Different forms of tomato pectinesterase have different kinetic properties

The kinetic properties of the three main forms (A-C) of pectinesterase (PE) found in pericarp of the tomato variety Ailsa Craig were investigated. Differences in the effects of salt on the three forms were correlated with the degree of charge carried by each form, as reflected in their isoelectric points (A:8.9, B:9.7 and C:9.9). All three forms showed Michaelis-Menten kinetics, with K m varying with salt concentration and degree of esterification (DE) of the substrate. PE-C generally exhibited lower values for K m than PE-A, and was more effective at de-esterifying pectin with low DE. A large difference was seen in the K i for inhibition by polygalacturonate (A:11.0 mM, B:1.2 mM, C:0.7 mM). Calcium was particularly effective in activating PE-C. PE-C was completely inactive once the pH was reduced to 5.0, while PE-A maintained more than 60% of maximum activity at this pH. These results suggest that the different kinetic properties of the three forms of PE reflect different physiological functions. PE-A should be able to continue working during the processes of disruption and degradation that typify fruit ripening, while the other forms (particularly PE-C) have their activity under continuous fine control by local ionic and pH conditions in the wall, and are more likely candidates for a role in the modulation of cell wall growth.