Structural Deterioration of the Produce: The Breakdown of Cell Wall Components - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 170

modifications of cell wall polysaccharides (Alba et al., 2005; Handa et al., 2007). For ex-
ample, the expression of LePG in ripening tomato is accompanied by increases in other
proteins whose functions are to depolymerize or modify polysaccharides in cell wall, such
as pectin methylesterase (PME) (Harriman et al., 1991), endo-1,4, -glucanase (Lashbrook

et al., 1994), xyloglucan endo transglycosylases (Arrowsmith and de Silva, 1995),

β

-
galactosidases (Smith et al., 1998; Carey et al., 2001), and pectate lyases (Marin-Rodriguez
et al., 2002).
Pectin modification provides an excellent example of the modification of polysaccha-
ride domains. A number of enzymes modify or degrade pectin including enzymes that act
on pectins such as exo- and endo-acting PGs, PMEs,

β

-galactosidases, and pectate lyases
(PLs) (Table 8.2). Most of these enzymes exist in multigene families with a subset of one
or more gene family members regulating the cell wall modification processes associated
with fruit ripening. Complex interactions between gene family members of each enzyme
class and between different enzymes may also occur. A well-characterized cooperative re-
lationship exists between PG and PME. PME plays critical roles in establishing structurally
and functionally distinct classes of pectin, important in many stages of plant growth and
development (Ridley et al., 2001; Willats et al., 2001a).
Pectin is the most abundant class of macromolecules within the cell wall matrix and in
the middle lamella between primary cell walls. During fruit softening, pectin typically un-
dergoes solubilization and depolymerization processes that are thought to contribute to wall
loosening and disintegration. PG, which catalyzes the hydrolytic cleavage of galacturonan
linkage of pectin, is the most abundant pectin-degrading enzyme whose action coincides

β

Pectin-modifying and degrading enzymes in fruits

Table 8.2

Substrate Enzymes Products

Pectin Pectin methyl esterase Pectic acid + methanol

Endopolymethyl galacturonase Methyl oligogalacturonides

Rhamnogalacturonase α -(1,2)Linked L -Rha, α -(1,4)

linked D -Gal

Endopectin lyases Unsaturated oligogalacturonides

Hairy pectin Rhamnogalacturonan acetylesterase Pectin + acetic acid

Pectin acetyl esterase Pectin + acetic acid

Smooth pectins Lyases Oligogalacturonides

Protopectin Protopectinase Pectin

Pectic acid Endo-PG Oligogalacturonides

Exo-PG Monogalacturonides

Endopectate lyases Oligogalacturonides

Exopectate lyases Unsaturated digalacturonides

Trigalacturonic acid Oligogalacturonide hydrolase Monogalacturonides

4:5 (galacturonide) n 4:5 unsaturated

oligogalacturonide hydrolase

Unsaturated monogalacturonide +

galacturonides ( n − 1)

Unsaturated digalacturonate Oligogalacturonide lyases Unsaturated monogalacturonides

α α

Arabinans - L -Arabinofuranosidase - L -Arabinose

α

(1,5)- -Arabinans Endoarabinanase Arabinose and higher

oligosaccharides

Galactans β - D -Galactanase β - D -Galactose