1998

Luonteri, Elina

Various natural polymers, including hemicelluloses and pectic polysaccharides, contain arabinose and galactose residues either as main or side chain components. The properties and functions of three Aspergillus terreus alfa-L-arabinofuranosidases and three alfa-galactosidases of Penicillium simplicissimum were studied in this work. In addition, the effects of their action on different fibre matrices were evaluated. The three A. terreus alfa-L-arabinofuranosidases (alfaAra), A, B1 and B2, had very similar molecular and functional properties and almost identical N-terminal amino acid sequences. All three enzymes liberated arabinose as the only product from various arabinans, arabinogalactans and arabino(glucurono)xylans. Pectic polymers (branched arabinans and arabino-b-1,4-D-galactans) appeared to be the best substrates for them. alfaAras were also capable of releasing arabinose from softwood kraft pulp and from different wheat and rye flour fractions. Simultaneous application with xylanase had only minor effect on the activity. alfaAras were only poorly capable of removing arabinose residues attached to the internal xylose moieties of short arabinoxylo-oligosaccharides of DP 3-5, and they were unable to hydrolyse arabinose linked to C-2 of non-reducing terminal xylose residues. Feruloyl substituents limited the hydrolysis of arabinoxylan and arabinan oligosaccharides, but only if the feruloyl group was esterified to the terminal non-reducing arabinose. Surprisingly, a model lignin-carbohydrate compound, in which an arabinose residue linked to C-3 of methyl-xylopyranoside was substituted with a bulky dilignol group at O-5, was degraded by alfaAra B2. P. simplicissimum produces at least four alfa-galactosidases (AGL), of which three were characterized in this study. alfa-Galactosidases AGLI and AGLIII had rather similar molecular properties and N-terminal amino acid sequences, whereas AGLII differed from the other two enzymes in both respects. Both AGLI and AGLIII were inactiviated at rather low substrate (p-nitrophenyl-alfa-D-galactopyranoside) and galactose concentrations, whereas AGLII was highly resistant to both substrate and end product inhibition. The gene encoding the alfa-galactosidase AGLI was cloned and sequenced. On the basis of the similarities of the complete amino acid sequence with those of some other glycosyl hydrolases, AGLI was classified into the glycosyl hydrolase family 27. AGLIII had a similar N-terminal amino acid sequence to that of AGLI and other members of the family 27, but that of AGLII had no any similarity with any of the published sequences of glycosyl hydrolases. AGLI efficiently removed galactose side groups from polymeric galacto(gluco)mannans but it was also active for various oligomeric substrates. AGLII was highly specific towards the liberation of galactose residues attached to the non-reducing ends of various galactose-containing oligomers. AGLIII had inferior activity towards the substrates tested and its action was clearly augmented by the depolymerizing enzymes. The purified enzymes were also able to act on different fibre matrices. alfaAras released 20-25% of arabinose residues, and AGLI about 10% and 22% of the galactose residues alone and in combination with mannanase, respectively, from softwood kraft pulp. Surprisingly, the alfaAra treatment decreased the extractability of lignin and the pulp bleachability. In addition, the beneficial effects obtained with xylanase were diminished when the two enzymes were applied simultaneously. The overall effects of alfaAra treatment on the other pulp properties were of minor importance. In wheat bread baking, the major effect obtained by alfaAra addition was the more homogeneous pore structure and distribution without loss of the volume increase caused by xylanase.