Catalytic subunit of the polygalacturonase isozyme 1 and 2 (PG1 and PG2). Acts in concert with the pectinesterase, in the ripening process. Is involved in cell wall metabolism, specifically in polyuronide degradation. The depolymerization and solubilization of cell wall polyuronides mediated by PG2 during ripening seems to be limited by the beta subunit GP1, probably by recruiting PG2 to form PG1. (1,4-alpha-D-galacturonosyl)n+m + H2O = (1,4-alpha-D-galacturonosyl)n + (1,4-alpha-D-galacturonosyl)m. The ratio of Abs(276)/Abs(260)=1.35. These values are for PG1. Optimum pH is 4.4-4.8 at 35 degrees Celsius. PG1 is resistant to acidic but not to alkaline conditions, at which PG2 is released from the beta subunit. Optimum temperature is 55-60 degrees Celsius at pH 4.4. PG1 is more thermostable than PG2. Monomer PG2 (isoenzymes PG2A and PG2B). Also forms heterodimers called polygalacturonase 1 (PG1) with the beta subunit GP1. Associated to the cell wall. Expressed only in ripening fruits (at protein level). PG1 appears when fruits start to be coloured. When fruits are orange, both PG2 and PG1 are present. In fully ripe fruit, mostly PG2 is expressed. By ethylene. N-glycosylated. PG2B isozyme has a greater degree of glycosylation than PG2A. The effect of PG can be neutralized by introducing an antisense PG gene by genetic manipulation. The Flavr Savr tomato produced by Calgene (Monsanto) in such a manner has a longer shelf life due to delayed ripening. To avoid liquid rheology of tomato juice, temperature and pressure can be increased to inactivate selectively PG2 during the process. Belongs to the glycosyl hydrolase 28 family.