Food Biochemistry and Food Processing (2 edition)

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12 Pectic Enzymes in Tomatoes 233

Key:

Cellulose microfibril

Xyloglucans

Extensin

Arabinosides

RG I

with arabinogalactans

Junction zone

Pectins

Arabinose-rich runs

that open pores

Glucuronoarabinoxylan

Region of

H-bonding

β-Glucans

Cellotriosyl- and

cellotetraosyl-rich

Longer

cellodextrins

H-bonding

Phenolic

network

Ca^2 ++-linked

RG I

with arabinans

Figure 12.1.Current structural model of cell walls of flowering plants. (Reprinted from Buchanan et al. 2000).

be identified (Fig. 12.1), the cellulose–matrix glycan network,

consisting of cellulose microfibrils held together by matrix gly-

cans (formerly referred to as hemicelluloses), and the pectin

network (Carpita and Gibeaut 1993). The way by which these

polymers interact with each other is still unclear. Several cell

wall models are proposed in an attempt to explain measured

physical properties of the wall (Cosgrove 2000, Vincken et al.

2003, Thompson 2005). The cell wall provides shape and struc-

tural integrity to the cell, functions as a protective barrier to

pathogen invasion and environmental stress, and provides sig-

nal molecules important in cell-to-cell signaling, plant–symbiot,

and plant–pathogen interactions.

Pectin is an elaborate network of highly hydrated polysac-

charides rich in galacturonic acid (GalA). Pectins can be

classified in four major groups: homogalacturonan (HGA),

rhamnogalacturonan I (RGI), rhamnogalacturonan II (RGII),

and xylogalacturonan (XGA; Mohnen 2008). HGA is a ho-

mopolymer of 1,4-α-d-GalA (Fig. 12.2A) usually forming long

chains of approximately 100 GalA residues. Galacturonosyl

residues are esterified to various extents at the carboxyl group

with methanol and may be acetylated atO-2 orO-3. Non-

esterified regions of HGA chains can be associated with each

other by ionic interactions via calcium ion bridges, forming

egg-box-like structures (Fig. 12.2A). At least ten contiguous

unesterified galacturonyl residues are required in order to form

a stable cross link between HGA chains (Liners et al. 1992).

RGI is composed of repeating disaccharide units of 1,2-α-d-

rhamnose–1,4-α-d-GalA. Rhamnosyl residues on the backbone

of this polymer can be further substituted withβ-d-galactose-

andα-l-arabinose-rich side chains (Fig. 12.2B). RGII is a minor