Pectinase (PG2) Enzyme

Function: 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. Ref.13 Ref.15 Ref.18

Catalytic activity: Random hydrolysis of (1->4)-alpha-D-galactosiduronic linkages in pectate and other galacturonans. Ref.19

Subunit structure: Monomer PG2 (isoenzymes PG2A and PG2B). Also forms heterodimers called polygalacturonase 1 (PG1) with the beta subunit GP1. Ref.9 Ref.10 Ref.17

Subcellular location: Secreted › extracellular space › apoplast. Secreted › cell wall. Note: Associated to the cell wall. Ref.13

Tissue specificity: Expressed only in ripening fruits (at protein level). Ref.2 Ref.4 Ref.16 Ref.17

Developmental stage: 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. Ref.9 Ref.15 Ref.16

Induction: By ethylene. Ref.16

Post-translational modification: N-glycosylated. PG2B isozyme has a greater degree of glycosylation than PG2A. Ref.6 Ref.11 Ref.13

Biotechnological use: 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.

Miscellaneous: To avoid liquid rheology of tomato juice, temperature and pressure can be increased to inactivate selectively PG2 during the process.

Sequence similarities: Belongs to the glycosyl hydrolase 28 family.Contains 4 PbH1 repeats.

Biophysicochemical propertiesAbsorption:Abs(max)=276 nm Ref.9 Ref.11 Ref.12 Ref.20The ratio of Abs(276)/Abs(260)=1.35. These values are for PG1.Kinetic parameters:KM=38 µM for polygalacturonic acid (for PG2 at pH 4.6 and 35 degrees Celsius)KM=75 µM for polygalacturonic acid (for PG1 at pH 4.6 and 35 degrees Celsius)Vmax=58.8 µmol/min/mg enzyme (for PG2 at pH 4.6 and 35 degrees Celsius)Vmax=7 µmol/min/mg enzyme (for PG2 at pH 3.8 and 25 degrees Celsius)Vmax=27.7 µmol/min/mg enzyme (for PG1 at pH 4.6 and 35 degrees Celsius)Vmax=4 µmol/min/mg enzyme (for PG1 at pH 3.8 and 25 degrees Celsius)pH dependence: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.Temperature dependence:Optimum temperature is 55-60 degrees Celsius at pH 4.4. PG1 is more thermostable than PG2.

Sequence caution: The sequence CAD44521.1 differs from that shown. Reason: Frameshift at positions 156 and 161.