**6. Concluding Remarks**

β-Xylosidases are highly diverse in their amino acid sequence. Currently, enzymes with β-xylosidase activity can be found in 11 different glycoside hydrolase families in the CAZy database, i.e., in GH families 1, 3, 5, 30, 39, 43, 51, 52, 54, 116, and 120. They fold into several distinct three-dimensional structures. While the enzymes from GH families 1, 3, 5, 30, 39, and 51 all show a (β/α)8 TIM-barrel structure, those from families 43, 52 and 116, 54, and 120 have as their main structural feature a 5-bladed β-propeller, a (α/α)6-barrel, a β-sandwich, and a right-handed parallel β-helix, respectively. Likewise, the catalytic mechanism of β-xylosidases is also varied. Although generally β-xylosidases hydrolyze their substrates with retention of the substrate's anomeric carbon configuration, the enzymes from GH43 invert the anomeric configuration. However, despite their diversity in overall folds, all structurally characterized β-xylosidases have a typical pocket-shaped active site with two carbohydrate-binding subsites, which bind a xylobiosyl moiety of the xylooligosaccharide substrates. Unfortunately, the active sites of many β-xylosidases also possess a relatively high affinity for monosaccharides, such as xylose, arabinose, and erythrose, that competitively inhibit the enzymes' activity. Moreover, some β-xylosidases are also non-competitively or un-competitively inhibited by monosaccharides. Such product inhibition limits the application of β-xylosidases in xylan saccharification, since high monosaccharide concentrations may easily be generated during the process. Therefore, β-xylosidases with low monosaccharide affinity are highly desirable for various applications. A random mutagenesis approach has already shown success in reducing the affinity of a β-xylosidase for d-xylose. Furthermore, with 3D structures available for a variety of β-xylosidases, rational site-directed mutagenesis may also be a good approach to render the enzymes less prone to product inhibition.

**Author Contributions:** Conceptualization, A.R., B.W.D., and N.N.T.P.; software, A.R.; validation, A.R., B.W.D., and N.N.T.P.; data curation, A.R.; writing—original draft preparation, A.R.; writing—review and editing, B.W.D., and N.N.T.P.; visualization, A.R.; supervision, B.W.D., and N.N.T.P.; funding acquisition, A.R. and N.N.T.P.

**Funding:** This work was funded by Universitas Airlangga to A.R. and N.N.T.P. (Hibah Riset Mandat No. 624/UN3.14/LT/2017). N.N.T.P is a receiver of a Tahir Professorship of Universitas Airlangga (SK Rektor Unair No. 1149/UN3/2018).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
