Detailed Mode of Action of Arabinan-Debranching α-L-Arabinofuranosidase GH51 from Bacillus velezensis

The gene encoding an alpha-L-arabinofuranosidase (BvAF) GH51 from Bacillus velezensis FZB42 was cloned and expressed in Escherichia coli. The corresponding open reading frame consists of 1,491 nucleotides which encode 496 amino acids with the molecular mass of 56.9 kDa. BvAF showed the highest activity against sugar beet (branched) arabinan in 50 mM sodium acetate buffer (pH 6.0) at 45 degrees C. However, it could hardly hydrolyze debranched arabinan and arabinoxylans. The time-course hydrolyses of branched arabinan and arabinooligosaccharides (AOS) revealed that BvAF is a unique exo-hydrolase producing exclusively L-arabinose. BvAF could cleave alpha-(1,2)- and/or alpha-(1,3)-L-arabinofuranosidic linkages of the branched substrates to produce the debranched forms of arabinan and AOS. Although the excessive amount of BvAF could liberate L-arabinose from linear AOS, it was extremely lower than that on branched AOS. In conclusion, BvAF is the arabinan-specific exo-acting alpha-L-arabinofuranosidase possessing high debranching activity towards alpha-(1,2)- and/or alpha-(1,3)-linked branches of arabinan, which can facilitate the successive degradation of arabinan by endo-alpha-(1,5)-L-arabinanase.