Galacturonans

Pectic enzymes are classified according to their mode of attack on the galacturonan part of... 

Pectins are heteropolysaccharides with axial-axial a(1, 4)-D galactopyranosyluronic acid units condensed in the Ci conformation and interrupted by a-(1,2)-L rhamnopyranosyl residues. The structure is based on blocks of galacturonan partly esterified (the smooth zones) and blocks of highly ramified rhamnogalacturonan regions (hairy zones). The exact structure depends on the sources but also on the methods used to isolate the pectins [1,5]. Few data are given in Table 1. 

From disaccharide analysis, few authors predict the conformation of the poly a-D galacturonan as well as the role of the charge density [35,36] they determine the persistence length Ip that will allow us to explain the behaviour in solution. 

In this review we fitcus on the biosynthetic mechanisms by vririch the methyl-esterification of the galacturonan chains in pectins is controlled. We go on to examine the structure of the pectic gels that are formed and their relationsh with the stresses carried by specific parts of the cell wall. 

The data reported in table 1 show that the pectin fi action in active cells is rich in methyl-esterified galacturonans, whereas the resting cells contain mostly low-ester acidic pectins. 

Figure 7. The egg-box structure for the junction zones of dilute calcium pectate gels two galacturonan chains in the twofold (2i) helical conformation with calcium ions (shaded circles) locked between them.

Figure 8. The Walkinshaw Amott model [46] for solid calcium pectate, with the galacturonan chains in the right-handed (ii) helical conformation.

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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