Nanostructural modification of a model homogalacturonan with a novel pectin methylesterase: Effects of pH on nanostructure, enzyme mode of action and substrate functionality

A Carica papaya pectin methylesterase (CpL-PME) present in a commercial papain preparation was used to modify the amount and distribution of anionic charge in a population of a model pectic homogalacturonan (HG) at pH 4.5 and pH 7.5. Introduced negatively charged demethylesterified blocks (DMB) were excised as oligomers with limited endo-polygalacturonase (EPG) digestion, and then separated and quantified by high performance anion exchange chromatography. Exhaustive EPG digestion was also performed and the concentrations of mono-, di- and tri-galacturonic acid (GalA) produced were estimated. The CpL-PME mode of action was modeled while the HG populationʹs nanostructure and the resulting functionality of the modified pectin were characterized. The results indicated that average DMB size and number of these “average” blocks per molecule increased as the overall degree of methylesterification (DM) decreased by CpL-PME treatment (P < 0.05). Average DMB size differed depending on the reaction pH at each DM. Absolute degree of blockiness (DBabs) increased as DM decreased and demonstrated a linear relationship with the DM at both reaction pH values. Significant positive correlations between DBabs and average DMB size were observed for both pH series (P < 0.05). The enzyme mode of action, modeled in silico, demonstrates a processive multiple attack mechanism. Functionally, the storage modulus of ionotropic gels formed from the processed substrates increased as DM decreased, and as DBabs and average DMB size increased with highly significant correlations (P < 0.001). These results indicate it is feasible to engineer pectin nanostructure and functionality with CpL-PME.