Demethylation of a model homogalacturonan with the salt-independent pectin methylesterase from citrus: II. structure function analysis

Authors: Luzio, Gary; Cameron, Randall - Randy

Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2007
Publication Date: 1/2/2008
Citation: Luzio, G.A., Cameron, R.G. 2008. Demethylation of a model homogalacturonan with the salt-independent pectin methylesterase from citrus: Part II. Structure-function analysis. Carbohydrate Polymers. 71:300-309.

Interpretive Summary: Citrus peel is largely composed of pectin, a water soluble polysaccharide. Polysaccharides are typically sold based on their rheological properties such as viscosity. With pectin, rheology can be controlled by the manner or patterns in which this polysaccharide is deesterified using enzymes or chemical conditions. The difficulty is the lack of assay that can reveal these patterns of deesterification. In a jointly issued publication a procedure was published that can quantify these structures. In the publication discussed here these patterns are matched up to the functional or rheological properties of the pectin. This data has revealed new insights into how the pectins can crosslink with calcium to product unique products such as suspension aids.

Technical Abstract: A non-calcium sensitive pectin, with a degree of esterification (DE) of 94%, was demethylated at pH 4.5 and 7.5 with a monocomponent preparation of a pectin methylesterase isolated from citrus fruit tissue. Endo-polygalacturonase (EPG) was used to digest the pectin samples and to release block regions. In a co-publication, block size and number for pectins at each particular DE was characterized. Herein, rheology and calcium sensitivity measurements on these pectins indicate that both block number and block size need to be considered for crosslinking with calcium ion. For pectin deesterified to a DE of 55 at pH 4.5, greater than 95% of the molecules crosslinked with calcium ion with a yeild point of 0.381 Pa. For this sample the average block size was 9 and block number was 4. For the same pectin deesterified to a DE of 61 at pH 7.5, 45.6% of the molecules crosslinked with calcium ion with a YP of 0.248 Pa. For this sample the average block size was 10 and block number was 2. Competition experiments indicated that even small oligomer GA fragments with DPs of less than 10 can interfere with crosslinking which suggests that all block sizes greater than a DP of approximately 8 to 10 need to be considered when matching pectin block structure to functional properties.