Enzymic degradation of sorghum glucuronoarabinoxylans leading to tentative structures

Three glucuronoarabinoxylan (GAX) populations, obtained from water-unextractable cell wall material from sorghum by different alkali extractants, were digested by combinations of endo-xylanases (Xyl I, Xyl III and GXH), arabinofuranosidases (AXH and AraB) and an alpha-glucuronidase (GlcAase). All three GAX populations were shown to be rather poorly degradable, due to the very high degree of substitution, as well as the substitution pattern. The barium hydroxide-extracted GAX showed a maximum degree of degradation of almost 12%, using Xyl I combined with GXH and AXH. The GAX population extracted by 4 M KOH was hardly degraded by any of the tested combinations. In all cases, Xyl III showed lowest activity upon the three extracts. Synergistic effects were observed between Xyl I and AXH. Both neutral and acidic arabinoxylan oligomers were formed. The GlcAase acted only upon oligomeric material released by Xyl I. No synergistic effects were observed between the GXH and AXH. Combining the patterns of degradation with the modes of action of the enzymes, structures were proposed for the different populations of sorghum GAX. Evidence was obtained that the xylan backbone of especially the GAX extracted by 4 M KOH, is substituted by arabinose and glucuronic acid according to a strict pattern, which hinders the enzymes to act.