Coffee bean arabinogalactans: acidic polymers covalently linked to protein

The arabinogalactan content of green coffee beans (Coffea arabica var. Yellow Caturra) was released by a combination of chemical extraction and enzymatic hydrolysis of the mannan-cellulose component of the wall. Several arabinogalactan fractions were isolated, purified by gel-permeation and ion-exchange chromatography and characterised by compositional and linkage analysis. The AG fractions contained between 6 and 8% glucuronic acid, and gave a positive test for the beta-glucosyl-Yariv reagent, a stain specific for arabinogalactan-proteins. The protein component accounted for between 0.5 and 2.0% of the AGPs and contained between 7 and 12% hydroxyproline. The AG moieties displayed considerable heterogeneity with regard to their degree of arabinosylation and the extent and composition of their side-chains. They possessed a MW average of 650 kDa which ranged between 150 and 2000 kDa. An investigation of the structural features of the major AG fraction, released following enzymatic hydrolysis of the mannan-cellulose polymers, allowed a partial structure of coffee arabinogalactan to be proposed.