Functional Analysis of Degradative Enzymes for Hydroxyproline-linked β-L-Arabinofuranosides in Bifidobacterium longum

beta-L-Arabinofuranosides are hydroxyproline (Hyp)-linked sugar chains of extensin observed in plant cell wall fractions. Despite the broad distribution of beta-L-arabinofuranosyl residues in plants, degradative enzymes have not yet been identified. In 2011, we cloned and characterized the first degradative enzymes for Hyp-linked beta-L-arabinofuranosides from Bifidobacterium longum. These enzymes were composed of a glycoside hydrolase (GH) family 43 alpha-L-arabinofuranosidase (HypAA) releasing L-arabinose from Araf alpha 1-3Araf beta 1-2Araf beta 1-2Araf beta-Hyp, a GH121 beta-L-arabinobiosidase (HypBA2) releasing Araf beta 1-2Araf (beta-Ara(2)) from Araf beta 1-2Araf beta 1-2Araf beta-Hyp, and a GH127 beta-L-arabinofuranosidase (HypBA1) degrading beta-Ara(2) to L-arabinose. These enzymes are encoded in a conserved gene cluster on several B. longum genomes, but not in genome sequences of other intestinal bacteria. Hyp-linked beta-L-arabinofuranosides were utilized as carbohydrate sources by B. longum. This review presents the functional features of enzymes for Hyp-linked beta-L-arabinofuranosides and the predicted metabolic pathway in B. longum.