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The high catalytic potential of PG evident in reactions with soluble pectic polymers is typically not expressed in vivo. In this study, the binding and catalytic properties of PG isozyme 2, and the influence of the  B -subunit protein, were investigated using cell walls prepared from tomato fruit expressing the  B -subunit antisense gene. Cell walls were prepared from mature-green fruit and treated to remove/inactivate endogenous enzymes. Walls were then preloaded with rate-limiting quantities of purified PG 2, and incubated under catalysis-promoting conditions over the range of pH from 4.5 to 6.0. Cell walls of both  B -subunit antisense and wild-type fruit retained comparable quantities of loaded PG 2. The enzymic release of pectin from PG-loaded walls was proportional to the quantity of wall-bound PG 2. In walls lacking the  B -subunit protein, the quantity of pectin released by a given dose of wall-associated PG was as much as 2-fold higher than from wild-type walls. The  B -subunit protein also influenced the extent of pectin depolymerization during ripening. The release of pectin from cell walls during periods of catalysis was not the sole indicator of the range of pectins hydrolyzed. Treating postcatalytic loaded cell walls to inactivate PG, and subsequent extraction of residual wall pectins using 50 m m  CDTA solutions solubilized polymers of significantly lower mol mass compared with pectins solubilized directly from nonloaded cell walls.

Polygalacturonase Isozyme 2 Binding and Catalysis in Cell Walls From Tomato Fruit: Control and B-subunit Effects