*5.1. Inhibition by* d*-xylose*

Many β-xylosidases are inhibited to varying degrees by their main product d-xylose (Table 2). For example, the β-xylosidase from the fungus *Trichoderma harzianum* C is very sensitive to d-xylose inhibition; its activity is completely inhibited by the presence of only 2 mM d-xylose [83]. In contrast, the GH39 β-xylosidase from the extreme thermophilic bacterium *Dictyoglomus thermophilum* DSM 3960 is very resistant to d-xylose, with only 40% inhibition in the presence of 3 M of the sugar [84].


**Table 2.** Examples of microbial β-xylosidase inhibition by d-xylose.


**Table 2.** *Cont*.

#: GH family is not assigned in the CAZy database; §: Protein symbol.

Most characterized β-xylosidases have significant affinity for d-xylose, with reported inhibition constants (*K*i) of less than 10 mM (Table 3). To our knowledge, the β-xylosidase from *Talaromyces emersonii* has the highest affinity for d-xylose with a *K*<sup>i</sup> value as low as 1.3 mM [112], suggesting that the enzyme is very sensitive to inhibition by this monosaccharide. Other β-xylosidases are much less prone to d-xylose inhibition, such as those from *B. pumilus* 12 [113], *G. thermoleovorans* IT-08 [114], and uncultured bacterium [115] with *K*i's of 26.2, 76, and 145 mM, respectively. So far, the β-xylosidase from the bacterium *Cellulomonas uda* [116] is the β-xylosidase with the highest reported *K*<sup>i</sup> value for d-xylose, i.e., 650 mM, with the caveat that this *K*<sup>i</sup> was determined using crude enzyme. Thus, in general, and as summarized in Table 3, β-xylosidases have relatively high affinity (low *K*i) for d-xylose and, therefore, they are susceptible to product inhibition by this sugar.

**Table 3.** Examples of inhibition constants for d-xylose of β-xylosidases.



**Table 3.** *Cont*.

#: GH family is not assigned in the CAZy database; §: Protein symbol.

Uncultured rumen bacterium GH30\_2 10.6 [157] Uncultured rumen bacterium GH43\_1 76.0 [157]

From Tables 2 and 3 it appears that no direct relationship exists between the inhibition of β-xylosidases by d-xylose and their organismal origin or GH family. Many β-xylosidases from different bacteria and fungi suffer from such product inhibition. Likewise, product inhibition is observed for β-xylosidases belonging to different GH families. This is reasonable because all β-xylosidases bind the same substrate (d-xylose oligomers), necessitating affinity for xylosyl residues and some commonality in their active site structure (see above).

Interestingly, the activity of several β-xylosidases is stimulated by d-xylose, particularly at low concentration. The β-xylosidase from *Thermotoga thermarum* DSM 5069 was stimulated by d-xylose concentrations of up to 500 mM; its maximum activity was observed in the presence of 200 mM d-xylose, which was ~20% higher than in the absence of the sugar [96]. A similar stimulatory effect has been reported for the bifunctional β-xylosidase/α-l-arabinofuranosidase from *Phanerochaete chrysosporium* BKM-F-1767 [43] and the β-xylosidase from *Dictyoglomus thermophilum* DSM 3960 [84,158]. However, the mechanism of such stimulation is currently not known.
