There are two forms of polygalacturonases in peaches: the exo- and endopolygalacturonases.
The endopolygalacturonases (endo-PG) are the predominant forms in the freestone
type of peaches, whereas the exopolygalacturonases (exo-PG) are observed in the mesocarp
of both freestone and clingstone varieties of peaches. As the name implies, exopolygalacturonases
remove galacturonic acid moieties of pectin from the terminal reducing end of the chain, whereas the endopolygalacturonases can cleave the pectin chain at random within
the chain. The activities of these enzymes increase during the ripening and softening of
the fruit. Two exo-PG isozymes have been identified in peach, having a relative molecular
mass of near 66 kDa. The exo-acting enzymes are activated by calcium. Peach endo-PG is
observed to be similar to the tomato endo-PG. The peach endo-PG is inhibited by calcium.
The freestone peaches possess enhanced activities of both exo-PG and endo-PG leading to a
high degree of fruit softening. However, the clingstone varieties with low levels of endo-PG
activity do not soften as the freestone varieties. In general, fruits such as peaches, tomatoes,
strawberries, and pears, which soften extensively, possess high levels of endo-PG activity.
Apple fruits, which remain firm, lack endo-PG activity.