Cartoon representing the inhibition mechanism of xylan oligosaccharide with different substrates. (A) Xylan oligosaccharide (XOS) inhibition mechanism with para-nitrophenyl-lacoside (pNP-lac) used as the substrate. (B) XOS inhibition mechanism with PASC used as the substrate. In both cases, the activity of cellobiohydrolase I (TrCel7A) was decreased by the presence of XOS. The active site was most likely occupied by unproductively bound XOS, preventing the substrate from entering the active site tunnel. The degree of inhibition is dependent on the ratio of the binding affinities between the substrate and the inhibitor.