Behavior Of Pectin Methylesterases In Pectic Gels

Pectic gel has been used as a simplified solid matrix to study the behaviour of two pectin methylesterases (PMEs) - one from Aspergillus aculeatus and one from orange. Gel was obtained during demethoxylation of a high methoxy lime pectin with a initial degree of methylation of 71 by PME action in the presence of calcium ions. Orange PME action led to moderate decrease of the degree of methylation and rheological measurements showed the formation of strong gels even when the degree of methylation was around 70. On the opposite, fungal PME treatment led to fast decrease of the degree of methylation and, this decrease induced a gel appearance only at the highest activities or for prolonged time, when DM decreased below 20. Degree of blockiness determination showed the presence of block structures of free galacturonic acid residues after treatment by orange PME whereas it was characterized with a rather random pattern after treatment by fungal PME. To explain these results, behaviour of the two PMEs was compared in gel (i.e. in the presence of calcium) and solution (i.e. in the absence of calcium). Fungal PME treated-pectins have similar DM in the two conditions. However in the gel, pectins showed higher degree of blockiness, suggesting a less random mode of action of the enzyme. For the plant PME treated-pectins calcium affected not only the degree of blockiness but also the degree of methylation. Indeed in presence of calcium, plant PME treatment led to a lower decrease of degree of methylation but this was accompanied with a more blockwise mode of action since the degree of blockiness was higher in the presence of Ca than in the absence. Possible explanations for calcium-dependent mode of action of PME are changes in visco-elastic properties of the medium, gel formation or influence of Ca on the enzyme-substrate interactions.