In cell walls, proteins and polysaccharides may interact both non-covalently and covalently. Thus non-covalent associations between positively charged lysine and histidine residues on HRGP and negatively charged non-cellulosic polysaccharides, e. g., pectins and GAX have been postulated (122). Covalent intermolecular isotyrosine (phenyl ether) bridges between HRGPs have been proposed to account for the ability of acidified chlorite (123) to allow solubilization of the HRGP by cleavage of these linkages, but have not been identified, however, intramolecular isotyrosine linkages are found in HRGPs.

GAX in secondary walls of grasses may also be covalently linked to protein though dimer­ization of FA on GAX to tyrosine on proteins as proposed by Geissmann and Neukom (124) (Figure 4.3c). Such a mixed dimer cross-link has been isolated from an endosperm prolamin-AX complex formed during bread making (125) and has been proposed to occur in walls of lignified pericarp cells in wheat bran (126). In aleurone walls from wheat bran, a fraction that resists digestion by hydrolases for AX and (1—>3, 1—4)-p-D-glucan, con­tains a highly branched AX that appears to be linked to a protein, supposedly through an FA-tyrosine bridge (75).