Food Biochemistry and Food Processing

12 Pectic Enzymes in Tomatoes 273

homogalacturonan (HGA), rhamnogalacturonan I
(RGI), and rhamnogalacturonan II (RGII). Homo-
galacturonan is a homopolymer of (1 → 4) -D-
galacturonic acid (Fig. 12.2A). Galacturonosyl res-
idues are esterified to various extents at the carboxyl
group with methanol. Nonesterified 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). Rhamnogal-
acturonan I is composed of repeating disaccharide
units of (1 → 2)--D-rhamnose –(1 → 4)--D-
galacturonic acid. Rhamnosyl residues on the back-
bone of this polymer can be further substituted with

-D-galactose- and -L-arabinose-rich side chains
(Fig. 12.2B). Rhamnogalacturonan II (RGII) is a
minor constituent of the cell wall consisting of an
HGA backbone that exhibits a complex substitution
pattern with many diverse sugars, including the
unique sugar aceric acid. RGII can form dimers via
borate diesters at the apiose residues. Pectins are

covalently interlinked, in part via diferulic acid, and

further cross-linked to cellulose, matrix glycans, and

other cell-wall polymers (Thompson and Fry 2000).

Pectin often consists more than 50% of the fruit

cell wall. During ripening, pectin is extensively mod-

ified by de novo synthesized enzymes that bring

about fruit softening. The pectin backbone is de-

esterified, allowing calcium associations and gelling

to occur (Blumer et al. 2000). Pectin is solubilized

from the wall complex, followed by a decrease in

the average polymer size (Brummell and Labavitch

1997, Chun and Huber 1997). These changes are

accompanied by loss of galactosyl and arabinosyl

moieties from pectin side chains (Gross 1984).

FRACTIONATION OFPECTICSUBSTANCES

Methods for analyzing cell-wall polymeric sub-

stances are based on deconstruction of the cell wall

by sequentially extracting its components. Tomato

Figure 12.2. A.Structure of homogalacturonan showing calcium ion associations. B.Structure of rhamnogalacturo-
nan I, with 5-arabinan (A),4-galactan (B),and type I arabinogalactan (C)side chains. (Reprinted from Buchanan et
al. 2000.)
(Continues)