ENZYMATIC DEGRADATION OF SORGHUM GLUCURONOARABINOXYLANS LEADING TO TENTATIVE STRUCTURES

Three glucuronoarabinoxylan (GAX) populations, obtained from water-une xtractable cell wall material from sorghum by different alkali extract ants, were digested by combinations of endo-xylanases (Xyl I, Xyl III and GXH), arabinofuranosidases (AXH and AraB) and an ru-glucuronidase (GlcAase). All three GAX populations were shown to be rather poorly de gradable, due to the very high degree of substitution, as well as the substitution pattern. The barium hydroxide-extracted GAX showed a maxi mum 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 b etween Xyl I and AXH. Both neutral and acidic arabinoxylan oligomers w ere formed. The GlcAase acted only upon oligomeric material released b y 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 so rghum GAX. Evidence was obtained that the xylan backbone of especially the GAX extracted by 4 M KOH, is substituted by arabinose and glucuro nic acid according to a strict pattern, which hinders the enzymes to a ct. (C) 1998 Elsevier Science Ltd. All rights reserved.