Partially esterified oligogalacturonides are the preferred substrates for pectin methylesterase of Aspergillus niger

Investigations on the mode of action of Aspergillus niger pectin methylesterase (PME) towards differently C6- and C1-substituted oligogalacturonides (oligoGalpA) are described. De-esterification of methyl-esterified (un)saturated oligoGalpA proceeds via a specific pattern, depending on the degree of polymerization. Initially, a first methyl ester of the oligomer is hydrolysed, resulting in one free carboxyl group. Subsequently, this first product is preferred as a substrate and is de-esterified for a second time. This product is then accumulated and hereafter de-esterified further to the final product, i.e. oligoGalpA containing one methyl ester located at the non-reducing end residue for both saturated and unsaturated oligoGalpA, as found by post-source decay matrix-assisted laser-desorption/ionization–time-of-flight MS. The saturated hexamer is an exception to this: three methyl esters are removed very rapidly, instead of two methyl esters. When unsaturated oligoGalpA were used, the formation of the end product differed slightly, suggesting that the unsaturated bond at the non-reducing end influences the de-esterification process. In vivo, PME prefers methyl esters, but the enzyme appeared to be tolerant for other C6- and C1-substituents. Changing the type of ester (ethyl esterification) or addition of a methyl glycoside (C1) only reduced the activity or had no effect respectively. The specific product pattern was identical for all methyl- and ethyl-esterified oligoGalpA and methyl-glycosidated oligoGalpA, which strongly indicates that one or perhaps two non-esterified oligoGalpA are preferred in the active-site cleft.