Valencia pectinmethylesterase isozymes result in pectins of unique charge domain and functionality
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Based on the hypothesis that PME fraction containing 36/27 kDa peptides (U-PME) will yield differently modified pectin than PME fraction containing 36/13 kDa peptides (B-PME), the objective of this study was to isolate the Valencia PME isozymes, de-esterify pectin to a target DE and characterize the resultant pectin products for charge and charge distribution. In addition, the calcium sensitivity was estimated by viscosity and gelling properties with CaCl2, as well by ?-potential, an indicator of the surface charge. Finally, interactions of individual caseins with modified pectins was compared by sedimentation, protein/pectin content, particle size and ? - potential. Valencia PMEs de-esterified pectin to 63% (B-Pec) and 61%DE (U-Pec) from 73%DE (O-Pec), did not decrease molecular weight, created more negative ?-potential, and had different charge distributions. Based on elution from ion exchange chromatography (IEX), chemical shifts in NMR spectra, and ?-potential, we observed a blockwise de-esterification pattern following a 10% decrease in DE. From elution profile of IEX, the peak of B-Pec and U-Pec widened and shifted to a higher ionic strength compared to O-Pec. Finally, we concluded that B-PME and U-PME had different action patterns based on the 2-fold increase in the frequency of contiguous carboxylic acid groups (FGGG ). Also, we concluded that U-Pec had less contiguous blocks of carboxylic acid groups than B-Pec but a greater population of pectin molecules were modified by U-PME. In the presence of 35 mM CaCl2, 2% B-Pec and U-Pec formed a gel, in contrast to O-Pec. B-Pec and U-Pec were 20 or 50-fold higher G’ (elastic element) than O-Pec with CaCl2 while OPec had higher viscosity than B-Pec or U-Pec without CaCl2. From the interaction of individual caseins with pectins, addition of pectin to the milk fractions had unique effect and ranged from increasing the extent of sedimentation to minimizing sedimentation at pH 3.8. ?-Casein had no precipitate initially, but addition of pectin resulted in sedimentation, and U-Pec resulted in the greatest precipitate. Based on particle size, ?-potential, and viscosity, U-Pec or B-Pec had more effect on milk dispersions than O-Pec. ?-Casein seemed to interact with pectins more like ?-casein than ?S1,2-casein.