BIOSYNTHESIS OF ARABINOXYLANS

7.5 BIOSYNTHESIS OF ARABINOXYLANS

Relatively little is known about the exact mechanism of arabinoxylan synthesis. Like other polysaccharides, arabinoxylans are products of synthases and glycosyl

transferases and, as such, are secondary gene products. 96 Arabinoxylans and other cell wall polysaccharides (except cellulose) are synthesized within the cell in the Golgi apparatus and endoplasmic reticulum. 97 The immediate donors of monosac- charides for synthesis of arabinoxylans are UDP-D-Xylp and UDP-L-Araf, formed from UDP-D-Glcp by the action of appropriate epimerases. 61 Some generalizations can be made concerning the mechanism of arabinoxylan polymerization (Figure 7.10). It is believed that distinct glycosyl transferases are necessary for each of the different monosaccharide and glycosidic linkages existing in arabinoxylan chains. The process of polymerization can be divided into three steps: chain initiation, elongation, and termination. Available evidence suggests that the first sugar donation is not to a free monosaccharide, but to a protein or lipid primer. Tailward growth, i.e., addition of the new residue to the nonreducing end of the chain, has been generally accepted as the direction of chain elongation. Recently,

a β-(1→4)-xylosyl transferase, isolated from the microsomal membranes of the developing barley endosperm, has been shown to transfer xylose from uridine 5'- diphosphoxylose (UDP-Xyl) into an exogenous xylooligosaccharide chain (deriva-

tized at the reducing end). 98 Repeated attachment of xylose residues occurred at the nonreducing end of the pyridylaminated-xylotriose chain through β-(1→4) linkages. During the stepwise addition of the xylose residues to the growing polymer chain, a stage must be reached that involves the addition of the branching

points. Based on the in vivo studies on xyloglucans and glucuronoxylans, 99 it is assumed that arabinose residues are incorporated simultaneously with the poly- merization of the xylan backbone. However, it is not clear whether separate arabinosyl transferases are required for substitution of arabinose at C-2 and C-3

Arabinoxylans

Cinn Cou Cou-CoA

Cell wall Caff-CoA

cisterna(e) F F A, A

A A H 2 O 2 A 2 Vesicular

H 2 O 2 Arabino- furanosidase

UDP F UDP-Ara

A AAA × ××××

× ×××× Ara-1-P

Ara

Addition of Araf Cross-linking of AX Removal of

chains

Araf

FIGURE 7.10 Biosynthesis of arabinoxylans. X, xylose residues; A, arabinose residues; F, ferulic acid residue; F2, diferulic acid; Cinn, cinnamic acid; Cou, coumaric acid; caff, caffeic acid; Fer, ferulic acid. (Adapted from Fry, S.C. et al., Planta, 211, 679, 2000. With permission.)

of the xylose residues. The attachment of feruloyl groups to arabinoxylans may occur by transacylation, and the polysaccharides are feruloylated co-instanta- neously with the polymerization processes within the endomembrane system. 61,97

Fry and coworkers 61 showed that in maize cell cultures, the coupling of the feruloyl groups, leading to cross-linking of arabinoxylans, can occur within 1 min of the attachment of the feruloyl group to the polymer chain. However, cross-linking of the feruloylated arabinoxylans can also occur after their deposition in the cell walls. It is also thought that when arabinoxylans are initially deposited into walls, the xylan backbone is heavily substituted with arabinosyl residues. Subsequently, arabinosyl residues are removed by the action of arabinofuranohydrolases. 100 These postdeposition processes, debranching and cross-linking, lead to changes in phys- icochemical properties of arabinoxylans, such as solubility or capability to interact with other cell wall polysaccharides, thereby allowing the plant to control the tissue cohesion, cell expansion, and permeability of the cell walls to metabolites and pathogens.

The mechanism of chain termination, which directly controls the length of arabinoxylan chains, is the least known. It has been suggested that the rates of vesicle movement and fusion with plasma membrane play some role in determining

the degree of polymerization (DP) of cell wall polysaccharides; 97 however, no evidence exists to support this proposition. Because of the numerous and complex events involved in biosynthesis of arabinoxylans, the formation of these polymers is not strictly regulated and may depend on several factors. As a result, arabinox- ylans show a high degree of microheterogeneity and belong to the class of poly- dispersed polysaccharides. Their polydispersity can be reflected in the degree of

Functional Food Carbohydrates

polymerization of individual chains, in the abundance, distribution, and DPs of side-chain substituents, and in the degree of feruloylation and cross-linking.