Purification and characterization of Aspergillus β-d-galactanases acting on β-1,4- and β-1,3/6-linked arabinogalactans

Arabinogalactan and arabinan fractions were isolated from kraft pulping black liquor. Both type I and type II arabinogalactans consisting of 1,4- and 1,3-linked β-d-galactose backbones, respectively, were found. Samples contained more arabino-1,3/6-galactan than arabino-1,4-galactan. Arabinan was mainly 1,5-linked slightly branched polysaccharide. Two enzymes acting on galactans, an endo-β-1,4-d-galactanase and a β-1,6-d-galactanase, were isolated from commercial pectinase preparations produced by Aspergillus aculeatus and A. niger, respectively. The purified enzymes showed molecular masses of 38 and 58 kDa, respectively. Based on its N-terminal amino acid sequence the endo-β-1,4-d-galactanase was the same as the previously studied GAL1 from A. aculeatus. It acted on β-1,4-linked galactan, producing a range of galacto-oligosaccharides. It was also able to liberate galactose from a lignin–carbohydrate complex isolated from softwood kraft pulp. No activity was detected towards β-1,3-liked galactan. The β-1,6-d-galactanase was active on arabino-1,3/6-galactan, liberating galactose and 1,6-β-d-galactobiose. It was found to be active only on β-1,6-linkages and no detectable hydrolysis of β-1,3-galactose linkages occurred. It also showed no activity on 1,4-β-d-galactan. However, β-1,6-d-galactanase was able to liberate arabinose from arabinan. Although chemical pulps contain only a minute quantity of galactans, both galactanases have recently been shown to enhance the bleachability of spruce kraft pulp.