Structure of a Rhamnogalacturonan Fragment from Apple Pectin: Implications for Pectin Architecture

Abstract

A commercial apple pectin was sequentially digested with the cloned enzymes endopolygalacturonase, galactanase, arabinofuranosidase, xylogalacturonase, and rhamnogalacturonan hydrolase. The rhamnogalacturonan hydrolase-generated oligosaccharides were separated by ultrafiltration, anion exchange, and size-exclusion chromatography. Fractions from the ion exchange chromatography were pooled, lyophilized, and screened by MALDI-TOF MS. An oligosaccharide (RGP14P3) was identified and its structure, α -D-Gal p A- ( 1 → 2 ) - α -L-Rha p - ( 1 → 4 ) - α -D-Gal p A- ( 1 → 2 ) - α -L-Rha p - ( 1 → 4 ) - α -D-Gal p A, determined by 1D and 2D NMR spectrometry. This oligosaccharide probably represents a direct connection between homogalacturonan and rhamnogalacturonan in pectin. Alternatively, it could indicate that the nonreducing end of rhamnogalacturonan starts with a galacturonic acid residue.