Galacturonan from plant cell-walls

Pectic polysaccharides from plant cell walls are rarely simple (1 4)-linked a-D-galacturonans but contain varying proportions of neutral monosaccharides. L-Rhamnose is the only neutral sugar present in the... 

Pectins are probably the most complex polysaccharides known, in terms of their chemistry and are certainly so in terms of their biosynthesis. Classically they were regarded as al,4-galacturonans, with various degrees of methyl esterification, and the terms pectic acid and pectinic acid referred to the non-esterified and partially esterified forms respectively. A third term protopectin , was used of insoluble pectin that could not be extracted from plant cell walls by hot solutions of chelating agents. It was considered that these three classes of pectin constituted a pectic triad . This view is now known to be erroneous, but it is still frequently put forward, especially in botanical texts. Consequently any discussion of the synthesis of pectins must be prefaced by a description of their chemistry, as it is now understood. 

The plant cell wall biology studies require more sensitive and specific probes to target individual wall components. Traditionally, antibodies have been the primary workhorses for the spatial localization of cell wall polysaccharides. Currently, nearly 30 monoclonal antibodies directed toward specific arabinan, galactan, xylan, galacturonan, fucosylated xyloglucan, and cell wall glycoprotein epitopes are available, from academic and commercial sources [172], Nevertheless, CBMs may be used for this purpose, since they present intrinsic specificity for individual carbohydrates. 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

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