Action of three GH51 and one GH54 α-arabinofuranosidases on internally and terminally located arabinofuranosyl branches.

The action of three glycoside hydrolase family GH51 and one GH54 α-arabinofuranosidases (ABFs) was studied on polymeric arabinoxylan and arabinoxylooligosaccharides. The substrates covered all possible arabinofuranosyl (Araf) substituents, i.e., terminal and internal α(1→2) and α(1→3)-linked Araf monosubstitutions and disubstitutions. The GH51 ABFs removed nearly all mono- and disubstitutions from terminal non-reducing end xylopyranosyl (Xylp) residues, showing dual ABF-m/d activity. From internal Xylp, primarily monosubstitutions were removed, except after treatment with GH51 Aspergillus niger ABF. It degraded slowly internal disubstitutions as well, showing versatility in substrate specificity within GH51. GH54 Trichoderma reesei ABF core protein also presented dual ABF-m/d activity, slowly degrading Araf disubstitutions from both terminal and internal positions. Surprisingly, regioselectivity of the T. reesei ABF changed from α(1→3)-linked Araf on terminal Xylp to α(1→2)-linked Araf on internal Xylp on both mono- and disubstitutions. In conclusion, systematic analysis of natural substrates revealed interesting new details on the action T. reesei ABF and showed that the dual ABF-m/d activity could be more prevalent than previously thought, especially within GH51 ABFs.