's backbone are unusually acid-labile, especially the Api^GalA* and Kdo^GalA bonds; the side chains can therefore be pruned off the backbone by warm dilute acid. However, RG- I I is largely resistant to Driselase, which provides a useful method for its purification.

A convenient source from which to purify RG--I is red wine, since the yeasts used in its manufacture cannot hydrolyse grape RG-II (O'Neill et al. 2004 ).

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