Novel 3-O-α-D-Galactosyl-α-L-Arabinofuranosidase for the Assimilation of Gum Arabic Arabinogalactan Protein in Bifidobacterium longum subsp. longum

Gum arabic arabinogalactan (AG) protein (AGP) is a unique dietary fiber that is degraded and assimilated by only specific strains of Bifidobacterium longum subsp. longum. Here, we identified a novel 3-alpha-D-galactosyl-alpha-L-arabinofuranosidase (GAfase) from B. longum JCM7052 and classified it into glycoside hydrolase family 39 (GH39). GAfase released alpha-D-Galp-(1 -> 3)-L-Ara and beta-L-Arap-(1 -> 3)-L-Ara from gum arabic AGP and beta-L-Arap-(1 -> 3)-L-Ara from larch AGP, and the alpha-D-Galp-(1 -> 3)-L-Ara release activity was found to be 594-fold higher than that of beta-L-Arap-(1 -> 3)-L-Ara. The GAfase gene was part of a gene cluster that included genes encoding a GH36 alpha-galactosidase candidate and ABC transporters for the assimilation of the released alpha-D-Galp-(1!3)-L-Ara in B. longum. Notably, when alpha-D-Galp-(1 -> 3)-L-Ara was removed from gum arabic AGP, it was assimilated by both B. longum JCM7052 and the nonassimilative B. longum JCM1217, suggesting that the removal of alpha-D-Galp-(-> 3)-L-Ara from gum arabic AGP by GAfase permitted the cooperative action with type II AG degradative enzymes in B. longum. The present study provides new insight into the mechanism of gum arabic AGP degradation in B. longum. IMPORTANCE Bifidobacteria harbor numerous carbohydrate-active enzymes that degrade several dietary fibers in the gastrointestinal tract. B. longum JCM7052 is known to exhibit the ability to assimilate gum arabic AGP, but the key enzyme involved in the degradation of gum arabic AGP remains unidentified. Here, we cloned and characterized a GH39 3-O-alpha-D-galactosyl-a-L-arabinofuranosidase (GAfase) from B. longum JCM7052. The enzyme was responsible for the release of alpha-D-Galp-(1!3)-L-Ara and beta-L-Arap-(1 -> 3)-LAra from gum arabic AGP. The presence of a gene cluster including the GAfase gene is specifically observed in gum arabic AGP assimilative strains. However, GAfase carrier strains may affect GAfase noncarrier strains that express other type II AG degradative enzymes. These findings provide insights into the bifidogenic effect of gum arabic AGP.