Background:
The well studied cellulase mixture secreted by Trichoderma reesei (anamorph to Hypocrea jecorina) contains two cellobiohydolases (CBH, TrCel7A and TrCel6a) that are core enzymes for the solubilisation of cellulose. This has attracted significant research interest due the role of the CBHs in the conversion of biomass to fermentable sugars. However, the CHBs are notoriously slow and susceptible to inhibition, and this challenges the commercial utilization of biomass. One cause of reduced activity that has been suggested repeatedly is the xylans and xylan fragments that are also present in the biomass. Yet, the extent and mechanisms of this inhibition remains poorly elucidated. Therefore, we have studied xylan oligoscaccharides (XOS) of variable lengths with respect to their binding and inhibition of both TrCel7A and an enzyme variant without the cellulose binding domain (CBM).
Results:
The binding of xylan oligosaccharides (XOS) to TrCel7A is studied by isothermal titration calorimetry. It is shown that XOS bind to TrCel7A and that the affinity increases commensurate with the XOS length. The cellulose binding domain, on the other hand, does not affect the affinity significantly; this suggests that XOS may bind to the active site. Activity assays of TrCel7a clearly demonstrate the negative effect of the presence of XOS on the turnover number.
Conclusions:
On the basis of these binding data and a comparison of XOS inhibition of the activity of the two enzyme variants towards, respectively, soluble and insoluble substrates, we propose a competitive mechanism for XOS inhibition of TrCel7A with phosphoric swollen cellulose as a substrate.Source:
http://www.biotechnologyforbiofuels.com/rss/