Abstractβ-L-Arabinofuranosides are hydroxyproline (Hyp)-linked sugar chains of extensin observed in plant cell wall fractions. Despite the broad distribution of β-Larabinofuranosyl residues in plants, degradative enzymes have not yet been identified. In 2011, we cloned and characterized the first degradative enzymes for Hyp-linked β-L-arabinofuranosides from Bifidobacterium longum. These enzymes were composed of a glycoside hydrolase (GH) family 43 α-L-arabinofuranosidase (HypAA) releasing L-arabinose from Arafα1-3Arafβ1-2Arafβ1-2Arafβ-Hyp, a GH121 β-L-arabinobiosidase (HypBA2) releasing Arafβ1-2Araf (β-Ara 2 ) from Arafβ1-2Arafβ1-2Arafβ-Hyp, and a GH127 β-L-arabinofuranosidase (HypBA1) degrading β-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 β-L-arabinofuranosides were utilized as carbohydrate sources by B. longum. This review presents the functional features of enzymes for Hyp-linked β-L-arabinofuranosides and the predicted metabolic pathway in B. longum.
A. IntroductionL-Arabinose residues are the main constituents of sugars in plant cell walls, and they are mostly present in the form of α-L-furanose in polysaccharides such as arabinan, arabinoxylan, and pectic arabinogalactan. Glycoside hydrolases involved in the degradation of α-Larabinofuranosides are present in 6 glycoside hydrolase (GH) families (GH3, GH43, GH51, GH54, GH62, and GH93) in the CAZy database. In addition, degradative enzymes for α-and β-L-arabinopyranoside linkages were also found in the GH42 and GH27 families, respectively (1,2). On the other hand, β-L-arabinofuranoside linkages constitute sugar chains of hydroxyproline-rich glycoproteins (HRGPs), which are known