Homogalacturonan d omains

These quantitatively major domains comprise an unbranched chain of anionic a-GalA and uncharged MeGalA (= methyl ester of a-GalA) residues joined by (1^4)-bonds. The MeGalA residues tend to occur contiguously, as neutral blocks, along the homogalacturonan chain; this arrangement probably arises by the progressive action of pectin methylesterase on a more fully methylesterified precursor chain. The backbone also includes some 2AcGalA and/or 3AcGalA residues, which may affect the polysaccharide's solubility but not its charge. After de-esterification, e.g. by ice-cold dilute alkali, homogalacturonan can be cleaved by endo-polygalacturonase (EPG) to small fragments with degree of polymerization (DP) 1-3, which serve as a simple quantitative assay for this domain.

Homogalacturonans are water soluble at neutral and alkaline pH, but insoluble at mildly acidic pH (especially after removal of any methyl ester groups) and in the presence of Ca2+.

De - esterified homogalacturonans are digested by 'Driselase' (a mixture of numerous endo- and exo-hydrolases from the fungus Irpex lacteus; Fry 2000), giving a quantitative yield of free GalA. Driselase is preferred over acid hydrolysis for this purpose because (1) uronosyl linkages are only slowly acid-hydrolysed, and (2) the free GalA liberated is considerably less stable in hot acid than naturally occurring neutral monosaccharides, so recovery is low. Driselase also contains esterases that convert MeGalA to GalA unless a nearby O-acetyl group interferes. Driselase does not remove acetyl groups from homogalacturonan, and can thus yield structurally informative oligo-saccharides containing AcGalA residue(s) (Perrone et al. 2002 ).

Commercial homogalacturonan ('polygalacturonic acid') is a useful model, e.g. for practising analyses, but the commercial material is of much lower Mr than natural homogalacturonan.

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