a-D-galactosidase activity is required for hydrolysis of softwood mannans, specifically galac — tomannans and galactoglucomannans (71). The enzyme acts on the a — galactosyl side groups attached to the O-6 position of the backbone mannose units (65). Little work has been collected regarding this enzyme, though its importance in softwood pulping has been considered.

10.4.3 Acetyl xylan esterase

Acetyl groups occur on several hemicelluloses although the primary examples are xylan and galactoglucomannans. Cereal and hardwood xylans have much higher levels of acetylation than softwood xylans. Softwood acetylation occurs principally in the galactoglucomannans. The most likely reason for acetylation is to keep the hemicelluloses soluble and hydrated. Deacetylation ofxylan and glucogalactomannan results in significantly decreased solubility of the polymer. Acetyl groups also cause problems for microorganisms when they are released from the main chain, resulting in decreased pH. The release of acetate is inhibitory to many microbes and is a considerable problem in the conversion of biomass to fermentation products (72-75).

Acetyl groups are released from hemicelluloses both from high degree of polymerization (i. e., native) substrates and from acetylated oligomers resulting from depolymerization. Acetyl xylan esterases (AXEs) may exhibit a preference for one or the other form, or may act on both types (76). Synergy studies between AXE, xylanases, and other hemicellulase enzymes have repeatedly demonstrated that the most effect digestion occurs with the appro­priate ratios of all enzymes acting simultaneously (70, 76-78). Debranching in the absence of depolymerization results in insoluble long-chain hemicelluloses that can be more diffi­cult for the depolymerases to access (78). Depolymerization without debranching maybe limited by depolymerase access to the polymer main chain. Most AXEs are low molecular weight and may or may not have a carbohydrate-binding module.